An experimental study of cell death in the developing trochlear nucleus

Abstract

The development of the trochlear nucleus in the absence of its peripheral field of innervation was studied, from Day 7 of incubation age through hatching, in duck embryos ( Anas platyrhynchos). The optic primordium and the surrounding mesoderm were removed, usually on the right side, on the fourth day of incubation. Data on cell number and size on Day 12, when the nucleus attains maximum number of cells, indicate no significant differences between the control and the affected nucleus. Removal of the superior oblique muscle thus has no effect on proliferation and differentiation of the cells till Day 12. There is, however, a 50% cell loss in the control nucleus between Days 13 and 21. An additional hypoplasia in the affected nucleus is observed from Day 13 to hatching. Magnitude of cell loss in the affected nucleus varies from 85% loss to a virtual absence of the cells at hatching. Two morphologically distinct cell types, e.g., large well-differentiated, and small relatively undifferentiated cells are present in the control and the affected trochlear nucleus during the early period of morphogenesis. Cell death in the control is limited to the small cell population. It is suggested that the normally occurring cell death involves those cells (small cells) unable to send axons to the periphery to make connections. The additional hypoplasia in the affected nucleus is probably due to the failure of the growing axons to make appropriate synaptic contacts with the periphery.