Decidua

Decidua is the term for the uterine lining (endometrium) during a pregnancy. It is formed under the influence of progesterone and serves to support and interact with the gestation. The decidua represents the maternal portion of the placenta.

Etymology

The word comes from the Latin deciduus, meaning falling off or shedding.

Background

After ovulation, in mammals, the endometrial lining becomes transformed into a secretory lining in preparation to accept the embryo. Without implantation the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle). With implantation the lining now termed decidua evolves further during the pregnancy. The decidua is shed during the parturition process.

Structure

Different layers of the deciduas have been described, a compact outer layer (stratum compactum), an intermediate layer (stratum spongiosum), and a boundary layer adjacent to the myometrium. That part of the deciduas that interacts with the trophoblast is the decidua vera (“true decidua”), the remainder of the decidua is termed the decidua parietalis. The decidua has a histologically distinct appearance displaying large polygonal decidual cells in the stroma. Formation of a specialized decidua is called decidualization which is a special property of endometrium seen only in hemochorial placentation. Decidualization includes the process of differentiation of the spindle shaped 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 lareg granular lymphocytes(CD56 bright) are called "uterine NK cells" or uNK cells in Mice and decidual NK cells or dNK cells in humans. The border to the trophoblast is called Nitabuch’s layer.

Role

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. Pregnancy protein 14 (PP-14) also called placental protein 12 and Insulin like growth factor binding protein 1(IGFBP1) appears to be a specific product of the secretory and decidual lining. Other factors released include interleukin-15, vascular endothelial growth factor (VEGF), and insulin-like growth factor binding proteins. 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 women may shed the lining in the form of a decidual cast which may be mistaken as a miscarriage, when, in fact, the ectopic still persists.
A decidual reaction can be observed in tissue of the peritoneum and ovary during a pregnancy and represents a response of stromal tissue to progesterone.

External links

Anatomy | Obstetrics