Cell Lineage in the Development of the Leech Nervous System

Abstract

The intricate structure and function of the adult nervous system is the result of developmental interactions of factors both intrinsic and extrinsic to the embryonic neurons and their precursor cells. To fathom the mechanisms underlying these interactive processes, a detailed knowledge of the course of neurogenesis at the cellular level is essential. Once such knowledge is available, specific and well-focused questions can be formulated at the biophysical, biochemical, or genetic levels. One key aspect of the process of neurogenesis at the cellular level is cell lineage, i.e., the embryonic lines of descent of various types of neurons. The importance of cell lineage for understanding developmental processes was realized over a century ago by C. O. Whitman.¹ On the basis of his studies of the development of leeches, Whitman put forward the idea, then quite novel, that each identified cell of the early embryo, and the clone of its descendant cells, plays a specific role in later development. Cell lineage analyses were later extended to the embryos of other species, not only by direct observation but also by use of other techniques, such as selective ablation, application of extracellular marker particles, and, most importantly, production of chimera and genetic mosaics.^2–10 More recently, we have refined and extended Whitman’s century-old cell lineage studies in leech embryos, with particular emphasis on the cellular origins of the leech nervous system. As will be seen in this chapter, leeches are well suited for cellular investigations of neuronal development because both their early embryos and their adult nervous systems comprise identifiable cells accessible to experimental manipulation.