Human Reproduction and Development
40.1 From Embryology to Developmental Biology: A Descriptive Science Becomes Experimental
1. In the mid-eighteenth century, the theory of preformation gave way to the theory of epigenesis as biologists observed embryos specialize over time. Karl Ernst von Baer noted that early vertebrate embryos look alike but gradually become more distinctive.
2. Cells of the early embryo are totipotent. Gradually, biochemically distinct regions of the embryo develop and differentiate down specific developmental pathways.
3. A cell commits to a fate at a certain point in development. Before this time, a transplanted cell develops according to its new surroundings; after this point, it retains the specialization other cells exhibit in its original location. Differential gene expression underlies cell specialization.
40.2 Setting the Stage for Human Prenatal Development - The Reproductive System
4. Developing sperm originate in seminiferous tubules within the paired testes. Sperm mature in the epididymis and vasa deferentia, and they exit the body through the urethra during ejaculation. The prostate gland, seminal vesicles, and bulbourethral glands add secretions to sperm.
5. Oocytes originate in the female gonads, the ovaries. Each month after puberty, one ovary releases an oocyte into a fallopian tube, which leads to the uterus.
6. The ovaries secrete estrogen and progesterone, hormones that stimulate development of female sexual characteristics and together with GnRH, FSH, and LH control the menstrual cycle.
40.3 The Structures and Stages of Human Prenatal Development
7. Human prenatal development begins at fertilization. A single, capacitated sperm cell at a secondary oocyte burrows through the zona pellucida and corona radiata. The two pronuclei join. Cleavage ensues, and a 16-celled morula forms. Between days 3 and 6, the morula arrives at the uterus and hollows to form a blastocyst, made up of individual blastomeres. The trophoblast layer and inner cell mass form. Implantation occurs between days 6 and 14. Trophoblast cells secrete human chorionic gonadotropin (hCG), which prevents menstruation.
8. During the second week, the amniotic cavity forms as the inner cell mass flattens, forming the embryonic disc. The primitive streak appears. Ectoderm and endoderm form, and then mesoderm appears, establishing the germ layers of the gastrula. Cells in a particular germ layer develop into parts of specific organ systems.
9. During the third week the chorion starts to develop into the placenta, and the yolk sac, allantois, and umbilical cord form as the amniotic sac swells with fluid. Organogenesis occurs throughout this embryonic period. Gradually structures appear, including the notochord, neural tube, arm and leg buds, heart, facial structures, skin specializations, and skeleton. Structures continue to elaborate during the fetal period.
10. Labor begins as the fetus presses against the cervix. Uterine contractions expel the baby, placenta, and extraembryonic membranes.40.4 Growth and Development After Birth
11. Postnatal stages include the newborn, infant, child, adolescent, and adult. Drastic changes occur at birth, as the newborn takes on functions that the pregnant woman provided. Infancy and childhood are periods of rapid growth and maturation of organ systems. Hormonal changes dominate adolescence. Organ systems function during adulthood and begin to show signs of aging.
12. In passive aging, structures break down, and DNA repair becomes less efficient. In active aging, lipofuscin accumulates in cells and cells die.
13. The theoretical maximum human life span is 120 years. Life expectancy is a more realistic measure of how long someone will live, based on age and epidemiological information.