Schematic diagrams of sectioned blastocysts, a. A human blastocyst at about 4.5 days of development showing formation of the Inner cell mass. b. A monkey blastocyst at about 9 days of development. The trophoblastic cells of the monkey blastocyst have begun to invade the epithelial cells of the endometrium. In humans, the blastocyst begins to invade the endometrium at about the fifth or sixth day of de-
14 days velopment. c. A human blastocyst at 14 days of development. The small diagram shows the relationship of the embryo to the chorionic sac. At this stage, the trophoblast cells have differentiated into syn-cytiotrophoblasts and cytotrophoblasts. (Based on Sadler TW. Lang-man's Medical Embryology. 8th ed. Baltimore: Lippincott Williams & Wilkins, 2000.)
cavity. This event defines the beginning of the blastocyst. As the blastocyst remains free in the uterine lumen for 1 or 2 days and undergoes further mitotic divisions, the zona pellucida disappears. The outer cell mass is now called the tropboblast, and the inner cell mass is referred to as the embiyoblast.
Implantation occurs during a short period known as the implantation window
The attachment of the blastocyst to the endometrial epithelium occurs during the implantation windoiv, the period that the uterus is receptive for implantation of the blastocyst. This short period results from a series of programmed actions of progesterone and estrogens on the endometrium. Antiprogesterone drugs, such as Mifepristone (RU 486) and its derivatives, compete for the receptors in the endometrial epithelium, thus blocking hormone binding. The failure of progesterone to gain access to its receptors prevents implantation, thus effectively closing the window. In the human, the implantation window begins on day 6 after the LH surge and is completed by day 10.
As contact is made with the uterine wall by the trophoblastic cells over the embryoblast pole, the tropboblast rapidly proliferates and begins to invade the endometrium. The invading trophoblast differentiates into the syncy-tiotropboblast and the cytotropboblast.
• The cytotropboblast is a mitotically active inner cell layer producing cells that fuse with the syncytiotro-phoblast, the outer erosive layer.
• The syncytiotropboblast is not mitotically active and consists of a multinucleate cytoplasmic mass; it actively invades the epithelium and underlying stroma of the endometrium.
Through the activity of the trophoblast, the blastocyst is entirely embedded within the endometrium on about the 11th day of development (further development of the syn-cytiotrophoblast and cytotrophoblast is described in the section on the placenta).
The syncytiotrophoblast has well-developed Golgi complexes, abundant sER and rER, numerous mitochondria, and relatively large numbers of lipid droplets. These features are consistent with the secretion of progesterone, estrogens, hCG, and lactogens by this layer. Recent evidence indicates that cytotrophoblast cells may also be a source of steroid hormones and hCG.
After implantation, the endometrium undergoes decidualization
During pregnancy, the portion of the endometrium that undergoes morphologic changes is called the decidua or decidua graviditas. As its name implies, this layer is shed with the placenta at parturition. The decidua includes all but the deepest layer of the endometrium. The stromal cells differentiate into large, rounded decidual cells (see page 745). The uterine glands enlarge and become more coiled during the early part of pregnancy and then become thin and flattened as the growing fetus fills the uterine lumen.
Three different regions of the decidua are identified by their relationship to the site of implantation (Fig. 22.20):
• The decidua basalis is the portion of the endometrium that underlies the implantation site.
placenta decidua basalis
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