Although somites develop from the mesoderm in the tail of the chick embryo, they do not form to the tip of the tail. Previous work has shown that this terminal mesoderm possesses many of the characteristics of the segmental plate mesoderm which gives rise to the somites in the trunk. This investigation is aimed therefore at understanding why the terminal mesoderm fails to form somites.
Mitotic and pyknotic rates have been obtained for the tail region of chick embryos between stages 13 and 27. Embryos were treated with colchicine, so that the mitoses were blocked in metaphase, and counts were made on serial sections. The overall mitotic rates were highest between stages 15 and 18. Regions of high mitotic rate, which are an indication of cell synchrony, were found in the tail bud mesoderm though not in a consistent location, and only infrequently ner the anterior end of the tail segmental plate. In the trunk however (Stern and Bellairs 1984) a single peak of cell synchrony was routinely found near the cranial end of the segmental plate. It is concluded that the cells of the tail mesoderm are less synchronised in preparation for somitogenesis than are the corresponding mesoderm cells in the trunk. A further conclusion is that the tail bud is not per se a region of high proliferation, though there are patches of high mitotic rate.
The overall pyknotic rate reached a maximum at stage 25; peaks of pyknosis corresponded initially with the mitotic peaks and were associated with the ventral ectodermal ridge and the tail gut. By stage 25 however, the high levels of cell death were restricted mainly to the tip of the tail. It is concluded that cell death radually predominates over mitosis in the region of prospective somitogenesis. It is suggested moreover that the cells in the ventral ectodermal ridge may play a role in pattern formation similar to that of the apical ectodermal ridge in the limb.
Tail mesoderm was dissected from embryos at stage 13 (prior to cell death), and stage 16 (when patches of cell death were already established), and explanted in vitro. The cumulative cell death which occurred within the tissue from stage 13 was significantly less that from stage 16. It is concluded that the cells destined for death may be at least partially “rescued” if isolated in vitro.