Chapter 40. The Placenta: Decidual Formation - Review of Medical Embryology Book

Formation of the decidua results from the changes in the uterine mucosa that accompany pregnancy

THE 3 REGIONS OFTHE DECIDUA that are described according to the implantation site are

The decidua basalis: the portion underlying the conceptus and forming the maternal component of the placent Its compact layer is called the decidual plate
The decidua capsularis is the superficial portion overlying the conceptus
The decidua parietalis: all the remaining uterine mucosa

AT THE TIME OF IMPLANTATION, THE CONNECTIVE TISSUE CELLS OF THE MATERNAL MUCOSA undergo epithelioid transformation, the so-called decidual reaction, which forms the compact zone of the mucosa (here no trace of the surface portion of the uterine glands is seen)

The stromal cells increase in size and number, and blood vessels and glands undergo changes, all dependent on progesterone
In the layer just beneath this compact zone, the cul-de-sacs of the glands persist to form the so-called spongy zone through which the plane of cleavage will pass at the time of parturition

FROM MONTH 4 ON, THE DECIDUA CAPSULARIS bulges into the uterine cavity and becomes attenuated. The decidua parietalis and capsularis come in contact, fuse, and obliterate the uterine cavity

By week 22, reduced blood supply causes the decidua capsularis to degenerate and disappear
The amniotic sac enlarges faster than the chorionic sac, and their walls fuse to form the amniochorionic membrane. The latter fuses with the decidua capsularis and, following the disappearance of the capsularis, it fuses with the parietalis

There are up to 38 large villous trunks corresponding to the cotyledons (described earlier) and seen as lobes on the maternal side of the placenta at term. Each trunk with its branchings lies in a space partitioned laterally by the decidual septa which appear month 4 of development, starting out from the maternal (decidual) plate but not reaching the chorionic plate
The chorionic plate consists of the amnion, connective tissue, the syncytiotrophoblast, and the cytotrophoblast
The basal plate consists of the syncytiotrophoblast, cytotrophoblast, the compact zone (decidua), and the spongy layer or zone

At the time of delivery, the placenta separates from the uterus at the spongy zone