Worm Breeder's Gazette 5(1): 29
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 have continued experiments on the cleavage patterns of partial embryos and isolated blastomeres obtained by crushing early N2 eggs in embryonic culture medium supplemented with Ascaris coelomic fluid (W.B. G. June 1978). Neither isolated somatic precursor cells (blast cells) nor isolated P cells give rise to the spatial patterns formed by their progeny in intact embryos. However, all our results so far are consistent with the following simple hypothesis. The blast cells, and their progeny, are internally determined to cleave in a simple helical pattern. The P cells are internally determined to cleave linearly, maintaining the spindle orientation of preceding P-cell divisions parallel to the anterior-posterior axis. In the intact embryo, interactions of cells with each other (and perhaps with the eggshell) combine with these two intrinsic cleavage patterns to dictate normal cellular organization, which therefore depends upon both extracellular and intracellular cues. Cleavage of an isolated P2 blastomere results in an approximately linear chain of cells in the order C, C, D, P4. This result, together with the approximately anterior-posterior orientation of the preceding cleavages that give rise to the AB, EMSt, E, and MSt cells suggests the possibility that in the zygote, determinants for blast-cell primary differentiation are arranged along the posterior-anterior axis in the order C (ectoderm), D (mesoderm), P4 (germ line), E (endoderm), MSt (mesoderm), AB (ectoderm). This arrangement corresponds to that of a fate map proposed for the Ascaris des zygote by Zur Strassen (1896). It differs from that of the fate map proposed for the C. elegans zygote by Schierenberg (1978), who postulated the order P4, D, C, E, MSt, AB based on observations of division orientation in intact embryos,cytoplasmic streaming,and cell movements during early cleavage.