Chromatid behavior in late mitosis: a scanning electron microscopy analysis of mammalian cell lines with various chromosome numbers.

Chromatid activity during the process of nuclear reformation following metaphase is a period of mitosis where little precise information is available. Nuclear reformation requires that chromosomes, at metaphase and chromatids during anaphase and telophase align, position and associate in a clearly defined sequence to insure the specific design of each nucleus. Four cell lines with chromosome numbers ranging from seven to almost seventy were chosen to determine whether the process of nuclear assembly is the same throughout. Chromosomal alignment at metaphase is found to be radial in all four cell lines. Chromosome positioning is essentially the same in all four, where the smaller chromosomes are located centrally and longer ones are positioned peripherally in a radial alignment. Chromosomal association is directly related to chromosome number. The more chromosomes in a one dimensional plane occupying a given area, the closer the association. In comparing the HeLaS3 and muntjac chromatids, the former has the closer association at metaphase. Since association is the most important aspect of chromatid behavior in nuclear reformation, chromatid positioning becomes a vital process during anaphase movement. Chromatid positions established during anaphase determines later positioning in the interphase nucleus because of the subsequent interconnection of adjacent chromatids by the formation of a fibrous meshwork. This fibrous meshwork, formed in anaphase and early telophase, functions to stabilize chromatids following their positioning and it also serves as a substrate or matrix for the assembly of nuclear envelope.