The mechanism of chick blastoderm expansion

Abstract

At the time of laying, the domestic fowl blastoderm measures 4 mm across. After 4 days’ incubation, the extra-embryonic yolk-sac tissues have expanded to encompass the whole yolk mass. This expansion involves the migration over the inner surface of the vitelline membrane of a specialized band of ‘edge cells’ at the blastoderm periphery. As they move, they pull out the blastoderm behind them, setting up a considerable tension. Expansion also involves cell proliferation and changes in cell shape. This paper attempts to show how locomotion, tension, proliferation and changes in cell shape all contribute to the orderlyprocess of expansion. As a simplification, only the extra-embryonic epiblast is considered here. The findings are: 1. Expansion does not occur at a constant rate, but starts slowly, rises to a peak (over 500 μm/h) at around 3 days, and then slows as coverage of the yolk mass nears completion. 2. During the first day of incubation, edge-cell migration produces a tension in the blastoderm. This rises to a peak at 20–24 h, then declines. This tension may be due to an imbalance between expansion by migration and expansion by proliferation. 3. Migration of edge cells can be affected by tension in the blastoderm, i.e. very high tension may hold them back. However, the tension level normally found in the blastoderm seems not to do so. The low rate of expansion in the first day is therefore not due to the high level of tension. It may instead be due to changes in edge-cell organization. 4. Proliferation occurs throughout the extra-embryonic epiblast during the expansion period. It is not restricted to the blastoderm periphery. After the yolk has been covered, the epiblast continues to grow, with proliferation restricted largely to a band just distal to the advancing edge of the area vasculosa. 5. Cell shape and arrangement change considerably during expansion. The epiblast of the unincubated embryo is a monolayer of tall cells. During expansion, these become considerably flattened so that each contributes a larger amount to yolk-sac surface area.

The extra-embryonic area opaca plays a role in positioning the primitive streak of the early chick embryo

“…Based on a number of classical studies it had been generally thought that the peripheral region of the embryo, the area opaca, plays a role in providing nutrition to the embryo and in maintaining its tension, but that it does not have an instructive role in regulating cell fate or embryonic polarity ( Bellairs et al, 1967 ; Downie, 1976 ; Khaner et al, 1985 ; New, 1959 ; Spratt and Haas, 1960 ). Contrary to this conclusion, the present study uncovers three separable functions of the area opaca: induction of marginal zone properties, an influence on polarity of the marginal zone and determining the end of the period during which embryo fragments can ‘regulate’ (repolarise themselves and form a primitive streak from an isolated fragment lacking the primitive streak-forming region) ( Fig.…”

“…A second possibility is that tension generated by the expanding edges of the area opaca (the ‘margin of overgrowth’; Bellairs et al, 1967 ; Downie, 1976 ; New, 1959 ) plays a role. However, embryo fragments lacking the area opaca can regulate (albeit at reduced frequency) ( Spratt and Haas, 1960 , 1961 and results in the present paper) and, moreover, Spratt's experiments were performed by growing the embryo in the absence of the vitelline membrane and placing directly (in some cases with its ventral surface downwards) on a semi-soft agar substrate to which it cannot adhere and which promotes very limited expansion.…”

“…In contrast, the more peripheral region, area opaca, is generally believed to play a less active role in regulating embryonic patterning, its major functions at this early stage being providing a source of nutrition and maintaining the centrifugal tension of more central regions through adhesion of its edge to the vitelline membrane ( Bellairs et al, 1967 ; Downie, 1976 ; New, 1959 ). Indeed, the embryo can generate a primitive streak even when the area opaca is removed, which has been interpreted to imply that the area opaca has no role in embryonic polarity ( Spratt and Haas, 1960 ; Khaner et al, 1985 ).…”

Molecular characteristics of the edge cells responsible for expansion of the chick embryo on the vitelline membrane

“…This represents more than a 200-fold increase in surface area and a rate of expansion of 200–550 µm h −1 [1] (figure 1 a ).This expansion is driven by specialized cells located at the extreme edge of the embryo, which are the only cells that adhere to the inner (yolk-facing) surface of the vitelline membrane (VM). This narrow ring of cells has been called ‘margin of overgrowth’ [2] and spans approximately 40–60 µm [1,3–5]. Several requirements need to be met for proper expansion of the blastoderm and normal embryo development.…”

“…Cells at the edge of the embryo change their shape from initially cuboidal multiple layers to later a squamous and flattened monolayer [ 1 , 3 ]. The extreme edge region has larger cells with larger nuclei, while the immediately proximal region has densely packed cells [ 3 ].…”