Worm Breeder's Gazette 2(1): 12
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.
We are continuing our study on the development of C. elegans, concentration on embryogenesis. Cell lineage on living eggs has been followed past the 170-cell stage, using Nomarski optics and recordings on video tape. Selected cell types have been followed further, including the gut cells taking advantage of their large size and characteristic autofluorescence. We were able to identify cells uniquely by recording cell division, position of nuclei and cell size. Mitoses proceed in waves from anterior to posterior. From the 16-cell stage onwards groups of cells are clearly distinguishable. The members of each group form a clone. Again, these groups of cells divide largly sequentially from anterior to posterior. Within each group, cell division is essentially synchronous. Every complete wave starts with division of the AB descendants (classically designated Ectoderm), strictly alternating with the mitoses of the P1 descendants (classically called EMST, C and D). After four rounds of cell division a new cycle is initiated before the previous one has been completed. The length of the cell cycle in early cleavage is 15 to 20 minutes (at 22 C). The cycle lengthens gradually to 30 to 40 minutes at the 170-cell stage. We did not observe any cell death in the stages investigated. A coeloblastula with 2-3 very small cavities has been formed at the 24- cell stage. Cleavage and the formation of the three germ layers are not clearly separated in time. Gastrulation starts after the 24-cell stage when the two precursor cells of the endoderm begin to slide into the center. This movement is complete at the 51-cell stage. This type of gastrulation, known as morula delamination, is different from the types of gastrulation so far known in nematodes, as listed in Chitwood. Morula delamination also occurs in some Cnidaria (Clava Laomedea etc.) and also in some insects (e.g. some Collembola). Between the 24- and the 170-cell stage, bilateral symmetry as has been described for Ascaris is not apparent. In the fertilized egg the male and the female pronuclei meet in that part of the egg which at the 2- cell stage becomes the smaller P1-cell (situated at the posterior end). This allows early assignment of the anterior-posterior axis of the egg. The dorso-ventral assignment is possible at this stage by the asymmetry of the membrane of the first cell division. This membrane begins to form dorsally and the final hole in this 'asymmetric iris' closes well in the ventral part of the egg. So far 14 eggs in various stages have been serially sectioned for EM and are being reconstructed by hand or by computer for correlation with and extension of the Nomarski results. The fixation-embedding procedure works on all stages of embryogenesis, including prehatching. Computer capabilities have been increased this year by major hardware additions and by the installation of a new operating system ( RSX). Special attention is being given to computer-aided reconstruction of the embryo from EM series, from video film of light microscopic data, and to the correlation of the EM and LM data. Major enhancement in computer capabilities are in process along two lines. On the one hand the mechanical details associated with the recording process will be made as simple as possible for the user. On the other hand the analysis of the data involving abstraction and simplification will become much more efficient by organizing and storing the data in a 'tree-structured' data base. This will facilitate study not only of the cellular anatomy but also of subcellular structures. For example, we have begun to analyze the distribution of mitochondria in eggs and early cleavage stages. One ultimate goal is to display the important events in embryogenesis in a highly schematized movie on the screen of the graphics display. Some 10,000 EMS mutagenized clones have been tested for temperature sensitive mutants by methods similar to those of Hirsh and Vanderslice (1976), except that about half of the clones came from mutagenized hatchees. Over 100 ts mutants have been selected (designated ts G1, ts G2 etc. according to David Hirsh's nomenclature proposal, where G denotes Gottingen), including some 20% embryological arrest. Our first interest in these mutants is microscopic characterization. Dr. Johji Miwa joined the group at the end of the summer.