Decidua
The decidua is the modified mucosal lining of the uterus (that is, modified endometrium) that forms in preparation for pregnancy. It is formed in a process called decidualization under 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 word deciduous. Structure. The part of the decidua that interacts with the trophoblast is the "decidua basalis" (also called "decidua placentalis"), 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. The decidua has a histologically-distinct appearance, displaying large polygonal decidual cells in the stroma. These are enlarged endometrial stromal cells, which resemble epithelium (and are referred to as "epithelioid"). 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 (similar to epithelial cells). 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 (CD56 bright) are called "uterine natural killer cells" or "uNK cells" in mice, and "decidual NK cells" or "dNK cells" in humans. Development. After ovulation, in placental mammals, the endometrial lining becomes hypertrophic and vascular under the influence of the sex hormones, estrogen and progesterone. In animals exhibiting hemochorial placentation, the endometrium undergoes decidualization following implantation. If implantation does not occur, the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle). The decidua is shed with the placenta during birth. Function . 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 (IGFBP1) also called placental protein 12, and PAEP (glycodelin) appear to be specific products of the secretory and decidual lining. Other factors released include interleukin-15 and vascular endothelial growth factor (VEGF). A reasonable understanding of the role and interplay of these hormones and factors has not been evolved. Clinical significance. A long-lasting infection of the decidua, chronic deciduitis, is associated with pre-term labour. History. The word comes .