Appearance of tonofilaments and absence of microtubules in human endometrial glandular epithelium. A function of estrogen activity.

In 1966, Cavazos et al.(1) carried out electron-microscopic studies of human endometrium and reported the presence of microtubules in glandular epithelial cells. This study presents evidence that no microtubules are evident at any stage of the menstrual cycle. Instead, there are numerous tonofilaments that increase in number, forming bundles that relate to the formation of new desmosomes. They are under the influence of both endogenous and exogenous estrogen and dissipate under the influence of progesterone. This feature may be of submicroscopic importance in stabilizing the rigidity of cells prior to preparation for nidation under the influence of progesterone.