Gastrulation is complicated! Because of this, it is helpful to break the movements of gastrulation down into their component events wherever possible. In general, sheets of cells can engage in only a limited number of morphogenetic movements. This "morphogenetic repertoire" is helpful to keep in mind when we are presented with what seems to be an incomprehensible change in the shape of the embryo. Through careful observation and experimental manipulation that will be discussed in this section, gastrulation can be analyzed in convenient organisms such as sea urchins. On this page and the next, the various major morphogenetic movements that occur during gastrulation in diverse organisms are schematically represented. Some of these movements are only performed by epithelial cells, while others can be performed by both bona fide epithelial cells and by deeper, non-epithelial cells that nevertheless behave as integrated sheets of cells. The latter are poorly understood, but are common in amphibians as well as in higher vertebrates.
Click on a cell movement below to get a fuller description of what is going on.
During invagination, an epithelial sheet bends inward to form an inpocketing.
One way to think of this in three dimensions is to imagine that you are
poking a partially deflated beach ball inward with your finger. The resulting
bulge or tube is an invagination. If the apical side of the epithelium forms
the lumen (central empty space) of the tube, then the movement is termed
invagination. If the lumen is formed by basal surfaces, then the movement
is termed an evagination.
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During ingression, cells leave an epithellial sheet by transforming from
well-behaved epithellial cells into freely migrating mesenchyme cells. To
do so, they must presumably alter their cellular architecture, alter their
program of motility, and alter their adhesive relationship(s) to the surrounding
cells. Primary mesenchyme cells are an example of a mesenchymal cell type
that emigrates out of an epithelium (do you know which one?).
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During involution, a tissue sheet rolls inward to form an underlying
layer via bulk movement of tissue. One helpful image here is of a tank tread
or conveyor belt. As material moves in from the edges of the sheet, material
originally at the sites of inward rolling (shown in blue here) is free to
move further up underneath the exterior tissue.
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