The midblastula cell cycle transition and the character of mesoderm in u.v.-induced nonaxial Xenopus development

Xenopus embryos (UV embryos) resulting from u.v. (254 nm) irradiation to the vegetal egg hemisphere and thus developing little or no axial pattern (UV5-Scharf & Gerhart, 1983), have been compared histologically with synchronous normal siblings at each of three stages. In addition, the relative amounts of blood-forming tissue produced in normal and in UV embryos have been studied by Western blotting total protein from larval stages and by immunofluorescence on sections. The observations on midblastulae (around 5000 cells) were aimed at detecting any systematic retardation, due to u.v., of the slowing of the cell cycle that normally commences at the 2-4000 cell stage and makes possible zygotic transcription and the preparation for gastrulation. No such retardation was apparent. Observations on postgastrular stages gave an assessment of the size and character of the population of mesoderm founder cells, in relation to the control, for embryos visibly undergoing entirely nonaxial development. Little deficit in total mesodermal cell number was found, though the entire mesoderm adopted the histological character proper to only some 40% of that in the normal pattern i.e. trunk lateral plate. Blood-forming capacity appears to be enhanced out of all proportion to the size of the mesoderm as a whole. The results are discussed in terms of the probable nature of the primary positional system for axial pattern and the later mechanisms of mesodermal patterning.