It has been suggested that substrate adhesion molecules of the tenascin family may be responsible for the segmented outgrowth of motor axons and neural crest cells during formation of the peripheral nervous system. We have used two monoclonal antibodies (M1B4 and 578) and an antiserum [KAF9(1)] to study the expression of Jl/tenascin-related molecules within the somites of the chick embryo. Neural crest cells were identified with monoclonal antibodies HNK-1 and 20B4. Young somites are surrounded by Jl/tenascin immunoreactive material, while old sclerotomes are immunoreactive predominantly in their rostral halves, as described by other authors (Tan et al. 1987 - Proc. natn. Acad. Sci. U.S.A. 84, 7977; Mackie et al. 1988 - Development 102, 237). At intermediate stages of development, however, immunoreactivity is found mainly in the caudal half of each sclerotome. After ablation of the neural crest, the pattern of immunoreactivity is no longer localised to the rostral halves of the older, neural-crest-free sclerotomes. SDS–polyacrylamide gel electrophoresis of affinity-purified somite tissue, extracted using M1B4 antibody, shows a characteristic set of bands, including one of about 230×103, as described for cytotactin, Jl-200/220 and the monomeric form of tenascin. Affinity-purified somite material obtained from neural-crest-ablated somites reveals some of the bands seen in older control embryos, but the high molecular weight components (120-230X103) are missing. Young epithelial somites also lack the higher molecular mass components. The neural crest may therefore participate in the expression of Jl/tenascin-related molecules in the chick embryo. These results suggest that these molecules are not directly responsible for the segmented outgrowth of precursors of the peripheral nervous system.

The peripheral nervous system of higher vertebrate embryos becomes segmented because neural crest cells and motor axons emerging from the developing spinal cord are restricted in their migration to the rostral half of each adjacent sclerotome (Keynes and Stern, 1984; reviewed by Keynes and Stern, 1988). The study of the interactions between precursors of the peripheral nervous system and somite tissue therefore constitutes an apparently simple model system for the investigation of factors controlling patterning in the developing nervous system. Recently, our laboratories and others have set out to find macromolecules that are differentially ex pressed in the two halves of the sclerotome, with the aim of identifying those that have a function in controlling the segmental pattern (Stern et al. 1986; Tosney et al. 1987; Tan et al. 1987; Mackie et al. 1988; Layer et al. 1988; Norris et al. 1989; Halfter et al. 1989).

One recent paper (Tan et al. 1987) reported that the glycoprotein cytotactin (main Mr = 190–220×103; Grumet et al. 1985; Hoffman et al. 1988) is restricted to the rostral halves of the sclerotomes of the chick embryo, while a cytotactin-binding proteoglycan (CTB-proteoglycan; Hoffman and Edelman, 1987) is restricted to the caudal halves. Tan et al. (1987) suggested that cytotactin and its proteoglycan ligand ‘may control the pattern of neural crest migration’. However, several of their results suggest that the relationship between neural crest migration and the expression of these macromolecules is not straightforward. First, cytotactin is not localised to the rostral halves of the sclerotomes present before stage 16 (Hamburger and Hamilton, 1951) or so, at which time the neural crest has already entered the rostral halves of these sclerotomes (Thiery et al. 1982; Rickmann et al. 1985; Bronner-Fraser, 1986; Loring and Erickson, 1987; Teillet et al. 1987. cf. Crossin et al 1986); second, paradoxically, cytotactin substrates inhibit neural crest migration and spreading in vitro; third, it seems curious that while cytotactin is restricted to the rostral half of the sclerotome, its proteoglycan ligand becomes localised to the opposite half after the appearance of cytotactin.

Comparable results were obtained by another group of workers (Mackie et al. 1988), who investigated the distribution of tenascin (Chiquet and Fambrough, 1984; Mackie et al. 1987), a hexamer of glycoproteins of similar molecular mass to cytotactin (Vaughan et al. 1987), in the sclerotome of the quail embryo. They, too, found that tenascin is restricted to the rostral halves of the sclerotomes after stage 16 (Mackie et al. 1988), and that tenascin substrates inhibit neural crest migration and spreading in vitro when cultured on basal laminae (Mackie et al. 1988; Halfter et al. 1989).

It seems likely that cytotactin and tenascin, as well as two other glycoproteins, I1 200/220 (Kruse et al. 1985; Faissner et al. 1988) and GMEM (glioma extracellular matrix molecule; Bourdon et al. 1985) are related, if not identical (see Aufderheide and Ekblom, 1988; Faissner et al. 1988; Friedlander et al. 1988; Hoffman et al. 1988; Mackie et al. 1988; Halfter et al. 1989). Their relative molecular masses appear to be the same; all of them comprise several polypeptides of various relative molecular masses between 110 and 260 ×103, with a predominant band at Mr = 220×103, and both cytotactin and tenascin are assembled as a hexamer, called a ‘hexabrachiori’ (Friedlander et al. 1988). Tenascin and JI have been reported to be related immunologically, and cytotactin and tenascin show apparently identical patterns of immunohistological localisation in various tissues (see Mackie et al. 1988), as well as identical effects on neural crest cells in vitro. At least when isolated from brain, cytotactin and its proteoglycan ligand can carry the HNK-1/L2 carbohydrate epitope (Hoffman and Edelman, 1987; Künemund et al. 1988; Friedlander et al. 1988; Hoffman et al. 1988), as does JI (Kruse et al. 1985; Faissner et al. 1988).

In this study, we have used three different antibodies, two monoclonals (M1B4 – Chiquet and Fambrough, 1984 and MAb578 – Faissner et al. 1988 and in preparation) and one antiserum (KAF9(1); Faissner et al. 1988 and in preparation), to study the distribution of Jl/tenascin-related molecules in the sclerotome of the chick embryo in more detail. We performed neural crest ablations to examine the relationship between the position of neural crest cells and the expression of these molecules, and constructed an M1B4 affinity column to purify the antigens for subsequent identification by SDS–polyacrylamide gel electrophoresis.

Embryos and operations

Fertile hens’ eggs (Rhode Island Red hybrids) were incubated at 38°C for 2–3 days until they had reached Hamburger and Hamilton (1951) stages 10–20. Embryos destined for ablation of the neural crest were treated as follows: at about stage 10–12, a 1·5×l·5cm window was cut in the egg shell with a scalpel, the embryo floated up to the level of the shell with calcium- and magnesium-free Tyrode’s saline (CMF), and about 50 μl of a 1:10 solution of Indian ink (Pelikan Fount India) in CMF injected into the subblastodermic cavity, to improve contrast between the embryo and the underlying yolk. Removal of the neural crest was then performed, under a drop of CMF, using a microsurgical knife (Week, 15° angle), Dewecker’s scissors and a steel needle made from a N°1 entomological pin. The dorsal half of the neural tube spanning the region between the caudal end of the segmental plate and the 4th most recently formed somite (corresponding to some 14 segments in length) was excised. The ventral half of the tube and the underlying notochord were left in place. After the operation, the CMF was removed from above the embryo, and about 2 ml thin albumen withdrawn with a syringe from a hole in the blunt end of the egg; this lowered the embryo back into the shell. The shell was then sealed with PVC tape and the egg incubated at 38°C for a further 36–48 h, by which time the embryos had reached stages 17–22. Three additional embryos were incubated for a longer period, until they had reached stages 25–27.

Control embryos were treated in an identical manner: the vitelline membrane was cut above the embryo and, in most cases, deep cuts were made on either side of the neural tube. After these manipulations, the embryos were lowered into the shell, the eggs sealed and incubated exactly as for operated embryos. All of the 107 operated embryos and 116 controls survived the subsequent period of incubation.

Immunohistochemistry

Control and operated embryos destined for immunohistochemistry were fixed in 100% ethanol for Ih or in buffered formol saline (4 % formaldehyde in phosphate-buffered saline [PBS]) for 30 min, and washed well with PBS. They were then placed in a 5% solution of sucrose in PBS for 1–3 h, then in 20% sucrose/PBS for 8–24 h, and finally infiltrated for 2–6 h at 38°C with a solution containing 7–5% gelatin (Sigma, 300 Bloom) and 15 % sucrose in PBS. They were then brought to room temperature, to allow the blocks to set. These blocks were stored at 4 °C for no longer than a week prior to serial sectioning (5–10 μm) at −20°C in a Bright cryostat. The sections were collected on gelatinised glass slides, air dried and stored at 4°C until required.

Prior to immunohistochemical staining, the sections were rehydrated and the gelatin removed by placing them in PBS at 38°C for a few minutes, then washed extensively with PBS. They were then blocked with 3% bovine serum albumin (BSA, Sigma) for 20 min, placed in the appropriate antibody (see below) for Ih at room temperature, washed extensively with PBS, placed in the appropriate labelled secondary antibody for 1 h at room temperature, washed again, and then processed for immunofluorescence or immunoperoxidase.

For immunofluorescence, the slides were mounted in a nonquenching medium (14% Gelvatol 20/30 [Fisons] containing 8-5 mg ml-1 diazobicyclo-octane [DABCO, Aldrich, as anti-quenching agent], 30% glycerol and 350 wgml-1 sodium azide as preservative in PBS, pH6·8), and viewed and photographed under epifluorescence optics in an Olympus Vanox-T microscope, using supplementary exciter and barrier filters to minimise spill-over between fluorescein and rhodamine wavelengths. The supplementary filter combinations used were: excitation EY455/emission B460 for fluorescein (blue excitation, green pass) and excitation EO530/emission 0590 for rhodamine (green excitation, red pass). Lack of spill-over may be confirmed by the reader by viewing the colour figures with red/green glasses.

For immunoperoxidase, the slides were washed briefly in 0·1 M-Tris-HCl and then incubated in a 500 μg ml-1 solution of diaminobenzidine (Aldrich), to which H2O2 was added to a final concentration of 0·3 %. The slides were incubated in this for about 10 min at room temperature, then washed in running tap water for 30 min, rinsed in distilled water and mounted in Aquamount medium (BDH).

Antibodies

Neural crest cells present within the sclerotome of chick and quail embryos display the HNK-1/L2 epitope, and therefore monoclonal antibodies specific for this epitope (such as NC-1 or HNK-1; Tucker et al. 1984) are often used to detect the presence of neural crest cells in histological sections. Bronner-Fraser (1986) and Teillet et al. (1987) have investigated the expression of the HNK-1 epitope in neural crest cells in the chick embryo in some detail, conclude that almost all the crest cells that traverse the sclerotome express HNK-1 immunoreactivity (unlike some crest cells in the head and some that take the dorsolateral pathway between the ectoderm and the somite); HNK-1 is therefore a good marker for neural crest cells present in the sclerotome at this stage of development. Since Jl/tenascin-related molecules can carry the HNK-1/L2 epitope, we have used antibody 20B4 (which recognises a different, as yet uncharacterised, epitope expressed by neural crest cells; Dr J. Denburg, unpublished data) in addition to HNK-1 to detect neural crest cells and to control for the possibility of cross-reaction between the HNK-1 antibody and Jl/tenascin-related molecules.

The primary antibodies used (dilutions in 0·3 % BSA/PBS) were:

M1B4 supernatant (Mouse IgG, originally raised by Dr Douglas Fambrough and obtained from the Developmental Studies Hybridoma Bank). This was used at concentrations between 1 and 10 μg ml” ‘(between 1:10 dilution and undiluted). The antibody recognises the original ‘myotendinous antigen’ (= tenascin = hexabrachion) described by Chiquet and Fambrough (1984; see also Friedlander et al. 1988) and an epitope near the carboxy-terminal end of chain I of the cytotactin molecule (Friedlander et al. 1988).

578 purified ascites (Rat IgG monoclonal against JI 200/220; Faissner et al. 1988 and in preparation). Used at 1:100 (50 μg ml-1).

KAF9(1) (IgG fraction of rabbit antiserum directed against mouse JI 200/220-tenascin; Faissner et al. 1988 and in preparation). Used at 1:200 (25 μgml-1).

HNK-1 supernatant (Mouse IgM, originally raised by Abo and Balch, 1981). Used 1:2 or undiluted (about 50 μg ml-1). The neural crest specificity of this antibody is discussed by various authors (e.g. Tucker et al. 1984; Rickmann et al. 1985; Bronner-Fraser, 1986; Teillet et al. 1987; Canning and Stern, 1988).

20B4 supernatant (Mouse IgG, originally raised by Dr Jeff Denburg, to chick dorsal root ganglion cells and obtained from the Developmental Studies Hybridoma Bank). Used undiluted (20 μg ml-1).

In double-immunofluorescence studies, it was important to use secondary antibodies that did not cross-react across species or immunoglobulin types. After extensive tests to ascertain this, the following antibodies were selected:

For mouse IgG: (1) affinity-purified tetramethylrhodamine isothiocyanate (TRITC)-labelled goat anti-mouse IgG (Fc fragment specific; Sigma), used at 1:50 or (2) IgG fraction of affinity-purified fluorescein isothiocyanate (FITC)-labelled goat anti-mouse IgG (Fc fragment, γ-chain specific; Cappel). Used at 1:50 or 1:100.

For mouse IgM: (1) affinity-purified FITC-labelled goat antimouse IgM (μ-chain specific; Sigma), used at 1:100, or (2) TRITC-labelled sheep anti-mouse IgM (Fc fragment specific; Serotec), used at 1:40.

For rat IgG: affinity-purified TRITC- or FITC-labelled goat anti-rat IgG (Fc fragment specific, tested for the absence of cross-reactivity to mouse IgG by the manufacturers; Cappel 1213-1721). We confirmed the absence of cross-reactivity between this antibody and mouse IgG ourselves in sections that had been incubated with mouse monoclonals M1B4 or 20B4. Used at a dilution of 1:100 (30 μg ml-1).

For rabbit IgG: affinity-purified TRITC- or FITC-labelled goat anti-rabbit IgG (Fey chain specific; Cappel), used at 1:50 (20 μg ml-1).

To absorb out any remaining nonspecific binding, all secondary antibodies were made up in 0·3% BSA in PBS containing 1·5% normal goat serum, and the final working solution absorbed against either fresh or formalin-fixed 2-day chick embryo tissues for 30 min under gentle agitation at room temperature; after this, the working solution was centrifuged for 3 min at 100g in a Micro-Centaur centrifuge. These procedures were necessary to remove components of the secondary antisera that were found to bind to various chick tissues. In double-immunofluorescence experiments, the two primary antibodies were added together in 0·3 % BSA/PBS; the two secondary antibodies were also used in a single solution.

In each experiment, the entire control or neural-crest-ablated embryo (i.e. all the serial sections) was stained with one combination of antibodies. At least two control and two operated embryos were studied in this way for each antibody combination. Table 1 shows the combinations of primary antibodies used in single- and double-immunofluorescence experiments.

Table 1.

Combinations of antibodies used for immunofluorescence studies

Combinations of antibodies used for immunofluorescence studies
Combinations of antibodies used for immunofluorescence studies

Control sections were included in each experiment; in these, incubation in 3 % BSA/PBS replaced the incubation in primary antibody.

Electrophoretic characterisation of antigens

We were not able to detect antigens recognised by antibodies M1B4, 578 or KAF9(1) in immunoblots made from polyacrylamide gels of unfractionated somite tissue under either reducing or nonreducing conditions. For this reason, we had to resort to affinity-extraction of the antigens from these tissues. 45 μg of M1B4 IgG were purified by affinity from culture supernatant using goat anti-mouse IgG agarose (Sigma), following the manufacturer’s protocol. The bound IgG was eluted using 0·1 M-glycine and 0·15 M-NaCl (pH2·4), and the fractions collected were neutralised with a small volume of 1M-Trizma base (Sigma). This purified IgG was coupled to 0·5 ml CNBr-activated Sepharose-4B (Sigma) following the manufacturer’s instructions.

Strips of somite tissue (with some adherent ectoderm and endoderm) were dissected from unoperated embryos or from embryos from which the neural crest had been ablated 36–48 h earlier (as described above). Equivalent regions were taken from both operated and unoperated embryos, ignoring the first and last three pairs of somites opposite the operated region; both control and operated embryos were at stages 18–22. Two types of sample were dissected from younger embryos. In one, the last four pairs of somites plus some adherent tissues were dissected from embryos at stages 12–14 and treated identically to the other samples. In the other, the same region was dissected but the neural tube and notochord were included in the sample. In all cases, tissues were collected in PBS containing a cocktail of protease inhibitors: 1 mM-phenymethylsulphonylfluoride (PMSF; Sigma), ImM-N-ethyl maleimide (NEM; Sigma), 10 μgml-1 soybean trypsin/chymotrypsin inhibitor (STI; Sigma), ImM-EDTA and 1 mM-EGTA, and quickly frozen on solid CO2.

The frozen tissue samples were thawed and homogenised in 200 μl solubilization buffer, made up of 0·25 % sodium deoxycholate, 10mM-Tris, 150mM-NaCl (pH 8·3) containing the cocktail of protease inhibitors described above. They were then centrifuged at 11600g for 5 min and the supernatants collected for loading onto the affinity column. Samples were incubated in the column for 45 min at room temperature, after which the column was washed with 15 column volumes of solubilisation buffer containing 300mM-NaCl. Elution was then performed using a buffer containing 0·1 M-diethylamine, 150mM-NaCl and 0·25% sodium deoxycholate (pH 11-5). 100 μl fractions were collected and a protein assay performed as a dot blot on Immobilon membrane (Millipore) using AuroDye (Janssen). Protein-containing fractions were pooled and concentrated by pressure dialysis against 0-1% deoxycholate, 20mM-Tris, 150mM-NaCl, 1 mM -EGTA and ImM-EDTA (pH 8 · 3).

Approximately equal amounts of protein were loaded per track of 4 – 15% SDS-polyacrylamide gradient minigels (9 cm × 9 cm × 0 · 5 mm) using the Laemmli (1970) buffer system in an apparatus designed as described by Matsudaira and Burgess (1978). Electrophoresis was performed at 150 V for 90 min, or until the tracking dye was 1·5 cm above the bottom of the gel. The resulting gels were silver-stained as described previously (Canning and Stern, 1988; Norris et al. 1989).

14C-labelled, colour-coded molecular weight standards (Amersham, Rainbow Markers) were run in one or two tracks in each gel. A track containing a sample of purified mouse Jl/tenascin was also included in each gel for comparison. This preparation of Jl/tenascin was prepared by affinity purification as described previously (Faissner et al. 1988), using postnatal mouse brain tissue as starting material. The eluted fractions were dialysed against water and lyophilised.

Immunoblots of purified mouse brain tenascin were also made. Each blot included radioactive relative molecular mass markers and samples of purified Jl/tenascin, under reduced and non-reduced conditions. Transfer to Immobilon membranes was performed as described previously (Primmett et al. 1989). Each membrane was probed with one of the antibodies used in this study, namely M1B4, 578 or KAF9(1), and detected with either rabbit anti-rat followed by gold-labelled goat anti-rabbit Ig for MAb578, or gold-labelled goat antirabbit Ig for KAF9(1), or gold-labelled goat anti-mouse IgG for M1B4. All gold-labelled antibodies were purchased from Janssen (AuroProbe system).

Immunohistochemistry

Identification of neural crest cells with antibodies 20B4 and HNK-1

Monoclonal antibody 20B4 is reported to be specific for dorsal root ganglion cells and migrating neural crest cells; it also detects an antigenic determinant in the ectoderm of chick embryos (Dr J. Denburg; data from sheet obtained with supernatant supplied from the Developmental Studies Hybridoma Bank). In our hands, 20B4 was not as useful as HNK-1 in revealing the distribution of neural crest cells in the sclerotome because 20B4 staining was always of lower intensity and it only recognised a subset of cells stained by HNK-1. Nevertheless, the overall pattern of staining seen with this antibody in embryo sections was similar to that seen with HNK-1.

JI / tenascin-immunoreactivity

Most of the immunocytochemical procedures involved double staining, using one antibody directed to Jl/ tenascin-related molecules and one directed to neural crest cells. The combinations of antibodies used in these experiments are summarised in Table 1. Identical results were obtained using all three antibodies directed against Jl/tenascin [M1B4, 578 and KAF9(1)]. This was confirmed in double-staining experiments combining the three antibodies, one pair at a time: in every case, the staining patterns in the rhodamine and fluorescein wavelengths were identical.

In all somites of the youngest embryos (stage-13 or earlier) and in the most caudal 2–3 pairs of (epithelial) somites of older embryos (stage 16–22), staining was restricted to the extracellular matrix surrounding each somite and to the matrix associated with mesenchymal cells within the lumen of the epithelial somites, as described by Crossin et al. (1986), Tan et al. (1987) and Mackie et al. (1988). Also in accordance with Tan et al. (1987) and Mackie et al. (1988), staining within the sclerotomes of the most cranial (older) somites of embryos older than stage 16 was restricted to the rostral half of each sclerotome; most of the cells expressing Jl/tenascin immunoreactivity were not HNK-1- or 20B4-positive (Fig. 1 E, F). In addition, Jl/tenascin-immunoreactive filamentous material was seen close to the border between adjacent sclerotomes; this material appeared to be associated with the intersegmental vessels (Fig. 1 B). Immunoreactive extracellular material was also seen between the dermomyotome and the ectoderm at all segmental levels. Strong staining was also seen between adjacent dermomyotomes, especially at the caudal margin of each (Fig. 1 A).

Fig. 1.

Changes in the expression of Jl/tenascin-related molecules during development of the sclerotome. (A) Coronal section through the two most caudal sclerotomes of a stage-14 embryo, stained with M1B4 (red) and HNK-1 (green). Note the intensely HNK-l-positive perinotochordal matrix, and the HNK-l-positive neural crest cells in the rostral (left) half of the sclerotomes. MlB4-positive material, on the other hand, is more intense in the caudal than the rostral halves. (B) Sagittal section through two caudal sclerotomes of a stage-15 embryo, stained with HNK-1 (red) and KAF9(1) (green). More intense KAF9(1) staining is seen in the caudal (right) halves of the sclerotomes, while many HNK-l-positive neural crest cells have already entered into the rostral halves. KAF9(1) staining is also seen surrounding each somite and near the intersegmental vessels. (C) Oblique transverse section through a stage-15 embryo, stained with KAF9(1) (green) and HNK-1 (red), about 9 segments cranial to the segmental plate. The section passes through the rostral half of the sclerotome on the left and through the adjacent caudal half on the right. Note the abundance of KAF9(l)-positive material between the dermomyotome and the ectoderm, between the neural tube and the sclerotome, and within both halves of the sclerotome. KAF9(l)-positive material is more abundant in the caudal half at this level. HNK-l-positive cells have entered deep into the rostral half of the sclerotome. (D) Coronal section through two more rostral sclerotomes (about 12 segments rostral to the segmental plate) of a stage-15 embryo, stained with HNK-1 (red) and MAb578 (green). MAb578-immunoreactivity begins to predominate in the rostral half. (E) Coronal section through three more rostral sclerotomes (about 15 segments rostral to the plate) of a stage-17 embryo, stained with M1B4 (red) and HNK-1 (green). Within the sclerotome, M1B4 immunoreactivity is now confined to the rostral (left) half, except in the most ventromedial portion of the sclerotome, adjacent to the notochord. (F) Higher power view of one of the sclerotomes shown in (E). HNK-1 (green) and M1B4 (red) immunoreactivity do not coincide except in very few areas. In each figure, the head of the embryo lies towards the left of the photograph. Scale bars: 50 μ m (A-E), 25 μm (F). d, dermomyotome; m, myotome; n, notochord; s, sclerotome; t, neural tube; v, intersegmental vessel.

Fig. 1.

Changes in the expression of Jl/tenascin-related molecules during development of the sclerotome. (A) Coronal section through the two most caudal sclerotomes of a stage-14 embryo, stained with M1B4 (red) and HNK-1 (green). Note the intensely HNK-l-positive perinotochordal matrix, and the HNK-l-positive neural crest cells in the rostral (left) half of the sclerotomes. MlB4-positive material, on the other hand, is more intense in the caudal than the rostral halves. (B) Sagittal section through two caudal sclerotomes of a stage-15 embryo, stained with HNK-1 (red) and KAF9(1) (green). More intense KAF9(1) staining is seen in the caudal (right) halves of the sclerotomes, while many HNK-l-positive neural crest cells have already entered into the rostral halves. KAF9(1) staining is also seen surrounding each somite and near the intersegmental vessels. (C) Oblique transverse section through a stage-15 embryo, stained with KAF9(1) (green) and HNK-1 (red), about 9 segments cranial to the segmental plate. The section passes through the rostral half of the sclerotome on the left and through the adjacent caudal half on the right. Note the abundance of KAF9(l)-positive material between the dermomyotome and the ectoderm, between the neural tube and the sclerotome, and within both halves of the sclerotome. KAF9(l)-positive material is more abundant in the caudal half at this level. HNK-l-positive cells have entered deep into the rostral half of the sclerotome. (D) Coronal section through two more rostral sclerotomes (about 12 segments rostral to the segmental plate) of a stage-15 embryo, stained with HNK-1 (red) and MAb578 (green). MAb578-immunoreactivity begins to predominate in the rostral half. (E) Coronal section through three more rostral sclerotomes (about 15 segments rostral to the plate) of a stage-17 embryo, stained with M1B4 (red) and HNK-1 (green). Within the sclerotome, M1B4 immunoreactivity is now confined to the rostral (left) half, except in the most ventromedial portion of the sclerotome, adjacent to the notochord. (F) Higher power view of one of the sclerotomes shown in (E). HNK-1 (green) and M1B4 (red) immunoreactivity do not coincide except in very few areas. In each figure, the head of the embryo lies towards the left of the photograph. Scale bars: 50 μ m (A-E), 25 μm (F). d, dermomyotome; m, myotome; n, notochord; s, sclerotome; t, neural tube; v, intersegmental vessel.

The pattern of staining seen in younger sclerotomes (the 4–6 most caudal sclerotomes, between 4 and 10 segments rostral to the segmental plate) of embryos beyond stage 13 was different from that described by Tan et al. (1987) and by Mackie et al. (1988). In these young somites, which have already begun to be colonised by migrating neural crest cells (Rickmann et al. 1985; Bronner-Fraser, 1986; Teillet et al. 1987), the caudal half of each sclerotome is more intensely stained by the antibodies than the rostral half (Fig. 1 A-C). In about half the cases, moreover, staining was restricted to the caudal half. The relationship between the distribution of migrating neural crest cells (as revealed by antibodies HNK-1 and 20B4) and that of Jl/tenascin immunoreactivity was revealed in double-immunofluorescence experiments (Fig. 1): at this stage in the development of the somite, when neural crest cells are starting to invade the sclerotome, Jl/tenascin immunoreactivity predominates in the opposite half of the sclerotome to that occupied by the neural crest cells (Fig. 1 A-C). Jl/tenascin immunoreactive material in the sclerotome was most abundant dorsally near the dermomyotome, where it had a filamentous appearance (Fig. 1 A, B), and ventrally in the vicinity of the notochord, where it was associated with the peri-notochordal sheath. It was also associated with the basal lamina surrounding the neural tube (Fig. 1 C).

The 4–5 sclerotomes rostral to the 4–6 most recently formed (situated between the 10th and 15th segment rostral to the segmental plate) displayed an intermediate pattern of staining: both halves of these sclerotomes showed approximately equal levels of immunoreactivity (Fig. 1 D). The region of the sclerotome that lies adjacent to the notochord is immunoreactive for Jl/ tenascin along the entire rostrocaudal axis (i.e. in both halves of the sclerotome) rostral to the 10th segment beyond the segmental plate (e.g. Fig. 1 E).

Neural crest removal

In a preliminary series of experiments, a strip of dorsal neural tube, 3–15 segments in length, was removed from a region opposite the caudal paraxial mesoderm. The operated embryos were then incubated for 36-48 h, fixed, sectioned and stained with monoclonal antibody HNK-1 to visualise the distribution of the neural crest cells and to assess the effectiveness of the operation. In agreement with earlier investigations (Yntema and Hammond, 1954; Rickmann et al. 1985; Teillet et al. 1987), we found that neural crest cells are capable of migrating along the rostrocaudal axis for a distance of three segments in each direction (e.g. Fig. 2 A). Therefore, the sclerotomes in the operated region of embryos in which the neural crest removal spanned six segments or less always became colonised by HNK-l-immuno-reactive cells.

Fig. 2.

Neural crest ablation and expression of Jl/tenascin-related molecules. (A) Low-power view of an oblique coronal section through a stage-15 embryo after ablation of six segments worth of dorsal neural tube at stage 11, stained with HNK-1 and TRITC-labelled secondary antibody. Note the presence of neural crest cells in most of the sclerotomes in the operated region. (B) Phase-contrast micrograph of a sagittal section through two neural-crest-free sclerotomes of a stage-16 neural-crest-ablated embryo. Note that the rostral (left) halves display a reduction in cell number as compared with the corresponding caudal halves. (C) Same section as in B above, stained with M1B4 (red) and HNK-1 (green). Note the absence of HNK-l-positive cells, and the predominance of M1B4 immunoreactivity in the caudal half-sclerotome. (D) Sagittal section through neural crest-free older sclerotomes (about 12 segments rostral to the segmental plate) of a stage-17 embryo, stained with HNK-1 (red) and KAF9(1). Note the predominance of KAF9(1) immunoreactivity in the caudal halves of the sclerotomes. [Compare the pattern of staining with that shown in Fig. 1E,F, which passes through the same level of a similarly-staged but unoperated embryo]. (E) Sagittal section through neural-crest-free sclerotomes of a stage-17 embryo stained with HNK-1 (red) and KAF9(1) (green). In this embryo, both halves of the neural-crest-free sclerotomes display KAF9(1) immunoreactivity. Scale bars: 100 μm in A 50 μ m in B–E. The head of the embryo lies towards the left of each photograph, a, aorta; d, dermomyotome; n, notochord; t, neural tube.

Fig. 2.

Neural crest ablation and expression of Jl/tenascin-related molecules. (A) Low-power view of an oblique coronal section through a stage-15 embryo after ablation of six segments worth of dorsal neural tube at stage 11, stained with HNK-1 and TRITC-labelled secondary antibody. Note the presence of neural crest cells in most of the sclerotomes in the operated region. (B) Phase-contrast micrograph of a sagittal section through two neural-crest-free sclerotomes of a stage-16 neural-crest-ablated embryo. Note that the rostral (left) halves display a reduction in cell number as compared with the corresponding caudal halves. (C) Same section as in B above, stained with M1B4 (red) and HNK-1 (green). Note the absence of HNK-l-positive cells, and the predominance of M1B4 immunoreactivity in the caudal half-sclerotome. (D) Sagittal section through neural crest-free older sclerotomes (about 12 segments rostral to the segmental plate) of a stage-17 embryo, stained with HNK-1 (red) and KAF9(1). Note the predominance of KAF9(1) immunoreactivity in the caudal halves of the sclerotomes. [Compare the pattern of staining with that shown in Fig. 1E,F, which passes through the same level of a similarly-staged but unoperated embryo]. (E) Sagittal section through neural-crest-free sclerotomes of a stage-17 embryo stained with HNK-1 (red) and KAF9(1) (green). In this embryo, both halves of the neural-crest-free sclerotomes display KAF9(1) immunoreactivity. Scale bars: 100 μm in A 50 μ m in B–E. The head of the embryo lies towards the left of each photograph, a, aorta; d, dermomyotome; n, notochord; t, neural tube.

In another series of preliminary experiments, a length equivalent to 3-15 segments of the entire dorso-ventral extent of the neural tube was removed, and the distribution of HNK-l-positive cells examined in sections after 36–48h incubation. In these embryos, the HNK-l-positive cells did not migrate rostrocaudally from the edges of the operation site. However, the integrity of the somites opposite the operated region was affected: the somites were always fused in the midline and the segmental arrangement of somitic mesoderm was often disrupted.

For these reasons, all our neural crest ablation experiments were performed on embryos from which the dorsal half of the neural tube was excised over 10–15 segments. In these embryos, after 36–48 h postoperative incubation, the somites appeared normal and at least five sclerotomes opposite the operated region were always found to be devoid of HNK-l-immuno-reactive cells.

The distribution of M1B4, MAb578 or KAF9(1) staining in sclerotomes devoid of neural crest cells at stages 18 –22 was rather different from that seen in unoperated sclerotomes, and different from the pattern reported by Tan et al. (1987). Young sclerotomes in the operated region displayed immunoreactivity predominantly in the caudal half of the sclerotome (Fig. 2C) (as in unoperated embryos). However, in most embryos (39/56; 70%), this pattern was unchanged in older sclerotomes opposite the operated region (Fig. 2 D). In some embryos (13/56; 23 %), immunoreactive material was seen in both halves of the sclerotomes (Fig. 2 E), in three embryos (5%) the sclerotomes appeared to be devoid of immunoreactivity in the operated region and in one embryo staining was restricted to the rostral halves of sclerotomes devoid of HNK-l-immuno-reactive cells.

To investigate whether the difference between operated and control embryos in the pattern of Jl/tenascin immunoreactivity is due to a contribution of the neural crest cells or to a developmental delay of the operated embryos, three embryos from which the neural crest had been removed as described above were incubated for a total of 3-5 days after the operation, until they had reached stages 25–27. The pattern of Jl/tenascin immunoreactivity in these embryos was similar to that seen in operated embryos incubated to stages 18–22: in every case, Jl/tenascin immunoreactive material was found in both halves of the neural-crest-free sclerotomes.

Biochemical characterisation of J1/tenascin-related antigens

Purification of J1/tenascin-related antigens was performed using a column of immobilised M1B4 IgG, followed by SDS–polyacrylamide gel electrophoresis (SDS-PAGE) to identify the eluted antigens under nonreducing conditions.

In preliminary experiments, the specificity of the three antibodies (M1B4, KAF9(1) and MAb578) for Jl/tenascin-related molecules was assessed in immunoblots of a sample of purified mouse brain tenascin separated by SDS-PAGE under nonreducing conditions. Silver-stained gels obtained from this sample showed a characteristic pattern of bands, summarised in Table 2. In immunoblots, both KAF9(1) and MAb578 recognised many of the bands seen in silver-stained gels, of which a band of Mr = 230 ×103 was most prominent (Fig. 3). The avian-specific antibody M1B4, by contrast, did not recognise any components in this mouse tenascin preparation.

Table 2.

Apparent relative molecular masses of J1 / tenascin-related molecules (Mr × 10 −3)

Apparent relative molecular masses of J1 / tenascin-related molecules (Mr × 10 −3)
Apparent relative molecular masses of J1 / tenascin-related molecules (Mr × 10 −3)
Fig. 3.

Western blots of mouse brain tenascin showing specificity of antibodies used for Jl/tenascin. Immunoreactivity was detected using the AuroProbe system. Odd-numbered lanes: non-reduced mouse brain tenascin. Even-numbered lanes: reduced mouse brain tenascin. Lanes 1 and 2 were probed with KAF9(1). Lanes 3 and 4 were probed with MAb578. Lanes 5 and 6 were probed with M1B4 (which is specific for avian tenascin).

Fig. 3.

Western blots of mouse brain tenascin showing specificity of antibodies used for Jl/tenascin. Immunoreactivity was detected using the AuroProbe system. Odd-numbered lanes: non-reduced mouse brain tenascin. Even-numbered lanes: reduced mouse brain tenascin. Lanes 1 and 2 were probed with KAF9(1). Lanes 3 and 4 were probed with MAb578. Lanes 5 and 6 were probed with M1B4 (which is specific for avian tenascin).

In samples of somite tissue obtained from unoperated embryos at stage 18–22, several bands were seen in silver-stained gels of eluted material. Their Mr ranged from 39 to 230×103 (Fig. 4). Table 2 summarises the relative molecular masses of the polypeptides seen in these gels. The highest relative molecular mass component seen appeared to be similar to the major component (c. 230 × 103) found in silver-stained gels and immunoblots of samples of purified mouse tenascin (Figs 3, 4). The sample of purified tenascin contained a higher relative molecular mass component which may represent a multimer (hexamer, Mr approximately 106 = ‘hexahrachion’; Chiquet and Fambrough, 1985; Mackie et al. 1988), but this was not seen in samples obtained from chick embryo somites.

Fig. 4.

Silver-stained gels of somite material purified by affinity to M1B4 antibody. Lane 1, relative molecular mass markers. Lane 2, mouse brain tenascin (nonreduced; not affinity purified through M1B4 column). Lane 3, sclerotomes from unoperated stage-18 embryos. Lane 4, sclerotomes from neural-crest-ablated stage-18 embryos. Lane 5, stage-13 epithelial somites with neural tube and notochord. Lane 6, stage-13 epithelial somites without other axial tissues. The molecular masses of the bands seen in these samples are given in Table 2.

Fig. 4.

Silver-stained gels of somite material purified by affinity to M1B4 antibody. Lane 1, relative molecular mass markers. Lane 2, mouse brain tenascin (nonreduced; not affinity purified through M1B4 column). Lane 3, sclerotomes from unoperated stage-18 embryos. Lane 4, sclerotomes from neural-crest-ablated stage-18 embryos. Lane 5, stage-13 epithelial somites with neural tube and notochord. Lane 6, stage-13 epithelial somites without other axial tissues. The molecular masses of the bands seen in these samples are given in Table 2.

Samples obtained from neural-crest-ablated embryos displayed a different pattern of silver-stained bands after elution from the M1B4 column (Table 2). The six bands of highest relative molecular mass (120, 130,140, 150, 195 and 230×103) seen in unoperated embryos were missing. Some lower relative molecular mass components were also absent. Some bands, however, were present in samples from both unoperated and neural-crest-ablated embryos (Mr 39, 49. 52, 58 and 70×103).

Antigens were also detected in samples of epithelial somites from stage-13 to -14 embryos subjected to affinity purification through the M1B4 column; some differences were found between those samples that included the neural tube and notochord and those that did not. Gels are shown in Fig. 4 and the molecular masses of the bands summarised in Table 2. The pattern of bands seen in this material was different from that seen in similarly treated sclerotome from unoperated embryos. The most conspicuous difference was the absence of the 230×103 component, as seen in neural-crest-ablated embryos. However, other components absent from neural-crest-ablated sclerotomes were seen in the epithelial somite preparations (Table 2).

We have used two monoclonal antibodies (M1B4 and 578) and one antiserum (KAF9(1)] to study the expression of Jl/tenascin-related molecules in somite tissue of the chick embryo. Young somites are surrounded by immunoreactive material, while old sclerotomes display expression of these molecules predominantly in their rostral halves, as previously described by other authors (Tan et al. 1987; Mackie et al. 1988). Sclerotomes at intermediate stages of development, however, often display immunoreactivity in the opposite (caudal) half of the sclerotome to that occupied by neural crest cells (Fig. 5). Our results therefore differ from those of Tan et al. (1987), who stated that ‘at the time of sclerotome formation, cytotactin began to be expressed in the rostral half of the sclerotome, and neural crest cell invasion occurred in this area; and from those of Mackie et al. (1988). Also in contrast to Tan et al.’s (1987) observations, we find that after neural crest ablation, immunoreactivity is not restricted to the rostral half of the sclerotome. SDS-PAGE of affinity-purified somite tissue reveals a characteristic pattern of bands of various relative molecular masses, including two bands at about 195 and 230×103, as described for cytotactin, Jl-200/220 and the monomeric form of tenascin (Chiquet and Fambrough, 1984; Grumet et al. 1985; Kruse et al. 1985; Vaughan et al. 1987; Faissner et al. 1988; Hoffman et al. 1988). After ablation of the neural crest, somite material analysed in the same way still shows some of the bands seen in unoperated embryos, but several components are missing, including the six of highest relative molecular mass (120–230 ×103).

Fig. 5.

Diagram summarising the changes in the distribution of Jl/tenascin immunoreactivity in relation to the onset of neural crest migration and motor axon outgrowth through the sclerotome in the presence and absence of neural crest cells. Each of the two embryos is depicted at about stage 16, with its caudal end towards the left of the picture, so that younger somites appear towards this end. The upper diagram shows the distribution of neural crest cells and of Jl/tenascin immunoreactivity in an unoperated embryo, while the lower cartoon illustrates the pattern of Jl/tenascin after neural crest removal. In this lower diagram, the results are presented as if the neural crest were absent over the entire extent of the embryonic axis. The line below the diagrams gives an approximate indication of the time scale of the changes that occur and points to the level at which the motor axons are starting to enter the sclerotome.

Fig. 5.

Diagram summarising the changes in the distribution of Jl/tenascin immunoreactivity in relation to the onset of neural crest migration and motor axon outgrowth through the sclerotome in the presence and absence of neural crest cells. Each of the two embryos is depicted at about stage 16, with its caudal end towards the left of the picture, so that younger somites appear towards this end. The upper diagram shows the distribution of neural crest cells and of Jl/tenascin immunoreactivity in an unoperated embryo, while the lower cartoon illustrates the pattern of Jl/tenascin after neural crest removal. In this lower diagram, the results are presented as if the neural crest were absent over the entire extent of the embryonic axis. The line below the diagrams gives an approximate indication of the time scale of the changes that occur and points to the level at which the motor axons are starting to enter the sclerotome.

Ablation of the neural crest

In agreement with other workers (e.g. Yntema and Hammond, 1954; Rickmann et al. 1985; Teillet et al. 1987), we have found that ablation of a short length (six segments or less) of the dorsal half of the neural tube allows neural crest cells from the cut ends of the tube to colonise the rostral halves of the sclerotomes in the operated region. Under these conditions, neural crest cells appear to be able to migrate for about three segments along the rostrocaudal axis. If the whole thickness of the neural tube is ablated, moreover, the integrity of the adjacent somites is disrupted, as reported by Strudel (1955) and Teillet and Le Douarin (1983). In order to obtain neural-crest-free and morphologically normal sclerotomes, therefore, it was necessary to extirpate a length of dorsal neural tube at least seven segments in length.

In Tan et al.’s (1987) experiments, the neural crest was ablated by removing a six-segment length of an unspecified depth of neural tube. In our hands, if only the dorsal portion of the tube was removed, such an operation would have allowed neural crest cells to colonise all the sclerotomes in the operated region. If, on the other hand, the whole depth of neural tube was removed, the operation would have caused the somites to display abnormal segmentation. Moreover, Tan et al. (1987) ablated the neural tube opposite the last six pairs of segments of 15- to 18-somite embryos; at this stage, the most rostral one or two of these pairs already contain neural crest cells (Rickmann et al. 1985).

Tan et al. (1987) argue that, after ablation of the neural crest, cytotactin expression in the sclerotomes is similar to that seen in unoperated embryos, that is, restricted to the rostral half of each sclerotome. Our results do not agree with this interpretation. While the pattern of expression of Jl/tenascin-related molecules is still segmental after ablation of the neural crest, immunoreactivity fails to become restricted to the rostral halves of the sclerotomes in the absence of neural crest cells.

Molecular nature of Jl/tenascin-related antigens

Removal of the neural crest leads to a change in the distribution of Jl/tenascin immunoreactivity and in the proportions of the various molecules extracted by affinity to M1B4 antibody. In the absence of neural crest cells, the higher relative molecular mass bands (120–230×103) characteristic of cytotactin, Jl–160/180, J1–200/220 and monomeric tenascin are absent, and lower relative molecular mass components predominate. Epithelial somites from younger embryos do not display the higher molecular mass (230×103) band. Many of the lower relative molecular mass components are present in a purified sample of tenascin prepared from mouse brain.

Several possibilities must be considered in attempting to identify the various components seen in gels. First, it is possible that at least some of them are degradation products of the larger macromolecule. Although this is probably responsible for some of the lower molecular mass components, it seems unlikely to be the only factor accounting for our results, because expression of the higher relative molecular mass components can be dissociated from expression of the lower relative molecular mass bands (in the absence of neural crest and in younger somites). While it might be argued that the operation of neural crest removal itself releases proteolytic enzymes, control embryos destined for biochemical investigations were subjected to cuts in and around the neural tube (without removing the neural crest). Moreover, sections of operated embryos did not show any differences in the pattern of immunoreactivity outside the operated region, suggesting that diffusible proteases are not released from the operation site.

A second possibility to consider is that the smaller components are not related to Jl/tenascin directly, but are ligands for molecules containing the epitope recognised by M1B4, which copurify with M1B4 antigens during affinity extraction. However, staining is seen with all three antibodies in the absence of neural crest cells and in young somites, despite the finding that the higher molecular mass components are absent from these tissues.

A final possibility is that the molecules recognised by the antibodies used in this study are not single molecules but rather complexes, or associations of several molecules of various molecular masses. The possibility of a single promiscuous epitope present on many different molecules can probably be excluded because the two monoclonal antibodies (M1B4 and 578) recognise different epitopes. It is possible, however, that the multiple bands seen in silver-stained gels from affinity-purified Jl/tenascin-related molecules represent whole portions of the larger molecule(s), which may be used by the embryo in different contexts, for example as precursors for the assembly of the higher molecular mass components.

The results of this study therefore suggest that a group of molecules, immunologically related to Jl/ tenascin, is expressed in the sclerotome of the chick embryo, and that the expression of the characteristic 120-–230×103 components may be regulated independently of the expression of the lower relative molecular mass forms. Further speculations on their function will have to await characterisation by antibodies that can distinguish between them, as well as knowledge of their amino-acid sequence.

Are Jl/tenascin-related molecules produced by neural crest or by sclerotome cells?

It was found both by Tan et al. (1987) and by Mackie et al. (1988) that, in vitro, neural crest cells cultured alone do not produce cytotactin or tenascin, while somite cultures do. However, it is not clear whether Tan et al.’s (1987) somite cultures contained neural crest cells, which begin to colonise the somitic mesenchyme as soon as the sclerotome can be distinguished (see Rickmann et al. 1985; Bronner-Fraser, 1986; Teillet et al. 1987). Jl/tenascin immunoreactivity in older sclerotomes is localised both to some of the cells expressing the HNK-1 and/or 20B4 epitopes (the two neural crest markers) and to some not expressing them (Fig. 1 E, F). Since recently formed epithelial somites, which do not yet contain any neural crest cells, already contain cytotactin and tenascin immunoreactivity (Tan et al. 1987; Mackie et al. 1988) within the somite lumen, Mackie et al. (1988) suggested that the somite cells themselves secrete these molecules into the lumen. Our results suggest that the Jl/tenascin-immunoreactive material detected in and around epithelial somites is due to molecular components different from those detected in older sclerotome/neural crest tissue. Taken together, therefore, these findings suggest that the presence of neural crest cells is required for expression of the higher molecular mass Jl/tenascin-related components. One possibility, suggested by our results on the relative molecular masses of the components isolated from epithelial somite tissue samples with and without neural tube/notochord, is that the somite cells produce some of the precursor chains of the tenascin molecule while the neural crest or other notochord or neural tube-derived tissues supply other chains. This accounts for our conclusion that both the neural crest and the somite are required for expression of the higher molecular weight components of Jl/tenascin.

Do Jl /tenascin-related molecules play a controlling role in neural crest migration and/or axon ougtrowth through the sclerotome?

Our findings appear to resolve the paradox presented by the results of Tan et al. (1987), Mackie et al. (1988) and Halfter et al. (1989). These authors suggested that cytotactin and tenascin, respectively, are restricted to the rostral half of each sclerotome, but that these molecules are inhibitory to neural crest migration and spreading in vitro. Our results show that, at the time of onset of neural crest migration through the rostral halves of sclerotomes in embryos between stages 13 and 16, Jl/tenascin-related molecules are present in the caudal halves of the sclerotomes, a finding that is more consistent with an inhibitory role of these molecules for neural crest migration in vivo. It may be interesting to note in this context that the notochord, which inhibits the migration of neural crest cells (Newgreen et al. 1986; Guillory and Bronner-Fraser, in preparation; Stern and Bronner-Fraser, in preparation), is immunoreactive with antibodies that recognise Jl/tenascin-related molecules (Fig. 1 B-C, E-F); it is therefore possible that these molecules contribute to the notochordal inhibition of neural crest migration.

The distribution of the cytotactin-binding proteoglycan (‘CTB proteoglycan’), reported by Tan et al. (1987) to become localised to the caudal halves of the sclerotomes, should also be considered. Because CTB proteoglycan binds peanut lectin (PNA), and because this lectin has been shown to stain the caudal halves of the sclerotomes of the chick embryo (Stern et al. 1986), CTB proteoglycan could be responsible for the pattern of staining seen with PNA. However, peanut lectin binding is never seen in both halves of the sclerotome, as described for CTB proteoglycan at early stages of somite development. Furthermore, preliminary experiments have shown that at the stages of development when PNA binding to caudal sclerotome cells is seen, several PNA-binding molecules of diverse molecular masses from 17 ×103 upwards are expressed differentially in the two halves of the sclerotome (Davies, Cook, Norris, Keynes and Stern, in preparation). Neither of these observations supports the hypothesis that CTB proteoglycan is responsible for the pattern of PNA staining in the sclerotome.

If cytotactin/Jl/tenascin played a controlling role in determining the migration pathways of trunk neural crest cells, one would expect their temporal and spatial patterns to correlate. However, this is not the case. When the first neural crest cells enter the rostral halves of the first sclerotomes to form (at about the 8 somite stage, stage-9; Thiery et al. 1982; Rickmann et al. 1985; Bronner-Fraser, 1986; Lim et al. 1987; Loring and Erickson, 1987; Teillet et al. 1987; Tosney, 1988), these sclerotomes display very little, if any, cytotactin/Jl/ tenascin immunoreactivity (Crossin et al. 1987; Tan et al. 1987; Mackie et al. 1988 and the present observations). Neural crest cells entering newly formed sclerotomes between stage-13 and stage-16 do so when these sclerotomes display immunoreactivity mainly in their caudal halves (present observations). Finally, neural crest cells entering the sclerotomes that form after stage-16 do so when Jl/tenascin immunoreactivity begins to predominate in the rostral halves of these sclerotomes (Tan et al. 1987; Mackie et al. 1988 and the present observations). Thus, some neural crest cells enter sclerotomes devoid of Jl/tenascin or the rostral halves of sclerotomes expressing Jl/tenascin in their caudal halves, while yet others enter the Jl/tenascin-containing halves. These results argue strongly that cytotactin/Jl/tenascin-related molecules cannot be solely responsible for the segmented pattern of migration of neural crest cells through the sclerotome.

Motor axon outgrowth through the rostral halfsclerotome, however, begins at about stage 17 (Keynes and Stern, 1984), shortly after the onset of cytotactin and of Jl/tenascin expression in the rostral half-sclerotome. Is it possible that these molecules determine the segmented pattern of motor axon outgrowth? Our results argue strongly that this is not the case. Ablation of the neural crest leads to a change in the spatial pattern of expression of Jl/tenascin-related molecules in the sclerotome, but the same operation does not disturb the pattern of axon outgrowth through the rostral half-sclerotome (Rickmann et al. 1985). Therefore, it can be concluded that Jl/tenascin-related molecules are not responsible for the segmented pattern of either neural crest migration or motor axon outgrowth.

There remains the possibility, nevertheless, that Jl/tenascin-related molecules play a role in the subsequent morphogenesis of neural crest derivatives, for example during dorsal root ganglion formation. This possibility is suggested by our finding that the neural crest appears to participate in the production of molecules capable of inhibiting its own migration. The time interval between the entrance of neural crest cells into the rostral half of a sclerotome and the change in the localisation of Jl/tenascin-immunoreactivity appears to be equivalent to about 8-10 somites, in turn equivalent to about 12-15 h. It is possible that such a mechanism acts to limit the number of neural crest cells entering the sclerotome. It may also prevent the further migration of neural crest cells in preparation for dorsal root ganglio-genesis, which occurs at about stage 21 (Lallier and Bronner-Fraser, 1988), and may therefore determine a balance between the number of neural crest cells destined to contribute to the autonomic nervous system and the number destined to form sensory neurones.

Our results provide strong evidence that Jl/tenascin-related molecules are not responsible for the segmented pattern of neural crest cell migration or motor axon outgrowth. Rather, they suggest that the neural crest modulates the expression of these molecules in the sclerotome, although it is not clear whether the neural crest itself is responsible for their production. Jl/tenascin-related molecules seem to have an inhibitory effect on neural crest migration both in vivo and in vitro and could play a role in gangliogenesis and the later morphogenesis of neural crest derivatives. Our results suggest that the names ‘tenascin’, ‘Substrate Adhesion Molecules (SAMs)’ and ‘cytotactin’ given to these molecules (e.g. Grumet et al. 1985) do not suit their apparent functions as judged by their pattern of expression in the developing chick embryo. While these terms suggest strong adhesion, cellular affinity and attraction of migrating cells, our results point towards a possible inhibitory role of these molecules in patterning the peripheral nervous system after the initial stages of its development.

This work was supported by a grant from Action Research for the Crippled Child to CDS. MBF, who is a Sloan Foundation Research Fellow, is supported by grants from USPHS (HD-15527), March of Dimes (1-896), and the Muscular Dystrophy Association. AF and MS are supported by the German Research Society. MS is a member of the Swiss Federal Institute of Technology, Zürich.

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