Nucleolar channel system in the human fetal endometrium.

Endometrial tissue from 3 human fetuses was examined electron microscopically. In 2 of the fetuses the nucleolar channel system was found within the nuclei of the endometrial epithelium. This finding demonstrates that the development of the system is not dependent on ovulation. We speculate that an appropriate concentration ratio between estrogen and progesterone might induce the development of nucleolar differentiation.