[Ultrastructural observations on human endometrial stromal cells during the normal menstrual cycle--with special reference to so-called "predecidual cells"].

Human endometrial stromal cells during the normal menstrual cycle were observed with the transmission electron microscope, the scanning electron microscope and the freeze-fracture replica method. In the early proliferative phase, the stromal cells had the characteristics of mesenchymal cells but in the early secretory phase, they resembled fibroblasts. In the late secretory phase, the stromal cells (so called "predecidual cells") were characterized by their large size, rounded shape, and their abundant cytoplasms with well-developed organelles. Furthermore, "predecidual cells" also had the following conspicuous features: 1. In the cytoplasms numerous membrane-bounded collagen fibers were seen, and positive acid-phosphatase activity was detected in these fibers. 2. Neighbouring "predecidual cells" were often joined to each other by "gap junctions". 3. "Predecidual cells" had many "coated regions" on their cytoplasmic membranes. Structural changes in endometrial stromal cells were discussed in regard to the function of these cells.