If implantation does not occur, humans shed this lining each menstrual cycle. Two-thirds of it is simply reabsorbed back into the body, and the remaining one-third is shed via menstruation.
However if implantation occurs, the uterus lining (now termed decidua) evolves further during the pregnancy, and is only shed at the birth.The uterus lining (now called “decidua”) forms the maternal part of the placenta. It is formed under the influence of progesterone and forms highly characteristic cells.
Different layers of the deciduas have been described: Compact outer layer (stratum compactum) Intermediate layer (stratum spongiosum) Boundary layer adjacent to the myometrium (stratum basalis).
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.
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) appear to be specific products of the secretory and decidual lining.
Decidualization is a characteristic of the endometrium of the pregnant uterus. It is a response of maternal cells to progesterone. Decidualization may be used to describe any change due to progesterone. These changes include the eosinophilic proliferation around arterioles after ovulationor progesterone action on endometrium which increases glandular epithelial secretion, stimulates glycogen accumulation in stromal cell cytoplasm, and promotes stromal vascularity (spiral arterioles) and edema. The sum of all the changes in the endometrium is significant, thus justifying a new term. The process is decidualization, the endometrium is now called decidua and it is ready for the implantation of the embryo.
Part of the Endometrium is used to form the Embyro’s Placenta:
The function of placentation is to transfer nutrients from maternal tissue to a growing embryo. In placental mammals, the placenta forms after the embryo implants into the wall of the uterus. The developing fetus is connected to it via an umbilical cord. During pregnancy, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessels in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.
In humans, the placenta develops in the following manner. Chorionic villi (from the embryo) on the embryonic pole grow, forming chorion frondosum. Villi on the opposite side (abembryonic pole) degenerate and form the chorion laeve (or chorionic laevae), a smooth surface. The endometrium (from the mother) over the chorion frondosum (this part of the endometrium is called the decidua basalis) forms the decidual plate. The decidual plate is tightly attached to the chorion frondosum and goes on to form the actual placenta. Endometrium on the opposite side to the decidua basalis is the decidua parietalis. This fuses with the chorion laevae, thus filling up the uterine cavity.
Basically a part of the embryo and a part of the endometrium merge together and develop into a placenta. The placenta in very firmly attached to the uterus wall (and it is attached to both fetal and maternal blood vessels). To cope with the placenta’s strong/invasive attachment to the uterus wall, humans have to build up a thick uterus lining each menstrual cycle (which is why humans menstruate overtly). However the benefit of the placenta sharing maternal blood vessels is that it is more easy for the mother to transmit nutrients (e.g. from her own diet) to her baby.