Prenatal Nutrition

One of the important changes to occur in the embryonic phase is the mode of nutrition. In trophoblastic (deciduous) nutrition, the conceptus is nourished by digesting

Chapter 29 Human Development 1095

4 8 12 16 20 24 28 32 36 40 Weeks after implantation

4 8 12 16 20 24 28 32 36 40 Weeks after implantation

Trophoblastic phase

Placental phase

Figure 29.7 The Timetable of Trophoblastic and Placental Nutrition. Trophoblastic nutrition peaks at 2 weeks and ends by 12 weeks. Placental nutrition begins at 2 weeks and becomes increasingly important until birth, 39 weeks after implantation. At what point do the two modes contribute equally to prenatal nutrition?

endometrial tissue. Progesterone stimulates the development of decidual6 cells, which are rich in glycogen, proteins, and lipids. The trophoblast digests these cells and the embryoblast imbibes the resulting nutritious fluid. This is gradually followed by placental nutrition, in which the conceptus is nourished by the mother's bloodstream. Nutrients diffuse from her blood through the placenta,7 a vascular organ that develops on the uterine wall. Figure 29.7 shows the time course of the transition from trophoblastic to placental nutrition. Trophoblastic nutrition is the only means of nutrition for the first week after implantation. The placenta begins to develop about 11 days after conception, but trophoblastic nutrition remains dominant for 8 weeks, called the trophoblastic phase of the pregnancy. After that, the placenta provides most nutrition; the trophoblastic mode declines and, by 12 weeks, it ends. The placental phase of the pregnancy lasts from the beginning of week 9 until birth.

Placentation, the formation of the placenta, extends from 11 days through 12 weeks after conception. Most development occurs in the embryonic stage. It begins when extensions of the syncytiotrophoblast, called chorionic villi, penetrate more and more deeply into the endometrium, like the roots of a tree penetrating into the nourishing "soil" of the uterus (fig. 29.8). As they digest their way through uterine blood vessels, the villi become surrounded by pools of free blood that eventually merge to form the placental sinus. This maternal blood stimulates increasingly rapid growth of the chorionic villi. Mesenchyme grows into the villi and develops into blood vessels.

6decid = falling off

7 placenta = flat cake

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1096 Part Five Reproduction and Development

Endometrium (a)

Embryoblast Endoderm Ectoderm

Uterine cavity

- Uterine blood vessels --Trophoblast-

Amniotic cavity

Embryonic disc (ectoderm and endoderm)

- Uterine blood vessels --Trophoblast-

Yolk Sac Amniotic Cavity

Amnion -Yolk sac

Uterine cavity

Amnion -Yolk sac

Uterine cavity

Uterine cavity

Chorion

(partially formed)

Amnion

Chorionic villus

Maternal — blood

Chorion—

Amnion

Chorionic villus

Chorionic Villi Amnion Allantosis

Ectoderm

Mesoderm

Endoderm

- Connecting stalk (future umbilical cord)

Allantois

Ectoderm

Mesoderm

Endoderm

_ Embryonic disc

- Connecting stalk (future umbilical cord)

Allantois

Maternal blood

Uterus

Maternal blood

Uterus

Smooth Chorion

Developing placenta

Umbilical blood vessels

Chorion (d)

Developing placenta

Umbilical blood vessels

Chorion (d)

Yolk sac Amnion Amniotic cavity

Embryonic Membrane

Figure 29.8 Development of the Placenta and Embryonic Membranes. (a) Implantation nearly complete; an amniotic cavity has formed within the embryonic disc, lined by cells that will become the amnion. (b) Conceptus about 9 days after fertilization; embryonic disc cells have now formed the yolk sac. (c) Conceptus at 16 days; the allantois is beginning to form, and the chorion is forming from the trophoblast and embryonic mesoderm. (d) Embryo at 4.5 weeks, enclosed in the amnion and chorion. (e) Embryo at 13.5 weeks; placentation is complete. (f) A portion of the mature placenta and umbilical cord.

Figure 29.8 Development of the Placenta and Embryonic Membranes. (a) Implantation nearly complete; an amniotic cavity has formed within the embryonic disc, lined by cells that will become the amnion. (b) Conceptus about 9 days after fertilization; embryonic disc cells have now formed the yolk sac. (c) Conceptus at 16 days; the allantois is beginning to form, and the chorion is forming from the trophoblast and embryonic mesoderm. (d) Embryo at 4.5 weeks, enclosed in the amnion and chorion. (e) Embryo at 13.5 weeks; placentation is complete. (f) A portion of the mature placenta and umbilical cord.

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5weeks Placenta
Figure 29.9 The Placenta and Umbilical Cord. Viewed from the fetal side.

The fully developed placenta is a disc of tissue about 20 cm in diameter and 3 cm thick. At term, it weighs about one-sixth as much as the baby. The surface facing the fetus is smooth and gives rise to the umbilical cord (fig. 29.9), which contains three blood vessels that connect the fetus to the placenta. The surface attached to the uterine wall is rougher. It consists of the chorionic villi contributed by the fetus and a region of the mother's endometrium called the decidua basalis. The chorionic villi are extensively branched treelike structures surrounded by the maternal blood in the placental sinus.

The fetal heart pumps blood to the placenta by way of two umbilical arteries. Fetal blood flows into the capillaries of the villi and then back to the fetus by way of a single umbilical vein. The villi are filled with fetal blood and surrounded by maternal blood; the two bloodstreams do not mix unless there is damage to the placental barrier. The barrier, however—the membrane covering the villi— is only 3.5 pm thick, about half the diameter of a single red blood cell. Early in development, the chorionic villi have thicker membranes that are not very permeable to nutrients and wastes, and their total surface area is relatively small. As the villi grow and branch, their surface area increases and the membranes become thinner and more permeable. Thus there is a dramatic increase in placental conductivity, the rate at which substances diffuse through the membrane. Oxygen and nutrients pass from the maternal blood to the fetal blood, while fetal wastes pass the other way to be eliminated by the mother.

Gases, electrolytes, fatty acids, and steroids pass through the membrane by simple diffusion, glucose by facilitated diffusion, amino acids by active transport, and insulin by receptor-mediated endocytosis. Unfortunately, the placenta is also permeable to nicotine, alcohol, and

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Table 29.1

Functions of the Placenta

Nutritional roles

Transports nutrients such as glucose, amino acids, fatty acids, minerals, and vitamins from the maternal blood to the fetal blood; stores nutrients such as carbohydrates, protein, iron, and calcium in early pregnancy and releases them to the fetus later, when fetal demand is greater than the mother can absorb from the diet

Excretory roles

Transports nitrogenous wastes such as ammonia, urea, uric acid, and creatinine from the fetal blood to the maternal blood

Respiratory roles

Transports O2 from mother to fetus and CO2 from fetus to mother

Endocrine roles

Secretes estrogens, progesterone, relaxin, human chorionic gonadotropin, and human chorionic somatomammotropin; allows other hormones synthesized by the conceptus to pass into the mother's blood and maternal hormones to pass into the fetal blood

Immune role

Transports maternal antibodies (especially IgG) into fetal blood to confer passive immunity on fetus

most other drugs in the maternal bloodstream. Nutrition, excretion, and other functions of the placenta are summarized in table 29.1.

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  • ortensia
    When the trophoblastic nutrition and placenta contributed equally?
    9 years ago

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