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The Placenta (afterbirth)
The Placenta
The placenta which is also known as afterbirth is a fascinating vascular structured organ that attaches the embryo or foetus to the uterus of the mother. The placenta is a temporary structure which consists of the maternal and foetal tissues and through which metabolic exchanges between the embryo or foetus and the mother takes place. The placenta is temporal in the sense that shortly after birth, the placenta and the foetal tissues are expelled out of the mother through the birth canal.

The placenta itself attaches to the endometrium of the uterus of the mother, and at about the centre of the placenta arises the umbilical cord which on the other end is attached to the foetus through the navel, belly button region or the umbilicus at the abdomen. It is through the placenta that exchange of oxygen, nutrients, carbon dioxide, foetal metabolic waste, etc takes place. However, there is no direct contact between the foetal and maternal blood.

Formation of the placenta

The placenta begins to form or develop upon implantation of the blastocyst into the uterine wall or endometrium. The placenta is a fetomaternal organ, it is formed in part from the embryonic tissues and in part from the maternal tissues. Hence it's basic parts consist of,
  • The fetal part, and
  • The maternal part.
The fetal part which is also the embryonic part of the placenta is formed from the villous chorion, while the maternal part of the placenta is formed from the part of the uterine wall called decidua basalis. It is the decidua basalis of the uterine wall or endometrium that the chorionic villi from the fetus penetrates.

Again, it is pertinent to note that blood do not directly flow between these two distinct parts; the fetal and maternal parts. Since these membranes are thin, fine and extremely permeable and are in very close proximity, exchange of nutrients, gases and metabolic waste across these surfaces takes place either through simple diffusion, facilitated diffusion, active transport, etc.

The fully formed and mature placenta is reddish brown in colour and have an oval disc shape with a diameter of about 15 to 20cm and a thickness of about 2.5cm. The mature placenta weighs about 500 to 600g.

Circulation in the placenta
Embedded within the umbilical cord which connects the embryo to the placenta are blood vessels, two umbilical arteries and one umbilical vein. The two umbilical arteries carry and deliver the fetal or embryonic blood to vessels within the villous chorion of the placenta. This fetal blood circulates within the chorionic villi and return to the fetus as oxygenated blood through the umbilical vein.

Also, the maternal blood which is highly oxygenated and loaded with nutrients, mineral salt, etc is delivered to the cavities within the decidua basalis which are located between the chorionic villi of the placenta. The maternal blood is delivered through the maternal artery and is eventually drained from the cavities through the maternal vein. In this process, the maternal and foetal blood are brought very close together, but they do not mix or touch within the placenta.

The placenta serve as a site for the exchange of gases, nutrients, metabolic waste, etc between the maternal blood and the fetal blood. Since the walls of the cavities are very thin, fine and highly permeable, oxygen diffuses from the mother's blood to the fetal blood and carbon dioxide diffuses in the opposite direction. Nutrients also diffuses from the maternal blood into the fetal blood while waste products likewise diffuses from the fetal blood into the maternal blood.

Generally, the placenta is an effective barrier which protects the fetus against diseases of bacteria, viral, parasites, etc.

Functions of the placenta
The placenta perform several critically important functions, some of which are;
  • Endocrine function,
  • Metabolic function, and
  • Transport function.
1. Endocrine function: the placenta functions as an endocrine gland by producing both glycoprotein and steroid hormones. Using precursors derived from both the foetus and the mother, the syncytiotrophoblast of the placenta synthesize protein and steroid hormones. The glycoprotein or protein hormones synthesized by the placenta are;
  • Human chorionic gonadotropin (HCG),
  • Human chorionic somatomammotropin (HCS) or placenta lactogen,
  • Human chorionic corticotropin, and
  • Human chorionic thyrotropin.
While the steroid hormones synthesized by the placenta are;
  • Progesterone, and
  • Oestrogen.
2. Metabolic function: the placenta performs metabolic functions by synthesizing glycoprotein, fatty acids and cholesterol early in pregnancy. The synthesized substances serves as source of nutrients and energy for the fetus.

3. Transport function: the placenta also performs transport function. This transport function is in both sides between the fetus and the maternal blood. This is facilitated by the great surfaces area of the placental membranes. Substances are mainly transported across the placental membrane either by;
  • Simple diffusion,
  • Facilitated diffusion,
  • Active transport, and
  • Pinocytosis.
Some of the substances transported across the placenta membrane between fetus and mother are;
  • Nutritional substances such as water, glucose free fatty acids (FFA), amino acids, vitamins, mineral salt, etc.
  • Waste products such as urea and uric acid, 
  • Hormones,
  • Oxygen, 
  • Electrolytes,
  • Maternal antibodies,
  • Drug and drug metabolites, etc. 

The umbilical cord

The umbilical cord is the cord connecting the fetus to the placenta. It is formed as the yolk sac shrinks and the amnion expands to envelope the tissues on the underside of the embryo. The umbilical cord is basically attached to the placenta nearly at the center. The umbilical cord contains blood vessels, two umbilical arteries which carry deoxygenated blood from the fetus to the placenta and one umbilical vein which carries oxygenated blood from the placenta to the fetus. These vessels are together surrounded by embryonic connective tissues called Wharton's jelly or mucoid connective tissues.

When the umbilical cord is fully formed and mature, it is approximately about 60cm long and about 1.5 to 2cm in diameter. On the average, the umbilical cord of male fetuses are approximately about 5cm longer than those of female fetuses.

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