Concept 37.1   Sexual Reproduction Depends on Gamete Formation and Fertilization

• In sexual reproduction the parents produce gametes that have only half as many chromosomes as the other cells in the body. The gametes of the female parent are relatively large, nonmotile cells called eggs or ova. Those of the male are small cells that swim, typically using flagella, and are called spermatozoa or sperm. The gametes fuse to produce a single-celled zygote that has a full set of chromosomes and develops into a sexually produced offspring. Review Figure 37.1
• Gametes are produced by meiosis and therefore are diversified in their chromosomes and genes by independent assortment and crossing over. The new individual that is formed when gametes fuse is not genetically identical to either parent.
• In asexual reproduction offspring are produced by mitosis and are genetically identical to their parent and to one another. Asexual reproduction produces no genetic diversity but has the advantage of maintaining favorable combinations of genes. Review Figure 37.2
• In sexually reproducing animals, the male and female sexes differ in their primary reproductive organs, accessory reproductive organs, and secondary sexual characteristics. The primary reproductive organs are the gonads: the ovaries in females and testes in males. The accessory reproductive organs are the reproductive organs other than the gonads. The secondary sexual characteristics are sex-specific properties of nonreproductive tissues and organs, such as facial hair, chest hair, and a deep voice in human males. Review Figure 37.3
• Gametogenesis occurs in testes and ovaries, where the germ cells proliferate mitotically, then undergo meiosis to produce gametes. Review Figure 37.4
• The gonads, in addition to having germ cells and producing gametes, typically contain somatic cells that contribute to reproduction. In testes, for example, Sertoli cells assist sperm production and interstitial cells (Leydig cells) produce the sex hormone testosterone. Review Figure 37.5 and ACTIVITY 37.1
• An oocyte (developing ovum) and its support cells are together called an ovarian follicle. The release of an ovum from the ovary is called ovulation. Review Figure 37.6
• Fertilization—the fusion of sperm and ovum—can be external or internal. External fertilization is common among aquatic animals. To achieve fertilization, these animals simply release their gametes into the water, a process termed spawning. Fertilization then occurs in the water outside the female’s body. Internal fertilization occurs when a male inserts sperm inside the reproductive tract of a female, permitting fertilization to occur inside the female’s body.
• For fertilization to occur, several steps must occur. In many animals these steps include species-specific binding of sperm to protective material surrounding the ovum, the acrosomal reaction, the sperm’s passage through the protective layers covering the ovum, and the fusion of the sperm and ovum cell membranes. Review Figure 37.7 and ANIMATED TUTORIAL 37.1
• The entry of a sperm into the ovum triggers blocks to polyspermy, which prevent additional sperm from entering the ovum.
• Sex determination is the process by which the sex of an individual becomes fixed. In humans and other placental mammals, sex is determined at fertilization and depends on which sex chromosomes the zygote receives from its parents.
• In some animals, sex is not determined at fertilization. Some of these species exhibit environmental sex determination, in which the sex of an individual is determined by the temperature it experiences during its embryonic development.
• Some species are sequential hermaphrodites; they can be males at one time and females at another during their lives. Review Figure 37.8 and Figure 37.9

Concept 37.2   The Mammalian Reproductive System Is Hormonally Controlled

• Somatic cells in the gonads produce steroid sex hormones. The principal sex steroid in males is testosterone, which is secreted by the interstitial (Leydig) cells in the testes. In females, the somatic cells of the ovarian follicles secrete feminizing steroids called estrogens. After ovulation, the follicular somatic cells remaining in the ovary reorganize to form a corpus luteum, which secretes another type of female sex steroid, progesterone, which is involved principally in coordinating processes associated with pregnancy.
• Ova mature in the female’s ovaries, and after release (ovulation), they enter the oviducts (fallopian tubes). Sperm deposited in the vagina during copulation move through the cervix and uterus into the oviducts, where fertilization occurs. Review Figure 37.10 and ACTIVITY 37.2
• If fertilization has occurred, the zygote undergoes its early development as it travels the length of the oviduct and arrives in the uterus as an early embryo called a blastocyst. The blastocyst buries itself in the endometrium of the uterus—a process called implantation.
• Ovulation is either induced or spontaneous. Induced ovulation is ovulation triggered by copulation. In spontaneous ovulation, the timing of ovulation is under control of endogenous hormonal cycles in the female.
• In most species of mammals that display spontaneous ovulation, a female ovulates in cycles. In primates these cycles are called menstrual cycles, in recognition of the fact that menstruation—the sloughing off of part of the endometrium—occurs in each cycle that does not result in pregnancy.
• Mammals other than primates do not menstruate. Often, however, they undergo dramatic changes in behavior: they cyclically enter estrus (heat). Their cycles accordingly are called estrous cycles.
• The human menstrual cycle is under the control of hormones secreted by the hypothalamus, anterior pituitary gland, somatic cells in the ovarian follicle, and the corpus luteum. Review Figure 37.11 and ANIMATED TUTORIAL 37.2
• The uterus requires progesterone to support a pregnancy. In humans, chorionic gonadotropin, secreted by the blastocyst, signals the corpus luteum not to degenerate and instead to provide adequate progesterone for the first 7–10 weeks of pregnancy. By then, the placenta—which also secretes progesterone—is sufficiently well developed to take on this task.
• The male reproductive organs produce and deliver semen. Semen consists of sperm suspended in a fluid that nourishes the sperm and facilitates fertilization. This fluid is produced by accessory reproductive glands, such as the prostate gland. Review Figure 37.12 and ACTIVITY 37.3
• Spermatogenesis depends on testosterone secreted by the interstitial (Leydig) cells of the testes, which themselves are under the control of hormones produced in the anterior pituitary gland.
• Methods of contraception (prevention of pregnancy) include abstention from intercourse and the use of technologies that decrease the probability of fertilization and implantation. Review Table 37.1 Part 1 and Table 37.1 Part 2

Concept 37.3   Reproduction Is Integrated with the Life Cycle

• Many animal species have evolved mechanisms of decoupling successive steps in the reproductive process. Such mechanisms increase options for certain steps to be coordinated with environmental conditions independently of other steps. These mechanisms include sperm storage and embryonic diapause, which in mammals is called delayed implantation. Review Figure 37.13
• Some animals are semelparous: each individual is physiologically programmed to reproduce only once in its lifetime. Examples include octopuses and Pacific salmon. Review Figure 37.14
• Most animal species are iteroparous, meaning that individuals are physiologically capable of two or more separate periods of reproductive activity during their lives. In iteroparous animals that live in environments with regular seasonal cycles, the reproductive cycle is nearly always timed to coordinate with the environmental seasonal cycle.