The course of endomitosis in human cells.

The course of endomitosis in human hydatidiform moles has been analyzed. It differs from the classical description of endomitosis in that endoprophase is completely missing and, very probably, so is a typical interphase. The chromosomes are even less synchronized in their replication and condensation cycle than in normal mitosis. At no point do all chromosomes decondense, but a part remains condensed while others are extended and in the process of synthesizing DNA. Even two paired sister chromosomes may replicate nonsynchronously. The latest replicating chromosome is usually a large darkly staining chromosome, which we tentatively identify as the inactive X. No DNA synthesis takes place during "endometaphase" or "endoanaphase" stages, when the chromosomes are most condensed. Some polyploid "endoanaphases" or "endotelophases" with stretched out chromosomes obviously represent end-stages of the endomitotic pathway, and the nuclei are in the process of reverting into evenly stained nuclei. In some "endometaphases," a near-haploid number of chromosomes can be counted. In others, the endochromosomes seem to be compound structures consisting of several chromosomes that have not separated during the previous endomitoses. This is seen also in normal trophoblast and cervical cancer. In large cancer cells, such bundles can be seen in the process of falling into individual chromosomes.