Ultrastructural observations of the human uterine smooth muscle cells during gestation.

At term pregnancy, the myometrium consists of bundles of smooth muscle cells bound together by varying amounts of connective tissue. Each bundle contains both dark and light muscle cells. During uterine contractions it is believed that the smooth muscle cells become darker, decrease in volume, and exhibit changes in diameter. This is accompanied by widening of the interspaces and by a decrease in the areas of cellular contact. Between contractions, there are more light cells which become arranged closer to each other and exhibit large areas of interdigitation. The significance of these observations in the mechanism of uterine contraction and retraction is discussed. Cell believed to be modified smooth muscle cells occupy the myoendometrial junction and the decidua basalis. They are irregular in shape, poor in myofilament content, and rich in other cytoplasmic organelles and form a loosely arranged layer of cells between the myometrium and the trophoblast. The possible functional significance of these cells is also discussed.