Body wall morphogenesis: Limb-genesis interferes with body wall-genesis via its influence on the abaxial somite derivatives

The vertebrate body wall is regionalized into thoracic and lumbosacral/abdominal regions that differ in their morphology and developmental origin. The thoracic body wall has ribs and intercostal muscles, which develops from thoracic somites, whereas the abdominal wall has abdominal muscles, which develops from lumbosacral somites without ribs cage. To examine whether limb-genesis interferes with body wall-genesis, and to test the possibility that limb generation leads to the regional differentiation, an ectopic limb was induced in the thoracic region by transplanting prospective limb somatopleural mesoderm of Japanese quail between the ectoderm and somatopleural mesoderm of the chick prospective thoracic region. This ectopic limb generation induced the somitic cells to migrate into the ectopic limb mesenchyme to become its muscles and caused the loss of distal thoracic body wall (sterno-distal rib and distal intercostal muscle), without causing any significant effect on the more proximal region (proximal rib, vertebro-distal rib and proximal intercostal muscle). According to a new primaxial–abaxial classification, the proximal region is classified as primaxial and the distal region, as well as limb, is classified as abaxial. We demonstrated that ectopic limb development interfered with body wall development via its influence on the abaxial somite derivatives. The present study supports the idea that the somitic cells give rise to the primaxial derivatives keeping their own identity and fate, whereas they produce the abaxial derivatives responding to the lateral plate mesoderm.     

Hox genes and morphological identity: axial versus lateral patterning in the vertebrate mesoderm

The successful organization of the vertebrate body requires that local information in the embryo be translated into a functional, global pattern. Somite cells form the bulk of the musculoskeletal system. Heterotopic transplants of segmental plate along the axis from quail to chick were performed to test the correlation between autonomous morphological patterning and Hox gene expression in somite subpopulations. The data presented strengthen the correlation of Hox gene expression with axial specification and focus on the significance of Hox genes in specific derivatives of the somites. We have defined two anatomical compartments of the body based on the embryonic origin of the cells making up contributing structures: the dorsal compartment, formed from purely somitic cell populations; and the ventral compartment comprising cells from somites and lateral plate. The boundary between these anatomical compartments is termed the somitic frontier. Somitic tissue transplanted between axial levels retains both original Hox expression and morphological identity in the dorsal compartment. In contrast, migrating lateral somitic cells crossing the somitic frontier do not maintain donor Hox expression but apparently adopt the Hox expression of the lateral plate and participate in the morphology appropriate to the host level. Dorsal and ventral compartments, as defined here, have relevance for experimental manipulations that influence somite cell behavior. The correlation of Hox expression profiles and patterning behavior of cells in these two compartments supports the hypothesis of independent Hox codes in paraxial and lateral plate mesoderm.     

Epaxial-adaxial-hypaxial regionalisation of the vertebrate somite: evidence for a somitic organiser and a mirror-image duplication

During vertebrate neural tube formation, the initially lateral borders between the neural and epidermal ectoderm fuse to form the definitive dorsal region of the embryo, while the initially dorsally located notochord-floor plate complex is being internalised. Along the definitive dorso-ventral body axis, one can distinguish an epaxial (dorsal to the notochord) and a hypaxial (ventral to the notochord) body region. The mesodermal somites on both sides of the notochord and neural tube give rise to the trunk skeleton and skeletal muscle. Muscle forms from the somite-derived dermomyotomes and myotomes that elongate dorsally and ventrally. Based on gene expression patterns and comparative embryology, it is proposed here that the epaxial (dermo)myotome region in amniote embryos is subdivided into a dorsalmost and a centrally intercalated subregion. The intercalated subregion abuts to the hypaxial (dermo)myotome region that elongates ventrally via the hypaxial somitic bud. The dorsalmost subregion elongates towards the dorsal neural tube and is proposed to derive from an epaxial somitic bud. The dorsalmost and hypaxial somite derivatives share specific gene expression patterns which are distinct from those of the intercalated somite derivatives. The intercalated somite derivatives develop adaxially, i.e. at the level of the notochord-floor plate complex. Thus, the dorsalmost and intercalated (dermo)myotome subregions may be influenced preferentially by signals from the dorsal neural tube and from the notochord-floor plate complex, respectively. These (dermo)myotome subregions are sharply delimited from each other by molecular boundary markers, including Engrailed and Wnts. It thus appears that the molecular network that polarises borders in Drosophila and vertebrate embryogenesis is redeployed during subregionalisation of the (dermo)myotome. It is proposed here that cells within the amniote (dermo)myotome establish polarised borders with organising capacity, and that the epaxial somitic bud represents a mirror-image duplication of the hypaxial somitic bud along such a border. The resulting epaxial-intercalated/adaxial-hypaxial regionalisation of somite derivatives is conserved in vertebrates although the differentiation of sclerotome and myotome starts heterochronically in embryos of different vertebrate groups.     

Perturbation of beta 1 integrin-mediated adhesions results in altered somite cell shape and behavior.

Cell contact and adhesion between somites and the axial extracellular matrix (ECM) is likely to play a fundamental role in vertebrate development. In a preliminary report we showed that injection of the monoclonal antibody CSAT, which recognizes the avian beta 1 integrins, causes a lateral separation of both somites and segmental plate tissue from the embryonic axis (Drake and Little, 1991). In this study we addressed the cell biological response to CSAT injection, particularly the cell-ECM interactions involved in maintaining normal somite-axial relationships. A total of 150 stage 7-10 quail embryos have been injected with CSAT and then cultured for varying periods (1-30 hr). CSAT caused somitic cells to behave abnormally. Changes include, rounding-up, extensive blebbing, and formation of retraction fibers. A majority of separated somites were able to assume normal axial position with further time culture. Whether a somite subsequently aligned at the axis was dependent on the amount of CSAT injected and the postinjection culture period. Embryos in which somites remained separated from the axis after relatively long culture intervals (18-24 hr) displayed abnormal sclerotomal cell migrations. In no case did control injected embryos exhibit cellular alterations. Similarly, the injection of RGD-containing peptides had no detectable effect on somitogenesis or somite/segmental plate adhesion to the axis. On the basis of these data, we conclude that beta 1 integrins are necessary for normal somitic cell adhesions to the axis, but not somite segmentation and differentiation.     

Formation of the dorsal root ganglia in the avian embryo: Segmental origin and migratory behavior of neural crest progenitor cells

The segmental origin and migratory pattern of neural crest cells at the trunk level of avian embryos was studied, with special emphasis on the formation of the dorsal root ganglia (DRG) which organize in the anterior half of each somite. Neural crest cells were visualized using the quail-chick marker and HNK-1 immunofluorescence. The migratory process turned out to be closely correlated with somitic development: when the somites are epithelial in structure few labeled cells were found in a dorsolateral position on the neural tube, uniformly distributed along the craniocaudal axis. Following somitic dissociation into dermomyotome and sclerotome labeled cells follow defined migratory pathways restricted to each anterior somitic half. In contrast, opposite the posterior half of the somites, cells remain grouped in a dorsolateral position on the neural tube. The fate of crest cells originating at the level of the posterior somitic half was investigated by grafting into chick hosts short segments of quail neural primordium, which ended at mid-somitic or at intersomitic levels. It was found that neural crest cells arising opposite the posterior somitic half participate in the formation of the DRG and Schwann cells lining the dorsal and ventral root fibers of the same somitic level as well as of the subsequent one, whereas those cells originating from levels facing the anterior half of a somite participate in the formation of the corresponding DRG. Moreover, crest cells from both segmental halves segregate within each ganglion in a distinct topographical arrangement which reflects their segmental origin on the neural primordium. Labeled cells which relocate from posterior into anterior somitic regions migrate longitudinally along the neural tube. Longitudinal migration of neural crest cells was first observed when the somites are epithelial in structure and is completed after the disappearance of the last cells from the posterior somitic region at a stage corresponding to the organogenesis of the DRG.     

T-cadherin expression alternates with migrating neural crest cells in the trunk of the avian embryo.

Trunk neural crest cells and motor axons move in a segmental fashion through the rostral (anterior) half of each somitic sclerotome, avoiding the caudal (posterior) half. This metameric migration pattern is thought to be caused by molecular differences between the rostral and caudal portions of the somite. Here, we describe the distribution of T-cadherin (truncated-cadherin) during trunk neural crest cell migration. T-cadherin, a novel member of the cadherin family of cell adhesion molecules was selectively expressed in the caudal half of each sclerotome at all times examined. T-cadherin immunostaining appeared graded along the rostrocaudal axis, with increasing levels of reactivity in the caudal halves of progressively more mature (rostral) somites. The earliest T-cadherin expression was detected in a small population of cells in the caudal portion of the somite three segments rostral to last-formed somite. This initial T-cadherin expression was observed concomitant with the invasion of the first neural crest cells into the rostral portion of the same somite in stage 16 embryos. When neural crest cells were ablated surgically prior to their emigration from the neural tube, the pattern of T-cadherin immunoreactivity was unchanged compared to unoperated embryos, suggesting that the metameric T-cadherin distribution occurs independent of neural crest cell signals. This expression pattern is consistent with the possibility that T-cadherin plays a role in influencing the pattern of neural crest cell migration and in maintaining somite polarity.     

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.     

Impact of node ablation on the morphogenesis of the body axis and the lateral asymmetry of the mouse embryo during early organogenesis.

The node of the mouse gastrula is the major source of the progenitor cells of the notochord, the floor plate, and the gut endoderm. The node may also play a morphogenetic role since it can induce a partial body axis following heterotopic transplantation. The impact of losing these progenitor cells and the morphogenetic activity on the development of the body axes was studied by the ablation of the node at late gastrulation. In the ablated embryo, an apparently intact anterior-posterior body axis with morphologically normal head folds, neural tube, and primitive streak developed during early organogenesis. Cell fate analysis revealed that the loss of the node elicits de novo recruitment of neural ectoderm and somitic mesoderm from the surrounding germ-layer tissues. This leads to the restoration of the neural tube and the paraxial mesoderm. However, the body axis of the embryo was foreshortened and somite formation was retarded. Histological and gene expression studies reveal that in most of the node-ablated embryos, the notochord in the trunk was either absent or interrupted, and the floor plate was absent in the ventral region of the reconstituted neural tube. The loss of the node did not affect the differentiation of the gut endoderm or the formation of the mid- and hindgut. In the node-ablated embryo, expression of the Pitx2 gene in the lateral plate mesoderm was no longer restricted to the left side but was found on both sides of the body or was completely absent from the lateral plate mesoderm. Therefore, the loss of the node results in the failure to delineate the laterality of the body axis. The node and its derivatives therefore play a critical role in the patterning of the ventral neural tube and lateral body axis but not of the anterior-posterior axis during early organogenesis.     

REF-1, a protein with two bHLH domains, alters the pattern of cell fusion in C. elegans by regulating Hox protein activity

Hox genes control the choice of cell fates along the anteroposterior (AP) body axis of many organisms. In C. elegans, two Hox genes, lin-39 and mab-5, control the cell fusion decision of the 12 ventrally located Pn.p cells. Specific Pn.p cells fuse with an epidermal syncytium, hyp7, in a sexually dimorphic pattern. In hermaphrodites, Pn.p cells in the mid-body region remain unfused whereas in males, Pn.p cells adopt an alternating pattern of syncytial and unfused fates. The complexity of these fusion patterns arises because the activities of these two Hox proteins are regulated in a sex-specific manner. MAB-5 activity is inhibited in hermaphrodite Pn.p cells and thus MAB-5 normally only affects the male Pn.p fusion pattern. Here we identify a gene, ref-1, that regulates the hermaphrodite Pn.p cell fusion pattern largely by regulating MAB-5 activity in these cells. Mutation of ref-1 also affects the fate of other epidermal cells in distinct AP body regions. ref-1 encodes a protein with two basic helix-loop-helix domains distantly related to those of the hairy/Enhancer of split family. ref-1, and another hairy homolog, lin-22, regulate similar cell fate decisions in different body regions along the C. elegans AP body axis.     

Temporospatial patterns of apoptosis in chick embryos during the morphogenetic period of development

We examined the temporospatial pattern of naturally occurring apoptosis in chick embryos to five days of incubation (H.H. stages 1-25; Hamburger and Hamilton, 1951) using TUNEL labeling. The initial TUNEL-positive structure was the embryonic shield at stage 1. Apoptotic cells became ubiquitously present within embryos by stage 3, which is early in gastrulation. Until stage 6, TUNEL-positive cells were restricted to the headfold region. In embryos of stages 7-8, most cell death was localized at the most anterior neural plate. TUNEL-positive neural plate, notochord and somites appeared at stage 9. Otic and optic regions became TUNEL-positive at stage 11. The aggregation of cells from which the tail bud arises contains apoptotic cells from stage 11 onwards. At stage 16, scattered TUNEL-positive cells appeared in the branchial arches. Three streams of apoptotic neural crest cells in the cranial region became most clearly visible at stage 18. The secondary neural tube from which caudal structures develop contains apoptotic cells at stage 14. Apoptotic cells are present in the branchial arches and lateral body wall for extended periods, stages 16-25 and 25 respectively. At stages 24-25, intense positive regions of cell death were confined to the caudal regions of the arches, to limb and tail buds and to the lateral body wall, the latter in relation to body wall closure. The new findings in this study are discussed along with past studies to provide the temporospatial pattern of cell death during early chick development.     

Molecular characterization of the rostral-most somites in early somitic stages of the chick embryo

Segmentation consists on the progressive formation of repetitive embryonic structures, named somites, which are formed from the most rostral part of the presomitic mesoderm. Somites are subdivided into anterior and posterior compartments and several genes are differentially expressed in either compartment. This has provided evidence for the importance of establishing the anterior-posterior polarity within each somite, which is critical for the correct segmented pattern of the adult vertebrate body. Although all somites appear morphologically similar, fate map studies have shown that the first 4 somites do not give rise to segmented structures, in contrast to more posterior ones. Moreover, in several somitogenesis-related mutants the anterior somites are not affected while posterior somites present clear defects or do not form at all. Altogether these data suggest relevant differences between rostral and caudal somites. In order to check for molecular differences between anterior and posterior somites, we have performed a detailed expression pattern analysis of several Notch signalling related genes. For the first time, we show that the somitic expression pattern profile is not the same along the anterior-posterior axis and that the differences are not observed always at the same somite level.     

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.     

Three developmental compartments involved in rib formation

When the thoracic somitic mesoderm was separated from the neural tube and the notochord with a piece of aluminum foil in two-day chick embryos, seven days after the operation ribs lacked their proximal part. The embryos were rescued by co-transplanting the notochord, the ventral half of neural tube, or QT6 cells transformed with Shh, on the somite side of the aluminum foil insert. Thus, proximal rib development depends on the notochord and the ventral neural tube, an effect which might be mediated through Shh secreted by these axial tissues. On the other hand, when the thoracic somitic mesoderm was separated from the surface ectoderm by a piece of polyethylene terephthalate film, the distal parts of the ribs were missing, suggesting that distal rib development depends on surface ectoderm. In these embryos, expression of Pax3 was weak and perturbed showing that the dermomyotome developed abnormally. It is not clear whether the development of distal rib is mediated by the dermomyotome, or the ectoderm. It has previously been shown that sternal rib development depends on lateral plate mesoderm. As to the distal rib, it is considered to be composed of two parts. Thus, the rib is composed of three developmental compartments, in agreement with a recently presented classification of somite derivatives as primaxial and abaxial.     

Mediolateral cell intercalation in the dorsal, axial mesoderm of Xenopus laevis

The pattern of mediolateral cell intercalation in mesodermal tissues during gastrulation and neurulation of Xenopus laevis was determined by tracing cells labeled with fluorescein dextran amine (FDA). Patches of the involuting marginal zone (IMZ) of early gastrula stage embryos, labeled by injection of FDA at the one-cell stage, were grafted to the corresponding regions of unlabeled host embryos. The host embryos were fixed at several stages, serially sectioned, and examined with fluorescence microscopy and three-dimensional reconstruction. Patterns of mixing of labeled and unlabeled cells show that mediolateral cell intercalation occurs in the posterior, dorsal mesoderm as this region undergoes convergent extension and differentiates into somites and notochord. In contrast, it does not occur in any dorsoventral sector of the anterior, leading edge of the mesodermal mantle. These results, taken with other evidence, suggest that the mesoderm of Xenopus consists of two subpopulations, each with a characteristic morphogenetic movement, cell behavior, and tissue fate. The migrating mesoderm (1) does not show convergent extension; (2) migrates and spreads on the blastocoel roof; (3) is dependent on this substratum for its morphogenesis; (4) shows little mediolateral intercalation; (5) consists of the anterior, early-involuting region of the mesodermal mantle; and (6) differentiates into head, heart, blood island, and lateral body wall mesoderm. The extending mesoderm (1) shows convergent extension; (2) is independent of the blastocoel roof in its morphogenesis; (3) shows extensive mediolateral intercalation; (4) consists of the posterior, late-involuting parts of the mesodermal mantle; and (5) differentiates into somite and notochord.     

Sfrp1 and Sfrp2 regulate anteroposterior axis elongation and somite segmentation during mouse embryogenesis

Regulation of Wnt signaling is essential for embryonic patterning. Sfrps are secreted Wnt antagonists that directly interact with the Wnt ligand to inhibit signaling. Here, we show that Sfrp1 and Sfrp2 are required for anteroposterior (AP) axis elongation and somitogenesis in the thoracic region during mouse embryogenesis. Double homozygous mutations in Sfrp1 and Sfrp2 lead to severe shortening of the thoracic region. By contrast, a homozygous mutation in one or the other exerts no effect on embryogenesis, indicating that Sfrp1 and Sfrp2 are functionally redundant. The defect of a shortened thoracic region appears to be the consequence of AP axis reduction and incomplete somite segmentation. The reduction in the AP axis is partially due to abnormalities in cell migration of pre-somitic mesoderm from the end of gastrulation. Aberrant somite segmentation is associated with altered oscillations of Notch signaling, as evidenced by abnormal Lfng and Hes7 expression during somitogenesis in the thoracic region. This study suggests that Wnt regulation by Sfrp1 and Sfrp2 is required for embryonic patterning.     

Distinct populations of endoderm cells converge to generate the embryonic liver bud and ventral foregut tissues

The location and movement of mammalian gut tissue progenitors, prior to the expression of tissue-specific genes, has been unknown, but this knowledge is essential to identify transitions that lead to cell type specification. To address this, we used vital dyes to label exposed anterior endoderm cells of early somite stage mouse embryos, cultured the embryos into the tissue bud phase of development, and determined the tissue fate of the dye labeled cells. This approach was performed at three embryonic stages that are prior to, or coincident with, foregut tissue patterning (1-3 somites, 4-6 somites, and 7-10 somites). Short-term labeling experiments tracked the movement of tissue progenitor cells during foregut closure. Surprisingly, we found that two distinct types of endoderm-progenitor cells, lateral and medial, arising from three spatially separated embryonic domains, converge to generate the epithelial cells of the liver bud. Whereas the lateral endoderm-progenitors give rise to descendants that are constrained in tissue fate and position along the anterior-posterior axis of the gut, the medial gut endoderm-progenitors give rise to descendants that stream along the anterior-posterior axis at the ventral midline and contribute to multiple gut tissues. The fate map reveals extensive morphogenetic movement of progenitors prior to tissue specification, it permits a detailed analysis of endoderm tissue patterning, and it illustrates that diverse progenitor domains can give rise to individual tissue cell types.     

Microinjection of synthetic Xhox-1A homeobox mRNA disrupts somite formation in developing Xenopus embryos

The structural similarity between Drosophila and vertebrate homeobox genes begs the question of whether the vertebrate gene products affect cell fate and pattern formation. To study the function of the Xenopus homeobox protein, Xhox-1A, we microinjected fertilized Xenopus eggs with an excess of synthetic Xhox-RNA and assayed for effects on development. The predominant phenotype is a disturbance in somite formation. When embryos are injected with Xhox-1A mRNA, but not with control mRNAs, morphogenesis of somites occurs chaotically and individual segments are lost. Histological staining, in situ hybridization, and immunohistochemistry indicate that the disorganized somitic tissue has differentiated into muscle cells. Overall, these results suggest that correct regulation of the Xhox-1A gene may be important for the normal development of the segmented somite pattern in early embryos. Moreover, the inferred role of Xhox-1A in somite formation indicates that there may be molecular parallels between mechanisms of segmentation in flies and vertebrates.     

The somitogenetic potential of cells in the primitive streak and the tail bud of the organogenesis-stage mouse embryo.

The developmental potency of cells isolated from the primitive streak and the tail bud of 8.5- to 13.5-day-old mouse embryos was examined by analyzing the pattern of tissue colonization after transplanting these cells to the primitive streak of 8.5-day embryos. Cells derived from these progenitor tissues contributed predominantly to tissues of the paraxial and lateral mesoderm. Cells isolated from older embryos could alter their segmental fate and participated in the formation of anterior somites after transplantation to the primitive streak of 8.5-day host embryo. There was, however, a developmental lag in the recruitment of the transplanted cells to the paraxial mesoderm and this lag increased with the extent of mismatch of developmental ages between donor and host embryos. It is postulated that certain forms of cell-cell or cell-matrix interaction are involved in the specification of segmental units and that there may be age-related variations in the interactive capability of the somitic progenitor cells during development. Tail bud mesenchyme isolated from 13.5-day embryos, in which somite formation will shortly cease, was still capable of somite formation after transplantation to 8.5-day embryos. The cessation of somite formation is therefore likely to result from a change in the tissue environment in the tail bud rather than a loss of cellular somitogenetic potency.     

Segmental relationship between somites and vertebral column in zebrafish

The segmental heritage of all vertebrates is evident in the character of the vertebral column. And yet, the extent to which direct translation of pattern from the somitic mesoderm and de novo cell and tissue interactions pattern the vertebral column remains a fundamental, unresolved issue. The elements of vertebral column pattern under debate include both segmental pattern and anteroposterior regional specificity. Understanding how vertebral segmentation and anteroposterior positional identity are patterned requires understanding vertebral column cellular and developmental biology. In this study, we characterized alignment of somites and vertebrae, distribution of individual sclerotome progeny along the anteroposterior axis and development of the axial skeleton in zebrafish. Our clonal analysis of zebrafish sclerotome shows that anterior and posterior somite domains are not lineage-restricted compartments with respect to distribution along the anteroposterior axis but support a 'leaky' resegmentation in development from somite to vertebral column. Alignment of somites with vertebrae suggests that the first two somites do not contribute to the vertebral column. Characterization of vertebral column development allowed examination of the relationship between vertebral formula and expression patterns of zebrafish Hox genes. Our results support co-localization of the anterior expression boundaries of zebrafish hoxc6 homologs with a cervical/thoracic transition and also suggest Hox-independent patterning of regionally specific posterior vertebrae.     

Developmental Plasticity of the Prospective Dermatome and the Prospective Sclerotome Region of an Avian Somite

The ventro-medial wall of a somite gives rise to the sclerotome and then to cartilaginous axial skeleton, while the dorso-lateral wall differentiates into the dermomyotome to form dermal mesenchyme and muscle. Although previous studies suggested pluri-potency of somite cell differentiation, apparent pluri-potency may be the result of migration of predetermined cells. To investigate whether the developmental fate of any region is determined, I isolated fragments of a region of a quail somite and transplanted them into chick embryos. When a fragment of the ventral wall of a quail somite, the prospective sclerotome, was transplanted into a chick embryo between the ectoderm and a newly formed somite, the transplanted quail cells were shown to form myotome and mesenchyme in 4-day chimera embryos and to form muscle and dermal tissue in 9-day chimeras. On the other hand, when a fragment of the dorsal wall of a quail somite, the prospective dermomyotome, was transplanted into a chick embryo between the neural tube and a newly formed somite, the graft gave rise to mesenchyme around the neural tube and notochord and then to vertebral cartilage. Thus the developmental fate of a region of a somite was shown not to be determined at the time of somite segmentation, confirming previous observations.