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PubMed1997

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

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

Amino Acid Sequence↗
PubMed1993

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

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

Animals↗
PubMed1994

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

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

Animals↗
PubMed1995

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

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

Animals↗
PubMed1988

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

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

Animals↗
PubMed2000

Contribution of single somites to the skeleton and muscles of the occipital and cervical regions in avian embryos.

Controversy has surrounded the process of resegmentation of cervico-occipital somites. We have reinvestigated this topic by grafting single somites of quail embryos homotopically into chick embryos. Somites one to five contribute to the skull. Somites one and two contribute to the parasphenoid, which develops by direct ossification in a non-segmental fashion. All cartilaginous derivatives of the somites are segmental. Somite two forms a stripe of cells in the basioccipital, exoccipital and supraoccipital. Somites three to five give rise to the subsequent caudal parts of the basioccipital and exoccipital. Somite five forms the first motion segment including the occipital condyle, the cranial part of the atlas and the tip of the dens axis. Therefore, the border between head and neck is in the centre of somite five, and corresponds to the expression boundary of Choxb-3. Somite six forms the caudal part of the atlas and the cranial part of the axis. Somites two to eight all contribute to the cranio-cervical muscles with the exception of the Mm. rectus capitis dorsalis and ventralis and the M. biventer cervicis, which do not receive contributions from somite two. In contrast, the M. cucullaris capitis is exclusively formed by myogenic cells from somite two, which parallels its exclusive innervation by the accessory nerve. Our data confirm the segmental nature of the occiput, and show that resegmentation is a very regular process involving all except the four cranialmost somites. Except for somites one and two, all of the somites contribute to the muscles located at the appropriate levels.

Animals↗
PubMed1988

The somitic level of origin of embryonic chick hindlimb muscles.

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

Animals↗
PubMed1987

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

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

Animals↗
PubMed1976

[Contribution of somite cells to the development of posterior limb buds in mice].

1. The structural changes accompanying the early genesis of the mouse hindlimb bud have been studied in serial Epon thick sections at five successive somitic stages collected during the 10th developmental day. 2. As soon as the 14-somite stage, increased proliferative activities occurring in a limited area of the embryonic somatopleure close to the caudal end of the coelomic cavity point out the onset of limb morphogenesis. From its very beginning on, up to its ceasing at the 27-somite stage, the somatopleural proliferation is more active in the caudal part of the prospective limb territory and in the whole area, it appears stronger ventrally than dorsally. 3. Discrete structural changes occur during this period in the mesoderm interposed between the hindlimb bud territory and the overlying unsegmented sheet of somitic mesoderm, in relation with the genesis of the mesonephritic constituents and of the posterior cardinal vein. A detailed analysis of this area in serial sections reveals that the unsegmented somitic mesoderm does not provide any cell to the limb bud mesoderm. 4. As soon as the somitic mesoderm becomes organized into segmented somites, obvious histological changes located on the lateral aspect of the ventro-lateral somitic edge reveal that numerous undifferentiated somitic cells invade the proximal area of the limb bud mesoderm. Starting at the 27-somite stage in the first two metameres adjacent to the precordonal portion of the limb bud, this processus extends to the five somites of the limb territory at the 33-somite stage. In each metamere, the area of cell migration takes place near the caudal border of the somite and, from one somite to the other, the number of migrating cells increases in a cephalo-caudal direction. No sign of a similar cell migration was observed in the somites located cranially or caudally to the hindlimb bud. 5. The results are discussed in relation with the various experimental and descriptive evidences obtained in non-mammalian Vertebrates about the contribution of the somites to limb morphogenesis and about the factors involved in the onset of cell proliferation in the prospective limb mesoderm.

Animals↗
PubMed1997

Distinct signal/response mechanisms regulate pax1 and QmyoD activation in sclerotomal and myotomal lineages of quail somites.

Pax1 and QmyoD are early sclerotome and myotome-specific genes that are activated in epithelial somites of quail embryos in response to axial notochord/neural tube signals. In situ hybridization experiments reveal that the developmental kinetics of activation of pax1 and QmyoD differ greatly, suggesting that myotome and sclerotome specification are controlled by distinct developmental mechanisms. pax1 activation always occurs in somite IV throughout development, indicating that pax1 regulation is tightly coordinated with early steps in somite maturation. In contrast, QmyoD is delayed and does not occur until embryos have 12-14 somites. At this time, QmyoD is the first of the myogenic regulatory factor (MRF) genes to be activated in preexisting somites in a rapid, anterior to posterior progression until the 22 somite stage, after which time QmyoD is activated in somite I immediately following somite formation. Experiments involving transplantation of newly formed somites to ectopic sites along the anterior to posterior embryonic axis were performed to distinguish the contributions of axial signals and somite response pathways to the developmental regulation of pax1 and QmyoD. These studies show that pax1 activation is regulated by somite formation and maturation, not by the availability of axial signals, which are expressed prior to somite formation. In contrast, the delayed activation of QmyoD is controlled by developmental regulation of the production of axial signals as well as by the competence of somites to respond to these signals. These somite transplantation studies, therefore, provide a basis for understanding the different developmental kinetics of activation of pax1 and QmyoD during sclerotome and myotome specification, and suggest specific molecular models for the developmental regulation of myotome and sclerotome formation in somites through distinct signal/response pathways.

Actins↗
PubMed1995

Early stages of chick somite development.

We report on the formation and early differentiation of the somites in the avian embryo. The somites are derived from the avian embryo. The somites are derived from the mesoderm which, in the body (excluding the head), is subdivided into four compartments: the axial, paraxial, intermediate and lateral plate mesoderm. Somites develop from the paraxial mesoderm and constitute the segmental pattern of the body. They are formed in pairs by epithelialization, first at the cranial end of the paraxial mesoderm, proceeding caudally, while new mesenchyme cells enter the paraxial mesoderm as a consequence of gastrulation. After their formation, which depends upon cell-cell and cell-matrix interactions, the somites impose segmental pattern upon peripheral nerves and vascular primordia. The newly formed somite consists of an epithelial ball of columnar cells enveloping mesenchymal cells within a central cavity, the somitocoel. Each somite is surrounded by extracellular matrix material connecting the somite with adjacent structures. The competence to form skeletal muscle is a unique property of the somites and becomes realized during compartmentalization, under control of signals emanating from surrounding tissues. Compartmentalization is accompanied by altered patterns of expression of Pax genes within the somite. These are believed to be involved in the specification of somite cell lineages. Somites are also regionally specified, giving rise to particular skeletal structures at different axial levels. This axial specification appears to be reflected in Hox gene expression. MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed. During early maturation, the ventral wall of the somite undergoes an epithelio-mesenchymal transition forming the sclerotome. The sclerotome later becomes subdivided into rostral and caudal halves which are separated laterally by von Ebner's fissure. The lateral part of the caudal half of the sclerotome mainly forms the ribs, neural arches and pedicles of vertebrae, whereas within the lateral part of the rostral half the spinal nerve develops. The medially migrating sclerotomal cells form the peri-notochordal sheath, and later give rise to the vertebral bodies and intervertebral discs. The somitocoel cells also contribute to the sclerotome. The dorsal half of the somite remains epithelial and is referred to as the dermomyotome because it gives rise to the dermis of the back and the skeletal musculature. the cells located within the lateral half of the dermomyotome are the precursors of the muscles of the hypaxial domain of the body, whereas those in the medial half are precursors of the epaxial (back) muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1999

Sonic hedgehog enhances somite cell viability and formation of primary slow muscle fibers in avian segmented mesoderm.

Primary skeletal muscle fibers first form in the segmented portions of paraxial mesoderm called somites. Although the neural tube and notochord are recognized as crucial in patterning myogenic cell lineages during avian and mammalian somitic myogenesis, the source, identities, and actions of the signals governing this process remain controversial. It has been shown that signals emanating from the ventral neural tube and/or notochord alone or Shh alone serve to activate MyoD expression in somites. However, beyond a role in initiating MyoD expression, little is known about the effects of Shh on primary muscle fiber formation in somites of higher vertebrates. The studies reported here investigate how the ventral neural tube promotes myogenesis and compare the effects of the ventral neural tube with those of purified Shh protein on fiber formation in somites. We show that purified Shh protein mimics actions of the ventral neural tube on somites including initiation of muscle fiber formation, enhancement of numbers of primary muscle fibers, and particularly, the formation of primary fibers that express slow myosin. There is a marked increase in slow myosin expression in fibers in response to Shh as somites mature. The effects of ventral neural tube on fiber formation can be blocked by disrupting the Shh signaling pathway by increasing the activity of somitic cyclic AMP-dependent protein kinase A. Furthermore, it was demonstrated that apoptosis is a dominant fate of somite cells, but not somitic muscle fibers, when cultured in the absence of the neural tube, and that application of Shh protein to somites reduced apoptosis. The block to apoptosis by Shh is a manifestation of the maturity of the somite with a progressive increase in the block as somites are displaced rostrally from somite III forward. We conclude that purified Shh protein in mimicking the effects of the ventral neural tube on segmented mesoderm can exert pleiotropic effects during primary myogenesis, including: control of the proliferative expansion of myogenic progenitor cells, antagonism of cell death pathways within the precursors to muscle fibers, and during the crucial process of primary myogenesis, can exert an effect on diversification of muscle fiber types.

Animals↗
PubMed2002

Two linked hairy/Enhancer of split-related zebrafish genes, her1 and her7, function together to refine alternating somite boundaries.

The formation of somites, reiterated structures that will give rise to vertebrae and muscles, is thought to be dependent upon a molecular oscillator that may involve the Notch pathway. hairy/Enhancer of split related [E(spl)]-related (her or hes) genes, potential targets of Notch signaling, have been implicated as an output of the molecular oscillator. We have isolated a zebrafish deficiency, b567, that deletes two linked her genes, her1 and her7. Homozygous b567 mutants have defective somites along the entire embryonic axis. Injection of a combination of her1 and her7 (her1+7) morpholino modified antisense oligonucleotides (MOs) phenocopies the b567 mutant somitic phenotype, indicating that her1 and her7 are necessary for normal somite formation and that defective somitogenesis in b567 mutant embryos is due to deletion of her1 and her7. Analysis at the cellular level indicates that somites in her1+7-deficient embryos are enlarged in the anterior-posterior dimension. Weak somite boundaries are often found within these enlarged somites which are delineated by stronger, but imperfect, boundaries. In addition, the anterior-posterior polarity of these enlarged somites is disorganized. Analysis of her1 MO-injected embryos and her7 MO-injected embryos indicates that although these genes have partially redundant functions in most of the trunk region, her1 is necessary for proper formation of the anteriormost somites and her7 is necessary for proper formation of somites posterior to somite 11. By following somite development over time, we demonstrate that her genes are necessary for the formation of alternating strong somite boundaries. Thus, even though two potential downstream components of Notch signaling are lacking in her1+7-deficient embryos, somite boundaries form, but do so with a one and a half to two segment periodicity.

Animals↗
PubMed1994

Analysis of the in vivo myogenic status of chick somites by desmin expression in vitro.

Expression of the muscle specific intermediate filament protein, desmin, is an early marker for chick somitic myogenesis. Somites are transient, paired, mesodermal structures adjacent to the neural tube which are formed very uniformly in a cranial to caudal fashion. The developmental somitic expression of desmin in vivo has been reported previously (Holtzer et al. [1991] "Frontiers in Muscle Research." New York: Elsevier Science, pp 187-207; Borman and Yorde [1994] J. Histochem. Cytochem. 42:265-272). Here we explore the ability of those somitic cells which are desmin negative in vivo to successfully carry out a myogenic program of development in the absence of the surrounding embryonic microenvironment. Somites which are known to be overtly desmin negative in the embryo were explanted and cultured on collagen gels for 4 days. Immuno-detection of desmin identified a population of somites that could support desmin positive cells in vitro as well as a population of somites that could not. The cranially located somites must remain in the embryo for a greater length of time than the caudally positioned somites prior to each being able to express desmin in vitro. In embryos of many ages there is also a population of somites unable to support desmin expression in vitro. The rate at which this ability to support somitic desmin expression in vitro progresses caudally in the embryo is significantly greater than the rate at which somites form. Notably, the detected expression of desmin in somites in vitro is parallel to the rate at which overt expression of desmin in vivo is detected. The implication for these observations with regard to the regulation of somitic myogenesis is discussed.

Animals↗
PubMed1994

Analysis of chick somite myogenesis by in situ confocal microscopy of desmin expression.

We explored the relationship in chick embryos between somitogenesis and the onset of somite myogenesis by immunodetection of the muscle-specific intermediate filament protein desmin. Early somite desmin expression was detected by whole-mount in situ confocal microscopy. No detectable somite desmin was observed in embryos of 15 somites (Stage 12) or younger. In embryos having between 16 and 26 somites (Stages 12-15), desmin could be detected in somites positioned increasingly more caudal in the embryo. Finally, in embryos of 27 somites (Stage 16) and older, somite desmin expression was consistently present in all but the caudal-most six somites. Although the rate of somite formation is fairly constant, the rate of observed somite desmin expression progressing caudally in the embryo is greater initially than the rate of segmentation. After an embryo has formed about 27 somites, the rate of desmin appearance parallels the rate of segmentation at a distance of about six somites. This result suggests that very early somite myogenesis is not linked to somitogenesis.

Animals↗
PubMed1989

The contribution made by cells from a single somite to tissues within a body segment and assessment of their integration with similar cells from adjacent segments.

Somites represent the first visual evidence of segmentation in the developing vertebrate embryo and it is becoming clear that this segmental pattern of the somites is used in the initial stages of development of other segmented systems such as the peripheral nervous system. However, it is not known whether the somites continue to contribute to the maintenance of the segmental pattern after the dispersal of the somitic cells. In particular, the extent to which cells from a single somite contribute to all of the tissues of a single body segment and the extent to which they mix with cells from adjacent segments during their migration is not known. In this study, we have replaced single somites in the future cervical region of 2-day-old chick embryos with equivalent, similarly staged quail somites. The chimerae were then allowed to develop for a further 6 days when they were killed. The cervical region was dissected and serially sectioned. The sections were stained with the Feulgen reaction for DNA to differentiate between the chick and quail cells. The results showed that the cells from a single somite remained as a clearly delimited group throughout their migration. Furthermore, the sclerotome, dermatome and myotome portions from the single somites could always be recognised as being separate from similar cells from other somites. The somitic cells formed all of the tissues within a body segment excluding the epidermis, notochord and neural tissue. There was very little mixing of the somitic cells between adjacent segments. The segmental pattern of the somites is therefore maintained during the migration of the somitic cells and this might be fundamental to a mechanism whereby the segmentation of structures, such as the peripheral nervous system, is also maintained during development.

Animals↗
PubMed1992

Determination of epithelial half-somites in skeletal morphogenesis.

The segmental body plan of vertebrates arises from the metameric organization of the paraxial mesoderm into somites. Each mesodermal somite is subdivided into at least two distinct domains: rostral and caudal. The segmental pattern of dorsal root ganglia, sympathetic ganglia and nerves is imposed by differential properties of either somitic domain. In the present work, we have extended these studies by investigating the contribution of rostral or caudal-half somites to vertebral development using grafts of multiple somite halves. In both rostral and caudal somitic implants, the grafted mesoderm dissociates normally into sclerotome and dermomyotome, and the sclerotome further develops into vertebrae. However, the morphogenetic capabilities of each somitic half differ. The pedicle of the vertebral arch is almost continuous in caudal half-somite grafts and is virtually absent in rostral half-somite implants. Similarly, the intervertebral disk is present in rostral half-somite chimeras, and much reduced or virtually absent in caudal somite chimeras. Thus, only the caudal half cells are committed to give rise to the vertebral pedicle, and only the rostral half cells are committed to give rise to the fibrocartilage of the intervertebral disk. Each vertebra is therefore composed of a pedicle-containing area, apparently formed by the caudal half-somite, followed by a pedicle-free zone, the intervertebral foramen, derived from the rostral somite. These data directly support the hypothesis of resegmentation, in which vertebrae arise by fusion of the caudal and rostral halves of two consecutive somites.

Animals↗
PubMed1994

Myogenic specification in somites: induction by axial structures.

Specification of the myogenic phenotype in somites was examined in the early chick embryo using organotypic explant cultures stained with monoclonal antibodies to myosin heavy chain. It was found that myogenic specification (formation of muscle fibers in explants of somites or segmental plates cultured alone) does not occur until Hamburger and Hamilton stage 11 (12-14 somites). At this stage, only the somites in the rostral half of the embryo are myogenically specified. By Hamburger and Hamilton stage 12 (15-17 somites), the three most caudal somites were not specified to be myogenic while most or all of the more rostral somites are specified to myogenesis. Somites from older embryos (stage 13-15, 18-26 somites) showed the same pattern of myogenic specification--all but the three most caudal somites were specified. We investigated the effects of the axial structures, the notochord and neural tube, on myogenic specification. Both the notochord and neural tube were able to induce myogenesis in unspecified somites. In contrast, the neural tube, but not the notochord, was able to induce myogenesis in explants of segmental plate, a structure which is not myogenic when cultured alone. When explants of specified somites were stained with antibodies to slow or fast MyHC, it was found that myofiber diversity (fast and fast slow fibers) was established very early in development (as early as Hamburger and Hamilton stage 11). We also found fiber diversity in explants of unspecified somites (the three most caudal somites from stage 11 to 15) when they were recombined with notochord or neural tube. We conclude that myogenic specification in the embryo results in diverse fiber types and is an inductive process which is mediated by factors produced by the neural tube and notochord.

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