deciduum

The deciduum is a specialized layer of the uterine endometrium that forms during pregnancy. It develops after implantation of the blastocyst, which is the early-stage embryo. The deciduum serves as the maternal contribution to the placenta and is essential for the successful nourishment and development of the fetus. It is characterized by significant changes in the uterine lining, including increased vascularization and the presence of decidual cells, which are modified endometrial stromal cells.

There are three main regions of the deciduum, distinguished by their relationship to the implanted blastocyst. The decidua basalis lies directly beneath the blastocyst and becomes the maternal part of the placenta. The decidua capsularis encloses the developing embryo and is located on the surface of the blastocyst, facing the uterine cavity. The decidua parietalis lines the rest of the uterine cavity, excluding the areas occupied by the decidua basalis and capsularis. As pregnancy progresses, the decidua capsularis fuses with the decidua parietalis, obliterating the uterine cavity. At the time of birth, the deciduum, along with the placenta, is expelled from the uterus, a process commonly referred to as afterbirth. The deciduum plays a crucial role in maternal immune tolerance, preventing the rejection of the semiallogeneic fetus.