When semen is deposited in the vagina, sperm travel through the cervix and uterus and into the fallopian tubes, where fertilization may occur if an ovum is present. The majority of sperm deposited in the vagina die within 1 to 2 hours because of the normal acidic environment. The sperm are aided in their travel into the fallopian tubes by uterine and tubal contractions and favorable mucous conditions.
The fertilized ovum, or zygote, remains in the fallopian tube for about 3 days. While in the tube, the fertilized ovum divides repeatedly to form a round mass of cells called the morula. If there is an obstruction in the fallopian tube, the fertilized ovum may become trapped in the tube and attach itself to the lining of the tube, giving rise to an ectopic, or tubal, pregnancy. In a normal pregnancy, approximately 6 to 8 days after fertilization, the morula becomes a blastocyst, which migrates through the tube into the uterus, where it attaches itself to the endometrium (implantation), with the inner cell mass adjacent to the endometrial surface. Substances that destroy the surface epithelial cells are released, allowing the blastocyst to burrow into the endometrium. The endometrium then grows over the invading blastocyst.
The primitive chorion, the combination of trophoblast and primitive mesoderm, secretes a luteinizing hormone known as human chorionic gonadotropin (hCG), which controls the corpus luteum and inhibits pituitary gonadotropic activity. Quickly thereafter, as the invasion proceeds, maternal venous blood vessels are tapped to form lakes of blood, and chorionic villi develop. These can be identified as early as the 12th day after fertilization. These villi develop a leafy appearance and are called the chorion frondosum. By the 15th day after fertilization, the maternal arterial vessels are tapped, and by the 17th to 18th day, a functioning placental circulation is established. At term, the uteroplacental blood flow is estimated to be about 550 to 705 mL/minute. Figure 23-1A illustrates the path of sperm, fertilization, and implantation.
Decidua is the name given to the endometrium of pregnancy. There are three types, distinguished by location with regard to the growing embryo. The decidua capsularis is the
overlying endothelium that covers the conceptus, and the decidua basalis is the decidual tissue lying between the blastocyst and the myometrium. The decidua of the remainder of the endometrial cavity is the decidua vera. A cross section through the uterus of a pregnant woman and the different types of decidua in early pregnancy are illustrated in Figure 23-1B and C.
One of the first placental hormones produced by the developing trophoblastic tissue is hCG. This hormone is present as early as the 8th day after fertilization has taken place. The titers increase to a maximal level by about the 60th to 70th days after fertilization and then decrease. The primary function of hCG is to maintain the corpus luteum during the first 2 months of pregnancy until the placenta can produce enough progesterone by itself. Other hormones, such as human placental lactogen, human chorionic thyrotropin, and adrenocorticotropic hormone, and estrogens are also produced by the placenta. It is beyond the scope of this book to discuss the actions of these hormones;the reader is referred to the references at the end of this chapter for further information.
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