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M Bancroft

Publications and source records attributed to M Bancroft.

7 recordsLinked to original sources
PubMed1977

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.

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PubMed1976

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.

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PubMed1976

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.

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PubMed1975

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.

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PubMed1975

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.

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