During implantation, the blastocyst secretes enzymes that digest away a tiny portion of the endometrium, allowing the embryo to completely insert itself into the endometrium. Then, the cells of the trophoblast send out tiny projections called, unfortunately, microvilli, that hook onto the endometrium. These projections further secure the implantation. Now, some changes begin...
The trophoblast cells continue to divide. As they do so, they form a pocket of fluid next to the developing embryo (from the inner cell mass). This pocket of fluid is the amniotic cavity, and the trophoblast cells that surround it are the amnion. This is one of the membranes formed during embryonic development.
The trophoblast cells that continued to invade the endometrium now develop larger cellular folds to hook into the endometrium. These are the chorionic villi... they look somewhat similar to intestinal villi in being folds of the entire membrane (so that many cells are in each projection). These chorionic villi are present by the end of the 4th week of embryonic development. They are crucial for the development of the placenta!
You see, as blood vessels form in the embryo, they run out the embryonic stalk and into the chorionic villi. Meanwhile, the endometrium around the chorionic villi fills with blood. This happens as spaces around the chorionic villi arise in the endometrium as it is damaged by the development of these chorionic villi. The damaged blood vessels in the endometrium spill their blood into these spaces. The spaces are called lacunae. Therefore, the embryonic blood vessels within the chorionic villi are near the mother's blood in the lacunae, and can pick up nutrients that cross through the villi. To pass through the chorionic villi means passing through a membrane-- the layer of cells that comprise the outer edge of the villi and the endothelia of the blood vessels within the villi. This membrane is called the placental membrane. Eventually, the interaction between the villi and the endometrium becomes confined to a disk-shaped region, called the placenta.
There are two other embryonic membranes that develop... the yolk sac and the allantois, but these also disappear pretty quickly during embryonic development. Finally, the amnion and the chorion, the membranes discussed above, eventually merge into one as the amniotic fluid increases and the volume of the amniotic sac increases. The resultant membrane is the amniochorionic membrane, and this is the one that must rupture when a pregnant woman's water breaks. The water itself is simply the amniotic fluid from within the sac.
© 2011 STCC Foundation Press