Properties of the primary organization field in the embryo of Xenopus laevis: IV. Pattern formation and regulation following early inhibition of mitosis

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ABSTRACT Visual observation of early blastulae, and counting of cell suspensions from the late blastula and subsequent stages of Xenopus development, have shown that exposure to either colcemid or mitomycin C can sustain blockage of the mitotic cycle, and hence prevent the normal increase in cell number. Such blockage is essentially complete within a period, following introduction of the drugs, which is short compared with the cell-cycle time normal to post-blastular stages. Although the cell-counting data do not allow a statistical demonstration, there is a suggestion that mitomycin C allows a greater proportion of cells to divide once, after its introduction, than does colcemid. This is in accord with its putative mode of action, in causing only intrachromatid cross-linkage within chromosones, but there are no data on the status of DNA replication within nuclei of the non-dividing cells under either drug. If blockage is begun not earlier than stage 10+, some 20 min after dorsal lip formation, essentially normal morphogenesis ensues up to tail-bud stages around 27, including histodifferentiation of ectodermal structures and notochord, and twitching of somitic muscle. Such embryos are described. In describing those arresting at earlier stages following blockage in blastulae, the possibility is mentioned that morphogenesis may fail for mechanical reasons due to low cell number and large cell size, rather than to lack of a normal field as such. Cell counting reveals that blocked embryos with qualitatively normal pattern formation by stages 22-24, in having a total cell number appropriate to stage 10+ or 10J, show about a seventh or an eighth of the number of cells normal to their stage. Between then and stage 27 when histogenesis, particularly of twitching muscle, is more clearly seen, the direct method of estimating cell number can no longer be used, but histology reveals no mitotic metaphases in any blocked embryos. In the same experiments, operations of two types described in previous papers are performed on early gastrulae so that regulation is required in the embryonic field after the cessation of mitosis. The results are as seen in the normal presence of the cell cycle, revealing that, in response to changes of positional information value, host material may change its presumptive differentiation tendency and final commitment, in the absence of cell division as such. There is some indication that the proportional numbers of cells assigned to different structures in an individuation field may deviate from normal when overall cell number is limiting.