Decidualization

Decidualization is a process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy. This includes morphological and functional changes to endometrial stromal cells (ESCs), the presence of decidual white blood cells (leukocytes), and vascular changes to maternal arteries. The sum of these changes results in the endometrium changing into a structure called the decidua. In humans, the decidua is shed during the third phase of birth.

Decidualization plays an important role in promoting placenta formation between a mother and her fetus by mediating the invasiveness of trophoblast cells. It also triggers the production of cellular and molecular factors that result in structural changes, or remodeling, of maternal spiral arteries. Decidualization is required in some mammalian species where embryo implantation and trophoblast cell invasion of the endometrium occurs, also known as hemochorial placentation. This allows maternal blood to come into direct contact with the fetal chorion, a membrane between the fetal and maternal tissues, and allows for nutrient and gas exchange. However, decidualization-like reactions have also been observed in some species that don't display hemochorial placentation.

In humans, decidualization occurs after ovulation during the menstrual cycle. After implantation of the embryo, the decidua further develops to mediate the process of placentation. In the event no embryo is implanted, the decidualized endometrial lining is shed or, as is the case with species that follow the estrous cycle, absorbed.

After ovulation, the high levels of progesterone initiate the molecular changes leading to decidualization. The process triggers an influx of decidual leukocytes along with morphological and functional changes of ESCs. The changes in the ESCs result in the endometrium developing a secretory lining that produces a variety of proteins, cytokines, and growth factors. These secreted factors will regulate the invasiveness of trophoblast cells that eventually form the placental connection if an embryo implants into the decidua.

...
Wikipedia