[Fragmentation of polyploid nuclei in the giant cells of the rat trophoblast. I. The ultrastructure of the nuclear fragments].

Electron microscope study of the nuclear fragments in the rat trophoblast has demonstrated that the division of the trophoblast giant nucleus results first in the formation of a multinuclear cell. Each nuclear fragment is covered with its own nuclear envelope made of two membranes with numerous pore complexes. The chromatin in these nuclear fragments is condenced with various degrees of condensation, which depends on the step of placenta development, cell differentiation and the degree of nuclear fragmentation. The nuclear ultrastructure in nuclear fragments also depends on the degree of nuclear fragmentation and on the level of chromatin condensation. The nucleolus has no granular component. On large fragments, with lower chromatin condensation the nucleolus is not homogenous being made of fragments of more and of less electron dense fibrilles. Small light lacunae are seen in the nucleolus where chromatin threads and strands pass on. With a high chromatin condensation in the nucleus, round small nucleoli look homogenous being made of moderately electron dense fibrilles. Products of chromosome activity have been found in the nuclear fragments: accumulations of minute granules (d = 15--20 nm), perichromatinous granules (d = 35--40 nm), and fibrillar nucleolus-like bodies. In the multinuclear cell, made as the result of fragmentation of the initially giant nucleus, all the small nuclei are first arranged very close to each other, so that the contours of the neighbouring nuclei coincide.