Function Of The Placenta

Main functions of the placenta are (a) exchange of metabolic and gaseous products between maternal and fetal bloodstreams and (b) production of hormones.

Exchange of Gases

Exchange of gases, such as oxygen, carbon dioxide, and carbon monoxide, is accomplished by simple diffusion. At term, the fetus extracts 20 to 30 ml of oxygen per minute from the maternal circulation and even a short-term interruption of the oxygen supply is fatal to the fetus. Placental blood flow is critical to oxygen supply, since the amount of oxygen reaching the fetus primarily depends on delivery, not diffusion.

Exchange of Nutrients and Electrolytes

Exchange of nutrients and electrolytes, such as amino acids, free fatty acids, carbohydrates, and vitamins, is rapid and increases as pregnancy advances.

Transmission of Maternal Antibodies

Immunological competence begins to develop late in the first trimester, by which time the fetus makes all of the components of complement. Im-munoglobulins consist almost entirely of maternal immunoglobulin G (IgG) that begins to be transported from mother to fetus at approximately 14 weeks. In this manner, the fetus gains passive immunity against various infectious diseases. Newborns begin to produce their own IgG, but adult levels are not attained until the age of 3 years.

CLINICAL CORRELATES Erythroblastosis Fetalis and Fetal Hydrops

Over 400 red blood cell antigens have been identified, and although most do not cause problems during pregnancy, some can stimulate a maternal antibody response against fetal blood cells. This process is an example of isoimmunozation, and if the maternal response is sufficient, the antibodies will attack and hemolyze fetal red blood cells resulting in hemolytic disease of the newborn (erythroblastosis fetalis). The anemia may become so severe that fetal hydrops (edema and effusions into the body cavities) occurs, leading to fetal death. Most severe cases are caused by antigens from the CDE (Rhesus) blood group system. The D or Rh antigen is the most dangerous, since immunization can result from a single exposure and occurs earlier and with greater severity with each succeeding pregnancy. The antibody response occurs in cases where the fetus is D(Rh) positive and the mother is D(Rh) negative and is elicited when fetal red blood cells enter the maternal system due to small areas of bleeding at the surface of placental villi or at birth. Analysis of amniotic fluid for bilirubin, a breakdown product of hemoglobin, serves as a measure of the degree of red cell hemolysis. Treatment for the affected fetus involves intrauterine or postnatal transfusions. However, the disease is prevented by identifying women at risk using an antibody screen and treating them with anti-D-immunoglobulin.

Antigens from the ABO blood group can also elicit an antibody response, but the effects are much milder than those produced by the CDE group. About 20% of all infants have an ABO maternal incompatibility, but only 5 % will be clinically affected. These can be effectively treated postnatally.

Hormone Production

By the end of the fourth month the placenta produces progesterone in sufficient amounts to maintain pregnancy if the corpus luteum is removed or fails to function properly. In all probability, all hormones are synthesized in the syncy-tial trophoblast. In addition to progesterone, the placenta produces increasing amounts of estrogenic hormones, predominantly estriol, until just before the end of pregnancy, when a maximum level is reached. These high levels of estrogens stimulate uterine growth and development of the mammary glands.

During the first two months of pregnancy, the syncytiotrophoblast also produces human chorionic gonadotropin (hCG), which maintains the corpus luteum. This hormone is excreted by the mother in the urine, and in the early stages of gestation, its presence is used as an indicator of pregnancy. Another hormone produced by the placenta is somatomammotropin (formerly placental lactogen). It is a growth hormone-like substance that gives the fetus priority on maternal blood glucose and makes the mother somewhat diabetogenic. It also promotes breast development for milk production.

CLINICAL CORRELATES The Placental Barrier

Most maternal hormones do not cross the placenta. The hormones that do cross, such as thyroxine, do so only at a slow rate. Some synthetic progestins rapidly cross the placenta and may masculinize female fetuses. Even more dangerous was the use of the synthetic estrogen diethylstilbestrol, which easily crosses the placenta. This compound produced carcinoma of the vagina and abnormalities of the testes in individuals who were exposed to it during their intrauterine life (see Chapter 7).

Although the placental barrier is frequently considered to act as a protective mechanism against damaging factors, many viruses, such as rubella, cy-tomegalovirus, Coxsackie, variola, varicella, measles, and poliomyelitis virus, traverse the placenta without difficulty Once in the fetus, some viruses cause infections, which may result in cell death and birth defects (see Chapter 7).

Unfortunately, most drugs and drug metabolites traverse the placenta without difficulty, and many cause serious damage to the embryo (see Chapter 7). In addition, maternal use of heroin and cocaine can cause ha-bituation in the fetus.

Amnion and Umbilical Cord

The oval line of reflection between the amnion and embryonic ectoderm (amnio-ectodermal junction) is the primitive umbilical ring. At the fifth week of development, the following structures pass through the ring (Fig. 6.15, A and C): (a) the connecting stalk, containing the allantois and the umbilical vessels, consisting of two arteries and one vein; (b) the yolk stalk (vitelline duct), accompanied by the vitelline vessels; and (c) the canal connecting the in-traembryonic and extraembryonic cavities (Fig. 6.15C). The yolk sac proper occupies a space in the chorionic cavity, that is, the space between the amnion and chorionic plate (Fig. 6.15B).

During further development, the amniotic cavity enlarges rapidly at the expense of the chorionic cavity, and the amnion begins to envelop the connecting and yolk sac stalks, crowding them together and giving rise to the primitive umbilical cord (Fig. 6.15B). Distally the cord contains the yolk sac stalk and umbilical vessels. More proximally it contains some intestinal loops and the remnant of the allantois (Fig. 6.15, B and D). The yolk sac, found in the chorionic cavity, is connected to the umbilical cord by its stalk. At the end of the third month, the amnion has expanded so that it comes in contact with the chorion, obliterating the chorionic cavity (Fig. 6.10B). The yolk sac then usually shrinks and is gradually obliterated.

The abdominal cavity is temporarily too small for the rapidly developing intestinal loops and some of them are pushed into the extraembryonic space in the umbilical cord. These extruding intestinal loops form a physiological

Primitive Umbilical Ring

Figure 6.15 A. A 5-week embryo showing structures passing through the primitive umbilical ring. B. The primitive umbilical cord of a 10-week embryo. C. Transverse section through the structures at the level of the umbilical ring. D. Transverse section through the primitive umbilical cord showing intestinal loops protruding in the cord.

Figure 6.15 A. A 5-week embryo showing structures passing through the primitive umbilical ring. B. The primitive umbilical cord of a 10-week embryo. C. Transverse section through the structures at the level of the umbilical ring. D. Transverse section through the primitive umbilical cord showing intestinal loops protruding in the cord.

umbilical hernia (see Chapter 13). At approximately the end of the third month, the loops are withdrawn into the body of the embryo and the cavity in the cord is obliterated. When the allantois and the vitelline duct and its vessels are also obliterated, all that remains in the cord are the umbilical vessels surrounded by the jelly of Wharton. This tissue, which is rich in pro-teoglycans, functions as a protective layer for the blood vessels. The walls of the arteries are muscular and contain many elastic fibers, which contribute to a rapid constriction and contraction of the umbilical vessels after the cord is tied off.

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Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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Responses

  • liviano sabbatini
    Are some placentas slower in producing progesterone?
    8 years ago
  • HAZEL
    What is umbilical ring constriction?
    8 years ago
  • giovanna
    What structures form the umbilical cord?
    8 years ago

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