The decidualized endometrium is a specialized transformation of the uterine lining, known as the endometrium. This change prepares the uterus for a potential pregnancy by altering its cells and structure to support an embryo. In humans, this process occurs monthly, regardless of conception, reflecting a preparatory state for reproduction.
The Endometrial Transformation
Decidualization involves structural and biochemical changes in endometrial stromal cells, which are fibroblast-like cells within the uterine lining. These cells differentiate, enlarging and transforming into specialized decidual cells. This transformation includes the accumulation of substances like glycogen and lipids within the cells.
Differentiation also changes the extracellular matrix, the network of molecules surrounding the cells, making the endometrium denser. Uterine glands become more specialized and tortuous, increasing their secretory activity. These cellular and glandular changes are accompanied by alterations in the blood vessels within the endometrium.
Hormonal signals initiate and maintain decidualization. Progesterone, produced after ovulation, plays a primary role in driving this process. Its interaction with estrogen, another ovarian hormone, regulates the timing and extent of these transformations.
This hormonal regulation triggers genetic and protein level changes within endometrial cells. This ensures the uterine lining is prepared for embryo implantation.
Functions in Early Pregnancy
If a pregnancy occurs, the decidualized endometrium serves multiple functions. It provides a receptive bed for embryo implantation, allowing the early embryo to adhere and embed within the uterine wall. The decidua produces factors promoting the attachment of trophoblast cells, the outer layer of the early embryo that will later form the placenta.
The decidualized lining also regulates the invasion of these trophoblast cells, ensuring the embryo embeds appropriately without over-invading maternal tissues. This controlled invasion is important for placental development. The decidua also provides initial nutritional support to the early embryo before the placenta is fully functional, with decidual cells filled with glycogen and fat for nourishment.
The decidualized endometrium also creates an environment that tolerates the semi-allogeneic embryo, which carries genetic material from both parents. It helps prevent the maternal immune system from rejecting the embryo. This involves recruiting and modulating specialized immune cells, such as uterine natural killer (uNK) cells and macrophages, at the maternal-fetal interface.
When Decidualization Goes Awry
When decidualization is inadequate or abnormal, it can contribute to complications. Issues with decidualization are associated with recurrent pregnancy loss and implantation failure. A poorly decidualized endometrium may not provide the necessary support or regulatory signals for an embryo to implant or for early pregnancy to progress.
Defective decidualization can also be linked to placental disorders. For instance, it may lead to shallow trophoblast invasion, seen in conditions like preeclampsia, where the placenta does not develop properly. This suggests the quality of decidualization can influence pregnancy health.
In some cases, decidual changes can occur outside the uterus, a phenomenon known as ectopic decidualization. This can happen in conditions like endometriosis, where endometrial-like tissue grows in other body areas. Ectopic decidualization can mimic malignancy and may contribute to reproductive problems experienced by individuals with endometriosis, including infertility.