Development of smooth muscle in the human fetal uterus: an ultrastructural study.

Prenatal development of uterine smooth muscle was studied by light and electron microscopy in specimens obtained from ten human fetuses between 12 and 40 weeks gestation. Light microscopical observation of transverse sections of the body of the uterus revealed that the outer, subserosal layer of elongated cells was distinguishable from 14 weeks. This layer was more cellular than the inner layer and increased its thickness with advancing age. Ultrastructurally, the mesenchymal cells of the uterus did not contain myofilaments until 16 weeks. At 18 weeks, however, spindle shaped cells in the outer layer had a few filaments with dense bodies and well developed organelles and were identified as immature smooth muscle cells. By 31 weeks, the cells in this layer developed into almost mature smooth muscle cells, which contained abundant cytoplasmic filaments, dense plaques and surface vesicles along the cell membrane, and an external lamina. Therefore, in the human fetal uterus, smooth muscle differentiation begins at about 18 weeks and myometrium is formed in the outer layer of the wall by 31 weeks gestation. In the inner layer of the uterus that corresponds to the endometrial stroma, in addition to fibroblast-like cells, there were cells with plump cytoplasm which contained well developed granular endoplasmic reticulum and Golgi apparatus and which were identified around blood vessels by 26 weeks. These features resembled those of predecidual cells, and suggested the influence of sex steroids on the human fetal uterus.