The tubular system of human endometrium.

The so-called "intranuclear" tubular system (TS) of human epithelial endometrial cells has been investigated in 50 specimens and has been found in some instances of secretory endometrium and in one case of hyperplastic endometrium with secretory change. The TS was found both in nucleus and cytoplasm. The intranuclear TS can be associated with the nucleolus, connected to the nuclear envelope or lying free in the karyoplasm. When it is connected with the nuclear envelope an intimately related cytoplasmic invagination containing many ribosomes is frequently found. The cytoplasmic TS is smaller than that found in the nucleus and is usually connected with rough endoplasmic reticulum profiles. These findings allow us to make the suggestion that the TS could have a cytoplasmic origin and subsequently migrate into the nucleus. The function of the TS is unknown.