Cellular interactions in establishment of regional patterns of cell fate during development.

Abstract

The central problem in developmental biology remains: How does the fertilized egg develop into an embryo with the correct spatial pattern of cellular differentiation? Two general solutions to this problem have been proposed. The first involves cytoplasmic localization, wherein specific determinants are physically localized within the egg cell and differentially inherited by the cleavage products. Thus, cells that inherit a muscle cell determinant would form muscle, whereas those that inherit the neuroplasm would form nervous tissue. Organisms that use cytoplasmic localization as the major mechanism for specification of cell fate are relatively few, with ascidians being perhaps the best example (see Chapter 1). Most species therefore make use of a mechanism involving a sequence of inductive interactions, in which the generation of new cell types results from interactions between two existing cell types. This process requires that an initial polarity, or asymmetry, be present in the fertilized egg or imposed on the embryo by its environment. This asymmetry might occur as a cytoplasmic localization, but it need not be as specific as the ascidian system, which specifies individual cell types; it would be sufficient, for example, to imagine a gradient of a substance the high point of which defines the prospective dorsal side of the egg and whose minimum value defines the ventral side.