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PubMed1975

[Electron microscopic study of the penetration and distribution of somitic cells in the mesoblast of the limb buds of reptiles (Anguis fragilis and Lacerta viridis)].

Based on characteristics of mitochondria and on the amount of lipid inclusions, a distinction between somitic cells and mesoblastic somatopleural cells is possible, at the early stages of the development of the limb bud in Reptiles (Anguis fragilis and Lacerta viridis). The dislocation of the ventral processes of the somites and the localisation of the somitic cells in the mesoblast of the anterior limb buds could be studied.

Allantois↗
PubMed1979

Somitogenesis in amphibia. II. Origins in early embryogenesis of two factors involved in somite specification.

A somite pre-pattern is established shortly before visible segmentation. The pre-pattern results from the interaction of two components: a wave of cell behavioural change that passes along the axis, and, an underlying co-ordination of the cells that is the basis for their association into large somite-sized groupings. The evidence is derived from studies of the zones of abnormal segmentation that follow temperature shocks delivered between the neurula and tail-bud stages (Pearson & Elsdale, 1979). Temperature shock given earlier at the mid-gastrula stage is however ineffective in inducing abnormalities in somitogenesis. Shocks given before the mid-gastrula stage reveal a prior period of sensitivity stretching back into the blastula. Thus early and late sensitive periods can be defined separated by a short refactory period. Quite different patterns in the distribution of somite abnormalities characterize the results of shock during the two sensitive periods, suggesting different aetiologies. It is concluded that the wave of rapid cell change is set up early in embryogenesis during the blastula stage, and each cell of the prospective paraxial mesoderm carries a determination to change after a specific length of time, i.e. a countdown is set in each cell. As a result of the movements of gastrulation, the prospective paraxial mesoderm cells become laid out along the axis of the neurula in the order (antero-posterior sequence) in which they will change. The achievement of the correct redistribution of the cells depends crucially on the conservation of the sequence in the blastula by the maintenance of topological integrity throughout gastrulation. It is suggested that early shock disturbs gastrulation movements, causing some mixing up of the cells resulting in incoherence of the wavefront. Whereas early shocks are thus assumed to affect the wave, the evidence suggests that late shock undergoes co-ordination. It is concluded therefore that co-ordination is established later, after the refractory period, around the late gastrula stage.

Animals↗
PubMed1979

Muscle-forming potential of the non-somitic cells of the early avian limb bud.

It has recently been shown that the musculature of the chick wing arises by migration of cells from the somites, and that on morphological grounds this process begins at about stage 14. We have carried out grafts of wing anlagen separate from the somites from quail donors to the extra-embryonic coelom of chicks, and find that anlagen from as early as stage 10 (11 pairs of somites) can give rise to muscle. We discuss the possible reasons for this finding, and conclude that in the absence of the cells normally giving rise to the musculature, mesodermal cells themselves can give rise to muscle.

Animals↗
PubMed1976

[Organization and differentiation of the somite mesoderm cultured in the presence of adjacent tissues (notocord, spinal cord, ectoderm, endoderm) in anurans].

The explantation of pieces of somitic mesoderm alone or in association with neighbouring tissues (notochord, spinal cord, ectoderm, endoderm) shows slearly the power of self-differentiation of the somitic tissue. Only, the spinal cord does not stimulate the differentiation of the somitic mesoderm into muscular tissue. Only the notochord does not stimulate the formation of typical myotomes. It seems that ii does not possess biochemical activity, its effect if reduced to a mechanical role.

Age Factors↗
PubMed1976

Abnormalities in somite segmentation following heat shock to Xenopus embryos.

The typical abnormality induced by a 15 min shock at 37degreesC is a single discrete length along the somite file within which segmental boundaries are absent or irregular. The two sides of the same embryo present a similar but not necessarily identical appearance. Usually all the embryos in a treated batch show abnormalities of similar severity. Survival of treated embryos, the details of the visible malformations, and temporal aspects of the phenomenon have been studied. The results indicate a temperature sensitive period that traverses the neurula, from head to tail at about the same rate as the somites form, but some hours beforehand. The temperature sensitive process is not associated with cell determination and differentiation, and there are reasons for thinking that the specification of the normal somite number occurs independently. The results are discussed in relation to Cooke & Zeeman's model of a wave front interacting with an oscillator.

Animals↗
PubMedDate not supplied

Researches on the formation of axial organs in the chick embryo. IX. On the development of somites in axial-paraaxial segments explanted to the zona pellucida.

Axial-paraaxial segments (neural tube, chorda, unilateral meso- and endoderm) excized from explanted 36--40-hour-incubated chick embryos at the level of unsegmented mesoderm, after removal of the ectoderm, were grafted onto subectodermal pockets of the zona pellucida. Under these conditions somites develop and differentiate normally. Paraaxial segments (unilateral meso- and endoderm) grafted under the same conditions show (retarded) somitogenesis only in 15% of the cases. Pure paraaxial unsegmented mesoderm grafted under the same conditions develops somites in 14% of the cases. Since in situ, the removal of the axial organs and of the endo- and ectoderm does not inhibit somitogenesis, the above-mentioned results prove that under conditions of grafting, some additionary "factors of realization" necessary for normal somitogenesis are lacking.

Animals↗
PubMed1979

Somite formation in the early chick embryo following grafts of Hensen's node.

Quail grafts of Hensen's node were examined for their potential to induce somites in chick blastoderms. The origin of the structures induced depended on the distance of the graft from the host's midline. Nodes placed at the margin of the area pellucida resulted in structures differentiated from the cells of the graft, whereas medially the graft organized host cells to form rows of somites. The results are discussed in terms of competence of graft and host mesenchyme and a positional signal from the node.

Animals↗
PubMed1979

The migration of myogenic cells from the somites into the leg region of avian embryos. An ultrastructural study.

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

Animals↗
PubMed1978

Cell interactions in the developing somite; in vivo comparisons between amputated (AM/AM) and normal mouse embryos.

Light and electron microscopy coupled to a quantitative analysis form a powerful technique for the analysis of cell behaviour in mutant and normal tissues. Using this approach the morphology of the cells and their contacts are examined in the somites of a recessive mouse mutant, amputated, and of its normal littermates. The results of the analysis show that cell density is the same in both mutant and normal sclertome but that mutant cells tend to form small clumps whereas normal cells disperse individually. There is a correspondingly greater area of cell contact per cell in the mutant. Filopodia are equally numerous in the mutant and normal but where in the normal they stretch across wide intercellular spaces to make contact at their tips with other cells, in the mutant they form a tangled web sticking back on to the surface of the cell of origin and adjacent cells. The appearance of mutant and normal sclerotome is compared with presomitic cells and with the cells of other mutants whose abnormal development has been shown to depend on cell contact morphology and behaviour.

Animals↗
PubMed1979

Culture of early somite mouse embryos during organogenesis.

Early somite (2-4) mouse embryos were explanted and then maintained in culture for 24 or 48 h intervals. Various types of media were tested and it was determined that rat serum supported normal growth over a period of 48 h, based on total protein analysis and histological comparisons with in vivo specimens. Other media including fetal calf serum and fetal calf serum and Waymouth's (1:1) supported some growth, but did not equal the success of using rat serum alone. During the 48 h culture period in rat serum, embryos developed to stages indistinguishable from embryos maintained for a similar time in vivo.

Animals↗
PubMed1978

Tissue specificity for incorporation of [3H]thymidine by the 10- to 12-somite mouse embryo: alteration by acute exposure to hydroxyurea.

Radioautograms from 10- to 12-somite mouse embryos labeled for 30 min in vitro with [3H]thymidine were examined for frequency and intensity of incorporation. Results from ten tissues showed that values ranged from 82% of nuclei with a mean of 16.6 grains for visceral yolk sac to 17% of nuclei labeled with a mean of 4.4 grains for epithelium of the anterior gut tube. Labeling in the ten tissues indicated (1) a tissue-specific spectrum of incorporation of [3H]thymidine, (2) close correlation between frequency and intensity of labeling within a tissue and (3) asymmetrical quantities of incorporation between right and left somatopleure. Treatment with hydroxyurea in vitro reduced the frequency of labeled nuclei by 85% to 12% of control values. Mean numbers of grains over treated nuclei, 3.3-4.6 grains, were well above background but were clustered below the low end of the control range. Tissues exposed to hydroxyurea showed (1) labeling of significant numbers of nuclei, (2) inhibition of labeling in selected tissues and (3) equalization of bilateral asymmetry in quantity (frequency and intensity) of incorporation in somatopleure. The selective reduction of thymidine incorporation and equalization of asymmetrical rates of proliferation may constitute mechanisms by which hydroxyurea causes abnormal morphogenesis.

Animals↗
PubMed1978

Attraction of primordial germ cells by notochord in seven somites chick embryo.

Chemical studies in chick embryo have indicated the existence of proteoglycan in notochordal sheath. Primordial germ cells were observed with scanning electron microscope on the notochord dorsal face, surrounded with perichordal material. We postulate the identification of such a material with proteoglycan which could attract primordial germ cells to the notochord.

Animals↗