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In vertebrates, BMPs (bone morphogenic proteins) play critical roles in establishing the basic embryonic body plan and are involved in the development of a large variety of organs and tissues. Here, we analyzed the expression pattern of various BMPs ( 2, 4, 5 and 7) by whole mount in situ hybridization during chick limb development. In limb, expression of BMPs suggests evolutionary conserved mechanisms of BMP-dependent differentiation between lower and higher vertebrates. During the early developmental stages, BMP-2 and BMP-7 are expressed in the posterior distal mesenchyme leaving a less prominent expression anteriorly. BMP-4 is initially expressed in the anterior mesenchyme and spreads later to the whole mesenchyme leaving a stronger expression at the anterior side. From HH-stage 25, expression of BMP-4 is observed in the anterior–posterior margins of the limb bud. The BMPs 2, 4 and 7 are expressed strongly in the AER, whereas BMP-5 is expressed as a weak signal in the distal mesoderm during the early stages of limb development. Later from HH-stage 25 onwards, BMP-5 is expressed in the dorsal and ventral muscular mass of the developing limb. As digits become identifiable, expression of BMPs are observed in the interdigital mesenchyme and can also be detected along the contours of the developing phalanges and at the distal tips of the digits. All these BMPs are found to be expressed in the developing feather buds from day 8 onwards. [ABSTRACT FROM AUTHOR]

Prox1 is a transcription factor with two highly conserved domains, a homeobox and a prospero domain. It has been shown that Prox1 knock-out mice die during early embryonic stages and display a rudimentary liver. We have studied the expression of Prox1 at RNA and protein levels in chick, rat, mouse and human liver and in transformed and non-transformed hepatic cell lines. Prox1 is expressed in early embryonic hepatoblasts and is still expressed in adult hepatocytes. Prox1 protein is located in the nuclei of hepatoblasts, which grow into the neighboring embryonic mesenchyme. The expression pattern in chick, mouse, rat and human embryos is highly conserved. Besides albumin and α-fetal protein, Prox1 belongs to the earliest markers of the developing liver. In adult liver, Prox1 is expressed in hepatocytes but is absent from bile duct epithelial and non-parenchymal cells (Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells and myofibroblasts). Isolated primary hepatocytes and hepatoma cell lines (HepG2, Hep3B) are Prox1 positive, whereas the immortalized murine liver cell-line MMH, which constitutively expresses the receptor c-met, is Prox1 negative. Transfection of MMH with Prox1 cDNA increases the expression level significantly as compared to control transfectants. In HepG2 and Hep3B, the Prox1 levels are even up to 100 times higher. Our studies show that Prox1 is a highly conserved transcription factor, expressed in hepatocytes from the earliest stages of development into adulthood and over-expressed in hepatoma cell lines. Its absence from bile duct epithelial cells suggests a function for the specification of hepatoblasts into hepatocytes. The genes controlled by Prox1 need to be studied in the future. [ABSTRACT FROM AUTHOR]

Uncx4.1 is a homeobox containing transcription factor that determines the development of the pedicles of the neural arches, transverse processes and proximal ribs. In this paper we characterize the expression pattern of Uncx4.1 during chick embryogenesis with special focus on its expression in the paraxial mesoderm. In the presomitic mesoderm, Uncx4.1 is expressed in the caudal halves of the somites I and II. In the newly formed somites, Uncx4.1 expression remains in the caudal somite halves but becomes restricted to the somitocoele and the ventral epithelial wall. After somite compartmentalization, Uncx4.1 is expressed in the caudal half of the sclerotome in a well defined spatial and temporal pattern. Micromanipulations revealed that Uncx4.1 expression in the presomitic mesoderm is independent of signals from the axial structures and presumably induced by the intrinsic Notch/Delta driven oscillator activity that determines craniocaudal somite polarity. In contrast, in the maturing somite Uncx4.1 expression depends on signals from the axial structures. The notochord-floor plate complex is essential for maintaining Uncx4.1 expression in the caudal somite half. The neural tube is necessary for providing sufficient Uncx4.1 positive sclerotomal material to enable development of pedicles of the neural arches and transverse processes. [ABSTRACT FROM AUTHOR]

The growth and maintenance of the blood and lymphatic vascular systems is to a large extent controlled by members of the vascular endothelial growth factor (VEGF) family via the tyrosine kinase receptors (VEGFRs) expressed in angioblastic cells. Here, we analyzed the Quek1 (VEGFR-2) expression pattern by whole mount in situ hybridization during quail development. During early embryogenesis, Quek1 expression was detected at the caudal end of the blastoderm and primitive streak and in the head paraxial mesoderm. In somites, expression was observed from HH-stage 9 onwards in the dorsolateral region of both the forming and recently formed somites. During somite maturation, expression persists in the lateral portion of the somitic compartments, the dermomyotome and the sclerotome. Additionally, a second expression domain in the maturing somite was observed in the medial part of the sclerotome adjacent to the neural tube. This expression domain extended medially and dorsally and surrounded the neural tube during later stages. In the notochord, expression was observed from HH-stage 23 onwards. In the limb bud, expression was initiated in the mesenchyme at HH-stage 17. During organogenesis, expression was detected in the pharyngeal arches and in the anlagen of the esophagus, trachea, stomach, lungs, liver, heart and gut. Expression was also seen in feather buds from day 7 onwards. Our results confirm the angiogenic potential of the mesoderm and suggest that VEGFR-2 expressing cells represent multiple pools of mesodermal precursors of the hematopoietic and angiopoietic lineages. [ABSTRACT FROM AUTHOR]

This study analyzes the temporal and spatial expression pattern of Fgf-8 over a continuous series of developmental stages. Special emphasis is laid on the paraxial mesoderm where Fgf-8 expression is highly dynamic. Whereas the anterior portion of the unsegmented mesoderm is devoid of expression, Fgf-8 is upregulated in the posterior half of a newly formed somite. Soon after somite formation, this highly localized expression gives way to a more diffuse pattern of Fgf-8 expression at low levels in presumptive sclerotomal cells. During later somite maturation, transcripts become restricted to the myotome. Co-staining with the myotome marker MyoD reveals that Fgf-8 expression defines a subpopulation of muscle precursor cells. [ABSTRACT FROM AUTHOR]

The question of regeneration after experimental somite extirpation has been controversial in the literature. While all workers agree that repair of the defects occurs, results concerning the extent and mechanism of this process, as well as the origin of the cells filling the defect, show great discrepancies. Our approach towards a re-examination of this question involved microsurgical removal of individual somites in 2-day-chick embryos in combination with grafting of quail somites and lateral plate. We show that the defect in the paraxial mesoderm is filled within a day after extirpation and that the reconstituting cells are derived only from the cranial and caudal somites, but not from the contralateral somites or from the lateral plate. There are no indications of an increase of proliferation in the neighbouring somites. In order to examine the differentiation capacities of the cells that fill the defect, we used immunohistochemistry and in situ-hybridization. We show that the cells in the defect are mesenchymal in morphology and express Pax-1 and Twist. There are a few desmin-positive cells in the defect that can be shown to derive from adjacent somites. An epithelial dermomyotome and myotome are absent at the operation site. Neural crest cells do not participate in the reconstitution of the defect. We conclude that cells in the defect either already have or adopt a ventral somitic (sclerotomal) identity, whereas derivatives with dorsal identity are absent from the defect except for a few individual cells. [ABSTRACT FROM AUTHOR]

The segmental character of the vertebrate body wall is reflected by metamerically arranged tissues that are patterned during embryonic life as a consequence of somite formation, compartmentalization and differentiation. The somites bud off the paraxial mesoderm in a cranio-caudal sequence and are compartmentalized by local signals from adjacent structures. These signals may be mediated by diffusible substances such as Sonic hedgehog (Shh), Wnts and Bone morphogenetic protein (BMPs) or by cell–cell interactions via membrane-bound receptors and ligands such as Delta and Notch. Compartmentalization of the somites and their derivatives is reflected by the differential expression of developmental regulatory genes such as Pax-1, 3, 7 and 9, MyoD, paraxis, twist and others. Secondary segmentation is imposed upon other tissues, such as blood vessels and nerves, by the rearrangement and regionalization of the somitic derivatives, especially the sclerotome. Early cranio-caudal identity is determined by the expression of different Hox genes. Finally, fusion of segmental anlagen occurs to form segment-overbridging skeletal elements and muscles. The expression of homologous genes indicates that the process of segmentation in vertebrates and invertebrates is homologous, derived by descent from a common ancestor. [ABSTRACT FROM AUTHOR]

In avian embryos, the migration behaviour of several cell populations, melanoblasts, Schwann cells, myogenic cells and axons after application of antibodies directed against the cell-attachment fragment of fibronectin (α-CAF) was investigated. The migration of the different cell types was influenced in different ways. 1. Epidermal melanoblasts did not colonize areas into which the antibody had been injected, i.e. distal to the grafting site. They frequently spread proximally to the back and neck, sometimes even as far as to the ipsilateral leg. When grafted to the dorsal side of the wing bud, melanoblasts never spread to the ventral side after injection of the antibody. Non-epidermal melanoblasts continued to migrate distally. 2. Grafted Schwann cells and host axons were not noticeably affected by the antibody injections. Both were found proximally and far distally to the grafting site, i.e. also within the injected area. 3. Myogenic cells were immobilized near the grafting site, where they differentiated biochemically, but sometimes only partially underwent fusion into myotubes. They participated in the formation of host muscle blastemas only immediately adjacent to the non-migratory cell population of the graft such as fibroblasts and cartilage. 4. The injected antibody could be localized up to 5 h after the application in the distal third of the limb bud. We conclude that migrating cell populations show differences in their fibronectin-dependence which probably reflect their use of fibronectin during migration. [ABSTRACT FROM AUTHOR]

Cytokinetic studies on the aortic endothelium using the BrdU/anti-BrdU-method were carried out on 2.5- to 6-day chick and quail embryos. The mitotic activity of the aortic endothelium is related temporally to the age of the avian embryo and spatially to the embryonic region where the aorta originates. The mitotic activity of the aortic endothelium decreases with increasing age of the embryos. In the limb buds, however, the mitotic rate of the aortic endothelial cells increases independently of the age of the embryo. This increase in the mitotic activity of the aortic endothelium at the appropriate levels coincides with the vascularization of the outgrowing limb buds. We concluded therefore that the aortic endothelium probably supplies endothelial cells for the formation of limb vessels at this stage. Thus our results suggest that angiogenesis (sprouting of capillaries from pre-existing vessels) takes place during limb vascularization in avian embryos. On the other hand, immunohistochemical studies with QH-1 or MB-1 antibody show, beside a capillary network in the central core of the wing bud, individual immunolabelled cells of mesenchymal character within the primarily avascular subectodermal region from the onset of vascularization onwards. We suggest that these cells have partly to be regarded as endothelial precursor cells, which have differentiated in situ from the local limb mesenchyme, and which will contribute to the developing vascular plexus. This means that not only angiogenesis, but also vasculogenesis (in situ from mesenchymal precursors differentiated endothelial cells) appears to be involved in limb vessel formation. [ABSTRACT FROM AUTHOR]

The origin of cells covering the nervous system and the cutaneous receptors was studied using the quail-chick marking technique and light and electron microscopy. In the first experimental series the brachial neural tube of the quail was grafted in place of a corresponding neural tube segment of the chick embryo at HH-stages 10 to 14. In the second series the leg bud of quail embryos at HH-stages 18-20 was grafted in place of the leg bud of the chick embryos of the same stages and vice versa. It was found that all meningeal layers of the spinal cord, the perineurium and the endoneurium of peripheral nerves, as well as the capsular and inner space cells of Herbst sensory corpuscles, develop from the local mesenchymal cells. Schwann cells and cells of the inner core of sensory corpuscles are of neural crest origin. The precursors of Merkel cells migrate similarly to the Schwann cells into the limb bud where they later differentiate. This means that in addition to the Schwann cells and the melanocytes a further neural crest-derived subpopulation of cells enters the limb. [ABSTRACT FROM AUTHOR]

Changes in the distribution of extracellular matrix components have been investigated immunohistochemically during neural crest development in the rat. Inside the ectodermal epithelium basal lamina components are formed resulting in a separation of neurectoderm and epidermal ectoderm. Within the presumptive neural crest area fibronectin, hyaluronan and chondroitin sulphate become apparent. Upon subsequent neural crest migration the basal lamina becomes disrupted. As the neural crest cells take part in mesectoderm formation, fragments of the basal lamina remain attached to their surface, as is demonstrated with antibodies against laminin and collagen type IV. The extracellular matrix is therefore active both in the separation of neuroectoderm from epidermal ectoderm and in mesectoderm formation. [ABSTRACT FROM AUTHOR]

Migration of vascular endothelial cells was traced in quail-chick chimeras. After heterospecific transplantations of quail limb bud pieces, or other tissues containing blood vessels, into the limbs or the coelomic cavity, the immunohistochemically stained endothelial cells of the quail were found to invade the chick host vessels, favouring the arteries. Within these vessels the endothelial cells regularly reach the host aorta, where they contribute to the endothelium on the ipsilateral side. It is concluded that the endothelial cells activity migrate, because microinjections of a synthetic peptide which contains the RGD-sequence and mimics fibronectin, stop the invasion of endothelial cells. [ABSTRACT FROM AUTHOR]

The avian thymus and its myoid cells were investigated paying special attention to the developmental and morphological differences between chick and quail. By means of light- and electron microscopy, and immunofluorescence technique using an anti-myosin antibody, the myoid cells were found to express characteristics corresponding to those of skeletal muscle cells. They change their appearance during embryonic development. In the chick the myoid cells become located singly and rounded, and their cross-striation disappears. In the quail they remain small, elongated, cross-striated, and become arranged in long cords. The origin of myoid cells was studied using the quailchick marking technique: Cranial somites and the prechordal mesoderm were grafted from quail into chick embryos. After somite transplantation the host thymus does not contain graft-derived cells. The myoid cells are exclusively derived from the chick. After implantation of prechordal mesoderm, graft-derived quail cells are found in the central cores of all visceral arches and also within the early epithelial anlage of chimeric thymus. These findings indicate that the thymus myoid cells are derived from the axially located prechordal head mesoderm. [ABSTRACT FROM AUTHOR]

Interspecific grafting experiments between chick and quail embryos were carried out in order to investigate the mechanism controlling myogenic cell migration in the avian limb bud. In six series, various experimental set-ups were prepared involving different age combinations of donor and host. The migration of the myogenic cells contained nor and host. The migration of the myogenic cells contained in the quail donor could be traced due to the prominent perinucleolar heterochromatin of the quail nucleus. Irrespectively of the presence or absence of the apical ectodermal ridge (AER), myogenic cells were found to migrate distally when implanted at a more distal site or into a younger host. They were even found to migrate in the reverse direction when younger host tissue was located proximal to the graft. From these findings, we conclude that the state of differentiation ('juvenility') of the limb bud mesenchyme controls the directed migration of myogenic cells. [ABSTRACT FROM AUTHOR]

The extracellular matrix-component fibronectin (FN) was detected in close localisation to the vascular system (VS) of the quail chorioallantoic membrane (CAM). We have examined the role of cell-fibronectin interactions within the developing CAM. In two series of experiments the CAM was directly exposed to (1) an antibody against the cell-binding fragment of FN, and to (2) RGD-containing synthetic peptides which are recognized by the FN receptor. For controls an antibody against tubulin and a SHL VE-pentapeptide that does not interfere with the FN-receptor were applied. In the presence of anti-FN antibodies and RGD-sequences the CAM could not establish a normal vascular system. We observed hypo-and partially avascular regions; the resulting vascular pattern was atypically lacunar. None of the control substances affected the regular development of the chorioallantoic vascular system. These results demonstrate the essential role of FN in CAM angiogenesis. [ABSTRACT FROM AUTHOR]

Quail leg buds were grafted in place of chick leg buds or chick wing buds and vice versa at stages 18 to 21 after colonization by muscle precursor cells had been completed. Motor endplate pattern in the plantaris muscle of the grafts was analyzed before hatching by means of esterase and acetylcholinesterase staining techniques. Muscle fibre types were made visual using the myosin ATPase reaction. Investigations are based on the species-specific endplate pattern of the plantaris muscle: multiply innervated fibres in the chick and focally innervated fibres in the quail. Muscle pieces isolated from the adjacent medial gastrocnemius muscle of the grafted legs were histologically examined to judge their species-specific composition. Horseradish peroxidase was injected into the plantaris muscles of both the grafted and the opposite leg as well as in the plantaris muscle of normal quail embryos, in order to be sure that the plantaris muscle of the graft is innervated by appropriate motoneurons. This procedural design offers for the first time a possibility to test experimentally the influences of motoneurons on endplate pattern formation under conditions corresponding to those in normal ontogenesis. It is shown that such appropriate motoneurons of one species which project to the plantaris muscle of the other species dictate the endplate pattern. When the plantaris muscle is innervated by inappropriate motoneurons, the endplate pattern inherent in the muscle primordium itself becomes realized. A sequence of hierarchically acting factors is proposed to bring different results in line. According to this, the neuronally set programme has priority compared with that set in the muscle. This is true for the normal development and might generate the high neuro-muscular specificity. If under experimental conditions the neuronal programme and the peripheral programme differ, the axons and muscle fibres selectively interact with respect to their inherent characteristics and the muscle-specific programme becomes expressed. If there is a lack of a certain axon type, muscle fibres might become innervated by non-corresponding motoneurons which alter the muscle fibre type. [ABSTRACT FROM AUTHOR]

Pieces of quail embryos from various developmental stages ranging from unincubated blastoderms (before the appearance of a primitive streak) to embryos having formed somites were grafted to the wing buds or into the coelomic cavity of chicken embryos. The grafts, which can be identified on a cellular level by virtue of the prominent nucleolus-associated chromatin, present in the quail and absent in the chicken, were screened after suitable periods of reincubation for the presence or absence of skeletal myotubes containing quail nuclei. Grafts having contributed to such skeletal myotubes were considered as having contained determined myogenic cells at the time of the grafting procedure. Determined myogenic cell appeared first in the primitive streak and in the mesodermal cells formed by the invagination (gastrulation) of epiblastic cells through the primitive streak. This is true for both the head process and the paraxial mesoderm. Epiblastic cells never gave rise to skeletal myotubes. Therefore it can be said, that the onset of myogenic determination coincides with gastrulation. It remains, however, to be established, whether these two events are causally related to one another. [ABSTRACT FROM AUTHOR]

The onset of myotome formation in somites of chick embryos was studied by use of a polyclonal antidesmin antibody and by histochemical demonstration of acetylcholine esterase activity. The myotome cells originate from the dermatome only; sclerotome cells do not contribute to the myotome. The formation of the myotome starts in the craniomedial corner of the dermatome. From there the myotome formation continues simultaneously along the medial and the cranial edge of the dermatome. It was found that only the already longitudinally oriented cells of the cranial dermatome edge give rise to the myotome; the cells of the dorsomedial dermatome edge do not contribute to the myotome. Myotome cells do not originate directly from the surface of the overlying dermatome by delamination. [ABSTRACT FROM AUTHOR]

In avian embryos the formation of ventrolateral abdominal muscles was studied by (1) heterospecific grafting experiments between chick and quail embryos and (2) ultrastructural examinations of cells having part in this process. The results demonstrate that the muscle cells are of somitic origin while the connective tissue derives from the somatopleure. Somatopleural cells do not differentiate into myocytes, and somite cells which have entered the ventrolateral abdominal wall, do not contribute to the connective tissue. It is concluded that both dermatome and myotome cells undergo muscular differentiation. The formation of muscles is found to take place in four characteristic steps. During the 4th day of development, epithelially structured ventral somite buds enter the somatopleure. The light cells of the inner myotome layer are elongated in a cranio-caudal direction and contain randomly distributed microfilaments. On the 5th day, the buds lose their epithelial arrangement and change into compact processes in which cells intermingle. The myotome cells show short bundles of thin and thick microfilaments. The third step can be characterized by the appearance of intercellular spaces and the disaggregation of processes becoming invaded by somatopleural cells. Thus, subdivision in single muscle blastemata begins to occur. In 7-day embryos, the muscle anlagen are distinctly separated and the first myotubes containing regularly arranged myofibrils are found. Coincidentally, signs of cell death are observed. Up to the 10th day, the tendons being of somatopleural origin become plainly outlined and the muscle anlagen move to their definitive positions. It is assumed that the formation of muscle pattern is controlled by the somatopleure. [ABSTRACT FROM AUTHOR]

The state of determination of somites, parts of somites, unsegmented paraxial mesoderm and of somatopleure was investigated by grafting these tissues from quail embryos to the wing buds of chicken embryos. It was found that muscular and chondrogenic determination occur before the formation of somites. Muscular determination takes place earlier than previously assumed and ahead of chondrogenic determination. Somatopleure yields cartilage, but no skeletal muscle. Prospective sclerotomes are primarily capable of differentiating into muscle and loose this potency in the course of development. Myogenic cells extensively migrate within the wing bud in a proximo-distal direction, whereas chondrogenic cells both of somitic and somatopleural origin show no overt migratory tendency. [ABSTRACT FROM AUTHOR]

The quail-chick-marker technique has been employed to elucidate the determination of embryonic tissues in the avian wing bud. It was found that myogenic cells of somitic origin are determinated to give rise only to muscular tissue from HH-stage 19 on. Mesenchyme in the cartilage forming regions is determinated to form cartilage from HH-stage 20 on. Tissue from non-cartilage forming regions retains the option to form cartilage at least up to HH-stage 26. Whereas cells of somatopleural lineage do not migrate within the limb bud, cells of somitic origin do so up to at least HH-stage 28. [ABSTRACT FROM AUTHOR]

The migration of myogenic stem cells into the leg anlagen of chick embryos between stages 16-20 of Hamburger and Hamilton was examined. SEM and TEM studies reveal that cell migration starts at stage 16 from the just-formed somites 26-28. The migrating myogenic cells are elongated and oriented in a medio-lateral direction. The leading ends branch into filopodia which contact a fibrillar network. At first, single cells migrate; later on the cells leaving the ventro-lateral edge of the dermatome migrate in strands and have specialized contacts between them. After reaction with ruthenium red and concanavalin A the migrating cells show a thick surface coat to which ruthenium red-positive particles are attached. The surface coat may be important in the interactions among the migrating cells as well as between the cells and the substrate. The migration of myogenic stem cells was found to take place in a matrix of collagenous fibrils and ruthenium red-positive particles, probably containing glycosaminoglycans. At the onset of migration the fibrillar network exhibits a preferred mediolateral orientation. Therefore, it may be concluded that this alignment of the fibrils influences the direction of cell migration. [ABSTRACT FROM AUTHOR]

In chick embryos undifferentiated myogenic stem cells migrate from the ventrolateral somite respectively dermatome edge into the prospective wing region after the second day of incubation. At first, single cells that are elongated in mediolateral direction, later also small groups of cells, are found in the space between somites and somatopleura at the wing bud level. The leading ends of the migrating cells are formed like finger-shaped lobopodia as well as flattened lamellipodia from which thin filopodia arise. The main structural features of the cell processes are microtubules and microfilaments predominantly oriented parallel to the long axis of the cells. The filopodia are found to be in close connection with the surrounding network of collagen fibrils. Since the main strands of the fibrils show a mediolateral orientation, it may be assumed that the direction of cell migration depends on the arrangement of the collagen fibrils. [ABSTRACT FROM AUTHOR]

Interspecific grafts of somites, as well as parts of the somatic plate mesoderm, have been made between quail and chick embryos (stages 12-14 H.H.) at the level of the prospective wing bud in order to examine the relationship between somites and wing bud myogenesis. The stability of the natural quail nuclear labelling makes it possible to follow the developmental fate of grafted mesodermal cells in the host embryo. Embryos examined after subsequent incubation periods of 3-7 days show the following distribution of somatic and somitic cells within the wing bud: as soon as the three zones of different cell density within the mesoderm can be distinguished, cells of somitic origin are limited to the prospective myogenic are which is made up of a mixed population of somatic and somitic cells, whereas the prospective chondrogenic area as well as the subectodermal zone only consists of cells originated from the somatic plate mesoderm. After further incubation, single muscle blastema are present which were also seen to be a mixture of somatic and somitic cells. The cells of muscular bundles are of somitic origin, while the muscle connective tissue cells are derived from the somatic plate mesoderm. After grafting into the coelomic cavity or on the chorio-allantoic membrane, fragments of the somatic plate mesoderm previously isolated from quail embryos (stage 14 H.H.) at the level of the prospective wing bud exhibit well developed skeletal elements, but fail to differentiate any musculature. These experimental investigations support previous evidence for a somitic origin of wing bud myogenic cells. Histological and scanning electron microscopic studies of the brachial somites and the adjacent somatic plate mesoderm of chick embryo (stages 13-15 H.H.) reveal that migration of still undifferentiated somitic cells into the brachial somatic plate mesoderm begins to take place in embryos at stage 14. [ABSTRACT FROM AUTHOR]