Ultrastructural relationships between decidua, trophoblast and lymphocytes at the beginning of human pregnancy.

Between the 20th and 40th day of pregnancy, human decidual cells in the endometrial connective tissue come into direct contact with the trophoblastic cells eroding the stromal tissue. Fibrillar deposits of maternal origin were randomly distributed between the maternal and fetal sites and did not form a continuous barrier. Structural features were prominent in the maternal and embryonic cells. Large decidual cells were intermingled with trophoblastic cells, and there was little intercellular material. Small developing decidual cells arose around the maternal precapillary arterioles and capillaries and were closely accompanied by lymphocytes. Fully developed decidual cells exhibited a fine structure characteristic of a state of active secretion, and trophoblastic cells gave structural indications of producing protein. The aggregates of decidual and trophoblastic cells in the human endometrium during early pregnancy ('deciduotrophoblastic complex') may function as a local humoral regulator releasing chemical factors and partaking in the establishment of immunological privilege at implantation.