Placenta
The placenta is an organ that sustains and nourishes the fetus and is of both fetal and maternal origin. The placenta produce hormones and thus has an endocrine function. The placenta has an immunological role in the suppression of fetal rejection. Blood travels from the fetus to the placenta and returns to the fetus via the umbilical cord which attaches near the center of the placenta. The placenta is expelled from the uterus following birth of the baby and is thus known as the afterbirth. The fully developed placenta is discoidal in shape with a diameter of 6-8 in (15-20 cm) and a thickness of about 1 in (2-3 cm).
The fetus has lungs but they have no respiratory function. The placenta serves in lieu of lungs. Fetal blood moves to the placenta via two umbilical arteries. Fetal blood loses carbon dioxide in the placenta and oxygen derived from the maternal blood diffuses across placental membranes to oxygenate fetal blood. The oxygen restored blood returns to the fetal body via a single umbilical vein. During its nine month duration in the uterus, the fetus increases in mass from a tiny blastocyst to a newborn that may weigh an average of 7.5 lb (3.4 kg). All of this growth is sustained by nutrients, derived from the mother, which diffuse across the placental membranes. Living beings produce metabolic wastes and the fetus is no exception. Metabolic wastes in the blood of the fetus pass to the maternal blood via the placenta and are excreted by her kidneys. Perhaps the best known hormone produced by the placenta is human chorionic gonadotropin (HCG) because of its use in the diagnosis of pregnancy. The production of progesterone is stimulated by HCG. While ovaries are the usual source of progesterone, HCG of the placenta stimulates other placental cells to secrete progesterone also. Human chorionic somatomammotropin, also known as placental lactogen, aids in the preparation of the maternal mammary glands for milk production after the baby's birth.
Identical twins do not reject grafted tissue obtained from each other. Ordinarily, all other humans tend to reject tissue that is not identical with self. The rejection is a function of major histocompatibility antigens. This is a problem that must be controlled with immunosuppressive drugs for the surgical transplantation of donor organs. How then is it that mothers do not reject the fetus that must live in the uterus for nine months? Mothers ordinarily reject transplanted organs of their children (after they are born) because of the expression of paternal antigens. It has been suggested that the placenta secretes proteins that block the production of antibodies. Further, the placenta stimulates production of lymphocytes concerned with blocking the immune response of the uterus. Increased knowledge of placental physiology as it relates to immunobiology may someday lead to enhanced organ transplantation.
As stated, the placenta is of dual origin. It is derived of fetal trophoblast cells and maternal decidua. Fertilization of the human egg occurs in the Fallopian (uterine) tubes. Implantation into the endometrium of the uterus does not occur until the sixth day. This gives the zygote time to cleave into a small cluster of blastomere cells. The cluster of cells continues to develop until the sixth day, at which time there is an inner cell mass that will become the embryo proper, and the surrounding trophoblast. The trophoblast cells become the embryonic component of the placenta. For nourishment (and other functions) to occur, the trophoblast must invade the endometrial lining of the uterus. The trophoblast becomes differentiated into a cellular layer, the cytotrophoblast, and a layer of trophoblast cells without cell membranes known as the syncytiotrophoblast. The syncytiotrophoblast cells are aggressively invasive and quickly digest their way past the epithelial lining of the uterus until they access and digest the walls of maternal blood vessels. Maternal blood then bathes the syncytiotrophoblast. The trophoblast cells in contact with maternal blood become invaded by branches of umbilical arteries which give rise to capillaries. It is at the capillary level that physiological exchange occurs. The refreshed blood returns to the embryo via the umbilical vein. It should be noted that fetal blood remains within the capillaries. The capillaries are in connective tissue covered with trophoblast cells. There is ordinarily no exchange of blood between the fetus and the mother because of the integrity of the umbilical capillaries and the covering of connective tissue. These tissues form an effective barrier against blood cell exchange.
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