The decidua is the modified mucosal lining of the uterus that forms in preparation for pregnancy. It is formed in a process called decidualizationunder the influence of progesterone. Endometrial cells become highly characteristic. The decidua forms the maternal part of the placenta and remains for the duration of the pregnancy. It is shed off during childbirth — hence why the term is used, "decidua" having the meaning of falling away, as in the worddeciduous.
Structure
The part of the decidua that interacts with the trophoblast is the decidua basalis, while the decidua capsularis grows over the embryo on the lumenal side, enclosing it into the endometrium. The remainder of the decidua is termed the decidua parietalis or decidua vera, and it will fuse with the decidua capsularis by the fourth month of gestation. Three morphologically distinct layers of the decidua basalis can then be described:
Within the decidua, occasional fibrinoid deposits form where the syncytiotrophoblast is damaged. The region of fibrinoid deposition where trophoblasts meet the compact portion of the decidua basalis is called Rohr's layer, while the fibrinoid deposits that occur between the compact and spongy layer of the decidua basalis is termed Nitabuch's layer. This layer is absent in placenta accreta. of decidualized endometrium due to exogenous progesterone. H&E stain. The decidua has a histologically-distinct appearance, displaying large polygonal decidual cells in the stroma. These are enlarged endometrial stromal cells, which resemble epithelium. Decidualization includes the process of differentiation of the spindle-shape stromal fibroblasts into the plump secretory decidual cells, which create a pericellular extracellular matrix rich in fibronectin and laminin. Vascularity, as well as vascular permeability, is enhanced in the decidualizing endometrium. Its leukocyte population is distinct, with the presence of large endometrial granular leukocytes being predominant, while polynuclear leukocytes and B cells are scant. The large granular lymphocytes are called "uterine natural killer cells" or "uNK cells" in mice, and "decidual NK cells" or "dNK cells" in humans.
As the maternal interface to the embryo the decidua participates in the exchanges of nutrition, gas, and waste with the gestation. It also protects the pregnancy from the maternal immune system. Further, the decidua has to allow a very controlled invasion of the trophoblast. In invasive placental disorders like placenta accreta decidualization have been consistently found to be deficient.
Hormone production
The decidua secretes hormones, growth factors, and cytokines. It has receptors for estrogen, progesterone, growth hormone, and others. Among its products are hormones commonly associated with other organs such as cortisol, CRF, GnRH, prolactin, and relaxin. Decidual prolactin is not under dopaminergic control. Insulin-like growth factor-binding protein 1 also called placental protein 12, and PAEP appear to be specific products of the secretory and decidual lining. Other factors released include interleukin-15 and vascular endothelial growth factor. A reasonable understanding of the role and interplay of these hormones and factors has not been evolved.
Other
In case of an extrauterine pregnancy, the endometrium nevertheless becomes decidualized. A woman may shed the lining in the form of a decidual cast, which may be mistaken as a miscarriage, when, in fact, the ectopic pregnancy still persists.
