8 Embryology

Abstract

I. INTRODUCTION
Formal aspects of the embryonic cell lineage and some discussion of embryonic cell determination have been presented in Chapter 5. This chapter provides a more complete description of embryogenesis per se, summarizes the embryonic origins of major organs and tissues, discusses further evidence for early cell determination by intrinsic, cell-autonomous determinants versus extrinsic cues, and reviews what has been learned about genetic control of embryogenesis by analysis of embryonic lethal mutants. The chapter concludes with a brief discussion of possible determination mechanisms in the early embryo.

Embryogenesis in Caenorhabditis elegans, from fertilization to hatching, takes about 14 hours at 22°C. The process can be conveniently considered in three major stages (Fig. 1). The first, including zygote formation and early cleavage, establishment of the embryonic axes, and determination of the somatic and germ-line founder-cell fates, takes place during the first 2 hours after fertilization. The second, including gastrulation, completion of most cell proliferation, and the beginning of cell differentiation and organogenesis, continues until about halfway through embryogenesis. The third stage, including morphogenesis as well as completion of embryonic cell differentiation and organogenesis, occupies the remainder of embryogenesis and concludes with hatching. Early embryogenesis normally (although not necessarily) takes place in the uterus; eggs in healthy well-fed hermaphrodites are normally laid during gastrulation, at 2–3 hours after fertilization.

II. DESCRIPTION OF CELLULAR EVENTS
A. Stage 1: Zygote Formation and Establishment of Embryonic Axes
Mature oocytes pass from the oviduct into the spermatheca and become fertilized by fusion with sperm...