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PubMed1997

System to identify individual somites and their derivatives in the developing mouse embryo.

The identification of the axial levels of metameric elements along the rostro-caudal axis of vertebrates until now was not possible before late, fetal development, when the vertebral anlagen first appear. We developed a new system for the exact axial identification of somites and their derivatives from early, embryonic stages of mouse development on (Theiler stages (TS) 15 to TS18-19). The initial axial identification of the somites was performed by relating them to the rostral-most two cervical spinal ganglia (SG), that exhibited characteristic morphologies (SG-C1: bar-like, SG-C2: triangular). At all stages of somitic development, the most prominent somite along the rostro-caudal axis correlated with the bar-like SG-C1, and, therefore, we named it the first cervical somite (SO-C1). The next step, the axial identification of the somites independently from the SG, was based on the observation that after in situ hybridization to Myf5, Pax3, Pax1, and Mox1 riboprobes, a distinct and characteristic morphology of the last occipital somite (SO-O5) and the first two cervical somites (SO-C1, SO-C2) can be observed. From TS15 on, these three somites formed a triad of the most prominent somites along the rostro-caudal axis. Also, the dermomyotomal, myotomal, and sclerotomal derivatives of this somite triad were the most prominent in later somitic development. Furthermore, SG-C1 and SG-C2 exhibited a transient bipartite anlagen in their early development, suggesting a "resegmentation" during SG formation. Later, when somites started to dissolve, the caudal moiety of the bar-like SG-C1 anlagen fused to the anlagen of SG-C2.

Alcohol Oxidoreductases↗
PubMed1999

Dynamic expression of lunatic fringe suggests a link between notch signaling and an autonomous cellular oscillator driving somite segmentation.

The metameric organization of the vertebrate trunk is a characteristic feature of all members of this phylum. The origin of this metamerism can be traced to the division of paraxial mesoderm into individual units, termed somites, during embryonic development. Despite the identification of somites as the first overt sign of segmentation in vertebrates well over 100 years ago, the mechanism(s) underlying somite formation remain poorly understood. Recently, however, several genes have been identified which play prominent roles in orchestrating segmentation, including the novel secreted factor lunatic fringe. To gain further insight into the mechanism by which lunatic fringe controls somite development, we have conducted a thorough analysis of lunatic fringe expression in the unsegmented paraxial mesoderm of chick embryos. Here we report that lunatic fringe is expressed predominantly in somite -II, where somite I corresponds to the most recently formed somite and somite -I corresponds to the group of cells which will form the next somite. In addition, we show that lunatic fringe is expressed in a highly dynamic manner in the chick segmental plate prior to somite formation and that lunatic fringe expression cycles autonomously with a periodicity of somite formation. Moreover, the murine ortholog of lunatic fringe undergoes a similar cycling expression pattern in the presomitic mesoderm of somite stage mouse embryos. The demonstration of a dynamic periodic expression pattern suggests that lunatic fringe may function to integrate notch signaling to a cellular oscillator controlling somite segmentation.

Animals↗
PubMed1996

Participation of individual brachial somites in skeletal muscles of the avian distal wing.

In this paper we investigate the somitic origin of the individual muscles of the forearm and hand using quail-chick chimeras. Our results show that only somites 16-21 give rise to wing muscle, but they take part in muscle formation to different extents. Somite 21 does not always participate in the formation of muscle of the forearm and hand. The most cranial somite (16) takes part in the radial muscles and the most caudal somites (20, 21) in the ulnar muscles, reflecting their position with respect to the limb bud. The centrally located somites (17, 18, 19) are involved in all (18) or most (17, 19) muscle primordia. This pattern of distribution is clearest in the forearm, whereas the participation of somites in particular muscle groups is not so distinct in the hand. Hand muscles are mainly made up of cells from somites 18-20. All brachial somites participate in dorsal (extensor) as well as ventral (flexor) muscles of the forearm and hand. Each somite takes part in more than three muscle primordia in a reproducible fashion, and every muscle primordium is derived from at least three somites. Especially the M. ulnimetacarpalis ventralis takes origin from all somites involved in limb muscle formation (16-21). Apart from muscle cells, endothelial cells also and a few fibroblasts of quail origin are found in the limb bud after somite grafting.

Age Factors↗
PubMed1999

Adhesive subdivisions intrinsic to the epithelial somites.

Developing somites express two subtypes of classic cadherin adhesion receptors, N-cadherin and cadherin-11 (cad11). To investigate the role of these adhesion molecules in somite morphogenesis, we analyzed the somites of mice whose N-cadherin and cad11 genes were disrupted. The epithelial somites of N-cadherin null mutant mice were fragmented as reported, whereas those of cad11(-/-) mice showed no structural anomaly. In mice double homozygous for N-cadherin and cad11 mutation, however, somites were further fragmented into smaller clusters than in the N-cadherin-deficient mice, suggesting that these two cadherins cooperate in the maintenance of epithelial somites. Despite the disorganization of epithelial structures, dorsoventral polarity markers were expressed in their correct patterns in all of these mutant somites. Uncx4.1, whose expression is localized only in the caudal region of each somite, was also expressed in a normal pattern in the mutant somites. However, the staining for Uncx4.1 revealed that, in the N-cadherin mutants, each somite tended to be cleaved at the border between the Uncx4. 1-positive and -negative regions and that the cleaved subunits maintained the clustered state, often exhibiting epithelioid morphology. This separation of the rostral and caudal regions was observed as soon as the epithelial somites had been formed. In the N-cadherin/cad11 double-homozygous mutants, this tendency was also observed, although each half of the somite further disintegrated into randomly arranged cell clusters. These results suggest that cells of the rostral and caudal regions of each epithelial somite have an activity to aggregate independently or separate from one another and that one role of N-cadherin and cad11 is to connect the two halves into a single unit.

Animals↗
PubMed2003

The relationship between limb muscle and endothelial cells migrating from single somite.

Somites contribute myogenic and endothelial precursor cells to the limb bud. Transplantations of single somites have shown the pattern of muscle cell distribution from individual somites to individual limb muscles. However, the pattern of the endothelial cell distribution from individual somites to the limb has not been characterized. We have mapped quail muscle and endothelial cell distribution in the distal part of the chick limb after single somite transplantation to determine if there is a spatial relationship between muscle and endothelial cells originating from the same somite. Single brachial somites from quail donor embryos were transplanted into chick embryos, and, following incubation, serial sections were stained with a quail-endothelial cell-specific monoclonal antibody (QH-1), an anti-quail antibody (QCPN) and an anti-desmin antibody to distinguish the quail endothelial and muscle cells from chick cells. Our results show that transplants of somite 16-21 each gave rise to quail endothelial cells in the wing. The anterioposterior position of the blood vessels formed by somitic endothelial cells corresponded to the craniocaudal position of the somite from which they have originated. Endothelial cells were located not only in the peri- and endomysium but also in the subcutaneous, intermuscular, perineural and periost tissues. There was no strict correlation between the distribution of muscle and endothelial cell from a single transplanted somite. Blood vessels formed by grafted quail endothelial cells could invade the muscle that did not contain any quail muscle cells, and conversely a muscle composed of numerous quail muscle cells was lacking any endothelial cells of quail origin. Furthermore, a chimeric limb with very little quail muscle cells was found to contain numerous quail endothelial cells and vice versa. These results suggest that muscle and endothelial cells derived from the same somite migrate on different routes in the developing limb bud.

Animals↗
PubMed2001

Ablation of axial structures activates apoptotic pathways in somite cells of the chick embryo.

It has been known for some time that ablation of the neural tube and/or the notochord in the chick embryo leads to a massive wave of cell death in the adjacent somites. It is postulated that in the normal embryo, survival signals emanate from the neural tube and/or notochord that suppress apoptosis in the cells of the somites, except for a small population of sclerotome cells that are programmed to die naturally. In this study we show that axial ablation results in the death of sclerotome and not somitic neural crest cells, and we have examined the apoptotic response of these cells to the ablation. We show that several elements of the apoptotic cascade become detectable in somite cells in response to the withdrawal of survival signals. We demonstrate the down-regulation of bcl-2 protein in the somites adjacent to, and caudal to, the site of ablation, corresponding to the region that displays an elevated level of cell death. Although caspase-9 appeared to be activated in somites at all levels of the trunk, caspase-2 showed a clear response to the ablation of the axial structures. Removal of the neural tube and notochord produced an up-regulation of caspase-2 activity in somites in the region of the operation. Cleavage of two down-stream substrates of these caspases was examined. The cleavage of poly (ADP-ribose) polymerase (PARP) was apparent in somites at all levels of the trunk, and showed only a modest up-regulation after ablation. By contrast, the cleavage of DNA fragmentation factor (DFF45) showed a marked up-regulation in response to ablation, suggesting that this is a primary substrate for a caspase-dependent apoptotic mechanism. Evidence was also found for a caspase-independent mechanism, since the expression of apoptosis-inducing factor (AIF) was found to be very sensitive to, and up-regulated in somites by, axial ablation. Because the wave of apoptosis that is precipitated in somites by removal of the axial structures may be mediated by BMP-4, we examined the levels of BMP-4 in somites in response to axial ablation. BMP-4 expression was clearly up-regulated in somites adjacent to, or close to, the site of operation.

Animals↗
PubMed2004

Wnt 6 regulates the epithelialisation process of the segmental plate mesoderm leading to somite formation.

In higher vertebrates, the paraxial mesoderm undergoes a mesenchymal to epithelial transformation to form segmentally organised structures called somites. Experiments have shown that signals originating from the ectoderm overlying the somites or from midline structures are required for the formation of the somites, but their identity has yet to be determined. Wnt6 is a good candidate as a somite epithelialisation factor from the ectoderm since it is expressed in this tissue. In this study, we show that injection of Wnt6-producing cells beneath the ectoderm at the level of the segmental plate or lateral to the segmental plate leads to the formation of numerous small epithelial somites. Ectopic expression of Wnt6 leads to sustained expression of markers associated with the epithelial somites and reduced or delayed expression of markers associated with mesenchymally organised somitic tissue. More importantly, we show that Wnt6-producing cells are able to rescue somite formation after ectoderm ablation. Furthermore, injection of Wnt6-producing cells following the isolation of the neural tube/notochord from the segmental plate was able to rescue somite formation at both the structural (epithelialisation) and molecular level, as determined by the expression of marker genes like Paraxis or Pax-3. We show that Wnts are indeed responsible for the epithelialisation of somites by applying Wnt antagonists, which result in the segmental plate being unable to form somites. These results show that Wnt6, the only known member of this family to be localised to the chick paraxial ectoderm, is able to regulate the development of epithelial somites and that cellular organisation is pivotal in the execution of the differentiation programmes. We propose a model in which the localisation of Wnt6 and its antagonists regulates the process of epithelialisation in the paraxial mesoderm.

Animals↗
PubMed1995

Myogenic specification of somites is mediated by diffusible factors.

Specification of myogenesis in early chicken somites and segmental plate was studied using transfilter explant cultures to determine if myogenic specification by axial structures is mediated by cell-cell contact. Formation of muscle fibers that express myosin heavy chain was assessed in somites transfilter from neural tube and notochord. Either the neural tube or the notochord from early chick embryos (ED 2) induces myogenesis in unspecified somites (Hamburger and Hamilton (HH) stages 11-14) when these tissues are separated by a 0.2- or a 0.05-micron pore filter. The ventral neural tube is found to be a strong inducer of myogenesis, while the dorsal neural tube is found to have low inducing activity. The reduced myogenic inducing activity of the dorsal neural tube is associated with an activity that inhibits myogenic differentiation in specified somites, somites containing cells already committed to myogenesis. Only 6-8 hr of transfilter exposure to neural tube is required to initiate somite myogenesis that is sustained in the absence of neural tube. Other tissues such as specified somites, heart, or ectoderm from the same aged embryo do not induce myogenesis in unspecified somites. Removal of the prospective floor plate of the caudal neural plate at HH stage 8 or 9 does not eliminate the inducing activity of the neurectoderm on unspecified somites (HH stage 11-14). Recombination of somites with neural tube or spinal cord from progressively older embryos (ED 4-20) showed myogenesis inducing activity at all ages, though the activity waned as development proceeded. Paraxial mesoderm need not segment into somites to respond to the inducing activity of the neural tube. We conclude that induction of myogenesis in somites does not require cell-cell contact between either the neural tube or the notochord and therefore induction is mediated by diffusible factor(s); that the inducing activity is localized principally to the notochord and ventral half of the neural tube, but not necessarily to the floor plate; that the myogenic response is specific to neural tube and notochord; and that the dorsal neural tube weakly induces myogenesis and contains an activity that inhibits myogenic cells from differentiating.

Animals↗
PubMed1991

Segmentation of sensory and sympathetic ganglia: interactions between neural crest and somite cells.

The segmental pattern of peripheral ganglia in higher vertebrates is generated by interactions between neural crest and somite cells. Each mesodermal somite is subdivided into at least two distinct domains represented by its rostral and caudal halves. Most migratory pathways taken by neural crest cells in trunk regions of the axis, as well as the outgrowth of motoneuron fibers are restricted to the rostral domain of each somite. Experimental modification of the somites, achieved by constructing a mesoderm composed of multiple rostral half-somites, results in the formation of continuous and unsegmented nerves, dorsal root ganglia (DRG) and sympathetic ganglia (SG). In contrast, both neurites and crest cells are absent from a mesoderm composed of multiple-caudal half somites. However, the mechanisms responsible for gangliogenesis within the rostral half of the somite, appear to be different for DRG and SG. Vertebral development from the somites is also segmental. In implants of either multiple rostral or caudal somite-halves, the grafted mesoderm dissociates normally into sclerotome and dermomyotome. However, the morphogenetic capabilities of each somitic half differ. The lateral vertebral arch is continuous in the presence of caudal half-somite grafts and is virtually absent in rostral half-somite implants. Therefore, the rostrocaudal subdivision of the sclerotome determines the segmental pattern of neural development and is also important for the proper metameric development of the vertebrae.

Animals↗
PubMed1975

[Demonstration, by means of electron microscopy, of the penetration of somitic cells into the mesoblast of the limb buds of reptile embryos (Anguis fragilis, Lacerta viridis)].

An electron microscopic study of the components of anterior limb buds of the slow-worm (Anguis fragilis) and of the green lizard (Lacerta viridis) (embryos of Anguis whose allantoic bud reach 0,7 to 4 mm of length; embryos of Lacerta 2 to 7 days old) provides data on the cytological characteristics of the components of the limb bud at these early stages. 1. The cells of the distal extremity of the somitic processes extending in the limb bud of Anguis and Lacerta, are elongated cells with ovoid nuclei containing large nucleolus; they possess mitochondria always thin and with dense matrix; they are rich in lipid droplets; they possess cilia; they are devoid of myofilaments; endoplasmic reticulum, free ribosomes and polyribosomes are abundant. Golgi networks display signs of activity. These characteristics are also observed in the cells of the "dermatome" layer of the dermo-myotome; and so, it appears probable that the cells of the "dermatome". Furthermore, in Anguis embryos, the cells of the distal extremities of the somitic processes possess numerous lysosomes and a certain number of cells among them, degenerate early. 2. The somatopleural mesoblastic cells of the limb bud of Anguis and Lacerta embryos keep the characters of the cells of the mesodermic layer of lateral plate from which they originate; they have rounded nuclei, cilia, and their mitochondria are always larger and more transparent to electrons, than the ones of cells of the somitic processes and of cells of the epiblastic apical crest. Golgi networks are well developped, endoplasmic reticulum is abundant, lipid droplets are rare. 3. The processes of somites which extend in the dorsal part of the limb bud of Anguis embryos are cords of cells with thin lumina; at the stage of the allantoic bud of 0,6 to 0,8 mm long, the distal extremity of these processes dislocate in group of cells which afterwards dissociate, releasing individual somitic cells which are integrated among the mesoblastic somatopleural cells. In young lizard embryos (2 to 4 days old) the distal extremity of the somitic processes enlarges into a vesicle from which cells are released and penetrate in the mesoblast of the limb bud. 4. The somitic cells released from the somitic processes of Anguis and Lacerta keep--at least at early stages--the cytological characteristics they displayed when they were still in situ in the somitic processes: grounded on the presence or absence of lipid droplets, on the width and density of the mitochondria, the distinction, at these stages, between the somitic and mesoblastic somatoplerual cells is possible; and it is also possible to observe the integration of the somitic cells into the mesoblast. This study brings the demonstration of the cellular contribution of the somites to the formation of the limb bud in Reptiles. 5...

Allantois↗
PubMed1986

An in vitro assay for neural crest cell migration through the somites.

Neural crest cells in the trunk of the avian embryo come into contact with the somites and neural tube during the course of their migration. However, the relationship between the somites and the early migratory routes followed by these cells is not yet completely understood. Here, we use a tissue culture assay to examine if avian neural crest cells migrate through the somites. Cultures of quail somites were prepared from four adjacent regions along the neural axis in the trunk. Each region had four pairs of consecutive somites with region I being most anterior and region IV containing the last four segments. Within each region, the somites were separated from other tissues by enzymatic digestion and plated onto collagen-coated dishes. Immunocytochemical techniques were used to confirm that no neural crest cells, recognized by the HNK-1 antibody, were present on the surface of the somites at the time of explanation. After several days in culture, the explanted somites were screened to identify pigment cells. Because neural crest cells give rise to all of the melanocytes in the trunk, the presence of pigment cells indicated that neural crest precursors were contained within the initial explant. After 5-11 days in vitro, the percentage of somite cultures containing pigment cells in regions I through IV, respectively, was 36%, 51%, 31% and 1%. These results suggest that neural crest cells migrate through the somitic mesenchyme and first enter the somites between 5 to 9 segments rostral to the most recently formed somite.

Animals↗
PubMed2003

Somites, spinal Ganglia, and centra. Enumeration and interrelationships in staged human embryos, and implications for neural tube defects.

Serial sections of 99 human embryos from Carnegie stages 8-23 were investigated and 38 graphic reconstructions were evaluated. At stage 9 somite 1 is of appreciable size and is separated from the otic disc, as also in the next several stages by rhombomeres and pharyngeal arches 3 and 4, thereby differing from the chick. At stage 10 somite 1 begins to differentiate into sclerotome and dermatomyotome. At stage 11 spinal neural crest begins to develop. At stage 12 parts of somites 1-4 are being transformed into the hypoglossal cell cord. It is stressed that the numbers of somites present at stages 9-12 are part of the definition of those stages. At stage 13 dense and loose zones begin to be detectable rostrally in the sclerotomes and also, although out of phase, in the perinotochord. Spinal ganglia begin to develop in phase with the somites. At stages 14-16 the maximum number of somites observed was 38-39 rather than 42-44, as usually given. Moreover, they did not extend to the tapered end of the trunk, which is not a (vertebrated) 'tail'. At stages 17-23 the maximum number of centra was 38-39, including coccygeal vertebrae 4-5. Although most of the somites appear during primary development, all of the spinal ganglia develop during secondary development (stages 13-18). The number of ganglia was at a maximum of 35 at stage 18, but was reduced to 32 already by stage 23. Important points confirmed in this study are that the number of occipital somites in the human is four, and that the level of final closure of the caudal neuropore is future somite 31, which represents approximately future sacral vertebra 2. The interpretation of relevant neural tube defects is discussed in the light of the findings. The ascensus of the conus medullaris during the fetal period is well established, but a concomitant ascent of the situs neuroporicus is proposed here, and has implications for defects that involve tethering of the spinal cord. The main results are integrated in comprehensive graphic representations of the levels and the interrelationships of (a) somites and centra, and (b) somites, neural crest, and spinal ganglia. These may aid in the elucidation of some frequently occurring anomalous conditions.

Embryonic and Fetal Development↗
PubMed1996

Function of somite and somitocoele cells in the formation of the vertebral motion segment in avian embryos.

We have studied the distribution of thoracic somite and somitocoele-derived cells using homotopical grafting between quail and chicken embryos and reincubation periods of 2-6 days. Serial sections were evaluated with antibodies against quail cells, quail hemangiopoietic cells and desmin. With the exception of neural crest cells in the cranial sclerotome half, all cells of the operated segment are quail cells derived from a single somite. These cells differentiate into sclerotome, myotome and the anlage of the dermis of the back. After longer reincubation periods, the somite-derived quail cells form the neighboring halves of 2 adjacent vertebral bodies and the intervening (disc-homologous) tissue. Resegmentation is furthermore visible in the lamina and the spinous process. Somite cells also form the articular and transverse processes, and the intertransverse muscle including its insertion to the next cranial transverse process. One thoracic somite forms the proximal part of 1 rib. In more distal parts, 1 somite forms the cranial half of 1 rib and the caudal half of the next cranial rib, and the intercostal muscle and part of the connective tissue. Somite-derived quail cells are found in muscle that bridges over 2 segments cranial and caudal from the operated segment. The craniocaudal distribution of endothelial cells is approximately the same. Somitocoele cells that are located centrally in the epithelial somite express the sclerotome-markers Pax-1 and Pax-9. After 2-3 days of reincubation, grafted thoracic somitocoele cells are found mainly in the cranial part of the caudal sclerotome half. They form an area representing the anlagen of the intervertebral disc and the rib. After longer reincubation periods, the grafted quail somitocoele cells form the intervertebral disc-homologous tissue and the proximal part of the rib. In more distal parts of the rib they are located in the cranial half of 1 rib and the caudal half of the next cranial rib. The somitocoele cells also form the surface of the intervertebral joint, and give rise to a small number of endothelial cells that are found up to 1 segment cranial and caudal to the operation site. Our studies show that resegmentation is found in most parts of the vertebra and in the distal ribs. One somite forms the origin and insertion of the segmental muscle. Therefore, the somite can be regarded as the ancestor of the vertebral motion segment. Somitocoele cells are located centrally both in the epithelial somite and in the vertebral motion segment.

Animals↗
PubMed1992

Two myogenic lineages within the developing somite.

It is well known that the muscles of the vertebrate body are derived from the somite. Precursor cells within the somite proper form the back or axial muscles while other precursor cells migrate away from the somite to populate the muscle of the limbs and ventral body wall. Although both types of muscle are generally thought of as arising from a common progenitor population, the myotome, recent evidence points to developmental differences in these two groups of muscles which may reflect different developmental lineages. To test the lineage hypothesis, we used microsurgery and the chick-quail nucleolar marker system to follow the developmental fate of the lateral and medial halves of somites at the wing level. The results showed that the structures of the mature somite (myotome and sclerotome) are derived virtually exclusively from cells residing in the medial half of the newly formed somite. On the other hand, virtually all of the cells residing in the lateral half of the newly formed somite are destined to leave the somite proper and populate the limb muscle and, probably, other somite-derived mesenchymal structures in the limb and ventral body wall. Switch-graft experiments show that the two halves of newly formed somites are largely interchangeable demonstrating that their ultimate developmental fate is position-dependent and that it becomes fixed as a result of extrinsic influences which act during later stages of somitogenesis. We conclude that at least two distinct myogenic lineages exist in the somite; one giving rise to the muscles of the back and the other giving rise to the limb musculature.

Animals↗
PubMed1985

Myogenic differentiation in early chick wing mesenchyme in the absence of the brachial somites.

A controversy exists in the literature over the ability of wing mesenchyme of somatopleural origin to form skeletal muscle. Experimental approaches used in such studies leave open the possibility of postoperative accessibility of the experimental wings to somitic cell invasion. In the present study wing somatopleural tissue was isolated from HH stage-12 to -21 chick embryos and grown either in organ culture (OC) or on the chorioallantoic membrane (CAM) of host chicks, conditions under which postoperative entry of somitic cells is impossible. In the presence of axial and somitic tissues of the brachial region, the wing territories from all four stages underwent comparable growth and tissue differentiation. However, isolated wing regions showed a stage-dependency in the differentiation of skeletal muscle but not of other limb tissues. The incidence and amount of skeletal muscle was markedly reduced in HH stage-12 and -15 isolated wing regions while myogenesis in HH stage-18 and -21 wing buds was not affected by the absence of the somitic tissues. These results are consistent with reported stages of somitic cell migration into wings with the exception of HH stage-12 explants which should have been muscleless if somitic cells are the sole source of wing myofibres. The possibility that somitic cells had been included in these explants was investigated by testing the myogenic potential of lateral plate tissue adjacent to the wing, altering the dissection procedure for isolating wing territories and using antibodies to skeletal muscle myosin and actin to detect myotubes. The results from this series of experiments illustrate the need for extraordinary care in the isolation of wing regions when investigating limb-somite relationships and suggest that the myogenic capacity attributed to wing somatopleural cells in the past can be accounted for by either postoperative entry of somitic cells into experimental wings or inadvertent inclusion of somitic cells in the primordia when dissected. Overall, the results show that somitic cells are the sole source of wing myofibres for, in their absence, somatopleural cells from all mesodermally-derived wing cell types except skeletal myofibres.

Animals↗
PubMed1979

Role of the somitic mesoderm in the development of the thorax in bird embryos. II. Origin of thoracic and appendicular musculature.

The regional embryonic origin of trunk and limb musculature was determined through heterospecific homotopic or heterotopic transplantations of quail somitic or somiticsomatopleural mesoderm into chick hosts, and through localized X-irradiation of the somitic mesoderm. Experiments were performed on 2-day embryos. Results show that the myoblastic component of all truncal and appendicular muscles is of somitic origin. Intra- and perimuscular connective tissue as well as tendons are of somatopleural origin. X-ray destruction of the somitic mesoderm at and beyond the wing level resulted in the complete or almost complete absence of musculature in the wing and corresponding truncal region. The mapping of the cephalocaudal origin of the various muscles was found to be as follows: Intrinsic and extrinsic muscles of the wing and scapular girdle Somites 12-20 Grand pectoral muscle Somites 12-22 Intercostal muscles Somites 19-26 Abdominal muscles Somites 27-29 Intrinsic and extrinsic muscles of the leg and pelvic girdle Somites 26-32 Dorsal and intervertebral muscles Metameric level, specific origin In the heterotopic transplantations, the grafted somitic mesoderm gave rise to site-specific morphogenesis, irrespective of the cephalocaudal level of its origin. This result demonstrates that, at 2 days of incubation, the myogenic somitic cells are not regionalized.

Animals↗
PubMed1981

Effects of L-phenylalanine on somite formation in the early chick embryo.

Chick embryos were treated in ovo and in vitro with L-phenylalanine from the intermediate streak stage (Hamburger & Hamilton stage 3, 12-13 h of incubation) to the 7-somite stage (H & H stage 9, 29-33 h of incubation). Treatment in ovo resulted in a large number of embryos developing somite blocks, i.e. imperfectly segmented somites. In embryos treated at an early developmental stage (12-21 h of incubation), the blocks of unsegmented somite mesoderm occurred mostly in the somite pairs 1-5, whereas treatment that began at a later stage (24-30 h of incubation) caused blocks in the somite pairs 5-10, i.e. the appearance of blocks of unsegmented somite mesoderm is correlated in time with the onset of the treatment. No difference regarding mitotic indices could be distinguished between normally segmented somites and blocks of unsegmented somite mesoderm. Autoradiography based on tritiated L-phenylalanine showed no regional differences in labelling of the chick embryo body. Electronmicroscopical observations indicate a slightly suppressed formation of microvilli in the cells of the unsegmented mesoderm blocks compared with cells in normally segmented somites. The observed disturbances are probably caused by a suppressed yolk granule decomposition in the developing somite cells. The experiments in vitro support the findings in the in ovo material; at the same time, they reveal an unexpectedly slow diffusion of L-phenylalanine through the vitelline membrane.

Animals↗
PubMed2000

Somite development in zebrafish.

A full understanding of somite development requires knowledge of the molecular genetic pathways for cell determination as well as the cellular behaviors that underlie segmentation, somite epithelialization, and somite patterning. The zebrafish has long been recognized as an ideal organism for cellular and histological studies of somite patterning. In recent years, genetics has proven to be a very powerful complementary approach to these embryological studies, as genetic screens for zebrafish mutants defective in somitogenesis have identified over 50 genes that are necessary for normal somite development. Zebrafish is thus an ideal system in which to analyze the role of specific gene products in regulating the cell behaviors that underlie somite development. We review what is currently known about zebrafish somite development and compare it where appropriate to somite development in chick and mouse. We discuss the processes of segmentation and somite epithelialization, and then review the patterning of cell types within the somite. We show directly, for the first time, that muscle cell and sclerotome migrations occur at the same time. We end with a look at the many questions about somitogenesis that are still unanswered.

Animals↗