The origin of the prelung cells was determined by tracing the movements of [3H]thymidine-labelled grafts excised from medium-streak to 4-somite stage chick embryos and transplanted to the epiblast, streak, and endoderm-mesoderm of similarly staged recipient embryos.

At the medium-streak stage the prelung endoderm cells are in the anterior third of the primitive streak; they shortly begin to migrate anteriorly and laterally into the endoderm layer. They are folded into the gut beginning at about the 4-somite stage, and begin to reach their definitive position in the ventrolateral gut wall at the 10-to 16-somite stage. At the ± 22-somite stage the prelung endoderm begins to burrow into the overlying splanchnic layer of mesoderm, pushing the prelung mesoderm ahead of it.

At the medium-streak stage the prelung mesoderm is in the epi blast (dorsal) layer about half-way to the lateral margin of the area pellucida on either side of the streak, at a level about half-way between the anterior and posterior ends of the streak. From this position the prelung mesoderm migrates medially to the streak and is invaginated into the mesoderm layer at a position about half-way between the anterior and posterior ends of the streak. As a section of the dorsal mesentery, it migrates anteriorly and laterally from the streak into the splanchnic mesoderm lateral to the somites. From the head process stage to the early somite stages, the prelung mesoderm is located posterior to the prelung endoderm. The prelung mesoderm continues to migrate with the splanchnic mesoderm into the mesentery dorsal to the heart, where it invests the prelung endoderm after the 16-to 19-somite stage. Beginning at about the 22-somite stage, the prelung endoderm penetrates the prelung mesoderm and the bilateral bronchi are formed.

The radioautographic mapping of the primitive-streak to head-process stage chick embryo (Rosenquist, 1966) demonstrated that the endoderm which wilt form the ventral side of the gut originates in the anterior end of the streak, while the lateral plate mesoderm which covers the gut originates in the epiblast layer lateral to the primitive streak. In this previous mapping study the differentiation of the lung from these tissues was not described because embryos older than the 17-somite stage were not investigated.

The development of the lung in birds has been described by Locy & Larsell (1916). The lung primordia develop when endoderm buds arising from the ventral wall of the gut burrow into the adjacent splanchnic mesoderm and extend ventrally, laterally and posteriorly from their point of origin into the coelomic space dorsal to the heart (Figs. 1J, 3 J). Using radioautographic analysis, the present investigation traces the movements of [3H]thymidine-labelled transplants from their original positions in the epiblast and streak of recipient embryos into the lung buds of these embryos.

Fig. 1.

The location of grafts from embryos 1–19 is shown. At the medium-streak (MS) stage, the prelung endoderm (contained in the shaded areas which represent the endodermal part of each graft) is located in the anterior part of the streak. It migrates from the streak, beginning at about the late medium-streak (LMS) stage. By the head-process (HP) stage, the prelung endoderm has moved into area pellucida about half-way between the lateral margin and the streak, extending a short distance anteriorly from the anterior end of the streak (see also Fig. 5, left) into the endodermal region destined for the ventral wall of the gut. From the head-fold (HF) to the .16-to 18-somite (16–18 S) stage the prelung endoderm moves progressively closer to the anterior intestinal portal (E, F) and is invaginated into the ventrolateral gut wall dorsal to the heart (G, H). At the ± 22-somite stage, the prelung endoderm begins to burrow into the overlying prelung mesoderm. The beginnings of bronchus formation are noted at the early limb-bud (ELB) stage. As in the text, an asterisk (*) after the embryo number indicates that the position shown is that of the graft after its migration in the host embryo. Each graft is therefore shown in two positions: immediately after it was placed, and after incubation. DS = definitive-streak stage.

Fig. 1.

The location of grafts from embryos 1–19 is shown. At the medium-streak (MS) stage, the prelung endoderm (contained in the shaded areas which represent the endodermal part of each graft) is located in the anterior part of the streak. It migrates from the streak, beginning at about the late medium-streak (LMS) stage. By the head-process (HP) stage, the prelung endoderm has moved into area pellucida about half-way between the lateral margin and the streak, extending a short distance anteriorly from the anterior end of the streak (see also Fig. 5, left) into the endodermal region destined for the ventral wall of the gut. From the head-fold (HF) to the .16-to 18-somite (16–18 S) stage the prelung endoderm moves progressively closer to the anterior intestinal portal (E, F) and is invaginated into the ventrolateral gut wall dorsal to the heart (G, H). At the ± 22-somite stage, the prelung endoderm begins to burrow into the overlying prelung mesoderm. The beginnings of bronchus formation are noted at the early limb-bud (ELB) stage. As in the text, an asterisk (*) after the embryo number indicates that the position shown is that of the graft after its migration in the host embryo. Each graft is therefore shown in two positions: immediately after it was placed, and after incubation. DS = definitive-streak stage.

Fig. 2.

(A) Cross-section ( × 200) showing labelled cells (black dots over nuclei) from a graft placed originally in the endoderm-mesoderm layer of a 2-somite stage recipient embryo. The labelled endoderm in the graft has migrated from its original position lateral and slightly posterior to the anterior intestinal portal (embryo 19, Fig. 1F) into the ventrolateral wall of the gut at the early limb-bud stage, where some of the cells have formed part of the wall of the lung bud (embryo 19*, Table 1, Fig. 1 J). (B) Cross-section ( × 200) showing labelled cells from a graft placed originally in the endoderm-mesoderm layer of a head-fold stage recipient embryo. The labelled mesoderm in the graft has migrated from its original position slightly lateral and anterior to the anterior end of the streak (embryo 37, Fig. 3E) into the proximal part of the splanchnic layer of the lateral plate mesoderm, and has invested the unlabelled lung bud on the right side of the embryo (embryo 37*, Table 1, Fig. 3 J).

Fig. 2.

(A) Cross-section ( × 200) showing labelled cells (black dots over nuclei) from a graft placed originally in the endoderm-mesoderm layer of a 2-somite stage recipient embryo. The labelled endoderm in the graft has migrated from its original position lateral and slightly posterior to the anterior intestinal portal (embryo 19, Fig. 1F) into the ventrolateral wall of the gut at the early limb-bud stage, where some of the cells have formed part of the wall of the lung bud (embryo 19*, Table 1, Fig. 1 J). (B) Cross-section ( × 200) showing labelled cells from a graft placed originally in the endoderm-mesoderm layer of a head-fold stage recipient embryo. The labelled mesoderm in the graft has migrated from its original position slightly lateral and anterior to the anterior end of the streak (embryo 37, Fig. 3E) into the proximal part of the splanchnic layer of the lateral plate mesoderm, and has invested the unlabelled lung bud on the right side of the embryo (embryo 37*, Table 1, Fig. 3 J).

The methods of staging and preparation of recipient and [3H]thymidine-labelled donor embryos and of transplantation and radioautographic analysis of the grafts were identical to those described in a previous publication (Rosen-quist, 1970), and the description will not be repeated here.

Although part of each of the forty transplants illustrated in Figs. 1 and 3 lay in the prelung region, the number of embryos investigated was relatively small, and each transplant contained cells other than prelung cells (as indicated in Table 1). Therefore the positions of the transplants at each stage in Figs. 1 and 3 suggest the location of the prelung region at that stage, but do not define it precisely. The mapping of the prelung region is based upon the following assumptions: (1) that previous studies have established the general position of the endoderm which will form the gut and the lateral plate mesoderm in the early stages of development without mapping every part of these tissues at each stage (Rosenquist, 1966). Consequently, a small number of transplants, carefully placed, can demonstrate the position of more specific portions of the embryo, such as the lung. (2) That graft positions in different embryos at the same stage are homologous even if the embryos were incubated for different lengths of time, and that the pathways followed by more than one accurately placed graft can be used to follow movements of a group of cells through several stages of development. (3) That maps of presumptive organ-forming regions of the embryo are valid even if structures other than lung buds in the recipient embryos contain labelled cells.

Table 1.

Position of labelled cells in recipient embryos carrying tritiated thymidine-labelled grafts*

Position of labelled cells in recipient embryos carrying tritiated thymidine-labelled grafts*
Position of labelled cells in recipient embryos carrying tritiated thymidine-labelled grafts*
Fig. 3.

The location of grafts from embryos 20–40 is shown. At the medium-streak (MS) stage the prelung mesoderm (contained in the shaded areas which represent the mesodermal part of each graft) is located in the epiblast layer (left half of each figure). It migrates progressively toward the streak, which it reaches at about the late medium-streak (LMS) stage. By the definitive-streak (DS) stage, some of the prelung mesoderm has migrated through the streak into the lateral plate mesoderm (right half of each figure), reaching a position anterior to the anterior end of the streak some time after the head-process (HP) stage. As part of the most proximal lateral plate mesoderm (i.e. that which is adjacent to the nephrotome), it moves progressively toward the anterior intestinal portal, and is folded with the splanchnic layer to the midline of the embryo ventral to the gut, where by the 15-to 17-somite (15–17 S) stage, it has begun to invest the ventral wall of the gut. By the 22-somite stage the bulging anteroventral wall of the gut has begun to penetrate the prelung mesoderm; the latter invests the bronchi on each side of the embryo by the early limb-bud(ELB) stage. As in Fig. 1, each graft is shown in two positions: immediately after it was placed, and after incubation (marked with an asterisk). HF = headfold stage.

Fig. 3.

The location of grafts from embryos 20–40 is shown. At the medium-streak (MS) stage the prelung mesoderm (contained in the shaded areas which represent the mesodermal part of each graft) is located in the epiblast layer (left half of each figure). It migrates progressively toward the streak, which it reaches at about the late medium-streak (LMS) stage. By the definitive-streak (DS) stage, some of the prelung mesoderm has migrated through the streak into the lateral plate mesoderm (right half of each figure), reaching a position anterior to the anterior end of the streak some time after the head-process (HP) stage. As part of the most proximal lateral plate mesoderm (i.e. that which is adjacent to the nephrotome), it moves progressively toward the anterior intestinal portal, and is folded with the splanchnic layer to the midline of the embryo ventral to the gut, where by the 15-to 17-somite (15–17 S) stage, it has begun to invest the ventral wall of the gut. By the 22-somite stage the bulging anteroventral wall of the gut has begun to penetrate the prelung mesoderm; the latter invests the bronchi on each side of the embryo by the early limb-bud(ELB) stage. As in Fig. 1, each graft is shown in two positions: immediately after it was placed, and after incubation (marked with an asterisk). HF = headfold stage.

Throughout the text and figures, an asterisk (*) after the embryo number indicates that the position shown is that of the graft after its migration in the host embryo.

Prelung endoderm

At the medium-streak stage the presumptive prelung endoderm cells were in the anterior third of the primitive streak, as illustrated by the migration of the grafts in embryos 1 and 2 (Table 1, Fig. 1 A). At the late medium-streak stage there were still some prelung cells in the streak, but some also were found in the endoderm layer anterior and lateral to the anterior end of the streak (embryos 3-5, Table 1, Fig. 1B). At the definitive-streak stage some of the prelung endoderm remained clustered around the anterior end of the streak (embryos 1*, 6, 7, Table 1, Fig. 1C). At the head-process stage (embryos 4*, 5*, 8–16, Table 1, Fig. 1 D) the prelung endoderm had migrated away from the streak into the zone of endoderm which is destined for the ventrolateral wall of the gut (Fig. 5, left). By the head-fold to 1-somite stage, the prelung cells had moved away from the anterior end of the streak, either by actual migration in an anterior direction or as a result of the posterior regression of the streak. The transplants in embryos 7*, 9*, 11*, 17 and 18 (Table 1, Fig. 1E) were located between the head-fold and streak, but like the transplants at the 2-to 4-somite stage they had not as yet been invaginated into the gut (embryos 10*, 19, Table 1, Fig. 1F).

Fig. 4.

Hatched regions in each figure represent prelung material identified by previous investigators using grafts to chorioallantoic membrane. A and C, Rudnick (1933); B and D, Hunt (1937); E, Rawles (1936). (White area, C, was not tested.) The position of the presumptive prelung material shown here is consistent with the findings of the present study; however, these previous investigators used very large grafts and did not distinguish between the prelung cells of the epiblast and those of the hypoblast. Their experiments were therefore unsuitable for detailed tracing of the morphogenetic movements of the pulmonogenic cells. DS = definitive-streak stage; HP = head-process stage.

Fig. 4.

Hatched regions in each figure represent prelung material identified by previous investigators using grafts to chorioallantoic membrane. A and C, Rudnick (1933); B and D, Hunt (1937); E, Rawles (1936). (White area, C, was not tested.) The position of the presumptive prelung material shown here is consistent with the findings of the present study; however, these previous investigators used very large grafts and did not distinguish between the prelung cells of the epiblast and those of the hypoblast. Their experiments were therefore unsuitable for detailed tracing of the morphogenetic movements of the pulmonogenic cells. DS = definitive-streak stage; HP = head-process stage.

Fig. 5.

At the head-process stage (HP, left) most of the prelung endoderm (broken cross-hatching) is located at the posterior ends of the crescent-shaped region which has invaginated into the ventral gut by the 16-somite stage (Rosenquist, 1966; 16 S right). Solid cross-hatching lateral and posterior to the crescent (left) represents prelung endoderm (see also Fig. 1D) located in the yolk-sac surface near the anterior intestinal portal at the 16-somite stage (right); the present study suggests that this material is inverted into the prelung portion of the gut after the 16-somite stage. In right and left drawings the line ABCB′A′ represents the boundary between endoderm cells destined for yolk sac and ventral gut, while the line AD EFE′ D′ A′ represents the boundary between endoderm cells destined for ventral and dorsal gut. By the 16-somite stage (right) all of the endoderm cells along the line A BCB′A′ have converged to form the lip of the anterior intestinal portal; the material inside the crescent (left) has been inverted and compressed to form the thickened ventral wall of the gut. Thus, the triangular zones of prelung endoderm ABD and A′B′D′ (left) have inverted to form the triangles ADB and A′D′B′ (right). The crosssection through the gut at the level of D and D′ (center) illustrates the position of the prelung endoderm (cross-hatching) along the ventrolateral wall of the gut (see also Fig. 1H). The three figures are drawn to scale.

Fig. 5.

At the head-process stage (HP, left) most of the prelung endoderm (broken cross-hatching) is located at the posterior ends of the crescent-shaped region which has invaginated into the ventral gut by the 16-somite stage (Rosenquist, 1966; 16 S right). Solid cross-hatching lateral and posterior to the crescent (left) represents prelung endoderm (see also Fig. 1D) located in the yolk-sac surface near the anterior intestinal portal at the 16-somite stage (right); the present study suggests that this material is inverted into the prelung portion of the gut after the 16-somite stage. In right and left drawings the line ABCB′A′ represents the boundary between endoderm cells destined for yolk sac and ventral gut, while the line AD EFE′ D′ A′ represents the boundary between endoderm cells destined for ventral and dorsal gut. By the 16-somite stage (right) all of the endoderm cells along the line A BCB′A′ have converged to form the lip of the anterior intestinal portal; the material inside the crescent (left) has been inverted and compressed to form the thickened ventral wall of the gut. Thus, the triangular zones of prelung endoderm ABD and A′B′D′ (left) have inverted to form the triangles ADB and A′D′B′ (right). The crosssection through the gut at the level of D and D′ (center) illustrates the position of the prelung endoderm (cross-hatching) along the ventrolateral wall of the gut (see also Fig. 1H). The three figures are drawn to scale.

In contrast, at the 9-to 10-somite stage the grafts in embryos 2*, 3* and 8* (Table 1, Fig. 1G) had been partially folded into the gut. By the 16-to 18-somite stage most of the prelung endoderm had been invaginated into the gut, where it comprised the ventrolateral wall (embryos 12*, 13*, Table 1, Fig. 1H). At the ± 22-somite stage all of the prelung endoderm had been inverted from yolk sac to ventrolateral gut wall. The gut had been invested by the layer of splanchnic mesoderm which was the dorsal mesoderm of the heart prior to its looping. In embryos 6* and 14*–18* (Fig. II) there were bilateral bulges on the anterolateral wall of the gut which were the beginnings of bronchus formation. The labelled grafts participated in the development; however, in none of these embryos had these bulges branched to form bronchi at the time the embryo was fixed. At the early limb-bud stage bronchi had begun to form, as the prelung endoderm burrowed into the splanchnic mesoderm (embryo 19*, Table 1, Figs. 1 J, 2 A).

Prelung mesoderm

At the medium-streak stage the presumptive prelung mesoderm was in the epiblast layer at a level about one-third to one-half the distance from the anterior to the posterior end of the streak and about half-way between the streak and the lateral margin of the area pellucida (embryos 20 and 21, Table 1, Fig. 3 A). At the late medium-streak stage, prelung mesoderm was found at a level approximately one-third to one-half the distance between the anterior and posterior ends of the streak, either in the streak or in the adjacent epiblast (embryos 22-25, Table 1, Fig. 3B). At the definitive-streak stage, the prelung mesoderm had begun to migrate through the streak into mesoderm (embryo 26, Table 1, Fig. 3C), although some was still found in the streak and adjacent epiblast (embryos 21*, 27, Table 1, Fig. 3C). By the head-process stage, the prelung mesoderm had migrated away from the streak into the mesoderm destined for the part of the lateral plate between the nephrotome and the heartforming region (embryos 26* and 28-36, Table 1, Fig. 3D). At the head-fold to 1-somite stage, the prelung mesoderm was located even farther from the streak, but posterior to the prelung endoderm (compare embryos 22*, 24*, 28* and 37, Table 1, Fig. 3E, with embryos 7*, 9*, 11*, 17 and 18, Table 1, Fig. 1E). At the 2-to 3-somite stage, the prelung mesoderm was in the mesoderm layer lateral to the streak and slightly posterior to the last somite, i.e. in the part of the mesoderm layer which would form the most medial part of the lateral plate (embryos 29*, 34*, 38–40, Table 1, Fig. 3F). At the 7-to 10-somite stage the prelung mesoderm had migrated anteriorly into the splanchnic mesoderm dorsal to the sinus venosus (embryos 23*, 27*, 32*, 35*, Table 1, Fig. 3G). By the 15-to 17-somite stage the prelung mesoderm in the dorsal mesentery had begun to adhere more closely to the ventral wall of the gut, from its lateral margin to the ventral midline (embryos 20*, 25*, 33*, 36*, Table 1, Fig. 3H). By the ± 22-somite stage this section of the splanchnic mesoderm clung closely to the bilaterally bulging endodermal lung buds at the ventrolateral side of the gut, as illustrated in embryos 30*, 39* and 40* (Table 1, Fig. 31). At the early limb-bud stage the prelung mesoderm had been further displaced by the burrowing tips of the endodermal lung buds which had sprouted from the anterolateral gut wall (embryos 31*, 37*, 38*, Table 1, Figs. 3 J, 2B).

Previous attempts to locate the presumptive pulmonogenic regions of the chick embryo utilized the technique of transplanting fragments cut from donor embryos of primitive-streak to head-fold stages to the chorioallantoic membranes of host embryos, where differentiation of the fragments was observed after 7–9 additional days of incubation. Using this method of culture, Willier & Rawles (1931) and Hunt (1934) first noted that respiratory epithelium may differentiate from full-thickness (epiblast and hypoblast) grafts removed from definitive-streak and head-process-stage embryos. Rudnick (1933) systematically probed the potencies of anterior versus posterior, and lateral versus medial fragments in the definitive-streak and head-process-stage embryo, noting that pseudostratified epithelium of the respiratory type (without bronchi) was present in all fragments while trachea was present in all parts of the blastoderm tested except the midline of the embryo posterior to the anterior end of the streak (Fig. 4A, C). Unexplained was (1) the finding that the lung cells would not differentiate into bronchi unless the cultures were made from fragments which contained the rudimentary lung buds (i.e. cut from 3 to 4-day-old donor embryos), and (2) the presence of pretracheal cells in the midline fragment anterior to the streak. It had been supposed previously that this region of endoderm anterior to the streak was destined for the dorsal gut, and the trachea was known to develop from a groove along the ventral wall of the gut. The mapping of the head-process-stage blastoderm by Rawles (1936) gave essentially the same results as those of Rudnick (Fig. 4E), except those at the level slightly caudal to the anterior end of the streak. Rawles found respiratory tract cells in the right-but not in the left-hand fragment, a finding which she ascribed to right-sided dominance.

Hunt (1937) also found a widespread potential for development of respiratory epithelium anterior to the anterior end of the streak, but found it in the mesectoderm (epiblast) rather than in the mesentoderm (hypoblast), except possibly at the definitive-streak and head-process stages (Fig. 4B, D). Since it had already been established that the lung forms from an endodermal bud which burrows into a mesodermal sheath, Hunt’s finding of presumptive respiratory tube material in mesectoderm rather than in mesentoderm suggested that the endoderm and/or mesoderm cells which would form the lung were still in the epiblast layer at stages prior to definitive streak, and possibly later as well.

In radioautographic mapping studies which utilized [3H]thymidine-labelled grafts, Rosenquist (1964, 1965, 1966) confirmed the work of Hunt (1937), demonstrating that in the chick embryo (1) the hypoblast layer lateral and anterior to the streak contains no pregut endoderm at the early streak stages, and (2) beginning at about the medium-streak stage, the pregut endoderm migrates in an anterior and lateral direction from the anterior portion of the streak. Additional workers have independently come to this conclusion (Modak, 1965; Nicolet, 1965; Gallera & Nicolet, 1969). Furthermore, it is now known that by the head-process stage these pregut cells have moved into a semicircular zone of endoderm which extends across the midline of the area pellucida anterior and lateral to the anterior end of the streak (Fig. 5, left). This semicircular zone is divided into two parts: an inner semicircular zone which will form the dorsal wall of the gut and an outer crescent-shaped zone destined for the ventral wall of the gut. The gut is formed (1) by anteroposterior elongation of the dorsal gut zone, and (2) by inversion (invagination) of the ventral gut zone at the anterior intestinal portai (Rosenquist, 1966). The present study shows that the prelung region at the posterior ends of the crescent (Figs. 1D, 5, left) is inverted and compressed into bilateral sites along the ventral wall of the gut (Figs. II, J; 5, center, right). Since the prelung endoderm does not complete these movements until after the 16-to 19-somite stage, intimate contact between prelung endoderm and prelung mesoderm does not occur until about the third day of incubation. This probably explains why histologically and morphologically normal lung tissue did not differentiate from chorioallantoic membrane grafts unless the fragments were cut from the donor embryos after the lung buds had formed (third or fourth day of incubation, Rudnick, 1933). This delay in contact between prelung endoderm and prelung mesoderm may also explain why the fragments of definitive-streak and head-process stage embryos formed meandering respiratory tubes (but not lung) in chorioallantoic membrane cultures (Rudnick, 1933); the prelung endoderm from the posterior ends of the crescent may have burrowed into the non-specific mesoderm with which it came into contact in culture, rather than into the prelung mesoderm, from which it was separated.

Still unexplained is the finding that histologically and morphologically normal trachea will differentiate from blastoderm fragments transplanted to chorioallantoic membrane cultures at the definitive-streak and head-process stages, when bronchi will not (Rudnick, 1933). Radioautographic mapping of the pregut regions of the embryo (Rosenquist, 1966) has determined that the endoderm in the midline (between points C and F, Fig. 5) is inverted and compressed into the ventral midline of the gut between the anterior intestinal portal and the future stoma (the site of the future trachea). It may be that trachea differentiated in Rudnick’s cultures either because the pretracheal endoderm was isolated with the pretracheal mesoderm, or because pretracheal endoderm did not require an organ-specific mantle of mesoderm for differentiation.

Neither the present nor previous radioautographic studies has explained why respiratory tubules differentiated from mesectoderm anterior to the anterior end of the streak in Hunt’s chorioallantoic membrane cultures of head-process stage blastoderm fragments. Previous mapping of the head-process stage embryo has indicated there are no pre-endoderm cells in epiblast anterior to the streak at this stage (Rosenquist, 1966). However, it may be speculated that a few prelung endoderm cells adhered to the mesectoderm layer in Hunt’s fragments, or that the prelung mesoderm contained in such fragments was able to induce adjacent epithelium (in this case, ectoderm) to form respiratory tubules.

In future research it should be possible to segregate the prelung endoderm from the prelung mesoderm in vitro, so that their developmental potential may be studied prior to the formation of the lung. Such research also might offer insight into abnormal lung development, as well as reawakening interest in the early development of organs which arise from endoderm destined for other parts of the digestive system.

Mise en évidence, par marquage radioautographique, de l’origine et des mouvements des cellules présomptives de l’ébauche pulmonaire dans l’embryon de poulet

L’origine des cellules constituant l’ébauche présomptive pulmonaire a été déterminée en suivant les mouvements de greffes marquées à la [3H]thymidine, excisées à des stades s’étendant de la ligne primitive moyenne à celui de 4 somites et transplantés ensuite dans ‘l’épiblaste’ (ectophylle), la ligne primitive et l’endoderme-mésoderme d’embryons récepteurs du stade correspondant.

Au stade de la ligne primitive moyenne les cellules endodermiques correspondant au poumon se trouvent dans le tiers antérieur de la ligne primitive; rapidement elles se mettent à migrer vers l’avant et le dehors au sein du feuillet endodermique. Elles se replient dans l’ébauche de l’enteron vers le stade à 4 somites et commencent à atteindre leur position définitive dans la paroi entérique ventro-latérale aux stades de 10 à 16 somites. Au stade de plus ou moins 22 somites, l’ébauche présomptive du poumon commence à creuser la couche splanchnique du mésoderme qui la recouvre, repoussant ainsi le mésoderme présomptif du poumon.

Au stade de la ligne primitive moyenne le mésoderme présomptif du poumon se trouve dans la couche (dorsale) de l’épiblaste (ectophylle), à peu près à mi-chemin entre la limite latérale de l’area pellucida de chaque côté de la ligne primitive, à un niveau situé à mi-chemin entre les extrémités antérieure et postérieure de la ligne. A partir de cette position, l’ébauche mésodermique présomptive du poumon se déplace vers le dedans jusqu’à la ligne primitive et est invaginée dans le feuillet mésodermique à mi-hauteur de la ligne primitive. En tant que partie du mésentère dorsal, elle émigre vers l’avant et les côtés dans le mésoderme splanchnique en dehors des somites. Depuis le stade du prolongement céphalique jusqu’à celui des premiers somites, l’ébauche mésodermique présomptive du poumon est située en arrière de l’endodermique. Cette ébauche mésodermique continue à migrer avec l’ensemble du mésodermique splanchnique vers le mésentère situé dorsalement par rapport au coeur où il entoure l’ébauche endodermique après le stade du 16e au 19e somite. Commençant à le faire au stade de 22 somites, l’endoderme pré-pulmonaire pénètre dans le mésoderme correspondant et y forme les bronches bilatérales.

This investigation was supported by USPHS research grants HE 10191 and K3 HE 20074 from the National Heart Institute. The author wishes to thank James D. Ebert for his continued interest in this research, and Soame D. Christianson for help in the preparation of the manuscript.

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