The establishment and maintenance of cell and tissue polarity is crucial for a range of biological processes, such as oriented division, migration, adhesion and barrier function. The molecular pathways that regulate cell and tissue polarity have been extensively studied in lower organisms as well as in mammalian cell culture. By contrast, relatively little is still known about how polarization regulates the in vivo formation and homeostasis of mammalian tissues. Several recent papers have identified crucial roles for mammalian polarity proteins in a range of in vivo processes, including stem cell behavior, cell fate determination, junction formation and maintenance and organ development. Using the epidermis of the skin as a model system, this Commentary aims to discuss the in vivo significance of cell and tissue polarity in the regulation of mammalian tissue morphogenesis, homeostasis and disease. Specifically, we discuss the mechanisms by which the molecular players previously identified to determine polarity in vitro and/or in lower organisms regulate epidermal stratification; orient cell division to drive cell fate determination within the epidermal lineage; and orient hair follicles. We also describe how altered polarity signaling contributes to skin cancer.

Polarity is defined as the unequal distribution of molecules (RNAs, lipids, proteins) within a cell to produce asymmetry in its structure and function at the cellular, tissue and organismal level. Such asymmetry is a basic feature of almost all cells, and is important for a wide range of processes, including cell fate determination, adhesion, migration, differentiation and stem cell maintenance (reviewed by Nelson, 2003; St Johnston and Ahringer, 2010). Polarity comes in two main forms: cell polarity and tissue polarity. In cell polarity, asymmetry is achieved in individual cells, such as for instance the leading and trailing edge of a migrating cell or apicobasal polarity (Fig. 1A). In tissue polarity, also known as planar cell polarity (PCP), subcellular structures and/or cells are aligned in the plane of the tissue (reviewed by Bayly and Axelrod, 2011; Klein and Mlodzik, 2005), as seen, for example, in the positioning of actin-based hair in Drosophila wing cells (Fig. 2A). Although it was initially mainly studied in lower organisms or in epithelial cell culture systems, it is now clear that polarization is a fundamental requirement for the proper functioning of cells and tissues and that inappropriate function of polarity pathways disturbs tissue homeostasis, leading to a variety of human diseases, such as kidney disease, hearing impairment and cancer (Bulgakova and Knust, 2009; Huang and Muthuswamy, 2010; Lee and Vasioukhin, 2008; McNeill, 2010).

Fig. 1.

Functional similarities and differences of polarity in simple epithelia and stratifying epithelia. At present, it is unclear whether the same molecular mechanisms controlling apico-basolateral polarity in simple epithelia are involved in the establishment of epidermal polarity. (A) apico-basolateral polarity in simple epithelia. The apical junctional complex, consisting of tight junctions (TJs), adherens junctions (AJs) and desmosomes (DSs) forms a border to establish apico-basolateral polarity. Mutual interactions between polarity proteins and complexes regulate apico-basolateral polarity and barrier formation in simple epithelia. (B) Polarity in the mouse epidermis. In contrast to simple epithelia, the epidermis has no distinct apical and basolateral membrane domains, but displays apico-basolateral polarity over the multilayered tissue with the stratum granulosum forming the viable apical border. This is reflected in differential expression or localization of polarity proteins, for example PAR3, PAR6, LGL1, Crumbs and aPKC (black lines indicate localization patterns in the different layers of the skin), and adhesive junctions. Moreover, lamellar bodies (blue circles) and keratohyalin granules (purple asterisks) are targeted (indicated by arrows) towards the upper layers to form the cornified envelope. Par, partitioning defective; Dlg, discs large; Lgl, lethal giant larvae; aPKC, atypical protein kinase C; Patj, protein-associated with tight junction; Pals1, proteins associated with Lin Seven-1.

Fig. 1.

Functional similarities and differences of polarity in simple epithelia and stratifying epithelia. At present, it is unclear whether the same molecular mechanisms controlling apico-basolateral polarity in simple epithelia are involved in the establishment of epidermal polarity. (A) apico-basolateral polarity in simple epithelia. The apical junctional complex, consisting of tight junctions (TJs), adherens junctions (AJs) and desmosomes (DSs) forms a border to establish apico-basolateral polarity. Mutual interactions between polarity proteins and complexes regulate apico-basolateral polarity and barrier formation in simple epithelia. (B) Polarity in the mouse epidermis. In contrast to simple epithelia, the epidermis has no distinct apical and basolateral membrane domains, but displays apico-basolateral polarity over the multilayered tissue with the stratum granulosum forming the viable apical border. This is reflected in differential expression or localization of polarity proteins, for example PAR3, PAR6, LGL1, Crumbs and aPKC (black lines indicate localization patterns in the different layers of the skin), and adhesive junctions. Moreover, lamellar bodies (blue circles) and keratohyalin granules (purple asterisks) are targeted (indicated by arrows) towards the upper layers to form the cornified envelope. Par, partitioning defective; Dlg, discs large; Lgl, lethal giant larvae; aPKC, atypical protein kinase C; Patj, protein-associated with tight junction; Pals1, proteins associated with Lin Seven-1.

Fig. 2.

Mechanisms and mediators of planar cell polarity in the Drosophila wing and mouse epidermis. (A) Single actin-based hairs in wing cells of Drosophila polarize to localize at the proximal side in all cells of the tissue (shown on the left). On the right, a schematical top view on Drosophila wing cells at pupal stages is shown illustrating the asymmetric distribution of core PCP proteins Pk–Vang–Fmi group (proximal) and the Dsh–Dgo–Fz–Fmi group (distal). (B) Polarized distribution of PCP proteins mediate orientation of developing mouse hair follicles. In the embryonic epidermal basal layer, E-cadherin is expressed at all cell–cell contact sites, whereas CELSR1, VANGL2 and FZ are exclusively localized at anterior and/or posterior lateral membranes already at placode stage, whereas the budding hair follicle does not reveal any obvious asymmetry. At hair germ stage, PCP signaling determines anterior constriction of cells resulting in asymmetry, characterized within the hair germ by anterior localized ZO-1 and P-cadherin and posterior enrichment of NCAM and E-cadherin. (C) Orientation of hairs on dorsal skin and paws of control Fz6+/− and Fz6−/− (Frizzled-6 knockout) mice. Loss of FZ6 interferes with normal hair patterning and leads to random waves, swirls and whorls in back skin (top panel) and paws of mutant mice (bottom panel). Image kindly provided by Jeremy Nathans. (D) Orientation of hair follicles in the skin. In dorsal skin of newborn mice, hair grows uniformly in a posterior direction and hair follicle angles encompass a range of roughly ± 45° relative to the anterior–posterior (A-P) axis (left). In mice with mutations in Vangl2 or Celsr1, this planar orientation of hair follicles is lost (right). Pk, Prickle; Vang, Van Gogh; Fmi, Flamingo; Dsh, Dishevelled; Dgo, Diego; Fz, Frizzled; Celsr1, cadherin EGF-like, LAG-like, seven pass receptor (murine homolog of Drosophila flamingo); VANGL2, murine homolog of Van Gogh; ZO-1, zona occludens-1; NCAM, Neural cell adhesion molecule.

Fig. 2.

Mechanisms and mediators of planar cell polarity in the Drosophila wing and mouse epidermis. (A) Single actin-based hairs in wing cells of Drosophila polarize to localize at the proximal side in all cells of the tissue (shown on the left). On the right, a schematical top view on Drosophila wing cells at pupal stages is shown illustrating the asymmetric distribution of core PCP proteins Pk–Vang–Fmi group (proximal) and the Dsh–Dgo–Fz–Fmi group (distal). (B) Polarized distribution of PCP proteins mediate orientation of developing mouse hair follicles. In the embryonic epidermal basal layer, E-cadherin is expressed at all cell–cell contact sites, whereas CELSR1, VANGL2 and FZ are exclusively localized at anterior and/or posterior lateral membranes already at placode stage, whereas the budding hair follicle does not reveal any obvious asymmetry. At hair germ stage, PCP signaling determines anterior constriction of cells resulting in asymmetry, characterized within the hair germ by anterior localized ZO-1 and P-cadherin and posterior enrichment of NCAM and E-cadherin. (C) Orientation of hairs on dorsal skin and paws of control Fz6+/− and Fz6−/− (Frizzled-6 knockout) mice. Loss of FZ6 interferes with normal hair patterning and leads to random waves, swirls and whorls in back skin (top panel) and paws of mutant mice (bottom panel). Image kindly provided by Jeremy Nathans. (D) Orientation of hair follicles in the skin. In dorsal skin of newborn mice, hair grows uniformly in a posterior direction and hair follicle angles encompass a range of roughly ± 45° relative to the anterior–posterior (A-P) axis (left). In mice with mutations in Vangl2 or Celsr1, this planar orientation of hair follicles is lost (right). Pk, Prickle; Vang, Van Gogh; Fmi, Flamingo; Dsh, Dishevelled; Dgo, Diego; Fz, Frizzled; Celsr1, cadherin EGF-like, LAG-like, seven pass receptor (murine homolog of Drosophila flamingo); VANGL2, murine homolog of Van Gogh; ZO-1, zona occludens-1; NCAM, Neural cell adhesion molecule.

In this Commentary, we will briefly introduce the major molecular mediators that are important to establish cell and tissue polarity and then discuss how different types of polarity are established in a stratifying epithelium, the epidermis of the skin, and which molecular players have been implicated in the regulation of these different processes.

Polarity is established and maintained by the activity of a core set of polarity proteins, most of which were first identified in Drosophila melanogaster or Caenorhabditis elegans, and are highly conserved throughout metazoa. The basic molecular players that determine either cell or tissue polarity are not identical. Cell polarity is regulated by a core set of proteins and protein complexes (Table 1). The Par (partitioning-defective) proteins were the first cell polarity proteins to be identified based on their role in asymmetric cell division of the C. elegans zygote (Kemphues et al., 1988). The PAR genes encode either adaptor proteins, such as the PDZ-domain-containing proteins Par3 and Par6 or the 14-3-3 protein Par5, or serine and threonine kinases such as Par1 and Par4. Par3 and Par6 form a dynamic complex with atypical protein kinase C (aPKC) (Suzuki and Ohno, 2006; McCaffrey and Macara, 2009). Two additional cell polarity complexes regulate a range of asymmetric cell processes: the transmembrane protein Crumbs forms a complex with the PDZ adaptor proteins Patj (protein-associated with tight junction) and Lin7, as well as the membrane-associated guanylate kinase (MAGUK) scaffold Pals1 (proteins associated with Lin7) (Assémat et al., 2008; Bulgakova and Knust, 2009). The other complex consists of the leucine-rich repeat and PDZ (LAP) protein Scribble, the WD40 repeat-containing protein Lgl (lethal giant larvae) and the MAGUK protein Dlg (discs large) (Bilder, 2004; Humbert et al., 2008).

Table 1.

Cell polarity proteinsa

Original symbol Mammalian symbol Function 
Par1b MARK2 Ser/Thr kinase 
 MARK1, MARK3, MARK4, MARK5?  
Par3b PAR3A (ASIP) PDZ-containing scaffold 
 PAR3B  
Par4b LKB1 Ser/Thr kinase 
Par5b 14-3-3 family (7 members, β, γ, ε, ζ, τ, σ, η) PhosphoSer/Thr binding scaffold 
Par6b PAR6A  
 PAR6B PDZ-containing scaffold 
 PAR6C  
aPKC (atypical protein kinase C)d aPKCλ (m), aPKCι (h) Ser/Thr kinase 
 aPKCζ  
Lgl (Lethal giant larvae)c LGL1 (Hugl1) WD40-repeat-containing scaffold 
 LGL2  
Scribblec Scribble (Vartul, Crib1, LAP4) LAP protein or scaffold 
Dlg (Discs Large)c DLG1 (Sap97), DLG2 (PSD-93), DLG3 (SAP-102), DLG4 (PSD95, SAP90), DLG5 MAGUK protein or scaffold 
Crb (Crumbs)c CRB1 (RP12, LCA8), CRB2, CRB3 TM protein or scaffold 
Std (Stardust)c PALS1 (MMP5), PALS2 (MMP6, VAM-1, p55T) MMP1–MMP3, MMP7 MAGUK protein or scaffold 
Patj (Drosophila Pals1-associated tight junction protein)c PATJ (INADL), MUP1 PDZ-containing scaffold 
Lin7b LIN7A (Velis1, MALS-1), LIN7B (Velis2, MALS-2), LIN7C (Velis3, MALS-3) PDZ-containing scaffold 
Original symbol Mammalian symbol Function 
Par1b MARK2 Ser/Thr kinase 
 MARK1, MARK3, MARK4, MARK5?  
Par3b PAR3A (ASIP) PDZ-containing scaffold 
 PAR3B  
Par4b LKB1 Ser/Thr kinase 
Par5b 14-3-3 family (7 members, β, γ, ε, ζ, τ, σ, η) PhosphoSer/Thr binding scaffold 
Par6b PAR6A  
 PAR6B PDZ-containing scaffold 
 PAR6C  
aPKC (atypical protein kinase C)d aPKCλ (m), aPKCι (h) Ser/Thr kinase 
 aPKCζ  
Lgl (Lethal giant larvae)c LGL1 (Hugl1) WD40-repeat-containing scaffold 
 LGL2  
Scribblec Scribble (Vartul, Crib1, LAP4) LAP protein or scaffold 
Dlg (Discs Large)c DLG1 (Sap97), DLG2 (PSD-93), DLG3 (SAP-102), DLG4 (PSD95, SAP90), DLG5 MAGUK protein or scaffold 
Crb (Crumbs)c CRB1 (RP12, LCA8), CRB2, CRB3 TM protein or scaffold 
Std (Stardust)c PALS1 (MMP5), PALS2 (MMP6, VAM-1, p55T) MMP1–MMP3, MMP7 MAGUK protein or scaffold 
Patj (Drosophila Pals1-associated tight junction protein)c PATJ (INADL), MUP1 PDZ-containing scaffold 
Lin7b LIN7A (Velis1, MALS-1), LIN7B (Velis2, MALS-2), LIN7C (Velis3, MALS-3) PDZ-containing scaffold 
a

Original symbols of polarity proteins, the organism in which they were first identified, symbols of the mammalian orthologs and known structural function.

b

Identified in C. elegans.

c

Identified in Drosophila.

d

Identified in mammals.

N.D., not determined; TM, transmembrane protein; h, human; m, mouse.

To establish and maintain cell polarity, these cell polarity proteins function in an integrated network that controls their subcellular localization through multiple positive- and negative-feedback loops that are tightly regulated in a spatiotemporal manner (Fig. 1A). This network interacts with small GTPases, adhesive junctions, the cytoskeleton and vesicle transport to regulate the establishment and maintenance of cell polarity (Iden and Collard, 2008; Nelson, 2003). Although the basic mechanisms are similar, each type of cell polarity requires the engagement of different cell polarity proteins.

Tissue polarity is regulated by planar polarity proteins, which engage in signal transduction to coordinate changes in cell shape and polarity across cells in the plane of the tissue (McNeill, 2010; Simons and Mlodzik, 2008). Currently, two main core PCP pathways have been described, the Frizzled (Fz)–Dishevelled (Dsh)–Van Gogh (Vang) pathway and the Fat–Dachsous (Ds)–Four-joint (Fj) pathway, and their functions have been best characterized in Drosophila (McNeill, 2010; Simons and Mlodzik, 2008). Fz–Dsh–Vang proteins form asymmetric domains within cells in the plane of the tissue (Fig. 2A), whereas the Fat–Ds–Fj pathway establishes gradients over a range of cells (Table 2). Because tissue polarity requires the polarization of individual cells, the different polarity signal pathways are integrated to coordinate cell with tissue polarity (Simons and Mlodzik, 2008). For example, the cell polarity protein Scribble genetically interacts with the PCP protein vang-like 2 (VANGL2) in mice (Montcouquiol et al., 2003), and loss of Scribble results in classical PCP phenotypes, such as neural tube closure defects and impaired embryonic wound healing (Caddy et al., 2010; Dow et al., 2007; Murdoch et al., 2003). For an in-depth discussion of the function and interactions of core polarity proteins, we refer the reader to several excellent reviews that cover different aspects of polarity (Assémat et al., 2008; Bilder, 2004; Bulgakova and Knust, 2009; Goldstein and Macara, 2007; Humbert et al., 2008; Iden and Collard, 2008; Knoblich, 2008; Li and Bowerman, 2010; Martin-Belmonte and Mostov, 2008; McNeill, 2010; Nelson, 2003; Shin et al., 2006; Simons and Mlodzik, 2008; St Johnston and Ahringer, 2010).

Table 2.

Planar cell polarity proteins

Original symbol Mammalian symbol Function 
Fz (Frizzled)a FZ3 7-TM receptor 
FZ6 
Dsh (Dishevelled)a DVL1, DVL2, DVL3  
 PDZ, DIX and DEP domain scaffold 
  
Fmi (Flamingo), Starry knighta CELSR1 7-TM protein 
Vang (Van Gogh), Strabismusa VANGL1 4-TM protein 
VANGL2 (looptail mutant) 
Dg (Diego)a Diversin (ANKRD6) Ankyrin-repeat containing scaffold 
Inversin (INVS) 
Ft (Fat)a FAT4 Cadherin or TM protein 
Ds (Dachsous)a ND Cadherin or TM protein 
Fj (Four-jointed) ND TM serine/threonine kinase 
Pk (Prickle)a PRICKLE1, PRICKLE2 LI- and PET-domain-containing protein 
Original symbol Mammalian symbol Function 
Fz (Frizzled)a FZ3 7-TM receptor 
FZ6 
Dsh (Dishevelled)a DVL1, DVL2, DVL3  
 PDZ, DIX and DEP domain scaffold 
  
Fmi (Flamingo), Starry knighta CELSR1 7-TM protein 
Vang (Van Gogh), Strabismusa VANGL1 4-TM protein 
VANGL2 (looptail mutant) 
Dg (Diego)a Diversin (ANKRD6) Ankyrin-repeat containing scaffold 
Inversin (INVS) 
Ft (Fat)a FAT4 Cadherin or TM protein 
Ds (Dachsous)a ND Cadherin or TM protein 
Fj (Four-jointed) ND TM serine/threonine kinase 
Pk (Prickle)a PRICKLE1, PRICKLE2 LI- and PET-domain-containing protein 
a

Identified in Drosophila.

ND, not determined; TM, transmembrane protein.

Whereas in lower organisms most of the core polarity proteins are unique, in mammals, several orthologs exist for nearly all core polarity proteins (Tables 1, 2) (for references, see reviews above). Redundancy or compensation might thus obscure the in vivo relevance of a particular polarity protein when mouse knockout models of these proteins are examined. Nevertheless, the complete inactivation of several polarity proteins has already revealed their conserved role in mammalian polarity-dependent processes, such as FAT4, whose inactivation disturbs oriented cell division and tubule elongation, resulting in cystic kidney disease (Saburi et al., 2008).

Mammalian skin protects the organism from dehydration and provides a barrier against harmful influences, such as UV, temperature and microbes. The epidermis forms the outermost layer and consists of the interfollicular epidermis (IFE) and epidermal appendages, such as the hair follicles, and sebaceous and sweat glands (Koster, 2009). Epidermal keratinocytes balance life-long self-renewal in the proliferating basal layer with a strictly spatiotemporally regulated terminal differentiation program that is necessary to form the stratum corneum, a dead, cornified and water-impermeable cell layer (Koster, 2009). Different populations of stem and progenitor cells that are located in the basal layer of the IFE and in specific areas of hair follicles guarantee constant self-renewal under steady state conditions and ensure sufficient plasticity for the fast replacement of lost tissue in case of injury (Blanpain and Fuchs, 2009; Watt and Jensen, 2009).

The epidermis is therefore an excellent cell biology model system to study a range of processes that require polarization, such as (1) formation and maintenance of a life-long self-renewing stratifying epithelium and its appendages; (2) the behavior of different populations of stem or progenitor cells that drive epidermal self-renewal and cyclic regeneration of the hair follicle; (3) coupling of cell fate decisions to oriented cell division; (4) the highly organized spatial distribution of hair follicles and sebaceous glands; and (5) wound healing as a model for migration and tissue regeneration. Another advantage of this model system is that primary keratinoyctes can be used to study differentiation, stratification and junction formation. However, under 2D cell culture conditions, these cells will not form a stratum corneum. Therefore, using epidermis and keratinocytes, several key questions concerning polarity can be investigated, such as how a life-long self-renewing tissue regulates the balance between asymmetric and symmetric divisions and how this is coupled to cell fate; how multicellular structures, such as hair follicles, align in the plane of the tissue; and how apico-basolateral polarity is established over multiple layers. In the following sections, we highlight and discuss recent research on the epidermis that provides novel insight into these questions.

Simple epithelial cells polarize to establish two different membrane domains – the apical and basolateral membranes – which have distinct structural and functional characteristics and are separated by the apical junctional complex (Niessen and Gottardi, 2008). This complex consists of tight junctions, which form an ion and size barrier and provide a fence that prevents mixing of apical and basolateral membrane components, adherens junctions and desmosomes (Fig. 1A). Apico-basolateral polarity is essential for the formation of a protective barrier, and for vectorial functions, such as the directed secretion of components or transport of fluids. The establishment of apico-basolateral polarity in simple mammalian epithelial cells is achieved by complex agonistic and antagonistic interactions of cell polarity protein complexes that allow for the spatial targeting of polarity complexes to different domains within the cell (Fig. 1A) (Nelson, 2009; Roh and Margolis, 2003; St Johnston and Ahringer, 2010).

The stratifying epidermis is not a classically polarized epithelium in which tight junctions separate basolateral and apical membrane proteins and lipids. Instead, the epidermis establishes polarity along the basal to apical axis of the tissue, with the stratum granulosum forming the viable apical boundary (Fig. 1B). The formation of the uppermost layer, the stratum corneum, depends on the fusion of lamellar bodies (LBs) and keratohyalin granules with the more apical plasma membranes at the transition between stratum granulosum and corneum layers. Interestingly, functional tight junctions are found in the stratum granulosum and, as in simple epithelia, these might serve as a fence that is necessary for restricting ‘apical’ targeting of protein and lipid vesicles at the transition between the granular and cornified layer.

The mechanisms that regulate the formation of stratifying apico-basolateral tissue polarity are largely unknown. If mechanisms similar to those in simple epithelia are in place, the mutual antagonistic actions of polarity complexes have to be established over several cell layers. A relatively simple system could consist of counter-gradients of mutually inhibiting complexes over the basal–apical axis of the epidermis (Fig. 1B). From C. elegans to humans, in simple epithelia, the formation and maintenance of intercellular junctions and apical membrane domain identity is tightly linked to the activity of cell polarity proteins, such as Par4 (liver kinase B1 in mammals, LKB1), the Par–aPKC complex and the Crumbs homolog 1 (Crb1) complex (Goldstein and Macara, 2007; Nelson, 2003; Bulgakova and Knust, 2009). Loss of either Crb1 or LKB1 does not obviously impair epidermal barrier function, suggesting that these proteins are dispensable, although their barrier properties were not specifically assessed in these studies (Gurumurthy et al., 2008; van de Pavert et al., 2004). Interestingly, the small GTPase Rac and aPKC activity are necessary for tight junction barrier function in stratifying keratinocytes in vitro (Helfrich et al., 2007; Mertens et al., 2005). In addition, loss of cell adhesion molecules, such as E-cadherin (Tunggal et al., 2005) or CD44 (Kirschner et al., 2011), impairs skin barrier function that is associated with an altered activity or localization of the Par–aPKC complex and, in the case of CD44, with the loss of apical LBs. Taken together, these results suggest that adhesive contacts and polarity signal pathways cooperate in the skin in a similar manner to those in simple epithelia to drive epithelial barrier function.

Cell division not only generates daughter cells, but through the control of orientation can also regulate their position within the tissue, and/or their cell fate. Cells can divide either symmetrically (symmetric cell division, SCD) resulting in two daughter cells with similar fate, or asymmetrically (asymmetric cell division, ACD) leading to two daughter cells with differential fate (Fig. 3A,B). Organisms use this process not only to generate different cell types during development but also for tissue homeostasis and regeneration to replace cells that are turned over or lost (Knoblich, 2010). For example, different populations of stem and/or progenitor cells use ACD to allow for self-renewal and differentiation at the same time (Farkas and Huttner, 2008).

Fig. 3.

Mechanisms of asymmetric cell division. Schematic overview of asymmetric localization of polarity proteins and spindle orientation regulators during asymmetric cell division in Drosophila and in the interfollicular epidermis, illustrating that similar molecular mediators are involved in the establishment of asymmetric cell divisions of neuroblasts and keratinocytes. (A) Asymmetric cell division (ACD) in Drosophila neuroblasts. The apical aPKC–Baz–Par6 complex is connected to the Pins–Gα1–MUD complex through Inscuteable (Insc). This complex directs the asymmetric basal localization of the cell fate determinants Numb, Brat and Prospero. GMC, ganglion mother cell. (B) ACD in the developing IFE. ACD contribute to stratification by producing one basal, proliferating cell (light green) and one suprabasal cell (dark green), whereas symmetric cell divisions (SCD) result in two daughter cells residing in the basal layer. aPKC–Par3, INSC and Gα1–LGN–NUMA–DCTN1 localize to one side of the dividing cell and are important for the establishment of epidermal ACD, as reported for their Drosophila homologs in neuroblast ACD. Suprabasal activity of the Notch signaling pathway (indicated by nuclei positive for HES1, a well-known Notch target) are crucial for the regulation of this process. (C) Effects of deletion or overexpression of molecular mediators on the ratio between ACD and SCD and protein localization in the developing IFE. Epidermal deletion of aPKCλ (left panel) results in an increase in perpendicular spindle orientation in the epidermis. Forced induction of mouse INSC (middle panel) induces increased asymmetric spindle orientation in the epidermis shortly after induction (early), whereas 3 days after induction, this effect is reversed (late) (Poulson and Lechler, 2010). Upon in vivo knockdown of LGN or NUMA (right panel), epidermal basal cells are biased towards symmetric spindle orientation (Williams et al., 2011). Baz, Bazooka, Drosophila homolog of Par3; Par, partitioning defective; aPKC, atypical protein kinase C; Pins, partner of Inscuteable (Drosophila homolog of LGN); Mud, mushroom body defect (Drosophila homolog of NUMA); Brat, brain tumor; INSC, Inscuteable; NUMA, nuclear mitotic apparatus protein1; DCTN1, dynactin1.

Fig. 3.

Mechanisms of asymmetric cell division. Schematic overview of asymmetric localization of polarity proteins and spindle orientation regulators during asymmetric cell division in Drosophila and in the interfollicular epidermis, illustrating that similar molecular mediators are involved in the establishment of asymmetric cell divisions of neuroblasts and keratinocytes. (A) Asymmetric cell division (ACD) in Drosophila neuroblasts. The apical aPKC–Baz–Par6 complex is connected to the Pins–Gα1–MUD complex through Inscuteable (Insc). This complex directs the asymmetric basal localization of the cell fate determinants Numb, Brat and Prospero. GMC, ganglion mother cell. (B) ACD in the developing IFE. ACD contribute to stratification by producing one basal, proliferating cell (light green) and one suprabasal cell (dark green), whereas symmetric cell divisions (SCD) result in two daughter cells residing in the basal layer. aPKC–Par3, INSC and Gα1–LGN–NUMA–DCTN1 localize to one side of the dividing cell and are important for the establishment of epidermal ACD, as reported for their Drosophila homologs in neuroblast ACD. Suprabasal activity of the Notch signaling pathway (indicated by nuclei positive for HES1, a well-known Notch target) are crucial for the regulation of this process. (C) Effects of deletion or overexpression of molecular mediators on the ratio between ACD and SCD and protein localization in the developing IFE. Epidermal deletion of aPKCλ (left panel) results in an increase in perpendicular spindle orientation in the epidermis. Forced induction of mouse INSC (middle panel) induces increased asymmetric spindle orientation in the epidermis shortly after induction (early), whereas 3 days after induction, this effect is reversed (late) (Poulson and Lechler, 2010). Upon in vivo knockdown of LGN or NUMA (right panel), epidermal basal cells are biased towards symmetric spindle orientation (Williams et al., 2011). Baz, Bazooka, Drosophila homolog of Par3; Par, partitioning defective; aPKC, atypical protein kinase C; Pins, partner of Inscuteable (Drosophila homolog of LGN); Mud, mushroom body defect (Drosophila homolog of NUMA); Brat, brain tumor; INSC, Inscuteable; NUMA, nuclear mitotic apparatus protein1; DCTN1, dynactin1.

One of the mechanistically best understood types of ACD occurs in the Drosophila neuroblast, in which all divisions are asymmetric, generating another neuroblast progenitor and a ganglion mother cell. Neuroblast ACD requires the differential partitioning of fate-determining proteins (Fig. 3A) (Knoblich, 2008), which is achieved by differential polarization of two opposite membrane domains within the cell coupled with correct positioning and orientation of the mitotic spindle. This will ensure the appropriate partitioning of fate-determining factors during cell division (Pearson and Bloom, 2004). Recently, many authors have also demonstrated the importance of spindle orientation in regulating morphogenetic processes in mammals. For example, ACD promotes the exit of one daughter cell from the epicardium into the myocardium during heart morphogenesis (Wu et al., 2010), and regulate the differentiation of mammalian neuronal progenitors (Zhong and Chia, 2008).

Asymmetric and symmetric divisions in the epidermis

In stratifying epithelia, spindle orientations that are either in parallel or perpendicular to the basement membrane were first observed in the IFE (Smart, 1970), and later in the cornea (Lamprecht, 1990), hair follicles (Blanpain and Fuchs, 2009; Zhang et al., 2009) and in sebaceous gland development (Frances and Niemann, 2012) (Fig. 3B, Fig. 4). In the cornea, perpendicular cell divisions are associated with a differential size and morphology of the daughter cells (Lamprecht, 1990), suggesting that these divisions are asymmetric ones that promote differential cell fate, whereas parallel divisions are symmetric and produce two basal daughters that would increase the surface area of the cornea. Lechler and Fuchs subsequently showed that the switch from predominant symmetric (parallel) divisions to asymmetric (perpendicular) divisions coincides with the onset of stratification, providing indirect evidence that ACD promotes epidermal differentiation (Lechler and Fuchs, 2005). Indeed, whereas the basal daughter remains positive for the basal cell marker keratin 14, the suprabasal daughter originating from ACD expresses keratin 10, a marker for suprabasal cell identity (Poulson and Lechler, 2010).

Fig. 4.

Symmetric and asymmetric divisions in different compartments of the epidermis. In addition to ACD occurring in the IFE (see Fig. 2B), ACD-like divisions have been observed in the region where hair follicle stem cells move outwards of the bulge region (hair germ) suggesting that they fuel hair follicle anagen growth (black arrows), whilst replenishing bulge cells. By contrast, inside the bulge, symmetric cell divisions serve to replenish the stem cell pool during anagen phases of the hair cycle. Asymmetric divisions are also found in the bulb of the anagen hair follicle positioned close to the dermal papilla and might drive differentiation of the different hair follicle layers. BM, basement membrane; SG, sebaceous gland; DP, dermal papilla.

Fig. 4.

Symmetric and asymmetric divisions in different compartments of the epidermis. In addition to ACD occurring in the IFE (see Fig. 2B), ACD-like divisions have been observed in the region where hair follicle stem cells move outwards of the bulge region (hair germ) suggesting that they fuel hair follicle anagen growth (black arrows), whilst replenishing bulge cells. By contrast, inside the bulge, symmetric cell divisions serve to replenish the stem cell pool during anagen phases of the hair cycle. Asymmetric divisions are also found in the bulb of the anagen hair follicle positioned close to the dermal papilla and might drive differentiation of the different hair follicle layers. BM, basement membrane; SG, sebaceous gland; DP, dermal papilla.

An elegant in vivo lentiviral knockdown system used to interfere with the molecular machinery that determines spindle positioning in Drosophila neuroblasts, such as microtubule-associated dynein binding protein (MUD) and partner of inscuteable (PINS), provided first mechanistic insight into how ACD is established in the murine epidermis. For example, knockdown of the mammalian PINS homolog LGN (ten leucine-glycine-asparagine tripeptides in its N-terminal region) or the MUD homolog NUMA (nuclear mitotic apparatus protein), results in a strong decrease of ACD, concomitant with an increase in SCD (Fig. 3C) (Williams et al., 2011). These changes are accompanied by a thinner epidermis that has a decreased number of suprabasal layers with impaired expression of differentiation markers, as well as a slight increase in basal cell number. This observation links key regulators of spindle positioning to differentiation in mammalian epithelial cells.

Unlike in Drosophila neuroblasts, not all epidermal divisions are asymmetric. Moreover, the decision to divide either symmetrically or asymmetrically is not predetermined in single epidermal progenitors (Poulson and Lechler, 2010), indicating that the microenvironment plays an important role in the overall outcome of cell divisions. In agreement with this model, loss of cell adhesive cues, such as β1 integrins or the adherens junction molecule α-catenin in mouse epidermis, results in a random spindle orientation that leads to altered differentiation patterns (Lechler and Fuchs, 2005).

Polarity proteins in asymmetric and symmetric cell division in skin

What is the importance of polarity signaling in regulating asymmetric divisions in mammals? In Drosophila neuroblasts, the initial polarization cue comes from the apical enrichment of the polarity proteins Par3, Par6 and aPKC. This apical distribution is essential for asymmetric localization of cell fate determinants, which is coupled to spindle orientation by binding to the adaptor protein Inscuteable (Insc) (Fig. 3A). Insc then recruits a protein complex consisting of the heterotrimeric G protein α1-subunit (Gα1), PINS and MUD, which provides attachment sites for astral microtubules (Knoblich, 2010). Polarized distribution of the aPKC–Par complex is inherited from the epithelial cell, from which the neuroblast arose after delamination (Prehoda, 2009). Similarly, in both mouse neurons and in the epidermis, PAR3 and aPKC show an apical distribution that is independent of cell division (Lechler and Fuchs, 2005). In the epidermis, this apical polarity might have been inherited from the polarized single layer epithelium before the onset of stratification. Whereas in Drosophila neuroblasts the apical membrane domain determines the future neuroblast stem cell, in the epidermis, the aPKC- and Par3-enriched domain marks the future differentiated suprabasal daughter cell (Poulson and Lechler, 2010). In addition, Par3 and aPKC also show an apical localization in cells that undergo symmetric divisions, indicating that differential Par3 and aPKC localization alone is not sufficient to drive ACD. In line with this observation is the finding that the mammalian PAR–PAR6–aPKC complex regulates spindle positioning in SCD, as recently demonstrated in 3D cell culture using knockdown approaches (Durgan et al., 2011; Hao et al., 2010).

In the developing mammalian brain, either loss or overexpression of PAR3 increases SCD, but whereas PAR3 overexpression increases the number of cells with progenitor radial glial cell fate, knockdown of PAR3 results in a greater number of cells with differentiated fate (Bultje et al., 2009). Interestingly, this indicates that levels of PAR3 might directly determine cell fate. Similarly, loss of LGL1 in mice causes brain dysplasia as a result of an increase in SCD (Klezovitch et al., 2004). Thus, as in lower organisms, LGL1 and PAR3 in the brain are essential for ACD and work in an antagonistic manner to regulate differential cell fate. It is not yet clear whether PAR3 and LGL1 play similar roles in the developing epidermis.

The role of the two mammalian isoforms of aPKC – aPKCλ and aPKCζ – in the regulation of ACD and SCD is more controversial. Knockdown of aPKCλ in ex vivo cultured mouse embryos alters cell fate (Dard et al., 2009), whereas in vivo inactivation of either aPKCλ in developing neurons (Imai et al., 2006) or both mammalian aPKC isoforms in the hematopoetic system (Sengupta et al., 2011) does not alter cell fate, suggesting that, in mammals, aPKCs do not control ACD and thereby cell fate in vivo. By contrast, epidermal inactivation of aPKCλ results in an increase in asymmetric divisions in the interfollicular epidermis and the hair follicle that is associated with altered differentiation in both epidermal compartment (M.T.N., J. Scott and C.M.N., unpublished results). This implies that aPKCλ might in fact be a negative regulator of asymmetric spindle orientation (Fig. 3C).

Knockdown of NUMA or LGN in the epidermis does not affect aPKC or PAR3 localization (Williams et al., 2011) (Fig. 3C). A similar observation was made in mice with an epidermal deletion of serum response factor (SRF), a transcription factor that activates gene expression of a range of actin regulators (Luxenburg et al., 2011). In these mice, spindle positioning is random and is accompanied by random cortical localization of LGN, even though PAR3 remains apical (Luxenburg et al., 2011). Together, these data suggest that, similar to the process in neuroblasts, aPKC and PAR3 either act upstream of LGN and NUMA, or that the apical localization of these two complexes serves different functions; for example, the Par–aPKC-mediated localization of thus far unknown cell fate determinants versus LGN–NUMA-mediated spindle orientation. SRF loss also alters the basal cell architecture that is associated with reduced cortical localization of the actinomyosin cytoskeleton and the phosphorylated forms of the actin-associated proteins ezrin, moesin and radixin (collectively termed ERM). Interestingly, loss of the PAR3 binding protein merlin (also known as NF2), an ERM-related protein, also results in altered spindle positioning that is accompanied by impaired differentiation (Gladden et al., 2010). Thus, perhaps PAR3 regulates the recruitment of spindle position regulators to the apical membrane through regulation of ERM–merlin and actinomyosin.

In Drosophila neuroblasts, the Par–aPKC complex is not absolutely required for correct spindle orientation, because flies have an alternative mechanism that involves microtubules and the MUD complex, which is activated in telophase when the apical determinant complex is either missing or does not function properly (reviewed by Knoblich, 2010). Recent data indicate that the epidermis might also correct potential imbalances in division orientation, albeit through a very different mechanism. Epidermal overexpression of mouse INSC initially promotes asymmetric divisions, but this effect is reversed upon prolonged INSC expression, and is accompanied by the dissociation of NUMA from the apically localized INSC and LGN proteins (Fig. 3C) (Poulson and Lechler, 2010). This result suggests that epidermal progenitors can sense and correct disturbances in the balance of ACD versus SCD (Ray and Lechler, 2011). Taken together, these recent findings suggest that although the core machinery that controls ACD in Drosophila is conserved in the mammalian epidermis, there are also important differences with regard to the regulation of ACD and SCD.

Asymmetric divisions and cell fate determination: outstanding questions

One important question that remains to be answered is whether epidermal ACD is directly coupled to the asymmetric segregation of cell fate determinants, as is the case in Drosophila neuroblasts, and/or whether differential cell fate is achieved through unequal positioning of cells, which then receive diverse signals from the local environment to regulate their fate. Some evidence for the latter possibility is provided by the observation that the strong decrease in ACD upon in vivo knockdown of LGN in the epidermis is associated with a reduction in Notch signaling, which is necessary in suprabasal layers to promote epidermal differentiation (Williams et al., 2011).

Another important remaining question is whether asymmetric divisions regulate cell fate decisions of stem or progenitor cell behavior in adult skin. Lineage-tracing studies in mice combined with mathematical modeling imply that random SCD and ACD occur in the adult IFE (Clayton et al., 2007). However, it has been debated whether these cells truly represent long-lived stem cells. In mice, the hair follicle bulge harbours the most pluripotent stem cells and lineage tracing experiments in this system suggest that symmetric divisions replenish these cells (Petersson et al., 2011; Zhang et al., 2010). This is consistent with findings that hair follicle stem cells do not oblige with the ‘immortal strand’ hypothesis (Sotiropoulou et al., 2008; Waghmare et al., 2008). This proposes that stem cells asymmetrically segregate their chromosomes to retain the same strand of DNA over several divisions in order to avoid accumulating DNA mutations. Progenitors located just below the bulge in the hair germ (Greco et al., 2009) can divide asymmetrically (Zhang et al., 2010) and some recent evidence suggest that these cells replenish bulge stem cells while at the same time providing enough cells to drive anagen hair follicle growth (Hsu et al., 2011) (Fig. 4). Perpendicular spindles have also been observed in the hair bulb (Blanpain and Fuchs, 2009), suggesting that such asymmetric divisions might regulate the differentiation of the various hair follicle layers. One important caveat in all these studies is that tracing cell divisions cannot yet be correlated with tracing of the differential cell fate. Nevertheless, indirect evidence from global RNA expression data suggests that dividing stem cells in the bulge retain their stem cell signature in agreement with SCD for these cells (Zhang et al., 2009). It will be important to further examine the in vivo role of polarity proteins in the regulation of cell fate in the epidermis and its appendages.

The orientation and alignment of hair follicles along the body axis is an example of tissue polarity in the adult skin. In mice, hair follicles and hairs are inserted with an angle of around 45–55° relative to the anterior–posterior (A-P) body axis (Fig. 2D). Unlike single-cell appendages, such as the hair on a Drosophila wing or the stereocilia in the inner ear sensory hair cell, hair follicles consist of hundreds of cells. The individual cell position not only needs to be aligned with their direct neighbours, but the cells of an individual hair follicle also have to act as a unit to communicate their position with adjacent hair follicles to achieve this global anterior-posterior hair follicle pattern. Hair follicle morphogenesis starts at embryonic day 14.5 (E14.5) with the budding of a placode from the basal epidermal layer in response to inductive signals from the dermis (Schneider et al., 2009). Initially, placode formation is symmetric with first signs of A-P asymmetry observed later at the hair germ stage, when developing follicles adopt their polarized position owing to basal constriction of HF cells at the anterior side, concomitant with posterior cells adopting a columnar shape (Fig. 2B). This is accompanied by differential localization of several proteins either at the anterior or posterior side of the hair follicle (Devenport and Fuchs, 2008).

Several recent papers have provided substantial evidence that this coordinated polarization of hair follicles is driven by the activity of core PCP proteins. Upon loss of one of the PCP transmembrane component, either mammalian FZ6 (Guo et al., 2004) or the mammalian Flamingo ortholog CELSR1 cadherin (Ravni et al., 2009), hair patterning is disturbed, resulting in hair swirls (Fig. 2C). Closer analysis of FZ6-deficient mice indicates that hair follicle and shaft orientation is regulated by at least two systems, an early-acting global orientation that depends on FZ6 signaling and a later-acting FZ6-independent local PCP system (Wang and Nathans, 2007). How is asymmetry established in the early hair follicle? Recent data indicate that FZ6 probably acts together with CELSR1 and VANGL2 in a non-cell-autonomous fashion early in hair follicle morphogenesis to globally orient hair follicles. One day before placode formation (E13.5), FZ6, VANGL2 and CELSR1 exhibit a distinct localization pattern, with VANGL2 enriched on anterior basal membranes, FZ6 on posterior ones and CELSR1 on both (Devenport and Fuchs, 2008), which is a similar pattern to that observed for these molecules in, for example, Drosophila wing cells (Fig. 2A,B). Mutations in genes encoding CELSR1 or VANGL2 not only disturb anterior and/or posterior localization of VANGL2, FZ6 and CELSR1 (Devenport and Fuchs, 2008), indicating that their localization is mutually dependent, but also prevent constriction and hair follicle asymmetry (Fig. 2D). PCP proteins are internalized during mitosis in a manner that is dependent on CELSR1, and segregate symmetrically upon division, after which they regain their polarized distribution (Devenport et al., 2011). This mechanism probably serves as a protective mechanism against improper planar signaling across different cells and hair follicles, when cells round up during mitosis. Prevention of their internalization during development interferes with the orientation of hair follicles (Devenport et al., 2011). Together, these results indicate that the core FZ6–VANGL2 pathway regulates the individual and global polarization of hair follicles.

Hallmarks of cancer are unrestricted cell division, an altered cyto-architecture and a gain of migratory and invasive capacities, all properties in which polarity proteins have a key role. In general, loss or reduced expression of polarity proteins (e.g. LKB1, Scribble, DLG1, LGL2, DSH, PAR3 and Crumbs) has been associated with tumor initiation and/or progression (Hawkins and Russell, 2008; Huang and Muthuswamy, 2010; Humbert et al., 2008; Lee and Vasioukhin, 2008), suggesting that they might have tumor suppressive function. The E6 oncoprotein of human papillomavirus (HPV) strains associated with high carcinogenesis risk target human Scribble and DLG1 for degradation (Gardiol et al., 1999; Nakagawa and Huibregtse, 2000), in agreement with such a tumor suppressive function. In mice, loss of Scribble indeed results in prostate neoplasia (Pearson et al., 2011). However, only LKB1 mutations have been directly linked to human cancer thus far (Jansen et al., 2009). Epidermal inactivation of LKB1 predisposes mice to non-melanoma skin cancer, thus providing direct evidence that LKB1 serves as a cell-autonomous tumor suppressor in the skin (Gurumurthy et al., 2008). Epidermal loss of PAR3 also inhibits formation and growth of papillomas. However, such mice are predisposed to the formation of keratoacanthomas (S.I. and J. G. Collard, unpublished). These tumors are thought to derive from a different cellular subpopulation within the epidermis than papilloma (Perez-Losada and Balmain, 2003), thus suggesting that the cellular context might determine whether PAR3 functions as a tumor promoter or suppressor.

An important question for the future is how altered polarity signaling inhibits and/or promotes (skin) carcinogenesis. This is likely to occur on different levels. For instance, tumor initiation might occur through alterations in the extent of ACD compared with SCD. In Drosophila neuroblasts, switching from ACD to SCD promotes unrestricted growth and allows for tumor formation (Januschke and Gonzalez, 2008). An increase in ACD is therefore predicted to inhibit mammalian tumorigenesis, whereas an increase in SCD could promote carcinogenesis (Lee and Vasioukhin, 2008). Although this has not been rigorously tested in mammals, it is interesting to note that overexpression of PAR6 or aPKCι is associated with a range of human carcinomas (Fields et al., 2007; Nolan et al., 2008). In addition, expression of a constitutive membrane-bound aPKC promotes symmetric divisions and overgrowth in Drosophila neuroblasts (Lee et al., 2006). By contrast, loss of aPKCλ almost completely prevents tumorigenesis in either mouse models of colon cancer (Murray et al., 2009) or lung carcinoma (Regala et al., 2009). PAR6–aPKC might also drive tumorigenesis through interactions with kinase receptors, because for example, transformation driven by the tyrosine kinase ERBB2 depends on PAR6–aPKC (Aranda et al., 2006). Similarly, many in vitro studies have shown that polarity proteins have a crucial role in the assembly and disassembly of cell–cell junctions (Feigin and Muthuswamy, 2009; Suzuki and Ohno, 2006), as well as in migration (Caddy et al., 2010; Dow et al., 2007; Pegtel et al., 2007; Tscharntke et al., 2007). This is not only essential for wound healing (Caddy et al., 2010), but also is a key determinant for invasion of tumor cells.

The last decade has brought tremendous progress in our insights into how cell and tissue polarity proteins regulate a range of biological essential processes. Only recently, in vivo tissue-specific mammalian models have become available that now allow us to tease out the cell-autonomous and non-cell-autonomous functions of these proteins in different tissues. As highlighted here for the epidermis, these studies started to reveal important insights into their function during morphogenesis. Because many core polarity proteins have multiple mammalian orthologs, a future challenge will be to identify their unique and overlapping roles, as well as to determine tissue-specific requirements and link them to human disease. In this context, it is important to note that polarity protein functions not only determine cell and tissue architecture, but have also been implicated in the regulation of a variety of cellular pathways that are crucial for growth, differentiation, metabolic activity and innate immunity. Thus, polarity signal pathways could turn out to be central integrators of cyto-architecture and tissue homeostasis.

We would like to apologize to all colleagues whose original work we could not cite because of space limitations. We would like to thank Jeremy Nathans (Johns Hopkins University, Baltimore) for providing us with the images for Fig. 2C. We thank Susanne Vorhagen and Rehan Villani for helpful discussions.

Funding

C.M.N. is funded by the Deutsche Krebshilfe and the Deutsche Forschunsgemeinschaft (DFG) [grant numbers SFB829 A1, Z2 and SFB832 A3 and Z3]. S.I. is supported by the DFG [grant numbers SFB829 and SFB832], and by the Stiftung Kölner Krebsforschung.

Aranda
V.
,
Haire
T.
,
Nolan
M. E.
,
Calarco
J. P.
,
Rosenberg
A. Z.
,
Fawcett
J. P.
,
Pawson
T.
,
Muthuswamy
S. K.
(
2006
).
Par6-aPKC uncouples ErbB2 induced disruption of polarized epithelial organization from proliferation control.
Nat. Cell Biol.
8
,
1235
1245
.
Assémat
E.
,
Bazellières
E.
,
Pallesi–Pocachard
E.
,
Le Bivic
A.
,
Massey–Harroche
D.
(
2008
).
Polarity complex proteins.
Biochim. Biophys. Acta
1778
,
614
630
.
Bayly
R.
,
Axelrod
J. D.
(
2011
).
Pointing in the right direction: new developments in the field of planar cell polarity.
Nat. Rev. Genet.
12
,
385
391
.
Bilder
D.
(
2004
).
Epithelial polarity and proliferation control: links from the Drosophila neoplastic tumor suppressors.
Genes Dev.
18
,
1909
1925
.
Blanpain
C.
,
Fuchs
E.
(
2009
).
Epidermal homeostasis: a balancing act of stem cells in the skin.
Nat. Rev. Mol. Cell Biol.
10
,
207
217
.
Bulgakova
N. A.
,
Knust
E.
(
2009
).
The Crumbs complex: from epithelial-cell polarity to retinal degeneration.
J. Cell Sci.
122
,
2587
2596
.
Bultje
R. S.
,
Castaneda–Castellanos
D. R.
,
Jan
L. Y.
,
Jan
Y. N.
,
Kriegstein
A. R.
,
Shi
S. H.
(
2009
).
Mammalian Par3 regulates progenitor cell asymmetric division via notch signaling in the developing neocortex.
Neuron
63
,
189
202
.
Caddy
J.
,
Wilanowski
T.
,
Darido
C.
,
Dworkin
S.
,
Ting
S. B.
,
Zhao
Q.
,
Rank
G.
,
Auden
A.
,
Srivastava
S.
,
Papenfuss
T. A.
, et al. 
(
2010
).
Epidermal wound repair is regulated by the planar cell polarity signaling pathway.
Dev. Cell
19
,
138
147
.
Clayton
E.
,
Doupé
D. P.
,
Klein
A. M.
,
Winton
D. J.
,
Simons
B. D.
,
Jones
P. H.
(
2007
).
A single type of progenitor cell maintains normal epidermis.
Nature
446
,
185
189
.
Dard
N.
,
Le
T.
,
Maro
B.
,
Louvet–Vallée
S.
(
2009
).
Inactivation of aPKCλ reveals a context dependent allocation of cell lineages in preimplantation mouse embryos.
PLoS ONE
4
,
e7117
.
Devenport
D.
,
Fuchs
E.
(
2008
).
Planar polarization in embryonic epidermis orchestrates global asymmetric morphogenesis of hair follicles.
Nat. Cell Biol.
10
,
1257
1268
.
Devenport
D.
,
Oristian
D.
,
Heller
E.
,
Fuchs
E.
(
2011
).
Mitotic internalization of planar cell polarity proteins preserves tissue polarity.
Nat. Cell Biol.
13
,
893
902
.
Dow
L. E.
,
Kauffman
J. S.
,
Caddy
J.
,
Zarbalis
K.
,
Peterson
A. S.
,
Jane
S. M.
,
Russell
S. M.
,
Humbert
P. O.
(
2007
).
The tumour-suppressor Scribble dictates cell polarity during directed epithelial migration: regulation of Rho GTPase recruitment to the leading edge.
Oncogene
26
,
2272
2282
.
Durgan
J.
,
Kaji
N.
,
Jin
D.
,
Hall
A.
(
2011
).
Par6B and atypical PKC regulate mitotic spindle orientation during epithelial morphogenesis.
J. Biol. Chem.
286
,
12461
12474
.
Farkas
L. M.
,
Huttner
W. B.
(
2008
).
The cell biology of neural stem and progenitor cells and its significance for their proliferation versus differentiation during mammalian brain development.
Curr. Opin. Cell Biol.
20
,
707
715
.
Feigin
M. E.
,
Muthuswamy
S. K.
(
2009
).
Polarity proteins regulate mammalian cell-cell junctions and cancer pathogenesis.
Curr. Opin. Cell Biol.
21
,
694
700
.
Fields
A. P.
,
Frederick
L. A.
,
Regala
R. P.
(
2007
).
Targeting the oncogenic protein kinase Cι signalling pathway for the treatment of cancer.
Biochem. Soc. Trans.
35
,
996
1000
.
Frances
D.
,
Niemann
C.
(
2012
).
Stem cell dynamics in sebaceous gland morphogenesis in mouse skin.
Dev. Biol.
363
,
138
146
.
Gardiol
D.
,
Kühne
C.
,
Glaunsinger
B.
,
Lee
S. S.
,
Javier
R.
,
Banks
L.
(
1999
).
Oncogenic human papillomavirus E6 proteins target the discs large tumour suppressor for proteasome-mediated degradation.
Oncogene
18
,
5487
5496
.
Gladden
A. B.
,
Hebert
A. M.
,
Schneeberger
E. E.
,
McClatchey
A. I.
(
2010
).
The NF2 tumor suppressor, Merlin, regulates epidermal development through the establishment of a junctional polarity complex.
Dev. Cell
19
,
727
739
.
Goldstein
B.
,
Macara
I. G.
(
2007
).
The PAR proteins: fundamental players in animal cell polarization.
Dev. Cell
13
,
609
622
.
Greco
V.
,
Chen
T.
,
Rendl
M.
,
Schober
M.
,
Pasolli
H. A.
,
Stokes
N.
,
Dela Cruz–Racelis
J.
,
Fuchs
E.
(
2009
).
A two-step mechanism for stem cell activation during hair regeneration.
Cell Stem Cell
4
,
155
169
.
Guo
N.
,
Hawkins
C.
,
Nathans
J.
(
2004
).
Frizzled6 controls hair patterning in mice.
Proc. Natl. Acad. Sci. USA
101
,
9277
9281
.
Gurumurthy
S.
,
Hezel
A. F.
,
Sahin
E.
,
Berger
J. H.
,
Bosenberg
M. W.
,
Bardeesy
N.
(
2008
).
LKB1 deficiency sensitizes mice to carcinogen-induced tumorigenesis.
Cancer Res.
68
,
55
63
.
Hao
Y.
,
Du
Q.
,
Chen
X.
,
Zheng
Z.
,
Balsbaugh
J. L.
,
Maitra
S.
,
Shabanowitz
J.
,
Hunt
D. F.
,
Macara
I. G.
(
2010
).
Par3 controls epithelial spindle orientation by aPKC-mediated phosphorylation of apical Pins.
Curr. Biol.
20
,
1809
1818
.
Hawkins
E. D.
,
Russell
S. M.
(
2008
).
Upsides and downsides to polarity and asymmetric cell division in leukemia.
Oncogene
27
,
7003
7017
.
Helfrich
I.
,
Schmitz
A.
,
Zigrino
P.
,
Michels
C.
,
Haase
I.
,
le Bivic
A.
,
Leitges
M.
,
Niessen
C. M.
(
2007
).
Role of aPKC isoforms and their binding partners Par3 and Par6 in epidermal barrier formation.
J. Invest. Dermatol.
127
,
782
791
.
Hsu
Y. C.
,
Pasolli
H. A.
,
Fuchs
E.
(
2011
).
Dynamics between stem cells, niche, and progeny in the hair follicle.
Cell
144
,
92
105
.
Huang
L.
,
Muthuswamy
S. K.
(
2010
).
Polarity protein alterations in carcinoma: a focus on emerging roles for polarity regulators.
Curr. Opin. Genet. Dev.
20
,
41
50
.
Humbert
P. O.
,
Grzeschik
N. A.
,
Brumby
A. M.
,
Galea
R.
,
Elsum
I.
,
Richardson
H. E.
(
2008
).
Control of tumourigenesis by the Scribble/Dlg/Lgl polarity module.
Oncogene
27
,
6888
6907
.
Iden
S.
,
Collard
J. G.
(
2008
).
Crosstalk between small GTPases and polarity proteins in cell polarization.
Nat. Rev. Mol. Cell Biol.
9
,
846
859
.
Imai
F.
,
Hirai
S.
,
Akimoto
K.
,
Koyama
H.
,
Miyata
T.
,
Ogawa
M.
,
Noguchi
S.
,
Sasaoka
T.
,
Noda
T.
,
Ohno
S.
(
2006
).
Inactivation of aPKCλ results in the loss of adherens junctions in neuroepithelial cells without affecting neurogenesis in mouse neocortex.
Development
133
,
1735
1744
.
Jansen
M.
,
Ten Klooster
J. P.
,
Offerhaus
G. J.
,
Clevers
H.
(
2009
).
LKB1 and AMPK family signaling: the intimate link between cell polarity and energy metabolism.
Physiol. Rev.
89
,
777
798
.
Januschke
J.
,
Gonzalez
C.
(
2008
).
Drosophila asymmetric division, polarity and cancer.
Oncogene
27
,
6994
7002
.
Kemphues
K. J.
,
Priess
J R.
,
Morton
D G.
,
Cheng
N. S.
(
1988
).
Identification of genes required for cytoplasmic localization in early C. elegans embryos.
Cell
52
,
311
320
.
Kirschner
N.
,
Haftek
M.
,
Niessen
C. M.
,
Behne
M. J.
,
Furuse
M.
,
Moll
I.
,
Brandner
J. M.
(
2011
).
CD44 regulates tight-junction assembly and barrier function.
J. Invest. Dermatol.
131
,
932
943
.
Klein
T. J.
,
Mlodzik
M.
(
2005
).
Planar cell polarization: an emerging model points in the right direction.
Annu. Rev. Cell Dev. Biol.
21
,
155
176
.
Klezovitch
O.
,
Fernandez
T. E.
,
Tapscott
S. J.
,
Vasioukhin
V.
(
2004
).
Loss of cell polarity causes severe brain dysplasia in Lgl1 knockout mice.
Genes Dev.
18
,
559
571
.
Knoblich
J. A.
(
2008
).
Mechanisms of asymmetric stem cell division.
Cell
132
,
583
597
.
Knoblich
J. A.
(
2010
).
Asymmetric cell division: recent developments and their implications for tumour biology.
Nat. Rev. Mol. Cell Biol.
11
,
849
860
.
Koster
M. I.
(
2009
).
Making an epidermis.
Ann. N. Y. Acad. Sci.
1170
,
7
10
.
Lamprecht
J.
(
1990
).
Symmetric and asymmetric cell division in rat corneal epithelium.
Cell Tissue Kinet.
23
,
203
216
.
Lechler
T.
,
Fuchs
E.
(
2005
).
Asymmetric cell divisions promote stratification and differentiation of mammalian skin.
Nature
437
,
275
280
.
Lee
C. Y.
,
Robinson
K. J.
,
Doe
C. Q.
(
2006
).
Lgl, Pins and aPKC regulate neuroblast self-renewal versus differentiation.
Nature
439
,
594
598
.
Lee
M.
,
Vasioukhin
V.
(
2008
).
Cell polarity and cancer–cell and tissue polarity as a non-canonical tumor suppressor.
J. Cell Sci.
121
,
1141
1150
.
Li
R.
,
Bowerman
B.
(
2010
).
Symmetry breaking in biology.
Cold Spring Harb. Perspect. Biol.
2
,
a003475
.
Luxenburg
C.
,
Pasolli
H. A.
,
Williams
S. E.
,
Fuchs
E.
(
2011
).
Developmental roles for Srf, cortical cytoskeleton and cell shape in epidermal spindle orientation.
Nat. Cell Biol.
13
,
203
214
.
Martin–Belmonte
F.
,
Mostov
K.
(
2008
).
Regulation of cell polarity during epithelial morphogenesis.
Curr. Opin. Cell Biol.
20
,
227
234
.
McCaffrey
L. M.
,
Macara
I. G.
(
2009
).
Widely conserved signaling pathways in the establishment of cell polarity.
Cold Spring Harb. Perspect. Biol.
1
,
a001370
.
McNeill
H.
(
2010
).
Planar cell polarity: keeping hairs straight is not so simple.
Cold Spring Harb. Perspect. Biol.
2
,
a003376
.
Mertens
A. E.
,
Rygiel
T. P.
,
Olivo
C.
,
van der Kammen
R.
,
Collard
J. G.
(
2005
).
The Rac activator Tiam1 controls tight junction biogenesis in keratinocytes through binding to and activation of the Par polarity complex.
J. Cell Biol.
170
,
1029
1037
.
Montcouquiol
M.
,
Rachel
R. A.
,
Lanford
P. J.
,
Copeland
N. G.
,
Jenkins
N. A.
,
Kelley
M. W.
(
2003
).
Identification of Vangl2 and Scrb1 as planar polarity genes in mammals.
Nature
423
,
173
177
.
Murdoch
J. N.
,
Henderson
D. J.
,
Doudney
K.
,
Gaston–Massuet
C.
,
Phillips
H. M.
,
Paternotte
C.
,
Arkell
R.
,
Stanier
P.
,
Copp
A. J.
(
2003
).
Disruption of scribble (Scrb1) causes severe neural tube defects in the circletail mouse.
Hum. Mol. Genet.
12
,
87
98
.
Murray
N. R.
,
Weems
J.
,
Braun
U.
,
Leitges
M.
,
Fields
A. P.
(
2009
).
Protein kinase C βII and PKCι/λ: collaborating partners in colon cancer promotion and progression.
Cancer Res.
69
,
656
662
.
Nakagawa
S.
,
Huibregtse
J. M.
(
2000
).
Human scribble (Vartul) is targeted for ubiquitin-mediated degradation by the high-risk papillomavirus E6 proteins and the E6AP ubiquitin-protein ligase.
Mol. Cell. Biol.
20
,
8244
8253
.
Nelson
W. J.
(
2003
).
Adaptation of core mechanisms to generate cell polarity.
Nature
422
,
766
774
.
Nelson
W. J.
(
2009
).
Remodeling epithelial cell organization: transitions between front-rear and apical-basal polarity.
Cold Spring Harb. Perspect. Biol.
1
,
a000513
.
Niessen
C. M.
,
Gottardi
C. J.
(
2008
).
Molecular components of the adherens junction.
Biochim. Biophys. Acta
1778
,
562
571
.
Nolan
M. E.
,
Aranda
V.
,
Lee
S.
,
Lakshmi
B.
,
Basu
S.
,
Allred
D. C.
,
Muthuswamy
S. K.
(
2008
).
The polarity protein Par6 induces cell proliferation and is overexpressed in breast cancer.
Cancer Res.
68
,
8201
8209
.
Pearson
C. G.
,
Bloom
K.
(
2004
).
Dynamic microtubules lead the way for spindle positioning.
Nat. Rev. Mol. Cell Biol.
5
,
481
492
.
Pearson
H. B.
,
Perez–Mancera
P. A.
,
Dow
L. E.
,
Ryan
A.
,
Tennstedt
P.
,
Bogani
D.
,
Elsum
I.
,
Greenfield
A.
,
Tuveson
D. A.
,
Simon
R.
, et al. 
(
2011
).
SCRIB expression is deregulated in human prostate cancer, and its deficiency in mice promotes prostate neoplasia.
J. Clin. Invest.
121
,
4257
4267
.
Pegtel
D. M.
,
Ellenbroek
S. I.
,
Mertens
A. E.
,
van der Kammen
R. A.
,
de Rooij
J.
,
Collard
J. G.
(
2007
).
The Par-Tiam1 complex controls persistent migration by stabilizing microtubule-dependent front-rear polarity.
Curr. Biol.
17
,
1623
1634
.
Perez–Losada
J.
,
Balmain
A.
(
2003
).
Stem-cell hierarchy in skin cancer.
Nat. Rev. Cancer
3
,
434
443
.
Petersson
M.
,
Brylka
H.
,
Kraus
A.
,
John
S.
,
Rappl
G.
,
Schettina
P.
,
Niemann
C.
(
2011
).
TCF/Lef1 activity controls establishment of diverse stem and progenitor cell compartments in mouse epidermis.
EMBO J.
30
,
3004
3018
.
Poulson
N. D.
,
Lechler
T.
(
2010
).
Robust control of mitotic spindle orientation in the developing epidermis.
J. Cell Biol.
191
,
915
922
.
Prehoda
K. E.
(
2009
).
Polarization of Drosophila neuroblasts during asymmetric division.
Cold Spring Harb. Perspect. Biol.
1
,
a001388
.
Ravni
A.
,
Qu
Y.
,
Goffinet
A. M.
,
Tissir
F.
(
2009
).
Planar cell polarity cadherin Celsr1 regulates skin hair patterning in the mouse.
J. Invest. Dermatol.
129
,
2507
2509
.
Ray
S.
,
Lechler
T.
(
2011
).
Regulation of asymmetric cell division in the epidermis.
Cell Div.
6
,
12
.
Regala
R. P.
,
Davis
R. K.
,
Kunz
A.
,
Khoor
A.
,
Leitges
M.
,
Fields
A. P.
(
2009
).
Atypical protein kinase Cι is required for bronchioalveolar stem cell expansion and lung tumorigenesis.
Cancer Res.
69
,
7603
7611
.
Roh
M. H.
,
Margolis
B.
(
2003
).
Composition and function of PDZ protein complexes during cell polarization.
Am. J. Physiol. Renal Physiol.
285
,
F377
F387
.
Saburi
S.
,
Hester
I.
,
Fischer
E.
,
Pontoglio
M.
,
Eremina
V.
,
Gessler
M.
,
Quaggin
S. E.
,
Harrison
R.
,
Mount
R.
,
McNeill
H.
(
2008
).
Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease.
Nat. Genet.
40
,
1010
1015
.
Schneider
M. R.
,
Schmidt–Ullrich
R.
,
Paus
R.
(
2009
).
The hair follicle as a dynamic miniorgan.
Curr. Biol.
19
,
R132
R142
.
Sengupta
A.
,
Duran
A.
,
Ishikawa
E.
,
Florian
M. C.
,
Dunn
S. K.
,
Ficker
A. M.
,
Leitges
M.
,
Geiger
H.
,
Diaz–Meco
M.
,
Moscat
J.
, et al. 
(
2011
).
Atypical protein kinase C (aPKCζ and aPKCλ) is dispensable for mammalian hematopoietic stem cell activity and blood formation.
Proc. Natl. Acad. Sci. USA
108
,
9957
9962
.
Shin
K.
,
Fogg
V. C.
,
Margolis
B.
(
2006
).
Tight junctions and cell polarity.
Annu. Rev. Cell Dev. Biol.
22
,
207
235
.
Simons
M.
,
Mlodzik
M.
(
2008
).
Planar cell polarity signaling: from fly development to human disease.
Annu. Rev. Genet.
42
,
517
540
.
Smart
I. H.
(
1970
).
Variation in the plane of cell cleavage during the process of stratification in the mouse epidermis.
Br. J. Dermatol.
82
,
276
282
.
Sotiropoulou
P. A.
,
Candi
A.
,
Blanpain
C.
(
2008
).
The majority of multipotent epidermal stem cells do not protect their genome by asymmetrical chromosome segregation.
Stem Cells
26
,
2964
2973
.
St Johnston
D.
,
Ahringer
J.
(
2010
).
Cell polarity in eggs and epithelia: parallels and diversity.
Cell
141
,
757
774
.
Suzuki
A.
,
Ohno
S.
(
2006
).
The PAR-aPKC system: lessons in polarity.
J. Cell Sci.
119
,
979
987
.
Tscharntke
M.
,
Pofahl
R.
,
Chrostek–Grashoff
A.
,
Smyth
N.
,
Niessen
C.
,
Niemann
C.
,
Hartwig
B.
,
Herzog
V.
,
Klein
H. W.
,
Krieg
T.
, et al. 
(
2007
).
Impaired epidermal wound healing in vivo upon inhibition or deletion of Rac1.
J. Cell Sci.
120
,
1480
1490
.
Tunggal
J. A.
,
Helfrich
I.
,
Schmitz
A.
,
Schwarz
H.
,
Günzel
D.
,
Fromm
M.
,
Kemler
R.
,
Krieg
T.
,
Niessen
C. M.
(
2005
).
E-cadherin is essential for in vivo epidermal barrier function by regulating tight junctions.
EMBO J.
24
,
1146
1156
.
van de Pavert
S. A.
,
Kantardzhieva
A.
,
Malysheva
A.
,
Meuleman
J.
,
Versteeg
I.
,
Levelt
C.
,
Klooster
J.
,
Geiger
S.
,
Seeliger
M. W.
,
Rashbass
P.
, et al. 
(
2004
).
Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization and adhesion during light exposure.
J. Cell Sci.
117
,
4169
4177
.
Waghmare
S. K.
,
Bansal
R.
,
Lee
J.
,
Zhang
Y. V.
,
McDermitt
D. J.
,
Tumbar
T.
(
2008
).
Quantitative proliferation dynamics and random chromosome segregation of hair follicle stem cells.
EMBO J.
27
,
1309
1320
.
Wang
Y.
,
Nathans
J.
(
2007
).
Tissue/planar cell polarity in vertebrates: new insights and new questions.
Development
134
,
647
658
.
Watt
F. M.
,
Jensen
K. B.
(
2009
).
Epidermal stem cell diversity and quiescence.
EMBO Mol. Med.
1
,
260
267
.
Williams
S. E.
,
Beronja
S.
,
Pasolli
H. A.
,
Fuchs
E.
(
2011
).
Asymmetric cell divisions promote Notch-dependent epidermal differentiation.
Nature
470
,
353
358
.
Wu
M.
,
Smith
C. L.
,
Hall
J. A.
,
Lee
I.
,
Luby–Phelps
K.
,
Tallquist
M. D.
(
2010
).
Epicardial spindle orientation controls cell entry into the myocardium.
Dev. Cell
19
,
114
125
.
Zhang
Y. V.
,
Cheong
J.
,
Ciapurin
N.
,
McDermitt
D. J.
,
Tumbar
T.
(
2009
).
Distinct self-renewal and differentiation phases in the niche of infrequently dividing hair follicle stem cells.
Cell Stem Cell
5
,
267
278
.
Zhang
Y. V.
,
White
B. S.
,
Shalloway
D. I.
,
Tumbar
T.
(
2010
).
Stem cell dynamics in mouse hair follicles: a story from cell division counting and single cell lineage tracing.
Cell Cycle
9
,
1504
1510
.
Zhong
W.
,
Chia
W.
(
2008
).
Neurogenesis and asymmetric cell division.
Curr. Opin. Neurobiol.
18
,
4
11
.