Cell Behaviour in Chick Blastoderm Expansion

El-Gadi, Abdulgader Othman A (1991) Cell Behaviour in Chick Blastoderm Expansion. PhD thesis, University of Glasgow.

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The behaviour of the leading edge cells during the early stages of epiboly (stage 3 to 16, Hamburger and Hamilton, 1951) has been investigated in this study using light, transmission and scanning electron microscopy. The results showed a consistency in the distribution of junctions between these cells throughout these stages. However, there is a significant increase in both the width of the leading edge and the number of its cells around the second day of incubation (stage 11-13). The significance of this increase is discussed in relation to the reported increase in the rate of expansion of the blastoderm at around that time (Downie, 1976). As the leading edge migrates centrifugally over the vitelline membrane, it requires the addition of new cells to cover an increasingly wider area. This study provides evidence that cells recruited into the edge are the deep layer cells which are found on the basal lamina of the peripheral part of the blastoderm and not the ectodermal cells as is generally thought. The opposite task faces the leading edge as it passes the equator of the egg and occupies a smaller area of vitelline membrane, that is how to accommodate the large number of edge cells into a smaller space? The current study provides the first account of the unique strategy that the chick blastoderm uses to achieve this task. Some of the leading edge cells become stationary at points around the circumference of the edge. This allows just enough cells to continue their movement to cover the rest of the vitelline membrane and in turn the whole yolk. As more cells are added to the original stationary cells and become stationary themselves, long stationary streaks develop, giving the blastoderm edge a crescent-shaped appearance. The difference in the end of epiboly between the chick blastoderm and other embryos with similar systems is discussed in this study. The study also reports for the first time that cell death occurs in the leading edge cells during the late stages of development. This occurs in the oldest stationary cells then progresses centrifugally to the younger ones. As the stationary edge cells die, they are replaced by the nearby ectodermal cells which attach strongly to the vitelline membrane - an unusual behaviour for ectodermal cells which never attach to this membrane during the earlier stages of epiboly. The time and position of appearance of stationary points was found to be unrelated to the stage of development. The length of the stationary streaks and their complexity was found also to be different from one blastoderm to another even in blastoderms which have been incubated for the same length of time. Another peculiar finding of this study is that the extent of blastoderm expansion and the time it seals the vitelline membrane is also different from one embryo to another at any particular time of development. Indirect immunofluorescent staining of pieces of blastoderm edges cultured on glass confirmed previous reports that the shape of the epithelial cells does not depend on the integrity of an intact microtubule system and that microfilaments are the essential elements in maintaining the shape of the leading edge cells. Immunofluorescent staining for fibronectin distribution in the blastoderm edge showed it to be present in the basal lamina, and within a few attached edge cells during the first 2 days of expansion, but not between edge cells, or at the edge cell-vitelline membrane interface. However, extracellular fibronectin was detected in the stationary streaks after 3 days. These results differ from previous reports, and reasons for these differences are discussed. Experiments to study the behaviour of blastoderm cells lacking an edge on the inner surface of the vitelline membrane revealed that the blastoderm has the ability to form a regenerated edge which moves rapidly on the substratum. Cultures of blastoderm pieces with or without the leading edge on the outer surface of the vitelline membrane showed that these cells were unable to expand on this substratum. The reasons behind this behaviour are discussed.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Developmental biology
Date of Award: 1991
Depositing User: Enlighten Team
Unique ID: glathesis:1991-78273
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09
URI: http://theses.gla.ac.uk/id/eprint/78273

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