his is a specialized, highly modiied endometrium of pregnancy. It is essential for hemochorial placentation, that is, one in which maternal blood contacts trophoblasts. This relationship requires trophoblast invasion, and considerable research has focused on the interaction between decidual cells and invading trophoblasts. Decidualization, that is, transformation oO proliferating endometrial stromal cells into specialized secretory cells, is dependent on estrogen, progesterone, androgens, and factors secreted by the implanting blastocyst (Gibson, 2016). The decidua produces factors that regulate endometrial receptivity and modulate immune and vascular cell functions within the maternal-fetal microenvironment. The special relationship existing between the decidua and the invading trophoblasts ensures success of the pregnancy semiallograft yet seemingly deies the laws of transplantation immunology.
The decidua is classiied into three parts based on anatomical location. Decidua directly beneath blastocyst implantation is modiied by trophoblast invasion and becomes the decidua basalis. The decidua capsularis overlies the enlarging blastocyst and initially separates the conceptus from the rest of the uterine cavity (Fig. 5-5). This portion is most prominent during the second month of pregnancy and consists of stromal decidual cells covered by a single layer of lattened epithelial cells. Internally, it contacts the avascular, extraembryonic fetal membrane-the chorion laeve. he remainder of the uterus is lined by decidua parietalis. During early pregnancy, there is a space between the decidua capsularis and parietalis because the gestational sac does not fill the entire uterine cavity. The gestation sac is the extraembryonic coelom and also called the chorionic cavity. By 14 to 16 weeks' gestation, the expanding sac has enlarged to completely ill the uterine cavity. The resulting apposition of the decidua capsularis and parietalis creates the decidua vera, and the uterine cavity is functionally obliterated.
Decidua basalis Cervical canal parietal is Chorionic villi cavity
FIGURE 5-5 Three portions of the decidua-the basalis, capsularis, and parietalis-are illustrated.
In early pregnancy, the decidua begins to thicken, eventually attaining a depth of 5 to 10 mm. With magnification, furrows and numerous small openings, representing the mouths of uterine glands, can be detected. Later in pregnancy, the decidua becomes thinner, presumably because of pressure exerted by the expanding uterine contents.
he decidua parietalis and basalis are composed of three layers. here is a surface or compact zone-zona compacta; a middle portion or spongy zone-zona spongiosa-with remnants of glands and numerous small blood vessels; and a basal zone-zona basalis. The zona compacta and spongiosa together form the zona functionalis. The basal zone remains after delivery and gives rise to new endometrium.
In human pregnancy, the decidual reaction is completed only with blastocyst implantation. Predecidual changes, however, commence irst during the midluteal phase in endometrial stromal cells adjacent to the spiral arteries and arterioles. hereafter, they spread in waves throughout the uterine endometrium and then from the implantation site. The endometrial stromal cells enlarge to form polygonal or round decidual cells. he nuclei become vesicular, and the cytoplasm becomes clear, slightly basophilic, and surrounded by a translucent membrane.
As a consequence of implantation, the blood supply to the decidua capsularis is lost as the embryo-fetus grows. Blood supply to the decidua parietalis through spiral arteries persists. These arteries retain a smooth-muscle wall and endothelium and thereby remain responsive to vasoactive agents.