Amniotic Cavity and Placenta
Note in Figure 24-4 that the blastocyst consists of an outer layer of cells and an inner cell mass. As the blastocyst develops, it forms a structure with two cavities, the yolk sac and amniotic cavity.
FIGURE 24-4 Implantation and development. The hollow blastocyst implants itself in the uterine lining about 10 days after ovulation. Until the placenta is functional, nutrients are obtained by diffusion from uterine fluids. Notice the developing chorion and how the blastocyst eventually forms a yolk sac and amniotic cavity.
The yolk sac is most important in animals, such as birds, that depend heavily on yolk as the sole source of nutrients for the developing embryo. In these animals the yolk sac digests the yolk and provides the resulting nutrients to the embryo. Because uterine fluids provide nutrients to the developing human embryo until the placenta develops, the function of the yolk sac is not a nutritive one. Instead, it has other functions, including production of blood cells and stem cells that later form gametes in the gonads. The yolk sac is an important site of hematopoiesis in early development.
The amniotic cavity becomes a fluid-filled, shock-absorbing sac, sometimes called the bag of waters, in which the embryo 655floats during development. The chorion, shown in Figure 24-4 and Figure 24-5, develops into an important fetal membrane in the placenta. The chorionic villi shown in Figure 24-5 connect the blood vessels of the chorion to the rest of the placenta. The placenta (see Figure 24-5) anchors the developing fetus to the uterus and provides a “bridge” for the exchange of nutrients and waste products between mother and offspring.
FIGURE 24-5Placenta. The close placement of the fetal blood supply and the maternal blood in the placenta permits diffusion of nutrients and other substances. It also forms a thin barrier to prevent diffusion of most harmful substances. No mixing of fetal and maternal blood occurs. A, Diagram showing a cross section of the placental structure. B, Photograph of a normal, full-term placenta (fetal side) showing the branching of the placental blood vessels.
The placenta is a unique structure that has a temporary but very important series of functions during pregnancy. It is composed of tissues from mother and child and functions not only as a structural “anchor” and nutritive bridge but also as an excretory, respiratory, and endocrine organ (see Figure 24-5).
Placental tissue normally separates the maternal blood, which fills the lacunae of the placenta, from the fetal blood so that no intermixing occurs. The very thin layer of placental tissue that separates maternal and fetal blood also serves as an effective “barrier” that can protect the developing offspring from many harmful substances that may enter the mother’s bloodstream.
Unfortunately, toxic substances, such as alcohol and some infectious organisms, may nonetheless penetrate this protective 656placental barrier and injure the developing offspring. The cytomegalovirus (CMV), the Zika virus (ZV), or the bacterium that causes syphilis, for example, can easily pass through the placenta and cause tragic developmental defects in the fetus.
RESEARCH, ISSUES, AND TRENDS
IN VITRO FERTILIZATION
The Latin term in vitro means, literally, “within a glass.” In the case of in vitro fertilization, it refers to the glass laboratory dish where an ovum and sperm are mixed and where fertilization occurs.
In the classic technique, the ovum is obtained from the mother by first inserting a fiber-optic viewing instrument called a laparoscope through a very small incision in the woman’s abdomen. After it is in the pelvic cavity, the device allows the physician to view the ovary and then puncture and “suck up” an ovum from a mature follicle. Over the years, refinements to this technique have been made and less invasive procedures, such as the insertion of a needle through the vaginal wall, are currently being used. After the ovum is harvested, it is mixed with sperm so that fertilization occurs.
After about 2½ days’ growth in a temperature-controlled environment, the developing zygote, which by then has reached the 8- or 16-cell stage, is returned by the physician to the mother’s uterus. If implantation is successful, growth will continue and the subsequent pregnancy will progress. In the most successful fertility clinics in the United States, a normal term birth will occur in about 30% of in vitro fertilization attempts.
With the advent of genomics (see Chapter 25), we are now able to check embryos for genetic disorders before implanting them. However, the controversial practice of selecting embryos for gender has caused ethicists to debate whether the screening of embryos is appropriate medical practice.