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At least 19 recordsLinked to original sources
PubMed1995

Functional involvement of Pax-1 in somite development: somite dysmorphogenesis in chick embryos treated with Pax-1 paired-box antisense oligodeoxynucleotide.

The metameric pattern of the vertebrate axial skeleton, defined by structures such as the vertebral bodies and ribs, is a result of segmentation events that occur during embryogenesis. The key event in axial segmentation is somite formation. This study examines the role of Pax-1, a member of the paired-box containing Pax gene family, in chick somite development. To investigate whether misexpression of Pax-1 during somite development is functionally related to abnormal axial patterning, antisense methodologies were used to perturb Pax-1 expression. An antisense, phosphorothioate-modified oligodeoxynucleotide (ODN) was designed based on the mouse Pax-1 paired-box sequence, and was either injected into or directly applied topically to early, somitic stage chick embryos. Abnormalities in somite structure and pattern were subsequently observed and scored, including loss of somites (18% of injected embryos, 35% of embryos treated by topical application), fusion of somites (25% of injected, 6% with topical application), and shortened body axis (0% of injected, 11% with topical application). Control embryos receiving sense ODN or non-sense ODN (a scrambled sequence with base composition identical to the antisense ODN) showed substantially fewer somite anomalies, indicating that the effects were sequence-specific. These developmental abnormalities were analyzed using standard histological methods. Whole mount in situ hybridization was carried out to analyze the three-dimensional pattern of Pax-1 expression in whole embryos. In control, untreated embryos, the expression was localized to the entire epithelial somite, and as the somite matured, the expression was limited to its ventromedial region. With Pax-1 antisense ODN treatment, embryos with fused somites retained expression over the entire fused somite, and embryos that had complete loss of somites had greatly reduced expression of Pax-1 transcript. The results presented here provide strong evidence that Pax-1 is functionally important during somitogenesis and morphogenesis of the vertebral column. The spatial pattern of gene expression appears to delineate different populations of cells in the developing embryo (i.e., somite from somite, sclerotome from dermomyotome), and is consistent with the hypothesis that Pax-1 is involved in forming or maintaining boundaries at specific times and locations during development.

Abnormalities, Drug-Induced↗
PubMed1978

Chick somite determination: the role of factors in young somites and the segmental plate.

One class of hypotheses proposed to explain the anterior to posterior sequence of chick somite formation envisions the movement of morphogenetic substances or processes along the segmental plate prior to somite formation. An effort was made to demonstrate any such phenomena in the region of somite formation by placing grafts that contained forming somites into the segmental plates of cultured host embryos. No somite-inducing properties on the part of the grafts were detected. Host somite formation proceeded in a normal manner. In additional experiments, posterior portions portions of segmental plates were isolated from the more anterior parts of the segmental plate which might contain the hypothetical morphogenetic factors. It was found that anterior portions of the segmental plate representing as many as eight prospective somites could be removed without impairing the ability of the remaining posterior portion to form somites in vitro. It is concluded that other hypotheses of the type studied here are not likely to be useful and that the present data further strengthen the argument that the cells of the chick segmental plate present at any time between the 8- and 20-somite stages of development are committed to form somites. In addition, the explant experiments described here indicate that a somite pattern representing about 11 prospective somites, becomes established in the segmental plate.

Animals↗
PubMed1995

Halves of epithelial somites and segmental plate show distinct muscle differentiation behavior in vitro compared to entire somites and segmental plate.

Medial and lateral halves of the somite are known to differ with respect to their developmental fates: Cells from the medial half of the somite give rise to the epaxial muscle of the back and cells from the lateral half of the somite give rise to the skeletal muscles of the limbs and the ventrolateral body wall. To get a better insight into myogenic determination of somite hemispheres, isolated entire somites as well as medial and lateral parts of somites and of segmental plate from 2 day chick embryos were explanted in vitro. These parts of the paraxial mesoderm were also cocultured in contact with somite surrounding tissues such as neural tube lacking floorplate, neural tube including notochord-floorplate complex, and intermediate mesoderm, which were examined with respect to their muscle promoting or inhibiting influences. Skeletal muscle differentiation was monitored by the use of anti-myosin heavy chain antibody (MF20). It is shown that medial and lateral halves of segmental plate and epithelial somites are capable of undergoing myogenesis in the absence of axial organs. In contrast, cultures of intact segmental plate and epithelial somites from the same levels did not show muscle differentiation. Neural tube lacking floorplate promoted muscle differentiation in the medial halves especially of epithelial somites and also of segmental plate, but not in the lateral halves of the paraxial mesoderm at these levels. Intermediate mesoderm was found to inhibit muscle differentiation in medial and lateral halves of segmental plate and of epithelial somites. We further demonstrate that the arrangement of the myoblasts within tissue cultures is influenced by the presence or absence of axial organs.

Animals↗
PubMed1999

Plasticity of axial identity among somites: cranial somites can generate vertebrae without expressing Hox genes appropriate to the trunk.

Classic studies have shown that the presomitic mesoderm is already committed to a specific morphological fate, for example, the ability to generate a rib. Hox gene expression in the paraxial mesoderm has also been shown to be fixed early and not susceptible to modulation by an ectopic environment. This is in contrast to the plasticity of Hox expression in neuroectodermal derivatives. We reexamine here the potential of somites for morphological plasticity by transplanting the cranial (occipital) somites 1-4, that normally produce small contributions to the skull, to the trunk of avian embryos. Surprisingly, the transposed cranial somites are able to form reasonably normal vertebral anlage. In addition, the cranial somitic mesoderm produces intervertebral disks, structures not normally found in the skull. These somites are however unable to generate some elements of the vertebrae, such as the costal process. In contrast to the morphogenetic plasticity of the occipital somites, their characteristic inability to support survival of dorsal root ganglia was not significantly modified by posterior transplantation. Dorsal root ganglia initially developed and then degenerated with the same morphological stages as normally observed. In striking contrast to the plasticity of morphology, we found that all four members of the of the fourth paralogous group of Hox genes that are expressed endogenously at the level of the graft are not upregulated in the caudad-transposed cranial mesoderm. It therefore appears that genes other than those of the Hox family normally expressed at this axial level control the position-specific morphogenesis of ectopic vertebrae formed from cranial somites. In evolutionary terms, the present results imply that occipital somites that were incorporated into the "New Head" retain the ability to develop according to their original morphogenetic fate, into vertebrae.

Animals↗
PubMed1978

Limb-somite relationship: effect of removal of somitic mesoderm on the wing musculature.

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 replacemnt 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.

Animals↗
PubMed1993

Pair-rule gene expression in the somitic stage chick embryo: association with somite segmentation and border formation.

In vertebrates, metameric organization is high-lighted by the formation of somites from mesenchymal cells of the segmental plate which then differentiate into dermamyotomal and sclerotomal tissues. The resegmentation of the sclerotome into rostral and caudal halves follows, coincident with the production of specific extracellular matrix molecules at the abutment of these two cell types. Ultimately, cells from the caudal sclerotome migrate ventrally and contribute to the chondrogenic prevertebrae. The objective of this work is to investigate the molecular steps regulating these events. Our study is focused on the paired-box containing genes, which have been implicated in delineating boundaries early in development. A chick embryo system, which is readily accessible to manipulation and observation during early development, is used in this study. We have identified the existence of the paired-box motif in the chicken genome by polymerase chain reaction and hybridization with the mouse Pax 1 paired-box sequence. Expression of paired-box genes occurs early in development as shown by Northern analysis, and is localized by in situ hybridization to the edge of each somite, a patch at the central core of each somite, and the periphery of the neural tube. This specific spatial pattern of expression is consistent with the hypothesis that the pair-rule genes function as effecters of border formation in the early embryo. Moreover, the patch of positive cells at the center of a resegmenting somite appear to migrate ventrally, and may contribute to structures of the prevertebrae. These findings are relevant to our understanding of the mechanism of somite resegmentation and implicate the involvement of pair-rule genes in the process.

Animals↗
PubMed1999

[Somite patterning and segregation of different somite lineages].

The somite is a transient embryonic mesodermal structure, found only in vertebrates. In amniotes, somites give rise to the dermis of the back, to the striated skeletal muscles of the trunk and limbs and to the vertebral column and ribs. Segregation of these different lineages is linked to the establishment of two somitic polarity axes, a dorso-ventral one and a medio-lateral one. While the establishment of the former relies essentially on extrinsic cues, that of the latter obeys to both intrinsic and extrinsic mechanisms. Concerning the environmental cues regulating the establishment of both axes, somitic regionalisation results from antagonistic or combinatorial influences mediated by diffusible factors, such as Sonic Hedgehog, Wnt and Bmp-4, that act in gradients.

Animals↗
PubMed1999

F-Spondin, expressed in somite regions avoided by neural crest cells, mediates inhibition of distinct somite domains to neural crest migration.

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial-half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

Animals↗
PubMed1995

Somite differentiation. Sonic signals somites.

Sonic hedgehog, a secreted signalling molecule known to play a role in the patterning of the central nervous system and the limb in vertebrates, also controls differentiation of the somites.

Animals↗
PubMed1975

Origin and destination of the median germ cells in late somite stage and early post-somite stage duck embryos.

In continuation of preceding investigations, in 184 duck embryos of the developmental stages 18-27 according to Hamburger and Hamilton (about 85-140 hours of incubation) a study was done on the occurrence of median germ cells. These cells proved to be present in proportionally small numbers in the stages 19-26. Their numbers were independent of the sex of the embryos and did not show any relations whatever with changes in the numbers or distribution of the intragonadal germ cells. The temporary occurrence of the median germ cells could be shown to be due to the movement of the gonadal primordia from the splanchnopleure of the yolk sac to the ventro-medial surface of the mesonephros, and it was argued that ultimately all these germ cells reach the right of left gonad.

Age Factors↗
PubMed1997

Cloning and characterization of chicken Paraxis: a regulator of paraxial mesoderm development and somite formation.

To investigate the molecular regulation of embryonic somite formation and development, we have cloned the full-length cDNA and characterized the embryonic expression profile of chicken Paraxis, a member of a novel family of basic helix-loop-helix (bHLH) proteins, which has been suggested to play a role in paraxial mesoderm development. Chicken Paraxis encodes a 1.35-kb mRNA and contains a 53-amino-acid residue bHLH domain, identical in sequence to that found in the mammalian Paraxis genes of mouse, hamster, and human. Northern analysis revealed significant Paraxis expression in the early embryo up to the 30- to 35-somite stage, declining from Incubation Day 4 on and becoming undetectable by Day 5. By whole-mount in situ hybridization, Paraxis expression is first seen distinctly in the emerging paraxial mesoderm of the primitive streak stage chick embryo. During gastrulation, Paraxis expression in the mesoderm defines bilaterally symmetric crescents located immediately rostral to Hensen's node and appears to pre-configure the emerging somitic mesoderm. During somite development, Paraxis expression is evident in the rostral segmental plate and the newly formed somites, although the level of expression clearly decreases in the more mature somites. By the 10-12th pair of somites, counting from the caudal end, Paraxis expression appears to be preferentially localized to the medial aspect of individual somites. Histological analysis showed that Paraxis expression is evenly distributed in the newly formed caudal epithelial somites, then localized to the medial portion of maturing somites, and preferentially localized in the dermomyotome of more rostral somites before diminishing to undetectable levels in the most cranial somites. The functional involvement of Paraxis in somite development was assessed by perturbing its expression in somitic stage chick embryos using a Paraxis-specific antisense oligonucleotide. Disruption of somite formation from the paraxial mesoderm was observed in 67% of the surviving topically treated embryos, whereas control embryos treated with sense or random sequence oligonucleotides did not show similar effects. In addition, direct injection of Paraxis-specific antisense oligonucleotide into the paraxial mesoderm produced discrete segmentation anomalies which correlated spatially with the site of injection. Whole-mount in situ hybridization revealed that the regions defective in somite formation displayed perturbed Paraxis expression and a reduction of Pax-1 expression, a marker for epithelial somites and sclerotome. Histological analysis indicated poor condensation and/or epithelization of the somitic mesoderm. Finally, embryos treated with valproic acid, a known teratogen which affects somite segmentation, showed perturbed Paraxis expression, suggesting that the mechanism of action of this teratogen involves a pathway(s) requiring Paraxis activity. These data provide evidence that Paraxis acts as an important regulator of paraxial mesoderm and somite development and functions in axial patterning of the chick embryo.

Amino Acid Sequence↗
PubMed1993

Role of the brachial somites in the development of the appendicular musculature in rat embryos.

DiI, a fluorescent lipophilic dye, was micro-injected into the brachial somites of 10.5 day rat embryos to determine whether these somites can contribute cells to the development of the fore-limb bud. The injected embryos were cultured and harvested at the 20-25-somite stage. The dye did not interfere with somitogenesis because, at the injection site, the DiI-labelled somites were able to differentiate into dermomyotome and sclerotome. We have analyzed cryo-sections of 20-21-somite stage embryos and were unable detect the presence of DiI-labelled cells in the fore-limb buds. However, at the 22-somite stage, a few DiI-positive cells were found in the proximal region of the limb bud. These labelled cells had migrated into the limb from the lateral border of the dermomyotome. From the 23-somite stage onwards, there were even more DiI-positive cells inside the limb. We have performed an additional set of experiments to confirm that the somitic cells do have the ability to invade and colonize the limb bud. This was achieved by first labelling newly formed somites isolated from the caudal region of 10.5 day embryos with DiI and then grafting them into corresponding regions in 8-11-somite stage hosts. The donor somites were not orientated when they were implanted into the host. However, this did not disrupt their ability to undergo normal somitogenesis. We have detected the presence of DiI-positive cells in the limb buds of approximately 71% of the 19-30-somite stage embryos that have been examined. This is similar to what we obtained for the injected embryos. Nevertheless, there is one slight difference and that is the stage the somitic cells begin their invasion of the limb. For the injected embryos, migration began at the 22-somite stage but in the transplanted embryos, it commenced as early as the 18-somite stage. We have also investigated the myogenic potential of the fore-limb bud at various stages of development to ascertain whether there is a correlation between the stage the somitic cells first appear in the limb bud and the stage the bud acquires the capacity to form skeletal muscles. This was realized by culturing fore-limb buds excised from 18-30-somite stage embryos conventionally and in the kidney capsules of adult rats. In both methods, bone and cartilage were present in all of the cultures whereas skeletal muscles were only present in cultured explants older than the 21-22-somite stage.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Barrier inhibition of a temporal neuraxial influence on early chick somitic myogenesis.

Skeletal myogenesis in the chick embryo first occurs in the somite. Somites are transient, paired mesodermal structures adjacent to the neural tube. Somites form from the segmental plate mesenchyme at approximately 90-min intervals. We identify somitic myogenic cells by using confocal microscopy to detect the muscle specific intermediate filament protein, desmin, in whole mount chick embryo preparations. The appearance of desmin in somitic cells does not occur at a constant interval after the somite has formed. The rate of chick somitic myogenic onset, as evidenced by detection of desmin, is approximately 1.5 times faster than the rate of somitogenesis (Borman and Yorde [1994] J. Histochem. Cytochem. 42:265-272). Somitic myogenesis does not appear to be directly linked to somitogenesis but instead may be regulated by some influence external to the somite. Here we have specifically addressed the issue of whether an impermeable barrier placed between the neuraxis and the somites can prevent the onset of somitic myogenesis. When tantalum foil barriers are placed medial to the caudalmost 3-5 somites of embryos having up to 20 somites total (stage 13), the predominant result is an inhibition of myogenic cells lateral to the barrier. Conversely, when the tantalum foil is placed medial to the caudal somites of an embryo having 21 somites (stage 14) or more, desmin is detected lateral to the barrier in most cases. There is a temporal influence originating in the neuraxis which plays a role in the onset of somitic myogenesis. Although the nature of this interaction between the neuraxis and the somites is not yet clear, we have defined a precise temporal location within the developing embryo at which this tissue interaction is taking place.

Animals↗
PubMed1995

Neural tube and notochord promote in vitro myogenesis in single somite explants.

We have developed an in vitro assay for examining the neural tube/notochord influence on somite myogenesis. No cells committed to myogenesis (operationally defined as immunopositive staining for myosin heavy chain after a 48-hr culture period) are present in the most caudal somites (I-V) of stage 8-11 chicken embryos, whereas by stage 14-15, the most caudal somites occasionally produce differentiated muscle cells when cultured alone. To test for responsiveness to neural tube, one somite of each pair was cultured in contact with a piece of neural tube; as a control, the contralateral somite was cultured alone to test for prior myogenic commitment of somitic cells. Using this assay, we show that neural tube induces myogenesis in single somite explants, that close somite-neural tube proximity is required, and that the interaction can occur in defined medium. Neural tube induces myogenesis in the most recently formed somites and in segmental plate mesoderm, indicating that somite segmentation is not a prerequisite for neural tube mediated myogenesis in vitro. The assay was also used to determine whether continued exposure to neural tube is required for increasing myogenic cell numbers in somites which already contain cells committed to myogenesis. Since somites containing committed cells exhibit a fourfold increase in the number of myogenic cells when cocultured with neural tube, continued in vitro exposure to neural tube either recruits additional somite cells into the myogenic lineage, and/or it stimulates proliferation of existing myogenic cells. Consistent with this finding, the most rostral neural tube retains its inductive properties as evidenced by the induction of nascent myogenesis in the most caudal somites. Parallel studies of notochord mediated myogenesis indicate that the notochord promotes myogenesis within somites II-VIII, but not within somite I or the segmental plate. This observation suggests either a qualitative or quantitative difference in the mechanism by which the neural tube and notochord communicate with the youngest somite and presomitic mesoderm.

Animals↗
PubMed1988

Determination of somite cells: independence of cell differentiation and morphogenesis.

Somites are mesodermal structures which appear transiently in vertebrates in the course of their development. Cells situated ventromedially in a somite differentiate into the sclerotome, which gives rise to cartilage, while the other part of the somite differentiates into dermomyotome which gives rise to muscle and dermis. The sclerotome is further divided into a rostral half, where neural crest cells settle and motor nerves grow, and a caudal half. To find out when these axes are determined and how they rule later development, especially the morphogenesis of cartilage derived from the somites, we transplanted the newly formed three caudal somites of 2.5-day-old quail embryos into chick embryos of about the same age, with reversal of some axes. The results were summarized as follows. (1) When transplantation reversed only the dorsoventral axis, one day after the operation the two caudal somites gave rise to normal dermomyotomes and sclerotomes, while the most rostral somite gave rise to a sclerotome abnormally situated just beneath ectoderm. These results suggest that the dorsoventral axis was not determined when the somites were formed, but began to be determined about three hours after their formation. (2) When the transplantation reversed only the rostrocaudal axis, two days after the operation the rudiments of dorsal root ganglia were formed at the caudal (originally rostral) halves of the transplanted sclerotomes. The rostrocaudal axis of the somites had therefore been determined when the somites were formed. (3) When the transplantation reversed both the dorsoventral and the rostrocaudal axes, two days after the operation, sclerotomes derived from the prospective dermomyotomal region of the somites were shown to keep their original rostrocaudal axis, judging from the position of the rudiments of ganglia. Combined with results 1 and 2, this suggested that the fate of the sclerotomal cells along the rostrocaudal axis was determined previously and independently of the determination of somite cell differentiation into dermomyotome and sclerotome. (4) In the 9.5-day-old chimeric embryos with rostrocaudally reversed somites, the morphology of vertebrae and ribs derived from the explanted somites were reversed along the rostrocaudal axis. The morphology of cartilage derived from the somites was shown to be determined intrinsically in the somites by the time these were formed from the segmental plate. The rostrocaudal pattern of the vertebral column is therefore controlled by factors intrinsic to the somitic mesoderm, and not by interactions between this mesoderm and the notochord and/or neural tube, arising after segmentation.

Animals↗
PubMed1988

The somitic level of origin of embryonic chick hindlimb muscles.

Studies of avian chimeras made by transplanting groups of quail somites into chick embryos have consistently shown that the muscle cells of the hindlimb are derived from the adjacent somites, however, the pattern of cell distribution from individual somites to individual hindlimb muscles has not been characterized. I have mapped quail cell distribution in the chick hindlimb after single somite transplantation to determine if cells from an individual somite populate discrete limb muscle regions and if there is a spatial correspondence between a muscle's somitic level of origin and the known spinal cord position of its motoneuron pool. At stages 15-18 single chick somites or equivalent lengths of unsegmented somitic mesoderm adjacent to the prospective hindlimb region were replaced with the corresponding tissue from quail embryos. At stages 28-34, quail cell distribution was mapped within individual thigh muscles and shank muscle regions. A quail-specific antiserum and Feulgen staining were used to identify quail cells. Transplants from somite levels 26-33 each gave rise to consistent quail cell labeling in a unique subset of limb muscles. The anteroposterior positions of these subsets corresponded to that of the transplanted somitic tissue. For example, more anterior or anteromedial thigh muscles contained quail cells when more anterior somitic tissue had been transplanted. For the majority of thigh muscles studied and for shank muscle groups, there was also a clear correlation between somitic level of origin and motoneuron pool position. These data are compatible with the hypothesis that motoneurons and the muscle cells of their targets share axial position labels. The question of whether motoneurons from a specific spinal cord segment recognize and consequently innervate muscle cells derived from the same axial level during early axon outgrowth is addressed in the accompanying paper (C. Lance-Jones, 1988, Dev. Biol. 126, 408-419). Quail cell distribution was also mapped in chick embryos in which quail somites or unsegmented mesoderm had been placed 2-3 somites away from their position of origin. In all cases donor somitic tissues contributed to muscles in accord with their host position. These results indicate that muscle cell precursors within the somites are not specified to migrate to a predetermined target region.

Animals↗
PubMed1987

Autonomy of differentiation in avian branchial somites and the influence of adjacent tissues.

This study investigates the differentiative abilities of avian brachial somites at stages of development before, during and after the migration of somitic cells into the wing primordium. These somites are the source of cells that migrate into the forelimbs and there give rise exclusively, and totally, to the skeletal muscle lineage of the wing and yet show no morphological evidence of commitment to that fate when they leave the somites. The aim of the study was to see if the brachial somitic cells are committed to particular developmental pathways at these stages. The brachial somites were isolated from HH stage-12, -15 and -18 chick embryos, either by microdissection or enzymatic dissociation, and grown in organ culture, in explant culture on different substrata or on the chorioallantoic membrane (CAM) of host chicks, either alone or in combination with adjacent tissues. Myogenesis and chondrogenesis occurred in all stage-18 enzymatically separated somites, regardless of the growth environment. Myogenesis was reduced in stage-15 somites and unobservable in stage-12 somites; however, recombination of stage-12 somites with epithelium or neural tube increased the incidence of myogenesis at this stage. The incidence of chondrogenesis was also less in the younger explants. Unlike its effect on myogenic expression, recombination with epithelium resulted in a dramatic decrease in chondrogenesis in both stage-12 and -15 somites. The recombination experiments suggest that conditions that maintain the normal spatial relationships within isolated somites permit expression of a preexisting specification to a particular fate. They also show that the overlying epithelium can inhibit chondrogenesis in these somites. Overall, the results suggest that by the time migration of somitic cells into wing regions is finishing, brachial somitic cells have become stabilized in their ability to undergo both myogenesis and chondrogenesis for they will do so under a variety of growth conditions and independently of adjacent tissues. However, immediately before (stage 12) and shortly after (stage 15) the onset of migration, both myogenic and chondrogenic expression by brachial somitic cells are still under the influence of interactions with adjacent tissues.

Animals↗