Figure 2220

Development of the placenta. This schematic drawing shows growth of the uterus during human pregnancy and development of the placenta and its membranes. Note that there is a gradual obliteration of the uterine lumen and disappearance of the decidua capsularis as the definitive placenta is established. (Modified from Williams J. Am J Obstet Gynecol 1927;13:1.)

amnion decidua parietalis

placenta decidua basalis the uterine lumen and disappearance of the decidua capsularis as the definitive placenta is established. (Modified from Williams J. Am J Obstet Gynecol 1927;13:1.)

uterine cavity

uterine yolk sac chorionic villi chorion decidua basalis chorion decidua capsularis decidua basalis chorion decidua capsularis decidua parietalis

chorion decidua capsularis decidua parietalis uterine cavity chorion amnion decidua capsularis decidua basalis decidua parietalis uterine yolk sac chorionic villi chorion decidua basalis

* The decidua capsularis is a thin portion of endometrium that lies between the implantation site and the uterine lumen.

• The decidua parietalis includes the remaining endometrium of the uterus.

By the end of the third month, the fetus grows to the point that the overlying decidua capsularis fuses with the decidua parietalis of the opposite wall, thereby obliterating the uterine cavity.

By the 13th day of development, an extraembryonic space, the chorionic cavity, has been established (see Fig. 22.19c). The cell layers that form the outer boundary of this cavity, i.e., the syncytiotrophoblast, cytotrophoblast, and extraembryonic somatic mesoderm, are collectively referred to as the chorion. The innermost membranes enveloping the embryo are called the amnion (Fig. 22.20).

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