BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.8. Human Sexual Reproduction

21.8.7. Embryonic and Fetal Development

Some of the Cytology/cytology/16.html">Early stages of embryonic development are shown in Fig. 21.50. The timeframes are given from the moment of conception, i.e., Fertilization. After 6 weeks, when the embryo begins to display distinct human features, it is referred to as a fetus. Doctors and midwives calculate the gestational age from the last menstrual period, adding two weeks to the timeframes indicated below. Normally, a fetus reaches full development in 38 weeks. Most of the major Organs are already formed by the twelfth week of Pregnancy, with the remaining time dedicated primarily to fetal growth.

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Fig. 21.50. Timeline of human fetal development.

The Placenta is found only in placental mammals; it is the unique organ composed of Cells originating from two different organisms—the fetus and the mother. The function of the placenta is to facilitate Metabolic exchange between the mother and the fetus. It replaces the chorionic villi as the primary site of exchange between maternal and fetal Blood after 12 weeks of pregnancy.

Structure OF THE Placenta

The fetal portion of the placenta is formed by chorionic cells that develop into projections known as chorionic villi (Fig. 21.51). These villi significantly increase the surface area for absorption. They are permeated by an intricate network of capillaries branching from two major fetal Blood Vessels—the umbilical artery and the umbilical vein. These vessels run from the fetus to the uterine wall through the umbilical cord, a dense, cord-like structure approximately 40 cm long, covered with cells derived from the amnion and chorion.

The maternal portion of the placenta consists of endometrial tissue projections. Interspersed between these projections and the chorionic villi are spaces supplied with arterial blood from arterioles within the uterine wall. Blood drains from these spaces via venules, also located in the uterine wall. The placenta is a substantial structure; a fully developed placenta has an average mass of 600 g, a diameter of 15–20 cm, and a central thickness of 3 cm. About 10% of the mother's blood circulates through it during each cardiac cycle. Because maternal blood does not come into direct contact with the fetal Circulatory system, the fetus is protected from the relatively high maternal blood pressure. Another advantage of this Separation is that maternal and fetal blood might otherwise be incompatible due to different ABO blood group systems. Furthermore, blood mixing would complicate its safe separation at birth.

Fig. 21.51. The relationship between the umbilical blood vessels, chorionic villi capillaries, and blood-filled spaces in the human placenta. The placenta acts as the functional link between the fetal and maternal circulatory systems.