Persistent Nucleoli in Early Postimplantation of Rat Embryos

Ultrastructural changes of the nucleolus in mitotic embryonic ectodermal cells of 7 1/2-day and 7 2/3-day rat embryos were examined. It was found that the nucleolus was broken down into small fragments during late prophase and metaphase, and that some of these fragments persisted in the cytoplasm of telophase cell (persistent nucleoli). No interphase embryonic ectodermal cells contained persistent nucleoli. Persistent nucleoli were also found in telophase cells of extraembryonic ectoderm, extraembryonic visceral endoderm and parietal endoderm of the embryos, but they disappeared in interphase cells. Persistent nucleoli in telophase cells tended to decrease in size with embryonic age, and they had almost completely disappeared in neuroectodermal cells of the telencephalon in 14 1/2-day embryos. They were concluded to be remnants of disappearing nucleoli in embryonic cells that were cycling too rapidly to permit their nucleoli to disappear completely.