Chapter Summary
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42.1 Asexual Reproduction Is Efficient but Limits Genetic Variability
Asexual reproduction produces offspring that are genetically identical to their parent and to one another; it produces no genetic diversity.
Means of asexual reproduction include budding, regeneration, and parthenogenesis. Review Figures 42.1–42.3
42.2 Sexual Reproduction Involves Union of Haploid Egg and Sperm
Sexual reproduction involves three basic steps: gametogenesis, spawning or mating, and fertilization.
Gametogenesis and fertilization are similar in all animals, but spawning and mating include a great variety of anatomical, physiological, and behavioral adaptations.
Gametogenesis occurs in testes and ovaries. In spermatogenesis (the production of sperm) and oogenesis (the production of eggs), the germ cells proliferate mitotically, undergo meiosis, and mature into gametes.
Each primary spermatocyte can produce four haploid sperm through the two divisions of meiosis. Review Figure 42.4A
Primary oocytes immediately enter prophase of the first meiotic division, and in many species, including humans, their development is arrested at this point. Each oogonium produces only one egg. Review Figure 42.4B
Fertilization involves sperm activation, species-
specific binding of sperm to egg, the acrosomal reaction, digestion of a path through the protective coverings of the egg, and fusion of sperm and egg cell membranes. Fusion of these two membranes triggers blocks to polyspermy, which prevent additional sperm from entering the egg and, in mammals, signal the egg to complete meiosis and begin development. Review Figure 42.5, Animation 42.1 External fertilization is common in aquatic species. Internal fertilization is necessary in terrestrial species and usually involves copulation.
Hermaphroditic, or monoecious, species have both male and female reproductive systems in the same individual, either sequentially or simultaneously. Dioecious species have separate male and female individuals.
Animals can be classified as oviparous or viviparous, depending on whether the early stages of development occur outside or inside the mother’s body.
42.3 Male Sex Organs Produce and May Deliver Sperm
Males produce semen consisting of sperm suspended in seminal fluid (which nourishes the sperm and facilitates fertilization).
Sperm are generated in the seminiferous tubules of the testes, mature in the epididymis, and are delivered to the urethra through the vasa deferentia. Other components of semen are produced in the seminal vesicles, prostate gland, and bulbourethral glands. Review Figure 42.9, Activities 42.1, 42.2
All components of the semen join in the urethra at the base of the penis and are ejaculated through the erect penis by muscle contractions at the climax of copulation.
Spermatogenesis depends on testosterone secreted by the Leydig cells of the testes, which are under the control of hormones produced in the anterior pituitary and the hypothalamus. The production of these hormones is controlled by negative feedback from testosterone and from inhibin, a hormone produced by the Sertoli cells of the testes. Review Figures 42.10, 42.11
42.4 Female Sex Organs Produce Eggs and Nurture Embryos
Primary oocytes in the ovary are surrounded by granulosa and thecal cells that make up a follicle. Each month a small number of follicles respond to pituitary FSH by growing, producing estrogen, and promoting development of the oocyte.
Usually in each cycle only one follicle fully matures and releases its oocyte, which is picked up by the opening of the oviducts. Sperm deposited in the vagina during copulation move up through the cervix and uterus into the oviducts. Fertilization occurs in the upper regions of the oviducts. Review Figure 42.12, Activity 42.3
The maturation and release of eggs constitute an ovarian cycle. The uterine, or menstrual, cycle prepares the uterus for receipt of a blastocyst. If no blastocyst is implanted, the lining of the uterus sloughs off in the process of menstruation. Review Focus: Key Figure 42.13, Animation 42.2
Both the ovarian and the uterine cycles are under the control of hypothalamic and pituitary hormones, which in turn are under the feedback control of estrogen and progesterone. Review Figure 42.14
Childbirth is initiated by hormonal and mechanical stimuli that increase the contractility of uterine muscle. Review Figures 42.14, 42.15A
42.5 Fertility Can Be Controlled
Methods of contraception include abstention from copulation and the use of technologies that decrease the probability of fertilization. Review Table 42.1
Assisted reproductive technologies (ARTs) have been developed to increase fertility.
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