14 Specification of Cell Fates in the Early Embryo

Abstract

I. INTRODUCTION
Classical studies in embryology showed that in certain animals, the descent, or lineage, of cells was correlated strictly with cell fate; these animals were described as having a determinate, autonomous, or mosaic mode of development. Nematodes like Caenorhabditis elegans have an invariant lineage and have been a paradigm of the determinate mode. Results from experiments on C. elegans in the 1980s were, and remain, consistent with the classical notion that some specification of cell fates in nematodes occurs autonomously (Laufer et al. 1980; Cowan and McIntosh 1985; Edgar and McGhee 1986; Schierenberg 1988). For example, it was shown that certain cleavage-arrested blastomeres are nevertheless able to undergo tissue-specific differentiation. However, as described in this chapter, a large number of cell-cell interactions have now been identified in the C. elegans embryo. The invariant lineage of the embryo may result largely from the fact that invariant cleavage patterns set up reproducible patterns of cell-cell interactions.

All of C. elegans embryogenesis occurs within a transparent egg shell; the egg measures about 50 μm in length and 30 μm in diameter. The small size of the egg and the small number of cells at hatching (55) have made it possible to observe with the light microscope the pattern of cell cleavage and differentiation of all cells in the living embryo. A major conclusion from these studies is that this pattern, or lineage, is largely invariant between different embryos. Thus, it has been possible to construct a single diagram representing the entire cell...