Differentiation of the neural plate and neural tube in the young chick embryo. A study by scanning and transmission electron microscopy

The differentiation of the presumptive neural plate, the neural plate and the neural tube have been investigated in the chick embryo by SEM, TEM and histochemical techniques. The relationship of these tissues to neighbouring structures, including extracellular materials, has also been studied. When SEM micrographs of primitive streak stage embryos were examined in stereo, it was found that cells which had been invaginating at the time of fixation were similar in shape to fibroblasts migrating in vitro. It was concluded that SEM stereo pairs could provide evidence about the mode and direction of cell migration. Many more mid-bodies have been found associated with the developing neural tissue than with the lateral ectoderm. It was found possible to recognise mid-bodies not only by TEM but also by SEM. It is therefore proposed that SEM montages may be used for assessing which regions of a tissue have recently undergone extensive mitosis. The beads on the specialised threads seen in the early stages of development are now considered to be formed from mid-bodies. Similar, but unbeaded threads have been described which span the gap between the neural folds just prior to the dorsal closure of the neural tube and it seems probably that these threads help to close the neural tube. It is suggested that the beaded threads arise by incomplete separation of two daughter cells at mitosis, whereas the unbeaded threads form by outgrowth of cell processes.

Placodes of the chick embryo studied by SEM

The otic, the lens and the nasal placodes have been examined in chick embryos between stages 10 and 18 of Hamburger and Hamilton. At the stage when each placode first becomes visible conspicuous differences have been seen in the surface morphology between those cells which will invaginate and form the placode and those which will remain on the surface of the head, forming the epidermis. The differences become more pronounced with increasing development. The placode cells possess many surface projections whilst the epidermal cells do not. These differences in surface morphology are related to other differences which are visible in TEM sections, the placode cells being highly columnar and extending the full depth of the placode, whilst the epidermal cells are cuboidal or even squamous. This modification in cell shape of the placode cells is correlated with the presence of longitudinally orientated microtubules. The mechanism of invagination is discussed and evidence is presented which supports the idea that there is a migration of cells into the placode from one side. Such a phenomenon would help to explain the asymmetrical structure of the placode, including the presence of the overhanging lip.

Midbodies and beaded threads

When the dorsal surface of the young chick embryo is examined by SEM, long threads are visible, each of which appears to connect pairs of cells; these cells may be separated from each other by several intervening cells. Many of the threads possess a bead-like structure about half way along their length. When sections of the beads are examined by TEM they are found to resemble midbodies. Furthermore, the threads possess longitudinally arranged structures within them, which are probably the remnants of the microtubules which were part of a mitotic spindle. It is concluded that each bead is a midbody and that each beaded thread is the remains of a telophase bridge connecting two daughter cells which were incompletely separated after mitosis had taken place. The possible function of the beaded threads is considered.

The formation of the gonadal ridge in Xenopus laevis

This paper studies the surface morphology of the developing gonadal ridge in X. laevis between stages 44 and 49 (Nieuwkoop & Faber, 1956). During this period the primordialgerm cells (PGCs) move laterally from the dorsal mesentery of the gut to the position of thepresumptive gonadal ridge. As they do so the coelomic lining cells lateral to the mesenterydifferentiate into a specialized, longitudinally orientated band, stretching nearly the fulllength of the dorsal mesentery on each side. The PGCs migrate beneath this band of cells, which thus becomes the germinal epithelium of the gonadal ridge. We have demonstrated by irradiation experiments that this specialized band of cells can differentiate independently, in the absence of the PGCs.

A scanning electron microscope study of the early limb-bud in normal and talpid3 mutant chick embryos

Scanning electron microscope studies, supported by transmission electron microscope and light microscope observations, have been made of the wing-bud apex in the region of the apical ectodermal ridge (AER) in both normal and talpid3 mutant embryos.

In normal embryos: the ectoderm consists of two layers, the periderm and the basal layer, resting upon a basal lamina. The external surface of the periderm cells bears numerous villi, especially numerous at the cell boundaries. In the AER the cells of the basal layer are compacted into a fan-shaped form, remaining as a single layer, though displacement of their nuclei gives the appearance of a stratified epithelium; the periderm cells tend to round up and many are necrotic. Microfilaments and microtubules are present in greater numbers in the AER than in other ectoderm cells and are orientated along the long axis in the very narrow elongated cells which comprise the middle of the ridge. The mesenchymal cells of the mesoderm tend to be flattened, with extensive flattened surface areas separated from each other by long edges of lamellar cytoplasm, from which many long filopodial extensions arise. Often the cells are elongated, with the filopodia arising predominantly at the anterior and posterior ends.

In talpid3 embryos: when these were compared with normals no differences were detected in the general ectoderm or in the AER, but in the mesoderm a statistical analysis revealed significant differences in the occurrence of sections through very fine cytoplasmic processes in transmission electron microscope micrographs. Taken in conjunction with visual examination of scanning electron microscope pictures the differences suggested that talpid3 cells have filopodia distributed more extensively around the cell, but that these extensions do not extend so far through the intercellular spaces or produce such fine terminal arborizations as in normal mesenchyme.

At the ectodermal/mesodermal boundary there is a web of extracellular material under the basal lamina, separated from the underlying mesoderm by a very narrow gap in normal embryos. Fine cytoplasmic extensions from the mesenchyme cells extend across the gap towards the basal lamina and many are in contact with it. The gap is wider in talpid3 wing-buds and a statistical analysis confirmed that far fewer filopodia approach closely to the basal lamina or make contact with it.

The significance of these findings in relation to problems of AER formation and activity and motility of mesenchymal cells in the wing-bud is discussed.

The development of the notochord in the chick embryo, studied by scanning and transmission electron microscopy

The notochord of the chick embryo between stages 5 and 23 inclusive has been studied by scanning electron microscopy, supplemented by transmission electron microscopy. Three main phases of development are described, and these have been designated: bilaminar; rod-like, unvacuolated; rod-like and vacuolated. The changes in shape of the organ from bilaminar to rod-like is accompanied by changes in the shape, orientation and position of the cells, an increase in the complexity of the cell contacts, and the laying down of a basal lamina. The change from the unvacuolated to the vacuolated phase is accompanied by increasing complexity within the cytoplasm. Most of the vacuoles are intracellular and appear empty though some contain a granular material. The notochordal sheath appears to be secreted by the notochordal cells and fine fibrillar material has been seen in the intercellular spaces. By stage 23, most of the notochordal cells have become so highly vacuolated that the cytoplasm has become closely packed around the nucleus.

The formation of the gonadal ridge in Xenopus laevis. II. A scanning electron microscope study

This paper studies the surface morphology of the developing gonadal ridge in X. laevis between stages 44 and 49 (Nieuwkoop & Faber, 1956). During this period the primordial germ cells (PGCs) move laterally from the dorsal mesentery of the gut to the position of the presumptive gonadal ridge. As they do so the coelomic lining cells lateral to the mesentery differentiate into a specialized, longitudinally orientated band, stretching nearly the full length of the dorsal mesentery on each side. The PGCs migrate beneath this band of cells, which thus becomes the germinal epithelium of the gonadal ridge. We have demonstrated by irradiation experiments that this specialized band of cells can differentiate independently, in the absence of the PGCs.