Anatomical And Morphological Relationships

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A. Implantation

After fertilization, the zygote immediately initiates cell division by mitosis and passes through the 2-, 4-, and 8-cell stages until a tiny cluster of cells known as a morula is formed. After 3-4 days the morula reaches the uterus where it continues to grow. The morula now differentiates to create an outer shell, designated as the trophoblast, and an inner mass of cells that will ultimately become the embryo. This fluid-filled ball or blastocyst adheres to the surface of the uterine endometrium and begins the implantation process by the secretion of proteolytic enzymes, which digest a cavity in the endometrium (see Figure 14-1).

The sequelae of events leading from implantation of the blastocyst to formation of the fetal placental-decidual unit with its supporting fetal-maternal circulatory systems are summarized in Figures 14-2, 14-3, and 14-4A,B, respectively.

B. Fetal Placental-Decidual Unit

1. Background

The fetal placental unit collectively serves three important functions: (i) as a source of protein and steroid hormones, which are delivered to the maternal circulation; (ii) as a selective barrier that determines the nature of the communication and interaction between the maternal and fetal systems; and (iii) as a participant in the control of fetal growth, development, and endocrine function.

The subsequent stages of maturation of the blastocyst into the early fetus (56 days) are illustrated in Figure 14-2. The maternal-fetal circulatory system is established with the creation of an elaborate labyrinth network. As diagrammed in Figure 14-3, the fetus and maternal placenta have entirely separate circulatory systems; there is no direct communication between these two tissues.

Following implantation, the trophoblast of the blastocyst proliferates new villi rapidly. These villi infiltrate the endometrial decidua and generate the syncyti-

Uterine lumen Endometrium of uterus

Uterine lumen Endometrium of uterus

figure 14-1 Illustration of the blastocyst implantation site in the uterine endometrium at 12 days of gestation. The syncytiotrophoblast (stippled) layer of cells of the blastocyst invades and surrounds some endometrial capillaries. Columns of cytotrophoblast have begun to grow outwardly in a few areas to create primary villi (see Figure 14-3). AC is the amniotic cavity. Figure 14-8 also illustrates the close anatomical relationship of the cytotrophoblast and the syncytiotrophoblast as they collaborate to produce a variety of hormones. Modified with permission from Strauss III, J. F., Gafrels, M., and King, B. F. (1995). Placental hormones. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaux, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, Chapt. 124, pp. 2171-2206. W. B. Saunders Co., Philadelphia, PA.

figure 14-1 Illustration of the blastocyst implantation site in the uterine endometrium at 12 days of gestation. The syncytiotrophoblast (stippled) layer of cells of the blastocyst invades and surrounds some endometrial capillaries. Columns of cytotrophoblast have begun to grow outwardly in a few areas to create primary villi (see Figure 14-3). AC is the amniotic cavity. Figure 14-8 also illustrates the close anatomical relationship of the cytotrophoblast and the syncytiotrophoblast as they collaborate to produce a variety of hormones. Modified with permission from Strauss III, J. F., Gafrels, M., and King, B. F. (1995). Placental hormones. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaux, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, Chapt. 124, pp. 2171-2206. W. B. Saunders Co., Philadelphia, PA.

Amniotic cavity

Blast ocoel

Blast ocoel

Embryonic disc Entoderm Trophectoderm

Chorionic villi

Amniotic cavity

Yolk sac

Ectoderm Mesoderm

Yolk sac

Embryonic disc Entoderm Trophectoderm

Chorionic villi

Ectoderm Mesoderm

42 days

10.0 mm

Fused amnion and chorion figure 14-2 Representation of the various stages of development from blastocyst to early fetus. The blastocyst (upper left corner), which has resulted from the union of the spermatozoon and ovum, at ~7 days of age adheres to the surface of the maternal endometrium and initiates the implantation process, which ultimately leads to the development of the fetus (lower right).

42 days

10.0 mm

Fused amnion and chorion figure 14-2 Representation of the various stages of development from blastocyst to early fetus. The blastocyst (upper left corner), which has resulted from the union of the spermatozoon and ovum, at ~7 days of age adheres to the surface of the maternal endometrium and initiates the implantation process, which ultimately leads to the development of the fetus (lower right).

otrophoblast cells, which are in direct contact with the maternal blood, and the cytotrophoblast cells, which lie directly underneath (see Figure 14-3). Simultaneously the embryonic blood vessels develop in the tro-phoblast, and as soon as it is vascularized, it is termed the chorion (see Figure 14-3). Subsequently, another fetal membrane, the amnion, develops around the growing embryo; the amnion is connected to the chorion by the body stalk. The villi in this region enlarge to form the chorion frondosum, which ultimately develops into the fetal portion of the placenta. The smooth outer surface of the chorion is termed the chorion laeve. The body stalk then ultimately develops into the umbilical cord. Thus, the fully developed pla-

Medieval Knights Clothing Diagram
FIGURE 14-3 Diagram of the maternal and fetal circulating circulatory systems that emphasizes the entirely separate nature of the two systems.

centa consists of the maternal decidua basalis and the fetal chorion frondosum (see Figure 14-4A,B).

2. Placenta

The primary function of the placenta is to effect communication (both endocrine and nutrition) between the mother and the developing fetus and, at the same time, to maintain the genetic and immune integrity of both individuals. The rate of growth of the placenta is most rapid during the first trimester; by full term the placenta weighs between 500 and 600 g and measures 2-3 cm in thickness and 1520 cm in diameter.

One obvious function of the placenta is to provide the fetus with oxygen and all nutrients and to carry away excretory products. A second major role of the placenta is to serve as an endocrine gland to maintain pregnancy. In this respect, the placenta takes over the endocrine function of the ovary and the anterior pituitary.

Cholesterol from the mother is metabolized by the placenta into progesterone. In the interim, until placental production of progesterone is firmly established, the trophoblast secretes human chorionic gonadotropin, or hCG, which maintains the production of progesterone by the maternal corpus luteum. To form the other steroid hormones, placentally derived progesterone is transported to the fetal liver and fetal adrenal and then returned to the placenta for transformation into estrogens and a group of androgens, including testosterone.

3. Decidua

The endometrial stoma that surrounds the concep-tus shortly after implantation is referred to as the decidua. As the conceptus develops into a fetus, the decidua tissue that grows over the invaded blastocyst-fetus is termed the decidua capsularis, while the tissue beneath the blastocyst-fetus is termed the decidua basalis (see Figure 14-4A). Decidual cells are known to have some endocrine capabilities and can produce prolactin, relaxin, and, during labor, prostaglandins.

C. Mammary Glands

The breasts or mammary glands are functionally related to the female reproductive system in that they secrete milk for nourishment of the newborn; anatomically the breasts are related to the skin (see Figure 145). Each breast comprises 15-20 lobes, which overlie the major chest muscle, the pectoralis. They lie between the second and sixth ribs. Each lobe consists of glandular, lobular tissue that is individually drained by intralobular ducts, which in turn empty into the main lactiferous ducts. The breast tissue is responsive during pregnancy to estrogen, progesterone, prolactin, and oxytocin. The process of lactation is one of the most complex endocrine-mediated events in the body. The maturation of the mammary glands is initiated when the developing zygote is implanted in the uterus. Estrogens are responsible for the growth of the duct system, while progesterone mediates generation of the secretory system; then prolactin stimulates the production of milk.

In the first trimester of pregnancy, initial rapid growth and branching from the terminal portions of the gland are followed by the appearance of true glandular acini. The hollow alveolus becomes lined with a single layer of myoepithelial cells, ultimately enclosing the glandular alveolus in a loose network surrounded by capillaries so that the lumen of the alveolus is connected to an interlobular duct (see Figure 14-6). In the second trimester of pregnancy, the alveolar secretion begins. In the third trimester, particularly in the ninth month, breast enlargement occurs as a consequence of the hypertrophy of parenchymal cells and distention of the alveoli with colostrum.

In the human, the rate of milk production is normally, by 1 week postpartum, 500 ml/day; after 3 weeks milk production increases to 800-1000 ml/day. Table 14-1 tabulates the composition of human and bovine milk. The colostrum or milk obtained in the several days immediately following parturition differs from milk in its biological and physical properties. The

figure 14-4 Midsagittal section of a uterus with developing embryo, illustrating the placental and fetal membranes (panel A) at 1 month of gestation and (panel B) at 9 months of gestation. In A the villi adjacent to the decidua basalis are well-developed (chorion frondosum), whereas elsewhere the villi have regressed (chorion laeve). The chorion laeve is made up of the trophoblast (black) and mesoderm (stippled); at this stage it is covered by the decidua capsularis, but has not yet fused with the decidua parietalis. In B the chorion frondosum and decidua basalis have the same relation as in A; however, the chorion laeve and amnion have fused (chorioamnion) and the decidua capsularis has degenerated, causing the chorioamnion to become opposed to the decidua parietalis, which obliterates the uterine lumen. Abbreviations: AC, amniotic cavity; UC, umbilical cord; YS, yolk sac; EXO, exocoelom. Modified with permission from Strauss III, J. F., Gavels, M., and King, B. F. (1995). Placental hormones. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaux, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, pp. 2171-2206. W.B. Saunders, Philadelphia, PA.

figure 14-4 Midsagittal section of a uterus with developing embryo, illustrating the placental and fetal membranes (panel A) at 1 month of gestation and (panel B) at 9 months of gestation. In A the villi adjacent to the decidua basalis are well-developed (chorion frondosum), whereas elsewhere the villi have regressed (chorion laeve). The chorion laeve is made up of the trophoblast (black) and mesoderm (stippled); at this stage it is covered by the decidua capsularis, but has not yet fused with the decidua parietalis. In B the chorion frondosum and decidua basalis have the same relation as in A; however, the chorion laeve and amnion have fused (chorioamnion) and the decidua capsularis has degenerated, causing the chorioamnion to become opposed to the decidua parietalis, which obliterates the uterine lumen. Abbreviations: AC, amniotic cavity; UC, umbilical cord; YS, yolk sac; EXO, exocoelom. Modified with permission from Strauss III, J. F., Gavels, M., and King, B. F. (1995). Placental hormones. In "Endocrinology" (L. J. DeGroot, M. Besser, H. G. Burger, J. L. Jameson, D. L. Loriaux, J. C. Marshall, W. D. Odell, J. T. Potts, Jr., and A. H. Rubenstein, eds.), 3rd ed., Vol. 3, pp. 2171-2206. W.B. Saunders, Philadelphia, PA.

principal difference is a higher protein content (up to 8 and 20% in the human and bovine, respectively).

III. CHEMISTRY, BIOCHEMISTRY, AND BIOLOGICAL RESPONSES

A. Pregnancy

1. Introduction

The endocrine changes accompanying pregnancy are remarkable. A pregnant woman in the late phase of the third trimester produces, on a daily basis, some 250-300 mg of progesterone, 15-20 mg of 17/3-estradiol, 50-100 mg of estriol, 75-100 mg of Cortisol, 3-8 mg of deoxycorticosterone (DOC), and 1-2 mg of aldosterone. In addition, there is a massive production of human chorionic somatomammotropin (in excess of 1 g/day), human chorionic gonadotropin, human cho rionic thyrotropin, and chorionic ACTH, as well as increased plasma levels of the angiotensins and renin. Figure 14-7 summarizes the temporal changes of many of these hormones throughout gestation.

2. Peptide Hormones a. Background

Each of the morphological variants of the trophoblast cells of the chorionic villi (see Figure 14-3) has a distinctive capability to produce a wide variety of steroid, peptide, and protein hormones (see Figure 148). The syncytiotrophoblast cells are in direct contact with the maternal blood where chorionic gonadotropin (CG), chorionic somatomammotropin (CS), chorionic proopiomelanocortin, and progesterone are present. However, these hormones do not readily gain access to the fetus due to the several barriers, which include the cytotrophoblast, the mesenchymal stroma of the

FIGURE 14-5 Ducts and glandular tissue of the human mammary gland.
mammary gland during lactation

contains nucl., usual organelles, 5 nm filaments

FIGURE 14-6 Schematic diagram of a mammary gland alveolus and its duct: (A) mammary gland; (B) alveolus and its duct; (C) cellular organization of alveolar duct. (Drawn by Dr. L. Paavola, Department of Anatomy, Temple University Medical School.)

contains nucl., usual organelles, 5 nm filaments

FIGURE 14-6 Schematic diagram of a mammary gland alveolus and its duct: (A) mammary gland; (B) alveolus and its duct; (C) cellular organization of alveolar duct. (Drawn by Dr. L. Paavola, Department of Anatomy, Temple University Medical School.)

TABLE 14-1 Composition of Milk

Bovine Human

TABLE 14-1 Composition of Milk

Bovine Human

Water

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