13 Fertilization and Establishment of Polarity in the Embryo

Abstract

I. INTRODUCTION
The body plan of the Caenorhabditis elegans embryo is established during the first few cleavages. The reproducible orientations of these cleavages coupled with asymmetric localization of cytoplasmic components initiate processes that establish the three principal axes of the body and set the fates of the six founder cells. In this chapter, we review the current understanding of mechanisms controlling the early cleavages, and we address the following issues: (1) when and how embryonic polarity is established, (2) how cytoplasmic factors are differentially partitioned along an axis, and (3) how spindle positioning is controlled to generate cells of the correct sizes, in the correct positions, and with the correct contents.

A. Overview of Embryogenesis
For detailed descriptions of C. elegans embryogenesis, see Sulston et al. (1983), which describes the entire embryonic lineage, and Wood (1988) and Strome (1989). It takes 14 hours at 20°C for a newly fertilized embryo to complete embryogenesis and hatch from its eggshell into a juvenile worm. During the first few hours, the embryo undergoes a series of four unequal divisions, to produce five somatic founder cells (AB, E, MS, C, and D) and the primordial germ cell (P₄) by the 28-cell stage (Figs. 1 and 2a–i). Gastrulation begins at the 28-cell stage when the two daughters of E move to the interior of the embryo (Fig. 2i), followed later by P₄ and some of the descendants of MS, C, D, and AB. These cell movements, coupled with continued proliferation, result in a generally...