[
1983]
Most multicellular eukaryotes posses a distinct group of germ-line cells that produces oocytes in one sex and sperm in the other. The production of adult germ cells appears to involve several developmental steps. First, during early embryogenesis, one or a few cells are committd to become germ precursor cells. Secondly, after a period of proliferation, some or all germ line descendants of the germ precursor cell leave the mitotic cell cycle and enter meiotic prophase. Thirdly, the meiotic germ cell matures as either a sperm or an oocyte. In this paper, I will review our knowledge of how each of these steps might be controlled in the small non-parasitic soil nematode, Caenorhabditis elegans.
[
1979]
In many invertebrates, cell lineages are apparently invariant from individual to individual. A given precursor cell follows a specific pattern of cell divisions, and its descendants follow fates that correspond to their respective positions in the lineage tree. Such a reproducible sequence of events provides an excellent system for studying how cells come to pursue particular fates during development. We have been interested to know if a cell's fate is specified by factors intrinsic to the cell, or if it is influenced by interactions between the cell and its environment. C. elegans is a particularly suitable organism for lineage studies because it is transparent throughout its life cycle, and because it consists of relatively few cells. Furthermore, C. elegans is a favorable organism for genetics, so the control of cell lineages can be studied by characterizing mutations that are defective in known lineages. The cell lineages of C. elegans have been described in the embryo to the 182 cell stage and after hatching. Approximately 50 cells resume divisions post-embyronically. In the somatic tissues, the number of cells (or nuclei) is increased from about 550 to about 950 in hermaphrodites and to about 1025 in males. These post-embryonic lineages are essentially invariant from worm to worm. As the worm enlarges and matures sexually, cells (or nuclei) are added to previously existing tissues (hypodermis, muscle, gut, and nervous system), and structures necessary for reproduction are elaborated. The latter include a gonad in both sexes, a vulva in hermaphrodites, and a tail specialized for copulation in males. This paper summarizes the results of laser ablation experiments performed on cells in the post-embryonic lineages of C. elegans. In particular, we focus on those experiments that demonstrate a regulative capacity in the cells of this predominantly invariant system. The post-embyronic lineages have the practical advantage for these studies that they can be traced by direct observation of the cells as they divide and assume their final fate. The regulative response, therefore, can be described at a level of cellular detail that has not been possible in other deletion studies. Our aim in performing these experiments is to infer how cells are controlled during normal development from their behavior in