Ultrastructure of the microvasculature in the human endometrium throughout the normal menstrual cycle.
In the early proliferative phase endometrial capillary basal lamina is loosely formed and discontinuous. It becomes more distinct in the late proliferative phase and develops whorled extensions that include pericytes in the early secretory to midsecretory phase. Throughout the cycle both endothelial cells and pericytes possess cytoplasmic processes that extend through discontinuities in the basal lamina and make contact with each other. These contacts may be simple or complex. They become more elaborate as the cycle progresses to the midsecretory phase. Endothelial cells show progressive but heterogeneous differentiation into the midsecretory phase, with individual cell hypertrophy. In the late-secretory phase the endometrial stroma and the basal lamina are characterized by widespread degeneration, and the cell-to-cell contacts sharply decrease. However, endothelial cells remain relatively intact.