chap20outline - Chapter 20 Pregnancy Growth and Development...

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Chapter 20 – Pregnancy, Growth, and Development 20.1 Introduction (p. 551) A. Development, which includes an increase in size, is the continuous process by which an individual changes from one life phase to another. B. The life phases are the prenatal period, which begins at fertilization and ends at birth, and the postnatal period, which begins at birth and ends at death. 20.2 Pregnancy (p. 551) A. Pregnancy is the presence of developing offspring in the uterus, an event resulting from fertilization. B. Transport of Sex Cells (p. 551; Fig. 20.1) 1. Sperm cells must reach the upper one-third of the uterine tubes for fertilization to occur. 2. Under the influence of estrogen during the first half of the menstrual cycle, uterine secretions are thin, allowing sperm cells to swim easily toward their destination. C. Fertilization (p. 551; Fig. 20.2) 1. With the aid of the acrosomal enzyme, the sperms cells erode away the corona radiata and zona pellucida surrounding the secondary oocyte, and one sperm cell penetrates the egg cell membrane. 2. When penetration occurs, changes in the egg cell membrane and zona pellucida prevent the entry of additional sperm cells. 3. Fusion of egg and sperm nuclei completes fertilization. 4. Fertilization results in a diploid zygote. 20.3 Prenatal Period (p. 552) A. Early Embryonic Development (p. 552; Fig. 20.3-20.5) 1. Cells undergo a period of mitosis called cleavage, when cells become smaller and smaller. 2. The dividing mass of cells (morula) moves down the uterine tube to the uterus, where a stage called the blastocyst implants in the lining of the uterus. 3. The offspring is called an embryo during the first eight weeks of development, and a fetus thereafter. 4. Some of the cells of the blastocyst become the placenta. B. Hormonal Changes during Pregnancy (p. 554; Fig. 20.6; Table 20.1) 1. The outer layer of cells (trophoblast) of the blastocyst stage secrete the hormone human chorionic gonadotropin (HCG), which maintains the corpus luteum and thus also maintains the uterine lining and the pregnancy. 2.
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chap20outline - Chapter 20 Pregnancy Growth and Development...

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