Electron microscopic study on early decidualization of the endometrium of pregnant mice, with special reference to gap junctions.

Cytological changes in mouse decidual cells during the early stages from 4.5th to 11th day of pregnancy were examined by electron microscopy with special reference to gap junctions. The most noticeable feature of decidual cells was that there was an abundance of various-shaped gap junctions such as flat, undulated, omega-shaped, and annular-shaped gap junctions. Serial sectioning revealed that annular-shaped gap junctions were separated from the decidual cell plasma membrane. Intramembranous particles (IMPs) of the gap junctions as seen by freeze-fracture displayed a well-packed arrangement and a homogeneous size. Degenerative changes first appeared in decidual cells directly contiguous with trophoblasts on the 7th day, and spread to the deeper area as clearly seen on the 8th day. Along with degeneration of decidual cells, annular-shaped gap junctions were enfolded and gradually degraded by lysosomes. IMPs of such degenerative gap junctions were irregularly arranged and heterogenous in size. The decidual tissue became thin with elongated decidual cells as the placental disc grew on the 10th day. Extremely long undulated gap junctions were present in decidual cells, and a small number of annular-shaped gap junctions were also found in the cytoplasm. The presence of abundant gap junctions as demonstrated in the present study suggests that decidual cells will be able to develop as well as to function synchronously. In addition, it is suggested that redundant gap junctions become annular-shaped to be finally degraded by lysosomes.