Cell death in the development of the lateral motor column of the chick embryo.

Cell counts were made in the lumbar lateral motor column (l.m.c.) of chick embryos of 5.5, 6, 7, 8, 9, 12, 18 days of incubation and five days posthatching (n equal 68). Only nuclei with nucleoli were counted and corrections were made for double counting (Abercrombie, '46). The population attains a peak value of over 20,000 cells (corrected figure: over 17,000) at 5.5-6.5 days equal stages 28 and 29 (Hamburger and Hamilton, '51). The l.m.c. loses between 7,000 and 8,000 cells between days 6.5 and 9.5, (between stages 29 and 36). In other words, 60% of the population survive. A plateau of approximately 12,300 cells (corrected figure: 10,300) is maintained through five days posthatching. Massive cell degeneration was observed in 7- and 8-day embryos. Counts of distinctly pyknotic cells indicate that at least 5-6% of the total population is in the process of degeneration at any particular time. This figure is probably an underestimation; hence it is virtually certain that the depletion of the l.m.c. is due entirely to cell death. Arguments are preue to the failure of their axons to survive in a competition process at the periphery. Observations of the time pattern of muscle differentiation and their neurotization in the leg further endorse this hypothesis. However, it is not clear whether the axons compete for contact sites on muscle fibers or for a "trophic" agent.