Worm Breeder's Gazette 1(1): 15
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
After hatching, the number of somatic cells in the hermaphrodite of C. elegans increases from about 530 to about 780. The precise lineages which lead to this increase in cell number have been determined by direct observation of developing nematodes. Young worms are mounted on a thin block of agar on a glass slide under a cover slip which contains a small amount of E. coli spread at its center; the edges are sealed with silicone grease or Vaseline to prevent dehydration. When so mounted, the young nematode develops normally and can be observed in the light microscope. Under Nomarski differential interference contrast optics, which detects refractive index variations, nuclei (and often nucleoli) are very distinct and can be followed as they migrate, divide, and, in some cases, die. Using this technique to observe nuclei in developing larvae, we have determined all of the somatic cell lineages which occur after hatching. All tissues in the nematode undergo some post-embryonic divisions. Hypodermal cell divisions occur around the time of the first three larval moults; six cells on each lateral side of the newly-hatched animal follow the same basic program of divisions. (Two of these 12 cells modify this program to produce most of the posterior lateral ganglia as well as hypodermal cells). Two additional hypodermal blast cells on each side of the head follow a different program. Another blast cell on each side of the tail produces both hypodermal and nerve cells. Most of the intestinal cells divide once, just before the first moult. Substantial mesodermal development derives from a single cell present in the newly hatched larva; a series of divisions during the first larval stage produces 14 somatic muscle cells, 2 non-muscle cells (probably secretory in function) and 2 myoblasts; these myoblasts migrate anteriorly until they reach the position where the vulva will form; they divide just before the third moult to produce 16 sex muscles, which function in egg-laying. Much of the neuronal development occurs in the ventral nerve cord and its associated ganglia. At hatching only 33 neurons are present in these structures, but divisions of 12 precursor cells and one neuroblast (followed by a number of programmed cell deaths) increase this number to 89. Each of the 12 precursors divides to give a hypodermal cell and a neuroblast. All 13 neuroblasts then follow the same asymmetric program of divisions. Each of 5 of the distinct anatomical classes of neurons in the adult ventral cord seems to be derived from the equivalent progeny cells of the 13 neuroblast lineages (e.g. the posterior daughter of the posterior daughter of each neuroblast becomes a 'Class D' neuron.) Three of the 12 new ventral cord hypodermal cells divide further just before the third moult to produce the 22-cell vulva. A number of other cell lineages also occur in the hermaphrodite. In the male, this developmental program is altered. The number of somatic cells in the male increases from about 530 to about 950; the extra cells are mostly derived from new lineages which occur in the tail. In addition, some of the somatic lineages of the hermaphrodite are modified to generate male-specific structures. For example, the male mesodermal lineage produces 14 somatic muscle cells, one non-muscle cell, and 41 male specific muscles which control the tail before and during copulation. In the male ventral cord there are fewer programmed cell deaths and an extra round of cell divisions. Two posterior ventral cord hypodermal cells divide to form part of the male tail, whereas the three related cells which produce the vulva in the hermaphrodite do not divide. We have begun to isolate and characterize mutants which affect these cell lineages. Some of these mutants display general defects in postembryonic cell division and, because of their aberrant gonadal development, produce sterile animals when homozygous. Other mutants show more specific effects, such as blocks in the ventral hypodermal lineage which produces the vulva. One mutant is generally deficient in ectodermal divisions, with both the ventral and lateral hypodermis and part of the male tail affected. Some mutants show cell divisions which do not normally occur in wild-type individuals; in one, for example, extra posterior lateral neurons are generated. Two genes affect the migrations of the ventral cord precursor cell nuclei which normally precede their divisions. Using both mutants and the laser system developed by John White to disrupt the normal cell lineages, we hope to learn more about the regulation of these developmental processes.