ABSTRACT
The aim of this study is to test the ability of the intrinsic wing musculature to develop in the absence of somitic mesoderm.
The experiments were performed on 2-to 2·5-day chick embryos either by replacing the somitic mesoderm adjacent to the wing field with a piece of 9-day chick embryonic midgut or by destroying, through local X-irradiation, not only the somitic mesoderm of the wing level, but also at least three somites (or presumptive somites) anterior and/or three presumptive somites posterior to the wing level.
The replacement of somitic tissue scarcely affected the organogenesis of the forearm musculature, at least when both bones were present. In the other experiments, radio-destruction severely impaired the development of the forearm muscles, which were seldom all present and in most cases were entirely missing. The absence of a given muscle involves the simultaneous absence of the corresponding tendons.
The possible origins of the muscles that formed despite the removal of the somitic mesoderm are discussed.
INTRODUCTION
Up to a recent date, the outgrowing avian limb-bud has been thought to consist of a homogeneous somatopleural population of multipotential mesodermal cells. But more recently, investigations performed by Christ, Jacob & Jacob (1974, 1977), and by our group (Chevallier, Kieny & Mauger, 1976; Chevallier, Kieny, Mauger & Sengel, 1977; Chevallier, Kieny & Mauger, 1977; Chevallier, 1977), have shown that limb-bud mesodermal tissues originate from two different sources. Using the quail/chick marker system advocated by Le Douarin & Barq (1969) both groups came to the same conclusions when chick somitic mesoderm from the wing level was orthotopically replaced by quail somitic mesoderm, i.e. cells originating from the quail somitic mesoderm were found in the adjacently developing chick wing-bud and their location was restricted to the muscle bulk. Hence, the anatomical muscle appeared as a composite structure: tendons and connective tissues (perimysium and endo-mysium) were of somatopleural origin, whereas the muscle cells were of somitic origin.
Furthermore, we showed that the same conclusions held for the development of the leg-bud and that there was no regionalization in the participation of somitic myogenic cells during the formation of the limb (wing or leg) musculature.
Nevertheless, the findings of the exclusively somitic origin of the limb muscular cells could not be confirmed in the reverse grafting experiments of chick somitic mesoderm into quail hosts. In these cases, the myogenic tissue comprised both chick and quail cells in various proportions. This result raises the question of the origin of the host-type limb muscle cells. Are they of somitic origin, due to a compensatory migration of somitic mesoderm from levels anterior or posterior to the implantation site? Or are they of somatopleural origin, implying that, in certain particular conditions at least, the somatopleural mesoderm would be able to express myogenic properties? It is known, indeed, that mesenchymal cells of the limb-bud can switch from a chondrogenic to a myogenic fate, i.e. they express myogenic or chondrogenic properties according to their spatial location (Searls & Janners, 1969) or according to metabolic gradients (oxygen tension) (Caplan & Koutroupas, 1973).
In order to test whether a somitic contribution is actually required for the development of a normal limb musculature, we undertook new series of experiments to disrupt the relationships between the somitic mesoderm and the adjacent somatopleural mesoderm. This disruption can be produced by two means, either through the translocation of the wing somatopleure into an abnormal environment or through the elimination of the somitic mesoderm adjacent to the wing somatopleure. In this paper we report the experiments dealing with the second of the above-mentioned methods.
Previous findings have shown that somitic mesoderm regulates to a very great extent its dermatomal [spinal feather tract (Mauger, 1972a)] as well as sclero-tomal [vertebrae and ribs (Kieny, Mauger & Sengel, 1972)] and myotomal [trunk musculature (Chevallier, unpublished data)] derivatives, when a portion of its length is excised. Therefore, in order to study the effects of the absence of the portion of the somitic mesoderm adjacent to the wing level, it was necessary to impede these regulative processes. For that purpose we used two procedures which we had already tested. The first one involved replacing the somitic tissues by a piece of non-somitic tissue (Mauger, 1972b; Kieny et al. 1972); the second one involved the local destruction of a segment of somitic tissue left in place (Mauger, 1970).
MATERIALS AND METHODS
The experiments were carried out on 2-to 2·5-day chick embryos (Wyandotte x Rhode Island Red). The exact stage reached at the time of operation was specified by the number of pairs of somites.
The experiments were designed to prevent the somatopleural mesoderm of the wing field (level 15–20) from being invaded by myogenic cells from the adjacent somitic mesoderm. Two kinds of experiments were performed.
In the first series, the unsegmented or partially segmented somitic mesoderm of level 15 – 20 from the left side was excised and replaced by an equal length of 9-day chick embryonic gut according to the experimental procedure outlined in Fig. 1 (a). It was easy to completely remove the piece of somitic mesoderm when it was unsegmented. However, when the somitic mesoderm was excised in a partially segmented state, it was impossible to exclude the possibility that a few ventral and medial somitic cells were left in place within the already segmented portion. The midgut was cut longitudinally into quarters before strips were grafted with their intestinal epithelium facing the host’s endoderm.
Experimental schemes of somitic mesoderm removal at age of 16 pairs of somites, (a) Replacement of the left somitic mesoderm adjacent to the wing field (hatched area) by a strip of midgut, (b), (c), Destruction by local X-irradiation of a 1 · 6 mm long portion of right somitic mesoderm behind the 14th somite (1st category) (b) or behind the 11th somite (2nd category) (c). Stippled area: irradiated somitic mesoderm; hatched area: presumptive wing territory.
Experimental schemes of somitic mesoderm removal at age of 16 pairs of somites, (a) Replacement of the left somitic mesoderm adjacent to the wing field (hatched area) by a strip of midgut, (b), (c), Destruction by local X-irradiation of a 1 · 6 mm long portion of right somitic mesoderm behind the 14th somite (1st category) (b) or behind the 11th somite (2nd category) (c). Stippled area: irradiated somitic mesoderm; hatched area: presumptive wing territory.
In the second series, somitic mesoderm was destroyed on the right side by local X-irradiation, which was performed through a rectangular (1·6 mm long × 0·1 mm wide) slot cut in an 0·1 mm thick tantalum shield that protects the rest of the embryo and part of the area pellucida. The irradiation, made under 20 kV and 30 mA during 10 min, the embryo being placed at a distance of 37 cm from the anticathode, was localized on a long portion of unsegmented or partially segmented somitic mesoderm. The irradiated part always included somite level 15–20 and extended at least three somites or presumptive somites in front of and/or in the rear of the wing somite level (Fig. 1 b, c). For histological control of the X-irradiated somitic mesoderm, embryos were fixed 24, 48 and 96 h after irradiation, paraffin-embedded, sectioned at 5 μm and stained with haemalum-eosin.
The operated embryos of both series were sacrificed between days 9 and 13 of incubation, the majority at days 12 or 13. The operated areas were photographed (if necessary after part of the feather filaments of the spinal pteryla and those of the experimental and contralateral wings had been plucked). These embryos were classified according to both the effects on the morphology of the spinal tract, and the effects on the morphology of the wing on the operated side. For each category of results, experimental (and often also contralateral) wings of a certain number of embryos, taken at random, were paraffin-embedded, sectioned at 7·5 μm and stained with haemalum-eosin.
RESULTS
Since the purpose of this study was to analyse the organogenesis of the wing musculature after elimination of the somitic mesoderm of the wing level, it was necessary to perform the experiments before the normally invading somitic cells are supposed to start their emigration, that is, as shown by one of us (Chevallier, 1977) before the somitic mesoderm of the wing level is wholly metamerized (stage 21 pairs of somites). Two kinds of experiments were undertaken: (1) the excised portion of the somitic mesoderm of level 15–20 was replaced by a piece of non-somitic (and non-somatopleural) tissue; (2) the somitic mesoderm was left in place, but was destroyed by local X-irradiation.
The study of the muscular organogenesis has been limited to the intrinsic musculature of the wing, in particular to the muscles contained in the forearm.
(1) Replacement of somitic mesoderm by non-somitic tissue
The grafts used were strips of 9-day embryonic midgut, cut to the appropriate size so they would fit as tightly as possible within the gap created by the excision of the unsegmented or partially segmented somitic mesoderm. The grafts were placed in embryos which ranged in age from stages 13 to 17 pairs of somites.
Nineteen embryos constitute this series; 3 were sacrificed at 9 days and 16 were sacrificed at 12 days, the stage at which the muscular organogenesis is qualitatively completed. All of them presented plumage deficiencies at the operated level, which means that the graft had not been rejected. Indeed, each embryo presented an unilateral featherless area extending over a length of three to six lateral feather rows within the narrow postcervical and thoracic portion of the spinal pteryla (Fig. 2). Except in one case, all embryos developed a wing on the operated side. Their humeral tracts were distorted, usually rotated anticlockwise by ca. 45° and spreading into the bare indentation of the spinal tract (Fig. 2). Their extrinsic musculature that serves to attach the humerus to the scapular girdle was missing or very deficient (Chevallier, unpublished data).
Forearm muscle organogenesis at 12 days of incubation after replacement of the wing-level somitic mesoderm (level 15–20) by a piece of 9-day midgut at stage 13–17 pairs of somites. All histological sections are given at the same magnification. Figs. 2 and 3. Host: 16 pairs of somites. Note on the left side the featherless notch (Fig. 2). The left experimental wing is normally developed. Its forearm musculature is complete (Fig. 3) (1st line of Table 1). Fig. 4. Host: 15 pairs of somites. This histological section passes through the proximal portion of the forearm of a reduced wing in which only the distal half of the radius was formed. The musculature is nearly complete (4th line of Table 1). Fig. 5. Host: 13 pairs of somites. Deficient wing, the preaxial portion of which contains neither the radius nor its surrounding muscles, but only a loose unorganized mesenchyme. Figs. 6 and 7. Host: 17 pairs of somites. Deficient wing in which only the postaxial portion of the forearm was formed (Fig. 6). Compare with a section through the contralateral control forearm (Fig. 7). R, radius; U, ulna. For the muscle abbreviations, see Table 1.
Forearm muscle organogenesis at 12 days of incubation after replacement of the wing-level somitic mesoderm (level 15–20) by a piece of 9-day midgut at stage 13–17 pairs of somites. All histological sections are given at the same magnification. Figs. 2 and 3. Host: 16 pairs of somites. Note on the left side the featherless notch (Fig. 2). The left experimental wing is normally developed. Its forearm musculature is complete (Fig. 3) (1st line of Table 1). Fig. 4. Host: 15 pairs of somites. This histological section passes through the proximal portion of the forearm of a reduced wing in which only the distal half of the radius was formed. The musculature is nearly complete (4th line of Table 1). Fig. 5. Host: 13 pairs of somites. Deficient wing, the preaxial portion of which contains neither the radius nor its surrounding muscles, but only a loose unorganized mesenchyme. Figs. 6 and 7. Host: 17 pairs of somites. Deficient wing in which only the postaxial portion of the forearm was formed (Fig. 6). Compare with a section through the contralateral control forearm (Fig. 7). R, radius; U, ulna. For the muscle abbreviations, see Table 1.
These 18 experimental wings were classified as deficient, reduced or normal according to the following criteria. Cylindrical wings (nine cases) in which the prepatagium was absent or quasi-absent were designated as deficient (see Fig. 20); the autopod was malformed in three cases (embryos fixed at 9 days) either by lacking digit II (one case) or by the development of an extra digit II (two cases). In the other nine wings, the characteristic wing shape was retained ; their normality or reduction was attested by comparing the number of their parallel rows of secondary remiges and coverts, running transversely on the dorsal face of the forearm, to the number of those of the contralateral wings. A normal 12-day wing bearing 13 or 14 rows, when the number of rows differed by more than one unit (two or three) between the experimental and the contralateral wings, the experimental wing was designated as reduced. Thus six wings were reduced and three were normal.
Eleven (one normal, five reduced, five deficient) of the 15 12-day wings were examined histologically (Table 1). The forearms of the normal and reduced wings not only contained muscles but their muscular equipment was nearly normal (Figs. 3, 4). In four out of the five deficient wings (Figs. 5, 6) which contained only the ulnar bone, the forearm possessed six fully developed muscles, which characteristically surround the ulna in normal development (on the dorsal side: anconeus, extensor metacarpi ulnaris and extensor digitorum communis; on the ventral side: flexor digitorum profundus, flexor carpi ulnaris and ulnimetacarpalis ventralis) (Figs. 6, 7). Those which normally accompany the radius were missing (Figs 5, 6). In the remaining deficient wing equipped with an ulna and a filiform radius, only few slender muscles differentiated, surrounding both the ulna and radius.
It is clear that the replacement of the portion of somitic mesoderm strictly adjacent to the wing level does not lead to muscleless wings.
(2) Destruction of somitic mesoderm by local X-irradiation
In order to test whether the results just described could be attributed to a compensatory migration of the somitic mesoderm anterior and/or posterior to the wing level, -the somitic limb-myogenic properties being not regionalized, -it was necessary to eliminate more than the wing-level somitic mesoderm. Because a long-length microsurgical tissue elimination and subsequent replacement leads, in our hands, to a too high mortality or to a too severely disturbed development, we proceeded to an in situ destruction of a portion of the somitic mesoderm by means of a local X-irradiation.
The irradiation was performed at ages ranging from stage 11 to 21 pairs of somites. Its histological effects on the somitic mesoderm were in complete concordance with those previously described by one of us (Mauger, 1970) for smaller areas under the same irradiation conditions, i.e. within 24 h, the X-irradiation produces a disorganization of the somitic mesoderm; the myotome-dermatome-sclerotome organogenesis does not occur, and the whole tissue becomes necrotic. Two days after the irradiation, the site of the radiolesions become gradually repopulated by healthy mesenchymal cells, which however do not become reorganized into a somitic structure.
A total of 82 irradiated chick embryos were sacrificed at 13 days of incubation. The irradiated portion always exceeded the wing level portion. In 30 cases, the slotted tantalum shield was placed in such a way that a 1·6 mm long segment of somitic mesoderm behind the 14th somite or presumptive somite was irradiated (1st category: level 15–20 plus three to seven presumptive somites behind). In the remaining 52 cases, a length of 1·6 mm somitic mesoderm behind the 11th somite was irradiated (2nd category: level 15–20 plus three somites or presumptive somites ahead of and one to four somites or presumptive somites behind the wing level).
Because there were no major differences between these two categories, the embryos were pooled and classified according to the same criteria as those applied in the preceding series (Table 2). Thus, the spinal feather tracts of 17 embryos were undisturbed (13 cases of the 1st category and four cases of the 2nd category) and the wing on the operated side was generally normal or slightly reduced. Histological investigation (three cases of 1st category) showed the presence of a complete intrinsic and extrinsic wing musculature.
Whereas in 65 embryos (17 cases of the 1st category and 48 cases of the 2nd category), the spinal tract presented an unilateral apterium at the irradiated post-cervico-thoracical level, that extended over a length of 6–12 feather rows (Figs 8, 10, 12,15, 18, 20). The clockwise distorted right humeral tract protruded into this spinal featherless notch, so that the dorsal and caudal borderlines of the humeral tract were often in continuity with the mid-dorsal feather row of the spinal tract. Forty of these 65 wings were harmoniously reduced (Figs 15, 18); the other wings were in the same proportions either normal (13 cases) (Figs 8, 10, 12) or deficient (12 cases) (Fig. 20).
Forearm musculature of normal-looking experimental wings in 13-day embryos whose right somitic mesoderm had been X-irradiated at stages of 13–18 pairs of somites. All histological sections are at the same magnification. Figs. 8 and 9. Case of irradiation behind the 11th somite at stage 18 pairs of somites. Note the unilateral apterium that extends, at right, over eight feather rows and in which the distorted humeral tract tends to spread (Fig. 8). The forearm musculature is completely developed (Fig. 9) (compare with Figs. 7 and 14). Figs. 10 and 11. Case of irradiation behind the 11th somite at stage 13 pairs of somites. The right humeral tract spreads within the featherless spinal tract indentation (Fig. 10). Note the absence of muscles and tendons (Fig. 11). Figs. 12–14. Case of irradiation behind the 14th somite at stage 15 paiis of somites. The humeral tract of the right wing spreads into the right spinal apterium that extends over ten feather rows (Fig. 12). Neither muscles nor tendons formed in the forearm of the experimental wing (Fig. 13). For comparison, see Fig. 14, a section through the contralateral wing. R, radius; U, ulna. For the muscle abbreviations, see Table 1.
Forearm musculature of normal-looking experimental wings in 13-day embryos whose right somitic mesoderm had been X-irradiated at stages of 13–18 pairs of somites. All histological sections are at the same magnification. Figs. 8 and 9. Case of irradiation behind the 11th somite at stage 18 pairs of somites. Note the unilateral apterium that extends, at right, over eight feather rows and in which the distorted humeral tract tends to spread (Fig. 8). The forearm musculature is completely developed (Fig. 9) (compare with Figs. 7 and 14). Figs. 10 and 11. Case of irradiation behind the 11th somite at stage 13 pairs of somites. The right humeral tract spreads within the featherless spinal tract indentation (Fig. 10). Note the absence of muscles and tendons (Fig. 11). Figs. 12–14. Case of irradiation behind the 14th somite at stage 15 paiis of somites. The humeral tract of the right wing spreads into the right spinal apterium that extends over ten feather rows (Fig. 12). Neither muscles nor tendons formed in the forearm of the experimental wing (Fig. 13). For comparison, see Fig. 14, a section through the contralateral wing. R, radius; U, ulna. For the muscle abbreviations, see Table 1.
Forearm musculature of reduced (Figs. 15-19) and deficient (Figs. 20-22) wings in 13-day embryos whose right somitic mesoderm had been X-irradiated at stages of 17–20 pairs of somites. All histological sections are at the same magnification. Figs. 15–17. Case of irradiation behind the 14th somite at stage of 17 pairs of somites. In Fig. 15, at right, the spinal apterium extends over seven feather rows. Two sections of the right forearm are shown in Figs. 16 and 17. Note the sporadic muscles, mainly located between the two forearm bones (1st line, Table 3). Figs. 18 and 19. Case of irradiation behind the 11th somite at stage of 17 pairs of somites. The spinal tract bears a featherless notch that extends over six feather rows (Fig. 18), and the intrinsic forearm muscles are missing (Fig. 19). Figs. 20 and 21. Case of irradiation behind the 14th somite at stage of 20 pairs of somites. The featherless notch extends over nine feather rows (Fig. 20) and the forearm of the cylindrical deficient wing is non-muscled (Fig. 21). Fig. 22. Histological illustration of a case of irradiation behind the 11th somite at stage 18 pairs of somites. The muscleless forearm contains only the ulna and loose unorganized mesenchyme. R, radius; U, ulna. For the muscle abbreviations see Table 1.
Forearm musculature of reduced (Figs. 15-19) and deficient (Figs. 20-22) wings in 13-day embryos whose right somitic mesoderm had been X-irradiated at stages of 17–20 pairs of somites. All histological sections are at the same magnification. Figs. 15–17. Case of irradiation behind the 14th somite at stage of 17 pairs of somites. In Fig. 15, at right, the spinal apterium extends over seven feather rows. Two sections of the right forearm are shown in Figs. 16 and 17. Note the sporadic muscles, mainly located between the two forearm bones (1st line, Table 3). Figs. 18 and 19. Case of irradiation behind the 11th somite at stage of 17 pairs of somites. The spinal tract bears a featherless notch that extends over six feather rows (Fig. 18), and the intrinsic forearm muscles are missing (Fig. 19). Figs. 20 and 21. Case of irradiation behind the 14th somite at stage of 20 pairs of somites. The featherless notch extends over nine feather rows (Fig. 20) and the forearm of the cylindrical deficient wing is non-muscled (Fig. 21). Fig. 22. Histological illustration of a case of irradiation behind the 11th somite at stage 18 pairs of somites. The muscleless forearm contains only the ulna and loose unorganized mesenchyme. R, radius; U, ulna. For the muscle abbreviations see Table 1.
Of these 65 wings 27 (8 of the 1st category; 19 of the 2nd category) were examined at histology. Only two of them (one of each category) that were classified as normal-looking wings had a complete skeleton and intrinsic forearm (and arm) musculature (Figs. 8, 9). In the 25 remaining wings the skeleton was normal in the normal-looking and reduced wings. But in the deficient ones, besides a subnormal ulna, the radius was filiform (five cases) or even totally missing (one case) (Fig. 22). Regardless of the external morphology of the wing, the forearm did not contain any muscle at all (18 cases) (Figs. 11, 13, 19, 21, 22) or contained only some (not more than seven) of the 11 main muscles that normally (Fig. 14) constitute its musculature (seven cases) (Figs. 16, 17). The analysis of the poorly developed musculature (Table 3) suggests that the few forearm muscles that developed were not randomly located. Some of them were always present, while others were generally missing. Indeed, the extensor metacarpi radialis (EMR) and the flexor carpi ulnaris (FCU) appeared to be predominantly formed.
This experimental series shows clearly that local X-irradiation of wing-level somite portions and beyond leads, in the majority of the cases, to muscleless wings on the irradiated side. The remaining cases are in general poorly muscled; and a complete forearm-muscle organogenesis is seldom obtained.
DISCUSSION AND CONCLUSIONS
The results of the two experimental series are not identical. The replacement of the wing level somitic mesoderm by a strip of midgut did not prevent the adjacent wing from being muscled. Except one of the eleven cases that were examined histologically, the musculature developed in accordance with the skeleton formed. When both bones, ulna and radius, were present, the intrinsic forearm musculature developed normally. When the forearm contained only the ulna, the muscular equipment was limited to the six muscles that normally surround this bone. The forearm, then nearly cylindrical, was generally reduced to its proximal half. But the former may also have formed a preaxial half; in such a case, the preaxial mesoderm comprised exclusively a loose totally unorganized mesenchyme.
In contrast, the in situ destruction of a portion of the somitic mesoderm that exceeded the wing level anteriorly and/or posteriorly led to different results. Whether or not the embryo presented external signs of the radiolesions, the wing musculature was hypomorphic or normal. These signs appear in the spinal feather tract as a featherless notch at the wing level in which the distorted humeral tract tends to spread.
Previous findings by one of us (Mauger, 1970) have shown that within the X-irradiated somitic mesoderm, the dermatome fails to differentiate, and the corresponding portion of the spinal tract forms glabrous skin. Moreover, these spinal plumage deficiencies are always associated with more internal vertebral (and ribs) deficiencies : the anterior levels of these plumage and axial alterations are concordant with one another, whereas the posterior level of the axial deficiencies, which corresponds to the posterior edge of the irradiated surface, always extends more caudally than that of the plumage deficiencies (Mauger and Kieny, unpublished data). Therefore, the length of plumage deficiency can certainly be considered as a guarantee of the destruction of the corresponding length of the somitic mesoderm.
Thus, embryos (20 %) with an apparent undisturbed plumage and with normally muscled experimental wings could not be considered for the present analysis. Only those showing the above-mentioned plumage abnormalities were retained. Their experimental wings were normally muscled in 7 %, poorly muscled in 26 % and had no muscles in 67 % of the 27 histologically examined cases.
The muscular reduction was qualitative as well as quantitative. No more than 7 of the 11 forearm muscles were recognizable and their volume seldom reached half the normal volume of the corresponding muscles. Yet, even reduced to about 25 % of its volume, the muscle bulk was accompanied by its tendons; while, when totally absent, the corresponding tendons were missing, too. The myogenic area was then occupied by a loose unorganized mesenchyme. In the cases of total muscle absence, the forearm comprised cartilage, loose mesenchyme, dermis and epidermis. These observations held also for the reduced and absent muscles of the previously described replacement series.
Tendons being of somatopleural origin (Chevallier et al. 1976, 1977; Christ et al. 1974, 1977), the simultaneous absence of tendons and muscle observed at 12–13 days of incubation raises the question whether, during preceding development, the tendon blastemae did not form or whether they did form and disappeared because of the absence of myogenic cells. This point will be the object of new investigations.
In general, it seems that the shape of the wing does not depend on the differentiation of all of its constituting tissues. It is obvious that the skeleton represents the shape’s framework, whereas the presence or absence of the musculature has no or few repercussions on the wing’s form.
As regards the cellular origin of the muscles in the experimental wings, different possibilities can be postulated in the replacement series where the wings are normally or subnormally muscled.
(1) The muscle cells may originate from the few somitic cells that may have been left behind after the extirpation of the somitic mesoderm. As the histologically examined cases were operated at stages of 13–17 pairs of somites (Table 1) and led to the same results whether the wing level somite mesoderm was excised in an unsegmented or partially segmented state, this possibility cannot reasonably account for the development of a complete wing musculature.
(2) The muscle cells can originate from the midgut external muscle layers. This point can be rejected. Experiments in which, under the same conditions, chick somitic mesoderm of the wing level had been replaced by a piece of quail midgut, showed that only some scattered quail cells had penetrated the 4-to 6-day chick wing. They were not selectively located in the muscle areas, but found in the vicinity of the nerves; they were supposed to be Schwann cells (Chevallier, 1977).
(3) The muscle cells can originate from the somitic mesoderm situated ahead of/or behind the implantation site. Because the somitic replacement concerns only the narrow portion adjacent to the wing territory, such compensatory migration of somitic cells may take place, particularly since the myogenic potentialities of the somitic mesoderm are not regionalized. The final differentiation of the somite-emigrated cells occurs ‘ortsgemäss’ (Chevallier et al. 1977).
(4) The muscle cells can originate from the somatopleure. This possibility, although doubtful in these replacement experiments, cannot be discarded.
The latter hypothesis becomes more likely in the case of large somitic elimination through radiodestructions. In this type of experiment which in addition involved the somitic mesoderm ahead of and/or behind the wing level,, the majority of the wings were muscleless and only one third of the cases-compensated their adjacently-somite-originating muscle deficiencies, mainly in an incomplete way. Anyhow we have no argument to decide in favour of a somitic or somatopleural origin of this poor musculature. But it is worth mentioning here that the somatopleure can give rise to muscle fibres. Indeed, in preliminary experiments muscle fibres of convincing somatopleural origin have been obtained in the case of ectopic wings resulting from the translocation of quail wing somatopleural mesoderm into chick neural tube, at stages before the somatopleural graft had become invaded by somitic cells. Thus in these conditions, the somatopleure seems able to compensate for somitic deficiency.
In conclusion, the elimination of a portion of somitic mesoderm including that of the wing level and extending ahead of and behind the wing level leads in two thirds of the cases to muscleless wings. This confirms that the somitic contribution is required for the development of a normal wing musculature.
ACKNOWLEDGEMENTS
Le mémoire représente une partie de la thèse qui sera soutenue par A. Chevallier derant l’Université scientifique et médicale de Grenoble pour l’obtention du grade de docteur d’Etat es Sciences.
