Thedecidua is the modifiedmucosal lining of theuterus (that is, modifiedendometrium) that forms every month, in preparation forpregnancy. It is shed off each month when there is no fertilized egg to support.^[1] The decidua is under the influence ofprogesterone. Endometrial cells become highly characteristic. The decidua forms the maternal part of theplacenta and remains for the duration of the pregnancy. After birth the decidua is shed together with the placenta.^[1]

Decidua
Diagrammatic sections of the uterine mucous membrane: A. The non-pregnant uterus. B.Decidua parietalis; the mucous membrane in the pregnant uterus and not beneath the placenta.
Sectional plan of the gravid uterus in the third and fourth month
Identifiers
MeSH	D003656
TE	E6.0.1.4.0.0.7
FMA	85538
Anatomical terminology [edit on Wikidata]

The part of the decidua that interacts with thetrophoblast is thedecidua basalis (also calleddecidua placentalis), while thedecidua capsularis grows over theembryo on the luminal side, enclosing it into the endometrium. The remainder of the decidua is termed thedecidua parietalis ordecidua 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:

• Compact outer layer (stratum compactum)
• Intermediate layer (stratum spongiosum)
• Boundary layer adjacent to themyometrium (stratum basalis)

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 calledRohr's layer, while the fibrinoid deposits that occur between the compact and spongy layer of the decidua basalis is termedNitabuch's layer (forRaissa Nitabuch). This layer is absent inplacenta accreta.^[2]

The decidua has ahistologically-distinct appearance, displaying large polygonaldecidual cells in the stroma. These are enlarged endometrial stromal cells, which resembleepithelium (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 infibronectin andlaminin (similar to epithelial cells).

Vascularity, as well as vascular permeability, is enhanced in the decidualizing endometrium.

Itsleukocyte population is distinct, with the presence of large endometrial granular leukocytes being predominant, while polynuclear leukocytes andB cells are scant.

The large granular lymphocytes (CD56 bright) are calleduterine natural killer cells| (uNK cells).

Afterovulation, inplacental mammals, the endometrial lining becomes hypertrophic and vascular under the influence of thesex hormones,estrogen andprogesterone.

In animals exhibitinghemochorial placentation, the endometrium undergoesdecidualization followingimplantation. If implantation does not occur, the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle).

The decidua is shed with the placenta duringbirth.

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 maternalimmune system. Further, the decidua has to allow a very controlled invasion of the trophoblast.

In invasive placental disorders likeplacenta accreta decidualization have been consistently found to be deficient.

The decidua secreteshormones,growth factors, andcytokines. It has receptors forestrogen,progesterone,growth hormone, and others.

Among its products are hormones commonly associated with other organs such ascortisol,CRF,GnRH,prolactin, andrelaxin. Decidual prolactin is not underdopaminergic control.

Insulin-like growth factor-binding protein 1 (IGFBP1) also called placental protein 12, andPAEP (glycodelin) appear to be specific products of the secretory and decidual lining.

Other factors released includeinterleukin-15 andvascular endothelial growth factor (VEGF). A reasonable understanding of the role and interplay of these hormones and factors has not been evolved.

• In case of anectopic pregnancy, the endometrium nevertheless becomes decidualized. A woman may shed the lining in the form of a decidual cast, which may be mistaken as amiscarriage, when, in fact, the ectopic pregnancy still persists.
• Adecidual reaction can be observed in tissue of theperitoneum andovary during a pregnancy, and represents a response ofstromal tissue toprogesterone.
• Decidua in alymph node may mimicmetastaticcarcinoma.^[3]

A long-lastinginfection of the decidua,chronic deciduitis, is associated withpre-term labour.^[4]

•  
  Micrograph ofchronic deciduitis.H&E stain.
•  
  Acute choriodeciduitis, withneutrophils seen in the chorion and decidua.

The word comes fromLatin deciduus 'falling off / shedding'.

•  
  Scheme of placental circulation.
•  
  Gross pathology of fetal membranes versus decidua

• Histologic picture at med.utah.edu
• Histology image: 19904loa – Histology Learning System at Boston University