Chromosomes of mouse oocytes in maturation: differential trypsin sensitivity and amino acid incorporation.

In the period of maturation in vivo, the chromosomes of mouse oocytes display a spectrum of unique configurations that is postulated to be related to a sequence of turnover of chromosomal proteins. Evidence on behalf of that hypothesis is provided by the following cytologic observations: The chromosomes of the diakinesis-metaphase I complement are resistant to disruption by mild treatment with trypsin. Following metaphase I, the chromosomes become exceedingly compact and display correlated increased resistance to trypsin. At telophase I, when the complements of the secondary oocyte and the first polar body have each coalesced into a “chromatin mass,” the chromosomes are greatly sensitive to trypsin. Following separation from the mass, the definitive oocyte chromosomes decompact into a “relaxed coil” conformation and display moderate trypsin sensitivity comparable to that of mitotic metaphase chromosomes. Autoradiography of [³H]-arginine and [³H]tryptophan incorporation show that while both amino acids are incorporated into the ooplasm, arginine, but not tryptophan, is incorporated into the chromosomal material. Analysis of the data indicates that incorporation takes place as two separate events, one in late dictyotene and the other post-telophase I and that the arginine-containing proteins incorporated into the dictyate chromosomes are transient and are not retained on the metaphase II chromosomes.