Ultrastructural changes in human endometrium at different phases of the menstrual cycle and their functional significance.

A review of the ultrastructure of normal human endometrium at different phases of the cycle is presented. The structural changes are correlated to the functional implications in the light of the recent knowledge on the functioning of cell organelles. Relatively undifferentiated cells of the beginning of the menstrual phase become functionally matured, proliferate and later degenerate. The appearance of the nucleolar channel system during the secretory phase coincides with the rise of progesterone level and facilitates the nucleocytoplasmic exchange. The secretion of the endometrial glands consists of glycogen, glycoproteins, lipids and mucopolysaccharides. Large autophagic vacuoles (giant lysosomes) digest various cell components at the end of the cycle. All the lamina functionalis of the endometrium is not shed, a part of it is regressed by focal death (autophagic vacuoles) and by total death (apoptosis) of individual cells.