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Building the Egg's Extracellular Matrix

Fertilization not only activates the egg and provides a new assortment of genes for the developing organism, it also provides the embryo with an extracellular matrix that is extremely important in coordinating cell stability and migration during cleavage and gastrulation. This extracellular matrix is sometimes called the hyaline layer. The sea urchin extracellular matrix is not a simple structure, either. Rather, it contains at least five major components, each of which is secreted at a particular time during the first cell cycle. Moreover, the cortical granules are not the only secretory vesicle in the egg. Rather, there are at least four types of secretory vesicle, each containing its own particular set of proteins, and each specified to fuse with the cell membrane at a particular time after fertilization.

In the oocyte, the apical vesicles are associated with the cortex, while the other vesicles are spread throughout the cytoplasm. After the germinal vesicle breakdown reinitiates meiosis, the cortical granules move to the cortex, and the apical vesicles become spread throughout the cytoplasm. (The cortical granule is technically a vesicle—but the traditional name has remained). Beginning at the point of sperm entry, the basal vesicles secrete their basal lamina material, and immediately thereafter, the cortical granules undergo exocytosis. This displaces the vitelline envelope and creates the first extracellular matrix of the embryo. At about one minute, this matrix consists of a cortical granule-derived hyalin protein adjacent to the zygote cell membrane and a basal lamina on the outer surface. About ten minutes after fertilization, the apical vesicles secrete their material, and this material forms a layer between the hyalin and the zygote membrane. Ten or 15 minutes later, the echinonectin vesicles secrete their echinonectin onto the cell surface. The apical vesicles and echinonectin vesicles do not secrete their material in accordance with the point of sperm entry. Rather, the secretion from each of these vesicles occurs simultaneously in each part of the egg. Thus, by thirty minutes after fertilization, there is a series of concentric rings around the egg, placed there by the timed secretion of the vesicle contents. The innermost layer is that of echinonectin. Above that are the proteins from the apical vesicles. The cortical granule hyalin protein comes next, and the outermost proteins are the basal lamina proteins, secreted from the basal vesicles (Figure 1; After Matese et al., 1997).

Figure 1
Figure 1   The ordered assembly of the sea urchin egg hyaline layer by the sequential exocytoses of vesicles formed in the oocyte. This diagram was constructed using immunofluorescence photographs showing the locations of each protein-containing vesicle at each time period. (After Matese et al., 1997.)

Finally, maternal cadherin, which is stored in yet a fifth distinct vesicle, is not detected on the surface until at least 30 minutes following fertilization. It does not form part of the hyaline layer, but is on the cell membrane.

Literature Cited

Matese, J. C., Black, S., and McClay, D. R. 1997. Regulated exocytosis and sequential construction of the extracellular matrix surrounding the sea urchin zygote. Dev. Biol. 186: 16-26.

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