The temporal appearance of sex chromatin and of the late-replicating X chromosome in blastocysts of the domestic rabbit

Abstract

It is demonstrated that sex chromatin appears in cells of pre-implantation blastocysts of the domestic rabbit over a period from approximately 96 to 120 hours after mating of the parent animals. We are unable to confirm the observation of Melander (1962) that chromatin bridges between separating masses of daughter chromosomes at anaphase (‘anaphase bridges’) are present only in those blastocysts in which sex chromatin begins to appear. The interrelationships between nuclear size, mitotic index and the appearance of sex chromatin were studied. The sequence of DN A replication of the sex chromosomes was investigated in adult rabbit tissues and in rabbit morulae and blastocysts after, during and before the period during which sex chromatin appears for the first time. It is apparent that a late-replicating X chromosome is present in morulae before sex chromatin can be identified. Although the frequency of ceils showing a late-replicating X chromosome is correlated with stage of development, the rate of change of frequency with stage of development differs from that observed for the frequency of sex chromatin. It is suggested that the phenomena of late replication and of X-chromosome condensation in interphase are not as closely related as has previously been thought; that neither produces the other but, rather, that each is an independent effect of some physiological activity of the cell which is at present little understood.