Lecture Outline for Campbell/Reece Biology, 8th Edition

Chapter 46 Animal Reproduction Lecture Outline Overview: Pairing Up for Sexual Reproduction Individuals are transient, and a population transcends the finite life spans of its members only by reproduction, the generation of new individuals from existing ones. In a sense, all aspects of animal form and function are adaptations that contribute to reproductive success. Concept 46.1 Both asexual and sexual reproduction occur in the animal kingdom. Sexual reproduction is the formation of offspring by the fusion of haploid gametes to form a diploid zygote. The female gamete, the unfertilized egg, is usually large and nonmotile. The male gamete is the sperm, which is usually small and motile. Asexual reproduction is the formation of individuals whose genes come from a single parent. There is no fusion of sperm and egg. Reproduction relies entirely on mitotic cell division. Diverse mechanisms of asexual reproduction enable animals to produce identical offspring rapidly. Many invertebrates reproduce asexually by fission, in which a parent separates into two or more approximately equal-sized individuals. Budding is a form of asexual reproduction in which new individuals split off from existing ones. Stony corals, which can grow to be more than 1 m across, are cnidarian colonies of several thousand connected individuals. In fragmentation, the body breaks into several pieces, followed by regeneration of lost body parts. In sea stars (starfish) of the genus Linckia, each of the five arms, if broken off the body, can regenerate an entire sea star. Numerous sponges, cnidarians, bristle worms, and sea squirts reproduce by fragmentation and regeneration. In parthenogenesis, an egg develops without being fertilized. The progeny of parthenogenesis can be either haploid or diploid. If haploid, they develop into adults that produce eggs or sperm without meiosis. Male (drone) honeybees are fertile haploid adults that arise by parthenogenesis. Recently, a female Komodo dragon and hammerhead shark produced parthenogenetic offspring in captivity, despite being kept apart from males. Sexual reproduction is an evolutionary enigma. Sex must enhance reproductive success or survival because it would otherwise rapidly disappear. Consider an animal population in which half the females reproduce sexually and half reproduce asexually, producing two offspring each. The two offspring of the asexual female are both daughters, each able to give birth to more reproductive daughters. In contrast, half of the sexual female’s offspring are male. The number of offspring remains the same at each generation because both a male and a female are required to reproduce. Thus the asexual condition increases in frequency at each generation. Despite this “twofold cost,” sex is maintained in the vast majority of eukaryotic organisms. Most hypotheses about the advantages of sex focus on the unique combinations of parental genes formed during meiotic recombination and fertilization. By producing diverse offspring, sexual reproduction may enhance the reproductive success of parents when environmental factors, such as pathogens, change rapidly. Asexual reproduction would be most advantageous in stable, favorable environments. Beneficial gene combinations arising through recombination may speed up adaptation. The theoretical advantage of this is significant only when the rate of beneficial mutations is high and the population size is small. Shuffling of genes during sexual reproduction may allow populations to rid themselves of harmful genes more readily. Experiments to test these and a range of other hypotheses are under way in many laboratories. Most animals exhibit variation in reproductive activity. Most animals exhibit cycles in reproductive activity, usually related to changing seasons. Animals can thus conserve resources and reproduce when more energy is available and when environmental conditions favor the survival of offspring. For example, ewes (female sheep) give birth to lambs in the early spring, the time when their chances of survival are optimal. Reproductive cycles are controlled by hormones, which are regulated by environmental cues such as changes in day length, seasonal temperature, rainfall, and lunar cycles. Animals may reproduce exclusively asexually or sexually, or they may alternate between the two modes, depending on environmental conditions. Daphnia reproduce by parthenogenesis under favorable conditions and sexually during times of environmental stress. Several genera of fishes, amphibians, and lizards reproduce by a form of parthenogenesis that produces diploid “zygotes.” Fifteen species of whiptail lizards reproduce exclusively by parthenogenesis. There are no males in these species, but the lizards carry out courtship and mating behaviors typical of sexual species of the same genus. During the breeding season, one female of each mating pair mimics a male. An individual adopts female behavior prior to ovulation, when the level of the female sex hormone estradiol is high, then switches to male-like behavior after ovulation, when the level of progesterone is high. These parthenogenetic lizards evolved from species having two sexes, and they still require certain sexual stimuli for maximum reproductive success. Sexual reproduction presents a problem for sessile or burrowing animals or parasites that may have difficulty encountering a member of the opposite sex. An evolutionary solution to this problem is hermaphroditism, in which one individual functions as both a male and a female. Some hermaphrodites can self-fertilize, but most mate with another member of the same species. In such a mating, each individual receives and donates sperm, resulting in twice as many offspring as would be produced if only one set of eggs were fertilized. Another reproductive pattern involves sex reversal, in which an individual changes its sex during its lifetime. The bluehead wrasse (Thalassoma bifasciatum) exhibits sex reversal from female to male. This coral reef fish lives in harems consisting of a single male and several females. When the male dies, the largest (and usually oldest) female in the harem becomes the new male, producing sperm and defending the harem against intruders. Certain oyster species provide an example of sex reversal from male to female. Oysters reproduce as males and then later become female. Because the number of gametes produced generally increases with size much more for females than for males, sex reversal in this direction maximizes gamete production. Concept 46.2 Mechanisms for fertilization bring together sperm and eggs of the same species. The mechanisms of fertilization, the union of sperm and egg, play an important part in sexual reproduction. In external fertilization, eggs and sperm are both released into a wet environment. In species with internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract. Successful fertilization requires careful timing. A moist habitat is required for external fertilization, to prevent gametes from drying out and to allow the sperm to swim to the eggs. In species with external fertilization, timing is crucial to ensure that mature sperm and eggs encounter one another. Individuals clustered in the same area may release their gametes into the water at the same time in response to chemical signals or environmental cues, a process known as spawning. When external fertilization is not synchronous across a population, individuals may engage in courtship behavior that leads to fertilization of the eggs of one female by one male. Internal fertilization is an adaptation to terrestrial life that enables sperm to reach an egg in a dry environment. Internal fertilization requires cooperative and sophisticated reproductive systems, including copulatory organs that deliver sperm and receptacles for sperm storage and transport. Mating animals may use pheromones, chemical signals released by one organism that influence the behavior or physiology of other individuals of the same species. Pheromones are small, volatile, water-soluble molecules that disperse into the environment and, like hormones, are active in minute amounts. Many pheromones attract males. Developing embryos are protected in various ways. All species produce more offspring than survive to reproduce. Species with external fertilization tend to produce very large numbers of gametes, but few survive. Internal fertilization tends to involve the production of fewer zygotes. The internally fertilized eggs of many species of terrestrial animals exhibit adaptations that protect against water loss and physical damage during their external development. In birds and other reptiles, as well as monotremes (egg-laying mammals), the zygote is protected by an amniote egg with a shell and a set of internal membranes. The fertilized eggs of fishes and amphibians have a gelatinous coat and lack internal membranes. Some animals retain the embryo within the female reproductive tract. Marsupial mammals retain their embryos in the uterus for only a short time. The embryos crawl out and complete fetal development attached to a mammary gland in the mother’s pouch. The embryos of eutherian mammals develop entirely within the uterus, nourished by the mother’s blood supply through the placenta. The embryos of some fishes and sharks also complete development internally, but without nutrient exchange between mother and young. Many animals provide parental care to their offspring. Birds feed their young; mammals nurse their offspring. A male Gastric Brooding Frog carries his tadpoles in his stomach until they undergo metamorphosis and hop out of his mouth as young frogs. Many invertebrates also provide parental care. Animals show variation in reproductive systems. A group of cells dedicated to serve as precursors for ova and sperm is often established very early in embryogenesis. Cycles of growth and mitosis amplify the number of cells available for making eggs or sperm. The simplest reproductive systems do not even contain discrete gonads, the organs that produce gametes in most animals. Most polychaete worms have separate sexes but lack distinct gonads; eggs and sperm develop from undifferentiated cells lining the coelom. Mature gametes may be shed through the excretory openings, or the swelling mass of eggs may split a portion of the body open, spilling the eggs into the environment. Most animals possess sets of accessory tubes and glands that carry, nourish, and protect the gametes and sometimes the developing embryos. Most insects have separate sexes with complex reproductive systems. In the male, sperm develop in a pair of testes and are passed along a coiled duct to two seminal vesicles for storage. During mating, sperm are ejaculated into the female reproductive system. Eggs develop in a pair of ovaries and are conveyed through ducts to the vagina, where fertilization occurs. In many insect species, the female reproductive system includes a spermatheca, a sac in which sperm may be stored for extended periods and released under appropriate conditions. In many nonmammalian vertebrates, the digestive, excretory, and reproductive systems have a common opening to the outside, the cloaca, a structure present in the ancestors of all vertebrates. Most mammals have a separate opening for the digestive tract; most female mammals have separate openings for the excretory and reproductive systems. In vertebrates, the uterus is partly or completely divided into two chambers. In mammals (including humans) that produce only one or a few young at a time, birds, and many snakes, the uterus is a single structure. Male reproductive systems differ mainly in the copulatory organs. Many nonmammalian vertebrates lack a penis and turn the cloaca inside out to ejaculate. Animals often mate with more than one member of the other sex; monogamy is relatively rare. However, mechanisms have evolved to diminish the chance of a female mating successfully with another male. Some male insects transfer secretions that make a female less receptive to courtship, thus reducing the likelihood of her mating again. Researchers have found that females also influence the relative reproductive success of their mates. Concept 46.3 Reproductive organs produce and transport gametes. Human reproduction involves intricate anatomy and complex behavior. The reproductive anatomy of the human female includes external and internal reproductive structures. The external reproductive structures consist of two sets of labia surrounding the clitoris and vaginal opening. The internal reproductive organs consist of a pair of gonads, which produce eggs and reproductive hormones, and a system of ducts and chambers, which receive and carry gametes and house the embryo and fetus. The ovaries, the female gonads, flank the uterus and are held in place by ligaments. Each ovary contains many follicles, consisting of an oocyte surrounded by support cells. The follicles nourish and protect the oocyte during oogenesis, the formation and development of an ovum. At birth, a woman’s ovaries contain 1–2 million follicles; only about 500 fully mature between puberty and menopause. Usually one follicle matures and releases its egg during each menstrual cycle in the process of ovulation. Prior to ovulation, cells of the follicle produce the primary female sex hormone, estradiol. After ovulation, the remaining follicular tissue develops into the corpus luteum, which secretes additional estrogens and progesterone to help maintain the uterine lining during pregnancy. If pregnancy does not occur, the corpus luteum disintegrates and a new follicle matures during the next cycle. At ovulation, the egg is released into the abdominal cavity near the opening of the oviduct. The cilia-lined funnel-like opening of the oviduct draws in the egg. Cilia and the wavelike contractions of the oviduct convey the egg through the oviduct to the uterus. The highly vascularized inner lining of the uterus is called the endometrium. The neck of the uterus, the cervix, opens into the vagina. The vagina is a thin-walled chamber that forms the birth canal and is the repository for sperm during copulation. The vagina opens to the outside at the vulva, the collective term for the external female genitalia. Until ruptured by sexual intercourse or vigorous physical activity, the vaginal opening is partially covered by a thin sheet of tissue called the hymen. The vaginal and urethral openings are located within a recess called the vestibule, which is surrounded by a pair of slender folds called the labia minora. The thick, fatty labia majora enclose and protect the labia minora and vestibule. The clitoris is found at the front edge of the vestibule. During sexual arousal, the clitoris, vagina, and labia engorge with blood and enlarge. During sexual arousal, Bartholin’s glands secrete mucus into the vestibule, providing lubrication and facilitating intercourse. Mammary glands are present in both males and females but normally produce milk only in females. Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts opening at the nipple. The breasts contain connective and fatty (adipose) tissue in addition to the mammary glands. Because the low level of estradiol in males prevents the development of the fat deposits, male breasts remain small. The male’s external reproductive organs consist of the scrotum and penis. The internal reproductive organs consist of gonads that produce sperm and reproductive hormones, accessory glands that secrete products essential to sperm movement, and ducts to carry the sperm and glandular secretions. The male gonads, or testes, consist of highly coiled tubes surrounded by layers of connective tissue. The tubes are seminiferous tubules, where sperm are produced. Leydig cells scattered between the seminiferous tubules produce testosterone and other androgens. The scrotum, a fold in the body wall, holds the testes outside the body cavity at a temperature about 20C below that of the abdomen. The testicular temperature is cooler than the temperature in the body cavity. The testes develop in the body cavity and descend into the scrotum just before birth. A testis within a scrotum is often termed a testicle. In many rodents, the testes are drawn back into the abdominal cavity between breeding seasons, interrupting sperm maturation. Some mammals whose body temperature is low enough to allow sperm maturation—such as monotremes, whales, and elephants—retain the testes within the abdominal cavity at all times. From the seminiferous tubules of the testes, the sperm pass through the coiled tubules of the epididymis. It takes three weeks for sperm to pass through the 6-m-long tubules of each epididymis of a human male. As they pass through this duct, sperm complete their maturation and become motile. Sperm acquire the ability to fertilize an egg only when exposed to the chemical environment of the female reproductive system. Ejaculation propels sperm from each epididymis through a muscular duct, the vas deferens. Each vas deferens extends behind the urinary bladder and joins with a duct from the seminal vesicle to form an ejaculatory duct, which opens into the urethra. The urethra drains both the excretory and reproductive systems. Accessory sex glands add secretions to semen. A pair of seminal vesicles contributes about 60% of total semen volume. Seminal fluid is thick, yellowish, and alkaline. It contains mucus, fructose (an energy source for sperm), a coagulating enzyme, ascorbic acid, and prostaglandins. The prostate gland secretes directly into the urethra. Prostatic fluid is thin and milky, containing anticoagulant enzymes and citrate, a sperm nutrient. Prostate problems are common in men older than 40, with benign prostate enlargement in virtually all men older than 70. Prostate cancer is one of the most common cancers in men. The bulbourethral glands are a pair of small glands along the urethra below the prostate. Prior to ejaculation, the bulbourethral glands secrete clear mucus that neutralizes any acidic urine remaining in the urethra. Bulbourethral fluid also carries some sperm released before ejaculation. This is one reason the withdrawal method of birth control has a high failure rate. The human penis contains the urethra as well as three layers of spongy erectile tissue. During sexual arousal, the erectile tissue fills with blood from arteries. The resultant increased pressure seals off the veins that drain the penis, causing it to engorge with blood. The engorgement of the penis with blood causes an erection, which is essential for the insertion of the penis into the vagina. Temporary impotence can result from the consumption of alcohol or other drugs, emotional problems, and aging. Long-term erectile dysfunction can be treated with drugs such as Viagra, which promote the vasodilating action of the local regulator NO (nitric oxide), enhancing relaxation of smooth muscles in the blood vessels of the penis. This allows blood to enter the erectile tissue and sustain an erection. The penis of some mammals possesses a baculum, a bone that helps stiffen the penis. A male usually ejaculates 2–5 mL of semen, with each milliliter containing 70–130 million sperm. The main shaft of the penis is covered by relatively thick skin; the sensitive head, or glans penis, is covered by thinner skin. The glans is covered by the foreskin, or prepuce, which may be removed by circumcision. Circumcision, which arose from religious traditions, lessens a man’s risk of contracting AIDS from heterosexual intercourse, though not as much as condom use. Human sexual response is very complex. Human arousal involves a variety of psychological and physical factors. Reproductive structures in the male and female that are quite different in appearance often serve similar functions, reflecting their shared developmental origin. The same embryonic tissues give rise to the glans penis and the glans clitoris, the scrotum and the labia majora, and the skin on the penis and the labia minora. The general pattern of human sexual response is similar in both genders. Two types of physiological reaction predominate in both sexes: vasocongestion, the filling of tissue with blood, and myotonia, increased muscle tension. Both smooth and skeletal muscles may show sustained or rhythmic contractions, including those associated with orgasm. The sexual response can be divided into four phases: excitement, plateau, orgasm, and resolution. Excitement prepares the vagina and penis for coitus. Vasocongestion leads to erection of the penis and clitoris; enlargement of the testicles, labia, and breasts; and vaginal lubrication. Myotonia may result in nipple erection or tension in the arms and legs. In the plateau phase, these responses continue as a result of direct stimulation of the genitalia. In females, plateau includes vasocongestion of the outer third of the vagina, expansion of the inner two-thirds of the vagina, and elevation of the uterus to form a depression that receives sperm at the back of the vagina. Stimulation by the autonomic nervous system increases breathing and heart rate. Orgasm is characterized by rhythmic, involuntary contractions of the reproductive structures in both sexes. In male orgasm, emission is the contraction of the glands and ducts of the reproductive tract, which forces semen into the urethra, while ejaculation occurs with the contraction of the urethra and expulsion of semen. In female orgasm, the uterus and outer vagina contract, but the inner two-thirds of the vagina do not. Orgasm is the shortest phase of the sexual response cycle, usually lasting only a few seconds. In both sexes, contractions occur at about 0.8-second intervals and may also involve the anal sphincter and several abdominal muscles. Resolution completes the cycle and reverses the responses of earlier stages. Vasocongested organs return to their normal sizes and colors; muscles relax. Most of the changes of resolution are completed in 5 minutes, but some may take as long as an hour. Following orgasm, the male enters a refractory period, lasting anywhere from a few minutes to hours, during which erection and orgasm cannot be achieved. Female do not experience a refractory period, so multiple orgasms within a short period of time are possible. Concept 46.4 The timing and pattern of meiosis in mammals differ for males and females. Reproduction in mammals involves two distinct types of gametes. Sperm are small and motile and survive only long enough to attempt fertilization. Eggs, which provide the initial food stores for the embryo, are typically much larger. Eggs mature in synchrony with the tissues of the female reproductive system that support the fertilized embryo. Egg and sperm development involves distinct patterns of meiotic division during gametogenesis, the production of gametes. Spermatogenesis, the formation and development of sperm cells, is a continuous and prolific process in the adult male. Hundreds of millions of sperm are produced each day within the seminiferous tubules, coiled within two testes. For a single sperm, the process takes about seven weeks from start to finish. Primordial germ cells of the embryonic testes differentiate into spermatogonia, the stem cells that give rise to sperm. As spermatogonia differentiate into spermatocytes and then into spermatids, meiosis reduces the chromosome number from diploid to haploid. As spermatogenesis progresses, the developing sperm cells move from the wall to the lumen of a seminiferous tubule and then to the epididymis, where they become motile. The structure of sperm fits its function. A head containing the haploid nucleus is tipped with an acrosome, which contains enzymes that help the sperm penetrate to the egg. Behind the head are a large number of mitochondria (or a single large one) that provide ATP to power the flagellum. Oogenesis is the development of mature oocytes (eggs). Immature eggs form in the ovary of the female embryo, but they do not complete their development until years, and often decades, later. Oogenesis begins in the female embryo with the differentiation of primordial germ cells into oogonia, ovary-specific stem cells. An oogonium multiplies by mitosis and begins meiosis, but the process stops at prophase I. The primary oocytes remain quiescent within small follicles until puberty. Beginning at puberty, follicle-stimulating hormone (FSH) stimulates a follicle to grow and induces its primary oocyte to complete meiosis I and start meiosis II. The development is arrested at metaphase II as a secondary oocyte, which is released when the follicle breaks open at ovulation. When a sperm penetrates the oocyte, oogenesis is completed to produce an ovum. Oogenesis differs from spermatogenesis in three major ways. In spermatogenesis, all four products of meiosis develop into mature gametes. In oogenesis, unequal cytokinesis during meiosis results in the formation of a single large secondary oocyte, destined to become the egg, and smaller polar bodies that degenerate. Spermatogenesis, including the mitotic divisions of stem cells and differentiated spermatogonia, occurs throughout adolescence and adulthood. At birth, an ovary contains all the primary oocytes it will ever have, and the production of mature gametes ceases at about age 50. Spermatogenesis produces mature sperm from precursor cells in a continuous sequence, whereas oogenesis has long “resting” periods. Concept 46.5 The interplay of tropic and sex hormones regulates mammalian reproduction. The coordinated actions of hormones from the hypothalamus, anterior pituitary, and gonads govern male and female reproduction. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which directs the anterior pituitary to secrete the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones regulate gametogenesis directly, through target tissues in the gonads, and indirectly, through the regulation of sex hormone production. The principal sex hormones are steroid hormones: in males, androgens, especially testosterone, and in females, estrogens, especially estradiol, and progesterone. The sex hormones regulate gametogenesis directly and indirectly. In many vertebrates, androgens are responsible for male vocal behavior, such as the territorial songs of birds and the mating calls of frogs. During human embryogenesis, androgens direct the development of the primary sex characteristics of males, the structures directly involved in reproduction. These include the seminal vesicles and other ducts as well as the external reproductive anatomy. At puberty, sex hormones in both males and females induce the formation of secondary sex characteristics, the physical and behavioral features that are not directly related to the reproductive system. In males, androgens cause deepening of the voice, development of facial and pubic hair, and growth of muscle (through stimulation of protein synthesis). Androgens also promote specific sexual behaviors and sex drive as well as increase general aggressiveness. In females, estradiol stimulates breast and pubic hair development. Estradiol also influences female sexual behavior, induces deposition of fat in the breasts and hips, increases water retention, and alters calcium metabolism. Gametogenesis involves the same basic set of hormonal controls in males and females. In males, the FSH and LH secreted in response to GnRH are both required for normal spermatogenesis. FSH promotes the activity of Sertoli cells, cells within the seminiferous tubules that nourish developing sperm. LH regulates Leydig cells, interstitial cells located between the seminiferous tubules. In response to LH, Leydig cells secrete testosterone and other androgens, which promote spermatogenesis within the tubules. Two negative-feedback mechanisms maintain androgen production at an optimal level. Testosterone regulates blood levels of GnRH, FSH, and LH through inhibitory effects on the hypothalamus and anterior pituitary. Inhibin, a hormone that in males is produced by Sertoli cells, acts on the anterior pituitary gland to reduce FSH secretion. Upon reaching sexual maturity, human males carry out gametogenesis continuously, whereas human females produce gametes in cycles. Ovulation occurs only after the endometrium (the lining of the uterus) has started to thicken and develop a rich blood supply, preparing the uterus for the possible implantation of an embryo. If pregnancy does not occur, the uterine lining is sloughed off and another cycle begins. The cyclic shedding of the endometrium from the uterus, which occurs in a flow through the cervix and vagina, is called menstruation. Two closely linked cycles characterize reproduction in human females. The menstrual cycle refers to the changes that occur in the uterus, also called the uterine cycle. Menstrual cycles average 28 days in length (although cycles vary, ranging from about 20 to 40 days). Cyclic events that occur in the ovaries define the ovarian cycle. Hormone activity links the two cycles, resulting in the synchronization of ovarian follicle growth and ovulation with the establishment of a uterine lining that can support embryonic development. The reproductive cycle begins with the release from the hypothalamus of GnRH, which stimulates the pituitary to secrete small amounts of FSH and LH. FSH stimulates follicle growth, aided by LH, and the cells of the growing follicles start to make estradiol. There is a slow increase in the secretion of estradiol during the follicular phase, the part of the ovarian cycle in which follicles are growing and oocytes are maturing. Low levels of estradiol inhibit secretion of the pituitary hormones, keeping FSH and LH levels low. The secretion of estradiol by the growing follicle rises sharply, and levels of FSH and LH shoot up. The high concentration of estradiol stimulates the secretion of gonadotropins by acting on the hypothalamus to increase its output of GnRH, stimulating the secretion of FSH and LH. LH secretion is especially high because the high concentration of estradiol increases the sensitivity of LH-releasing cells in the pituitary to GnRH. Follicles also respond more strongly to LH at this stage because more of their cells have receptors for this hormone. The increase in LH concentration caused by increased estradiol secretion from the growing follicle is a rare example of positive feedback. LH induces the final maturation of the follicle and ovulation, which takes place about a day after the LH surge. The follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte. Following ovulation, during the luteal phase of the ovarian cycle, LH stimulates the transformation of the follicle into the corpus luteum, a glandular structure. Under continued stimulation by LH, the corpus luteum secretes progesterone and estradiol. As the levels of these hormones rise, they exert negative feedback on the hypothalamus and pituitary, inhibiting the secretion of LH and FSH. Near the end of the luteal phase, the corpus luteum disintegrates, causing concentrations of estradiol and progesterone to decline. The pituitary and hypothalamus are liberated from the inhibitory effects of these hormones. The pituitary begins to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle. Prior to ovulation, ovarian steroid hormones stimulate the uterus to prepare for the prospect of an embryo. Estradiol secreted in increasing amounts by growing follicles signals the endometrium to thicken. The follicular phase of the ovarian cycle is thus coordinated with the proliferative phase of the uterine cycle. After ovulation, the estradiol and progesterone secreted by the corpus luteum stimulate development and maintenance of the uterine lining, including enlargement of arteries and growth of endometrial glands. The glands secrete a nutrient fluid that can sustain an early embryo before it implants in the uterine lining. The luteal phase of the ovarian cycle is coordinated with the secretory phase of the uterine cycle. The rapid drop in ovarian hormones as the corpus luteum disintegrates causes arteries in the endometrium to constrict, depriving it of blood. Much of the lining of the inside of the uterus disintegrates, and the uterus, in response to prostaglandin secretion, contracts. Constriction of small blood vessels in the endometrium releases blood that is shed along with endometrial tissue and fluid, resulting in menstruation, or the menstrual flow phase of the uterine cycle, and the beginning of a new cycle. During menstruation, new ovarian follicles begin to grow. About 7% of reproductive-age women suffer from endometriosis, a disorder in which some cells of the uterine lining migrate to an abdominal location that is abnormal, or ectopic. In its new location, the ectopic tissue still responds to stimulation by hormones in the bloodstream, swelling and breaking down each ovarian cycle, and leading to pelvic pain and bleeding into the abdomen. Treatments for endometriosis, involving hormonal therapy or surgery, focus on lessening discomfort. Ongoing research is directed at learning how and why endometriosis occurs. Menopause, the cessation of ovulation and menstruation, usually occurs in women between ages 46 and 54. During these years, the ovaries lose their responsiveness to FSH and LH, and menopause results from a decline in estradiol production by the ovary. In most species, females and males retain their reproductive capacity throughout life. One evolutionary hypothesis for human menopause is that cessation of reproduction allowed a woman to provide better care for her children and grandchildren, thus increasing the chance of survival of individuals bearing her genes and increasing her fitness. All female mammals undergo a thickening of the endometrium prior to ovulation, but only humans and certain other primates have menstrual cycles. Other mammals have estrous cycles. If pregnancy does not occur, the uterus reabsorbs the endometrium with little fluid flow. Estrous cycles are associated with more pronounced behavioral cycles than are menstrual cycles. The period of sexual activity, estrus, is the only time the female is receptive to mating. In contrast, human females may be sexually receptive throughout their menstrual cycle. The length and frequency of reproductive cycles vary widely among mammals. Bears and wolves have one estrous cycle per year; elephants have several. Rats have estrous cycles throughout the year, each lasting only five days. Concept 46.6 In placental mammals, an embryo develops fully within the mother’s uterus. During human copulation, 2–5 mL of semen are transferred, with 70–130 million sperm per milliliter. The alkalinity of the semen helps neutralize the acidic environment of the vagina, thus protecting the sperm and increasing their motility. Ejaculated semen coagulates, which helps to keep the ejaculate in place until sperm reach the cervix. In the cervix, anticoagulants liquefy the semen, and the sperm begin swimming through the female tract. Fertilization or conception occurs in the oviduct. Twenty-four hours later, cleavage begins. Three to four days after fertilization, the embryo reaches the uterus as a ball of 16 cells. By one week after fertilization, the blastocyst forms as a sphere of cells containing a cavity. After a few more days, the blastocyst implants in the endometrium. The embryo secretes hormones to control the mother’s reproductive system. The embryonic hormone human chorionic gonadotropin (HCG) acts like pituitary LH to maintain secretion of progesterone and estrogens by the corpus luteum for the first few months of pregnancy. Some HCG is excreted in the urine, where it is detected by pregnancy tests. In placental mammals, pregnancy or gestation is the condition of carrying one or more embryos in the uterus. A human pregnancy averages 266 days. Many rodents have gestation periods of 21 days. Cows have a gestation period of 270 days, and elephant gestation lasts 600 days. Not all fertilized eggs are capable of completing development; many pregnancies terminate spontaneously due to chromosomal or developmental abnormalities. Occasionally, a fertilized egg lodges in the fallopian tube, resulting in a tubal, or ectopic, pregnancy. Such pregnancies cannot be sustained and may rupture the oviduct, resulting in serious internal bleeding. A number of conditions, including endometriosis, increase the likelihood of tubal pregnancy. Bacterial infections arising during childbirth, from medical procedures, or as a sexually transmitted disease can scar the oviduct, making ectopic pregnancy more likely. Human gestation is divided into three trimesters of three months each. For the first 2–4 weeks of development, the embryo obtains nutrients directly from the endometrium. The outer layer of the blastocyst, the trophoblast, invades the endometrium and later forms the placenta. The placenta allows diffusion of material between maternal and embryonic circulations, supplying nutrients, providing immune protection, exchanging respiratory gases, and disposing of metabolic wastes for the embryo. Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein. Splitting of the embryo during the first month of development can result in identical, or monozygotic (one-egg), twins. Fraternal, or dizygotic, twins arise when two follicles mature in a single cycle, leading to separate fertilization and implantation of two genetically distinct embryos. Organogenesis, the development of the body organs, occurs during the first trimester. By the end of week eight, all the major structures of the adult are present in rudimentary form, and the embryo is called a fetus. The heart begins beating by the fourth week; a heartbeat can be detected at 8 to 10 weeks. During organogenesis, the embryo is most sensitive to environmental insults such as radiation or drugs that can lead to birth defects. By the end of the first trimester, the fetus is well differentiated but only 5 cm long. The mother is also undergoing major changes. High levels of progesterone initiate changes in the maternal reproductive system, including increased mucus in the cervix to form a protective plug against infection, growth of the maternal part of the placenta, enlargement of the uterus, and cessation of ovarian and menstrual cycling. The breasts enlarge rapidly and may be tender. About three-fourths of all pregnant women experience nausea during the first trimester. During the second trimester, the fetus grows rapidly to 30 cm and is very active. The mother may feel movements during the early part of the second trimester, although fetal activity is not visible through the abdominal wall until one to two months later. Hormonal levels stabilize as HCG levels decline; the corpus luteum deteriorates; and the placenta takes over the secretion of progesterone, which maintains the pregnancy. During the third trimester, the fetus grows rapidly to about 3–4 kg in weight and 50 cm in length. Fetal activity may decrease as the fetus fills the space available to it. Maternal abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages, and back strain. A complex interplay of local regulators (prostaglandins) and hormones (estradiol and oxytocin) induces and regulates labor. Birth, or parturition, is brought about by strong, rhythmic uterine contractions. The process of labor has three stages. The first stage is the opening up and thinning of the cervix, ending in complete dilation. The second stage is the expulsion of the baby as a result of strong uterine contractions. The third stage is delivery of the placenta. Lactation is unique to mammals. In response to suckling by the newborn, as well as changes in estradiol levels after birth, the hypothalamus signals the anterior pituitary to secrete prolactin, which stimulates the mammary glands to produce milk. Suckling also stimulates the secretion of a posterior pituitary hormone, oxytocin, which triggers release of milk from the mammary glands. Reproductive immunologists are working to understand why mammalian mothers do not reject the embryo as a foreign body, despite its paternal antigens. The symptoms of rheumatoid arthritis, an autoimmune disease of the joints, become less severe during pregnancy, suggesting that regulation of the immune system may be altered by pregnancy. Contraception, the deliberate prevention of pregnancy, can be achieved in several ways. Some methods of contraception prevent the release of female or male gametes from gonads; others prevent fertilization by keeping sperm and egg apart; still others prevent implantation of an embryo. Fertilization can be prevented by abstinence from sexual intercourse or by any of several barriers that keep sperm and egg apart. Temporary abstinence is called the rhythm method of birth control. With natural family planning, couples refrain from intercourse during the time conception is most likely to occur. The egg can survive in the oviduct for 24–48 hours and sperm for as long as five days. Ovulation can be detected by noting changes in cervical mucus and body temperature during the menstrual cycle. Natural family planning brings a pregnancy rate of 10–20%. Some couples use ovulation timing methods to increase the probability of conception. Coitus interruptus, or withdrawal (removal of the penis from the vagina before ejaculation), is an unreliable method of preventing fertilization. Sperm may be present in secretions that precede ejaculation. Barrier methods of contraception that block sperm from meeting the egg have pregnancy rates of less than 10%. The condom is a thin latex or natural membrane sheath that fits over the penis to collect semen. Latex condoms are the only contraceptives that are highly effective in preventing the spread of sexually transmitted diseases, including AIDS. The diaphragm is a dome-shaped rubber cap that fits into the upper portion of the vagina before intercourse. Both condoms and diaphragms are more effective when used in conjunction with a spermicide. Other barrier devices include the cervical cap, which fits tightly around the opening of the cervix and is held in place by suction, and the vaginal pouch, or “female condom.” After complete abstinence from sexual intercourse, the most effective means of birth control are sterilization, intrauterine devices (IUDs), and hormonal contraceptives. Sterilization is almost 100% effective. The IUD, with a pregnancy rate of 1% or less, is the most commonly used reversible method of birth control outside the United States. Placed in the uterus by a doctor, the IUD interferes with fertilization and implantation. Pregnancy rates of 1% or lower are also achieved with birth control pills. The most commonly used birth control pills are a combination of a synthetic estrogen and progestin (progesterone-like hormone). This combination acts by negative feedback to stop the release of GnRH by the hypothalamus and thus of FSH and LH by the pituitary. The prevention of LH release prevents ovulation. The inhibition of FSH secretion by the low dose of estrogen in the pills prevents follicles from developing. A similar combination of hormones is also available as an injection, in a ring inserted into the vagina, and as a skin patch. Combination birth control pills can also be used in high doses as morning-after pills, which prevent fertilization or implantation with an effectiveness of about 75%. A different hormone-based contraceptive contains only progestin, which causes thickening of a woman’s cervical mucus to block sperm from entering the uterus. Progestin decreases the frequency of ovulation and causes changes in the endometrium that interfere with implantation if fertilization occurs. Hormone-based contraceptives have both beneficial and harmful side effects. Female smokers have an increased risk of dying from cardiovascular disease if they use oral contraceptives. Birth control pills slightly increase a woman’s risk of abnormal blood clotting, high blood pressure, heart attack, and stroke. Despite these increased risks, oral contraceptives eliminate the dangers of pregnancy, and women who take birth control pills have mortality rates about half those of pregnant women. The pill also decreases the risks of ovarian and endometrial cancers. Research aimed at finding a male contraceptive has focused on hormone combinations that suppress gonadotropin release and thereby block spermatogenesis. Testosterone included in such combinations inhibits reproductive functions of the hypothalamus and pituitary, while maintaining secondary sexual characteristics. Sterilization is the permanent prevention of gamete release. Tubal ligation in women usually involves cauterizing or tying off (ligating) a section of the oviducts to prevent eggs from traveling into the uterus. Vasectomy in men is the tying off or excision of a small section of each vas deferens to prevent sperm from entering the urethra. Both male and female sterilization procedures are relatively safe and free from harmful effects. Secretion of sex hormones and sexual function are unaffected by either procedure, with no change in menstrual cycles in females or ejaculate volume in males. Both procedures are difficult to reverse, so each should be considered permanent. Abortion is the termination of a pregnancy in progress. Spontaneous abortion, or miscarriage, occurs in as many as one-third of all pregnancies, often before the woman is even aware she is pregnant. Each year, about 850,000 U.S. women have abortions performed by physicians. RU486, or mifepristone, is a drug that terminates pregnancy within the first 7 weeks. RU486 blocks progesterone receptors in the uterus, preventing progesterone from maintaining the pregnancy. RU486 is taken with a small amount of prostaglandin to induce uterine contractions. Genetic diseases and developmental problems can be diagnosed while the fetus is still in the uterus. Many genetic diseases and developmental problems can now be diagnosed while the fetus is still in the uterus. Ultrasound imaging generates images using high-frequency sound waves. In amniocentesis and chorionic villus sampling, a needle is used to remove fetal cells from fluid or tissue surrounding the embryo. A blood sample from the mother can contain fetal cells, which can be identified with specific antibodies and then tested for genetic disorders. A few fetal blood cells leak across the placenta into the mother’s bloodstream. Diagnosing genetic diseases in a fetus poses ethical questions. Most detectable disorders cannot be treated in the uterus; many cannot be corrected even after birth. Parents must decide whether to terminate a pregnancy or to raise a child who may have profound defects and a short life expectancy. Reproductive technology can help with a number of fertility problems. An inability to conceive offspring affects one in ten couples. The causes of infertility vary, and reproductive defects are equally likely in men and women. For women, the risk of reproductive difficulties as well as genetic abnormalities of the fetus increases steadily past age 35; the prolonged period of time oocytes spend in meiosis is largely responsible. Hormone therapy may increase sperm or egg production; surgery can correct ducts that have failed to form properly or become blocked. Many infertile couples turn to assisted reproductive technologies, which involve the surgical removal of eggs (secondary oocytes) from a woman’s ovaries after hormonal stimulation. The eggs are fertilized and returned to the woman’s body. Unused eggs, sperm, and embryos from such procedures can be frozen for later attempts. In in vitro fertilization (IVF), oocytes are mixed with sperm in culture dishes, the fertilized eggs are incubated until the eight-cell stage, and then the eggs are transferred into the woman’s uterus. If mature sperm are defective, scarce (fewer than 20 million per milliliter of ejaculate), or even absent, fertility may be restored by intracytoplasmic sperm injection (ICSI). In this procedure, the head of a sperm is drawn up into a needle and injected directly into an oocyte to achieve fertilization. Though costly, IVF procedures have enabled hundreds of thousands of couples to conceive children. Abnormalities arising as a consequence of the IVF procedure are rare. Lecture Outline for Campbell/Reece Biology, 8th Edition, © Education, Inc. 46-18