Animal Reproduction

Chapter 46   Animal Reproduction
Lecture Outline
Overview: Doubling Up for Sexual Reproduction
Concept 46.1 Both asexual and sexual reproduction occur in the animal kingdom

• Asexual reproduction involves the formation of individuals whose genes come from a single parent.
• There is no fusion of sperm and egg.
• Sexual reproduction is the formation of offspring by the fusion of haploid gametes to form a diploid zygote.
• The female gamete, the unfertilized egg, or ovum, is usually large and nonmotile.
• The male gamete is the sperm, which is usually small and motile.
• Sexual reproduction increases genetic variation among offspring by generating unique combinations of genes inherited from two parents.

 Diverse mechanisms of asexual reproduction enable animals to produce identical offspring rapidly.

• Many invertebrates can reproduce asexually by fission, in which a parent separates into two or more approximately equal-sized individuals.
• Budding is also common among invertebrates. This is a form of asexual reproduction in which new individuals split off from existing ones.
• In fragmentation, the body breaks into several pieces, some or all of which develop into complete adults.
• Reproducing in this way requires regeneration of lost body parts.
• Many animals can also replace new appendages by regeneration.
• Asexual reproduction has a number of advantages.
• It allows isolated animals to reproduce without needing to find a mate.
• It can create numerous offspring in a short period of time.
• In stable environments, it allows for the perpetuation of successful genotypes.

Reproductive cycles and patterns vary extensively among mammals.

• Most animals exhibit cycles in reproductive activity, usually related to changing seasons.
• This allows animals to conserve resources and reproduce when more energy is available and when environmental conditions favor the survival of offspring.
• Reproductive cycles are controlled by a combination of environmental and hormonal cues.
• Environmental cues may include seasonal temperature, rainfall, day length, 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.
• Parthenogenesis is the process by which an unfertilized egg develops without being fertilized.
• Parthenogenesis plays a role in the social organization of some bees, wasps, and ants.
• Male honeybees (drones) are haploid, and female honeybees (queens and workers) are diploid.
• 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 this species, but the lizards imitate courtship and mating behavior typical of sexual species of the same genus.
• Sexual reproduction presents a problem for sessile or burrowing animals or parasites that may have difficulty encountering a member of the opposite sex.
• One solution 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 matings, each individual receives and donates sperm.
• This results in twice as many offspring as would be produced if only one set of eggs were fertilized.
• In sequential hermaphroditism, an individual reverses its sex during its lifetime.
• In some species, the sequential hermaphrodite is female first.
• In other species, the sequential hermaphrodite is male first.

Concept 46.2 Fertilization depends on mechanisms that help sperm meet 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 are released by the female into a wet environment, where they are fertilized by the male.
• In species with internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract.
• A moist habitat is almost always required for external fertilization, both 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 encounter ripe eggs.
• Environmental cues such as temperature or day length may cause gamete release by the whole 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 sophisticated reproductive systems, including copulatory organs that deliver sperm and receptacles for their storage and transport to ripe eggs.
• 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, or water-soluble molecules that disperse into the environment.
• Like hormones, pheromones are active in minute amounts.
• Many pheromones act as male attractants.
• All species produce more offspring than can survive to reproduce.
• Internal fertilization usually involves the production of fewer zygotes than does external fertilization.
• However, the survival rate is higher for internal fertilization.
• Major types of protection include tough eggshells, development of the embryo within the reproductive tract of the mother, and parental care of the eggs and offspring.
• Marsupial mammals retain their embryos for only a short period in the uterus.
• 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 through the placenta.
• Parental care of offspring can occur regardless of whether fertilization is external or internal.

 Reproductive systems produce gametes and make them available to gametes of the opposite sex.

• The least complex reproductive systems lack gonads, the organs that produce gametes in most animals.
• Polychaete worms lack gonads. Eggs and sperm develop from undifferentiated cells lining the coelom.
• As the gametes mature, they are released from the body wall and fill the coelom.
• In some species, the body splits open to release the gametes, killing the parent.
• Some reproductive systems, such as those of parasitic flatworms, are very complex.
• Most insects have separate sexes with complex reproductive systems.
• In many species, the female reproductive system includes a spermatheca, a sac in which sperm may be stored for a year or more.
• The basic plan of all vertebrate reproductive systems is very similar.
• However, there are variations.
• In many nonmammalian vertebrates, the digestive, excretory, and reproductive systems share a common opening to the outside, the cloaca.
• Mammals have separate openings for the digestive and reproductive systems.
• Female mammals also have separate openings for the excretory and reproductive systems.
• The uterus of most vertebrates is partly or completely divided into two chambers.
• Male reproductive systems differ mainly in copulatory organs.
• Many mammalian vertebrates do not have a well-developed penis and simply turn the cloaca inside out to ejaculate.

Concept 46.3 Reproductive organs produce and transport gametes: focus on humans
Human reproduction involves intricate anatomy and complex behavior.

• The reproductive anatomy of the human female includes external and internal reproductive structures.
• External reproductive structures consist of two sets of labia surrounding the clitoris and vaginal opening.
• Internal reproductive organs consist of a pair of gonads and a system of ducts and chambers.
• The role of the ducts and chambers is to conduct the gametes and house the embryo and fetus.
• The ovaries, the female gonads, lie in the abdominal cavity, attached to the uterus by a mesentery.
• Each ovary is enclosed in a tough protective capsule and contains many follicles.
• Each follicle consists of one egg cell surrounded by one or more layers of follicle cells.
• A woman is born with about 400,000 follicles.
• Only several hundred of these will release eggs during a female’s reproductive years.
• Follicles produce the primary female sex hormones, estrogens.
• Usually one follicle matures and releases its egg during each menstrual cycle in the process of ovulation.
• After ovulation, the remaining follicular tissue develops into the corpus luteum.
• The corpus luteum secretes additional estrogens and progesterone, hormones that 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 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.
• It opens to the outside at the vulva, the collective term for the external female genitalia.
• 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.
• The vestibule is surrounded by a pair of slender folds called the labia minora.
• The 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 function only in females.
• They are not a component of the human reproductive system but are important to mammalian reproduction.
• Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts opening at the nipple.
• Adipose tissue forms the main mass of the mammary gland of a nonlactating mammal.
• The low estrogen level in males prevents the development of the sensory apparatus and fat deposits, so that male breasts remain small, with nipples unconnected to the ducts.
• The male’s external reproductive organs consist of the scrotum and penis.
• The internal reproductive organs consist of gonads that produce sperm and 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 2°C below that of the abdomen.
• This keeps testicular temperature cooler than that in the body cavity.
• The testes develop in the body cavity and descend into the scrotum just before birth.
• From the seminiferous tubules of the testes, the sperm pass through the coiled tubules of the epididymis.
• As they pass through this duct, sperm become motile and gain the ability to fertilize an egg.
• Ejaculation propels sperm from the epididymis to the vas deferens.
• The vas deferens run from the scrotum and behind the urinary bladder.
• Each vas deferens joins with a duct from the seminal vesicle to form an ejaculatory duct.
• The ejaculatory ducts open 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, a coagulating enzyme, ascorbic acid, and prostaglandins.
• The prostate gland secretes directly into the urethra.
• Prostatic fluid is thin and milky.
• This fluid contains anticoagulant enzymes and citrate.
• Prostate problems are common in males older than 40.
• Benign prostate enlargement occurs in virtually all males 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, they secrete clear mucus that neutralizes any acidic urine remaining in the urethra.
• Bulbourethral fluid also carries some sperm released before ejaculation.
• This is one of the reasons the withdrawal method of birth control has a high failure rate.
• A male usually ejaculates about 2–5 mL of semen, with each milliliter containing about 50–130 million sperm.
• Once in the female reproductive tract, prostaglandins in semen thin the mucus at the opening of the uterus and stimulate uterine contractions that help move the semen.
• When ejaculated, semen coagulates, making it easier for uterine contractions to move it along.
• Anticoagulants then liquefy the semen, and the sperm begin swimming.
• The alkalinity of semen helps neutralize the acidic environment of the vagina, protecting the sperm and increasing their motility.
• The human penis is composed of 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.
• The penis of some mammals possesses a baculum, a bone that helps stiffen the penis.
• Temporary impotence can result from the consumption of alcohol or other drugs, and from emotional problems.
• Irreversible impotence due to nervous system or circulatory problems can be treated with drugs and penile implant devices.
• The oral drug Viagra acts by promoting the action of 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 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.
• There is no verifiable health benefit to circumcision, which arose from religious tradition.

 Human sexual response is very complex.

• Human arousal involves a variety of psychological and physical factors.
• Human sexual response is characterized by a common physiological pattern.
• Two types of physiological reaction predominate in both sexes:
• Vasocongestion, filling of tissue with blood, is caused by increased blood flow.
• Myotonia is increased muscle tension.
• Both smooth and skeletal muscle may show sustained or rhythmic contractions.
• The sexual response can be divided into four phases: excitement, plateau, orgasm, and resolution.
• Excitement prepares the vagina and penis for coitus.
• Vasocongestion is evident in the erection of the penis and clitoris; the enlargement of the testes, 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.
• Stimulation by the autonomic nervous system increases breathing and heart rate.
• 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.
• Orgasm is the shortest phase of the sexual response cycle.
• It 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.
• Ejaculation occurs with the contraction of the urethra and expulsion of semen.
• In female orgasm, the uterus and outer vagina contract.
• Resolution completes the cycle and reverses the responses of earlier stages.
• Vasocongested organs return to their normal sizes and colors; muscles relax.
•  
• Concept 46.4 In humans and other mammals, a complex interplay of hormones regulates gametogenesis

 Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways.

• Gametogenesis is based on meiosis.
• Spermatogenesis is the production of mature sperm cells from spermatogonia.
• Spermatogenesis is a continuous and prolific process in the adult male.
• Each ejaculation contains 100–650 million sperm.
• Spermatogenesis occurs in seminiferous tubules.
• 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 production of ova from oogonia.
• Oogenesis differs from spermatogenesis in three major ways.
• At birth an ovary may contain all of the primary oocytes it will ever have.
• However, in 2004, researchers reported that multiplying oogonia exist in the ovaries of adult mice.
• Researchers are looking for similar cells in human ovaries.
• Sperm are produced from spermatogonia throughout a man’s life.
• Unequal cytokinesis during meiosis results in the formation of a single large secondary oocyte and three small polar bodies.
• The secondary oocyte becomes the ovum, while the polar bodies degenerate.
• In spermatogenesis, all four products of meiosis become mature sperm.
• Oogenesis has long “resting” periods.
• Spermatogenesis produces mature sperm from spermatogonia in an uninterrupted sequence.
• Oogenesis begins in the female embryo with 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.
• It is arrested at metaphase II as a secondary oocyte.
• The secondary oocyte is released when the follicle breaks open at ovulation.
• Meiosis is completed when a sperm penetrates the oocyte.
• Oogenesis is completed, producing an ovum.
• The haploid nuclei of the sperm and ovum fuse in fertilization.
• The ruptured follicle develops into the corpus luteum.
• If the released oocyte is not fertilized, the corpus luteum degenerates.
• In females, the secretion of hormones and the reproductive events they regulate are cyclic.
• Hormonal control of the female cycle is complex.
• Humans and many other primates have menstrual cycles.
• If pregnancy does not occur, the endometrium is shed through the cervix and vagina in menstruation.
• Other mammals have estrous cycles.
• If pregnancy does not occur, the uterus reabsorbs the endometrium.
• Estrous cycles are associated with more pronounced behavioral cycles than are menstrual cycles.
• The period of sexual activity, estrus, is the only time the condition of the vagina permits mating.
• Human females may be sexually receptive throughout their menstrual cycle.
• The term menstrual cycle refers specifically to the changes that occur in the uterus, and is also called the uterine cycle.
• It is caused by cyclic events that occur in the ovaries, the ovarian cycle.
• The cycle begins with the release from the hypothalamus of GnRH or gonadotropin-releasing hormone, which stimulates the pituitary to secrete small amounts of FSH and LH.
• FSH stimulates follicle growth, aided by LH, or luteinizing hormone, and the cells of the growing follicles start to make estrogen.
• There is a slow rise in estrogen secreted during the follicular phase, the part of the ovarian cycle in which follicles are growing and oocytes maturing.
• The low level of estrogen inhibits secretion of the pituitary hormones, keeping FSH and LH levels low.
• The levels of FSH and LH shoot up when the secretion of estrogen by the growing follicle rises sharply.
• The high concentration of estrogen stimulates the secretion of gonadotropins by acting on the hypothalamus to increase its output of GnRH.
• This stimulates the secretion of FSH and LH.
• LH secretion is especially high, because the high concentration of estrogen increases the sensitivity of LH-releasing cells in the pituitary to GnRH.
• LH induces the final maturation of the follicle and ovulation.
• 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 the continued stimulation by LH during this phase, the corpus luteum secretes progesterone and estrogen.
• 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 estrogen 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.
• The follicular phase of the ovarian cycle is coordinated with the proliferative phase of the menstrual cycle.
• Secretion of estrogens during the follicular phase stimulates endometrial thickening.
• The estrogen and progesterone of the luteal phase stimulate development and maintenance of the endometrium, including the enlargement of arteries and the growth of endometrial glands.
• The glands secrete a nutrient fluid that can sustain an early embryo before it implants in the uterine lining.
• Thus, 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 spasms in the uterine lining, depriving it of blood.
• The upper two-thirds of the endometrium disintegrates, 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.
• Estrogen is also responsible for female secondary sex characteristics, including deposition of fat in the breasts and hips, increased water retention, and stimulation of breast development.
• It also influences sexual behavior.
• Menopause, the cessation of ovarian and menstrual cycles, usually occurs between ages 46 and 54.
• During these years, the ovaries lose their responsiveness to FSH and LH, and menopause results from a decline in estrogen production by the ovary.
• Menopause is an unusual phenomenon.
• In most species, females and males retain their reproductive capacity throughout life.
• There might be an evolutionary explanation for menopause.
• One hypothesis proposes that cessation of reproduction allowed a woman to provide better care for her children and grandchildren, increasing the survival of individuals bearing her genes and increasing her fitness.

 The principle sex hormones in the male are the androgens.

• The male sex hormones, androgens, are steroid hormones produced mainly by the Leydig cells of the testes, interstitial cells near the seminiferous tubules.
• Testosterone, the most important male androgen, and other androgens are responsible for the primary and secondary male sex characteristics.
• Primary sex characteristics are associated with the development of the vas deferens and other ducts, development of the external reproductive structures, and sperm production.
• Secondary sex characteristics are features not directly related to the reproductive system, including deepening of the voice, distribution of facial and pubic hair, and muscle growth.
• Androgens also affect behavior.
• In addition to specific sexual behaviors and sex drive, androgens increase general aggressiveness.
• They are responsible for vocal behavior, like singing in birds and calling by frogs.
• Hormones from the anterior pituitary and hypothalamus control androgen secretion and sperm production by the testes.

Concept 46.5 In humans and other placental mammals, an embryo grows into a newborn in the mother’s uterus

• In placental mammals, pregnancy or gestation is the condition of carrying one or more embryos.
• A human pregnancy averages 266 days.
• Many rodents have gestation periods of 21 days. Cows have a gestation of 27 days, and elephant gestation lasts 600 days.
• 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 cells.
• By one week past 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 signal its presence and control the mother’s reproductive system.
• Human chorionic gonadotropin (HCG) acts like pituitary LH to maintain secretion of progesterone and estrogens by the corpus luteum for the first few weeks of pregnancy.
• Some HCG is excreted in the urine, where it is detected by pregnancy tests.
• Human gestation is divided into three trimesters of three months each.
• For the first 2–4 weeks of development, the embryo obtains nutrients from the endometrium.
• The outer layer of the blastocyst, called the trophoblast invades the endometrium, eventually helping to form the placenta.
• The placenta allows diffusion of material between maternal and embryonic circulations, providing nutrients, exchanging respiratory gases, and disposing of metabolic wastes for the embryo.
• Blood from the embryo travels to the placenta and returns via the umbilical vein.
• Organogenesis occurs during the first trimester.
• By the end of week four, the heart is beating.
• By the end of week eight, all the major structures of the adult are present in rudimentary form.
• The rapidity of development makes this a time when the embryo is especially sensitive to environmental insults such as radiation or drugs.
• High levels of progesterone initiate changes in the maternal reproductive system.
• These include increased mucus in the cervix to form a protective plug, 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 very tender.
• 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.
• Hormonal levels stabilize as HCG declines, 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 (estrogen and oxytocin) induces and regulates labor.
• The mechanism that triggers labor is not fully understood.
• In one possible model, high levels of estrogen induce the formation of oxytocin receptors on the uterus.
• Oxytocin, produced by the fetus and the mother’s posterior pituitary, stimulates powerful contractions by the smooth muscles of the uterus.
• Oxytocin also stimulates the placenta to secrete prostaglandins, which enhance the contractions.
• The physical and emotional stress associated with the contractions stimulate the release of more oxytocin and prostaglandins, a positive feedback system that underlies the process of 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 the expulsion of the placenta.
• Lactation is unique to mammals.
• After birth, decreasing levels of progesterone free the anterior pituitary from negative feedback and allow prolactin secretion.
• Prolactin stimulates milk production 2–3 days after birth.
• The release of milk from the mammary glands is controlled by oxytocin.
• Reproductive immunologists are working to understand why mammalian mothers do not reject the embryo as a foreign body, despite its paternal antigens.
• The trophoblast may inhibit a maternal immune response against the embryo by releasing signal molecules with immunosuppressive effects.
• These include HCG, a variety of protein “factors,” a prostaglandin, several interleukins, and an interferon.
• Some combination of these substances may interfere with immune rejection by acting on the mother’s T lymphocytes.
• A different hypothesis suggests that the trophoblast and later the placenta secrete an enzyme that rapidly breaks down local supplies of tryptophan, an amino acid necessary for T cell survival and function.
• This enzyme is essential for maintaining pregnancy in mice.
• Another possibility is the absence of certain histocompatibility antigens on placenta cells and the secretion of a hormone that induces synthesis of a “death activator” protein (FasL) on placental cells.
• Activated T cells have a complementary “death receptor” (Fas), and the binding of FasL to Fas causes the T cells to self-destruct by apoptosis.
• Contraception can be achieved in several ways.
• Some methods prevent the release of mature secondary oocytes and sperm from gonads, others prevent fertilization by keeping sperm and egg apart, and 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.
• This means of natural family planning depends on refraining from intercourse when conception is most likely.
• 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%.
• As a method of preventing fertilization, coitus interruptus, or withdrawal (removal of the penis from the vagina before ejaculation), is unreliable.
• Sperm may be present in secretions that precede ejaculation.
• The several barrier methods of contraception that block sperm from meeting the egg have pregnancy rates of less than 10%.
• The condom used by the male is a thin latex or natural membrane sheath that fits over the penis to collect the semen.
• The diaphragm is a dome-shaped rubber cap that fits into the upper portion of the vagina before intercourse.
• Both methods are more effective when used in conjunction with a spermicide.
• Birth control pills are chemical contraceptives with a pregnancy rate of less than 1%.
• 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.
• As a backup mechanism, the inhibition of FSH secretion by the low dose of estrogen in the pills prevents follicles from developing.
• A second type of birth control pill, the minipill, contains only progestin.
• It does not effectively block ovulation, and it is not as effective a contraceptive as the combination pill.
• The minipill prevents fertilization mainly by causing thickening of a woman’s cervical mucus so it blocks sperm from entering the uterus.
• It also causes changes in the endometrium that interfere with implantation.
• Combination pills carry a slightly elevated risk of abnormal blood clotting, high blood pressure, heart attack, and stroke.
• However, they decrease the risk of ovarian and endometrial cancers.
• Sterilization is the permanent prevention of gamete release.
• Tubal ligation in women involves cauterization or ligation of a section of the oviducts to prevent the eggs from traveling into the uterus.
• Vasectomy in men is the cutting of each vas deferens to prevent sperm from entering the urethra.
• Abortion is the termination of a pregnancy.
• Spontaneous abortion or miscarriage occurs in as many of one-third of all pregnancies.
• In addition, 1.5 million American women choose abortions performed by physicians each year.
• A drug called mifepristone, or RU-486, enables a woman to terminate pregnancy nonsurgically within the first seven weeks.
• An analogue of progesterone, RU-486 blocks progesterone reception in the uterus, preventing progesterone from maintaining pregnancy.
• It is taken with a small amount of prostaglandin to induce uterine contractions.

 Modern technology offers solutions for some reproductive problems.

• It is now possible to diagnose many genetic and congenital abnormalities while the fetus is in the uterus.
• Amniocentesis and chorionic villus sampling are invasive techniques in which amniotic fluid or fetal cells are obtained for genetic analysis.
• Commonly used noninvasive techniques use ultrasound imaging to detect fetal conditions.
• A newer noninvasive method uses the fact that maternal blood contains fetal blood cells that can be tested.
• A maternal blood sample yields fetal cells that can be identified by specific antibodies and tested for genetic disorders.
• Reproductive technology can help with infertility treatments.
• Hormone therapy can increase sperm and egg production.
• Surgery can correct blocked oviducts.
• Many infertile couples use assisted reproductive technology (ART).
• These procedures involve surgical removal of secondary oocytes from a woman’s body, fertilizing them, and returning them to the woman’s body.
• With in vitro fertilization, the most common ART procedure, the oocytes are mixed with sperm in culture dishes and inserted in the woman’s uterus at the eight-cell stage or beyond.
• In ZIFT (zygote intrafallopian transfer), eggs are also fertilized in vitro, but zygotes are transferred immediately to the woman’s fallopian tubes.
• In GIFT (gamete intrafallopian transfer), the eggs are not fertilized in vitro.
• Instead, the eggs and sperm are placed in the woman’s oviducts in the hope that fertilization will occur there.
• These techniques are performed throughout the world and have resulted in thousands of children.