Chapter 43
Animal Reproduction
Animal Reproduction
Asexual Reproduction
Sexual Reproduction
The Human Reproductive System
Human Sexual Behavior
Asexual Reproduction
Asexual reproduction involves a single parent, and offspring are genetically identical.
Asexual reproduction is highly __________ because there is no mating, which requires energy, and all individuals in the population can convert resources into offspring.
However, asexual reproduction does not generate genetic diversity, which may be a disadvantage in changing environments.
Asexually reproducing species tend to be sessile (and cannot search for mates) or live in sparse populations (rarely encounter potential mates).
Asexual Reproduction
In budding a new, genetically identical individual develops as an outgrowth of the body of an older individual.
A bud grows by mitotic cell division, and the cells differentiate before the bud breaks away from the parent.
Asexual Reproduction
Regeneration serves to replace lost or damaged tissues, but in some cases pieces of an organism can __________ entire individuals.
Cut-up sea stars will regenerate whole bodies as long as a portion of the central disc remains intact.
Some species of segmented worms develop segments with rudimentary heads bearing sensory organs and then break apart, with each segment then forming a new worm.
Asexual Reproduction
__________ is the development of offspring from unfertilized eggs.
Arthropods, fish, reptiles, and amphibians engage in parthenogenesis.
Most species that reproduce parthenogenetically also use sexual reproduction or engage in sexual behavior.
Some species (some ticks, mites, and lizards) engage in __________ __________ even when egg fertilization is unnecessary. __________ __________ is needed only to stimulate ovulation; the egg then develops parthenogenetically.
Asexual Reproduction
In some species, parthenogenesis is part of the mechanism that determines sex.
In many species of ants, bees, and wasps, males develop from unfertilized eggs and are haploid; females develop from fertilized eggs and are diploid.
Most females are sterile workers, but a select few become fertile queens.
After a queen mates, she has a supply of sperm she controls, thus enabling her to produce either fertilized or unfertilized eggs and to determine how much of the colonys resources is expended on males.
Sexual Reproduction
Despite the time, energy, and risk required, sexual reproduction confers an overwhelming evolutionary advantage: the __________ of genetic diversity.
[and the repair of damaged genomes by crossing over-ask to see the fruit and vegetable video analogy]
Sexual reproduction requires the joining of two haploid cells into one, which becomes a diploid individual.
These haploid cells, or gametes, are produced by gametogenesis, involving meiotic cell divisions.
Two events in meiosis contribute to genetic diversity: crossing over of homologous chromosomes and independent assortment of the chromosomes.
Sexual Reproduction
The three fundamental steps of sexual reproduction are:
Gametogenesis (producing sex cells)
Mating (getting sex cells together)
Fertilization (fusion of sex cells)
Gametogenesis and fertilization are fairly similar in different groups of animals.
Mating behaviors, however, show incredible evolutionary diversity.
Sexual Reproduction
The gonads (testes and ovaries) are the sites of gametogenesis.
Male gametes (sperm) move by beating flagella; female gametes (eggs or ova) are nonmotile.
The gametes are formed by germ cells which originate very early in the developing embryo.
The germ cells proliferate by mitosis, producing the oogonia and spermatogonia. These multiply by mitosis, producing diploid primary oocytes and primary spermatocytes.
Sexual Reproduction
Spermatogenesis involves two meiotic divisions: Primary spermatocytes form secondary spermatocytes, and then secondary spermatocytes produce four haploid spermatids.
Spermatocytes remain in cytoplasmic contact throughout their development, probably so that all spermatocytes can share the gene products of the X chromosome, which only half of them have.
Through further development, spermatids become compact, streamlined, and motile.
Sexual Reproduction
Oogenesis produces eggs through two meiotic divisions.
The primary oocyte enters prophase of the first meiotic division, and then its development is arrested (for days, months, or for as long as 50 years).
During this phase, the primary oocyte grows and adds to its energy, ribosome, and organelle store. This permits the later embryo to have nourishment.
When it resumes meiosis, the primary oocyte completes the first division, resulting in two cells of unequal size, the secondary oocyte and first polar body.
Sexual Reproduction
A second period of arrested development occurs at this point. The egg may be expelled from the ovary in this condition.
With the second meiotic division, one daughter cell forms the large, haploid ootid, which eventually becomes the mature ovum.
The other daughter cell forms the second polar body, which degenerates. The mature ovum is thus large and well-provisioned
In many species, including humans, the second meiotic division is not completed until the egg is fertilized by the sperm.
Sexual Reproduction
Fertilization is the union of haploid sperm and haploid egg to produce a single diploid cell, the __________. It involves a complex series of events:
The sperm and egg recognize each other.
The sperm is activated so that it can gain access to the plasma membrane of the egg.
The plasma membranes of the sperm and egg fuse.
The egg blocks entry of additional sperm.
The egg is stimulated to start development.
The egg and sperm nuclei fuse.
Sexual Reproduction
Specific recognition molecules mediate interactions between sperm and eggs.
This ensures that activities of the sperm are directed toward eggs and not other cells and prevents eggs from being fertilized by sperm of the wrong species.
This latter function is particularly important in aquatic species, such as sea urchins, that release sperm and eggs into the surrounding water.
Sexual Reproduction
Eggs of sea urchins release species-specific peptides that increase the motility of sperm and cause them to swim toward the egg.
Upon reaching the egg, sperm must get through two protective layers: a jelly coat and a proteinaceous vitelline envelope.
The membrane-enclosed acrosome which forms a cap over the sperm nucleus makes this possible.
Substances in the jelly coat trigger the acrosomal reaction, which begins with the breakdown of the plasma membrane covering the sperm head and the underlying acrosomal membrane.
Sexual Reproduction
An acrosomal process, which is coated with a protein called bindin, extends out of the head of the sperm and through the remainder of the jelly coat to make contact with the vitelline envelope.
Bindin receptors extend through the eggs vitelline envelope and react with the bindin, stimulating the egg plasma membrane to form a fertilization cone that engulfs the sperm head and brings it into contact with the egg cytoplasm.
Sexual Reproduction
In animals with internal fertilization, eggsperm recognition mechanisms also exist.
The mammalian egg is surrounded by a thick layer called the cumulus. Beneath that is a protein envelope called the zona pellucida.
In the female reproductive tract, mammalian sperm are metabolically activated; they are capable of an acrosomal reaction.
A species-specific glycoprotein in the zona binds to the head of the sperm, triggering the acrosomal reaction and releasing acrosomal enzymes that digest a path through the zona.
Sexual Reproduction
The first response to fertilization are blocks to __________: If more than one sperm enters the egg, the resulting embryo is unlikely to survive.
In the sea urchin fast block to polypspermy, an influx of Na+ ions within seconds after the sperm enters changes the electric charge difference across the eggs plasma membrane.
The slow block to polyspermy takes about a minute. Calcium from the eggss endoplasmic reticulum causes the vitelline envelope to form a hardened fertilization envelope, which prevents additional sperm from entering.
Sexual Reproduction
In mammals, calcium is released from the endoplasmic reticulum, eventually causing destruction of the sperm-binding molecules in the zona pellucida, activating the cells metabolism, and signaling it to complete meiosis.
Sexual Reproduction
External fertilization requires an aquatic habitat in which the gametes are released for transport.
Timing and amount of release are important, as well as synchronization of male and female reproductive activities.
Day length, temperature changes, weather changes, and social stimulation are important regulators of external fertilization.
Behavior, such as congregation of potential mates in a suitable environment, may play a vital role in bringing gametes together.
Sexual Reproduction
Internal fertilization was an important evolutionary step in the colonization of land by animals.
To enable internal fertilization, many accessory sex organs have developed, such as the penis (a tube for depositing sperm in the female reproductive tract).
The physical joining of the male and female accessory sex organs is called __________.
Sexual Reproduction
Some arthropods and amphibians deposit spermatophores (protected sperm containers) in the environment to be later picked up by the female.
Some male insects have elaborate structures to ensure species-specific copulations and even sperm removal from previously mated females.
Sexual Reproduction
Species that have separate and distinct male and female individuals are dioecious (two houses).
Species that have male and female reproductive systems in the same body are monoecious, or __________.
An example is the earthworm, which is both male and female at the same time, or a simultaneous hermaphrodite.
Some animals are male at one time in the life cycle but female at another. These are __________ hermaphrodites.
Sexual Reproduction
Simultaneous hermaphrodites have a selective advantage in ensuring that some reproduction can occur even when potential mates are scarce.
Most must mate with another individual, so hermaphroditism doubles their probability of producing offspring. The tapeworm, however, can fertilize its own eggs.
Sequential hermaphroditism may reduce inbreeding with siblings, thus increasing genetic diversity.
Sexual Reproduction
Early vertebrates evolved in aquatic environments, and the closest living relatives (fishes) remain exclusively aquatic, most having external fertilization.
Mating behavior in most fishes brings the sexes into close proximity for gamete release.
Some sharks and rays have evolved fin structures to permit direct transfer of sperm from male to female.
Amphibians were the first vertebrates to move onto land environments, but still return to water to reproduce.
Sexual Reproduction
Reptiles were the first vertebrate group to solve the problem of reproduction in dry environments by producing shelled, __________ eggs.
A hard shell protects the embryo and impedes water loss while allowing the diffusion of O2 and CO2.
Sperm must penetrate the egg before the shell forms. Hence, the evolution of internal fertilization was necessary.
Sexual Reproduction
Male snakes and lizards have paired hemipenes, which are blood-inflated and inserted in the female cloaca (a common tube for wastes and gametes) for gamete release.
Some primitive species of birds have erectile penises to channel sperm into the female cloaca.
In bird species with more recent evolutionary origins, the male stands on the females back in order to bring their genital openings close together.
Sexual Reproduction
All mammals have internal fertilization and, with the exception of the monotremes, have done away with the shelled egg.
The young embryo remains in the female reproductive tract through the early developmental period.
Sexual Reproduction
Two patterns of embryo care and nurture have evolved in animals: oviparity (egg bearing) and viviparity (live bearing).
Oviparous animals lay eggs stocked with abundant nutrients outside the mothers body in the environment.
Insects, reptiles, and birds protect the eggs from desiccation with tough, waterproof membranes or shells.
In some __________ animals, parents protect the eggs, but until hatching, the embryos are entirely dependent on nutrients stored in the egg.
Sexual Reproduction
Viviparous animals retain the embryo within the mothers body during early embryo development.
Mammals (except monotremes) are viviparous, and there are examples of viviparity in all other vertebrate groups except crocodiles, turtles, and birds.
Viviparous mammals have a uterus that holds the embryo and interacts with it to produce a placenta.
In most non-mammalian viviparous animals, fertilized eggs are simply retained in the mothers body until hatching. This is called ovoviviparity.
The Human Reproductive System
Semen, a mixture of fluids and molecules to support sperm and facilitate fertilization, is the product of the male reproductive system.
Sperm is produced in the testes, which are located in a sac called the scrotum outside the body cavity.
This location outside the body ensures that the optimal temperature for spermatogenesis (slightly below body temperature) is maintained.
Muscles control contraction and relaxation of the testes, keeping them at the proper thermal location in relation to the body.
The Human Reproductive System
Spermatogenesis takes place in the seminiferous tubules.
Each tubule is lined with a stratified epithelium, within which spermatogonia reside.
The germ cells are protected from noxious substances in the blood by Sertoli cells, which also provide nutrients for the developing sperm and are involved in the hormonal control of spermatogenesis.
Male sex hormones are produced by clusters of Leydig cells lying between the seminiferous tubules.
The Human Reproductive System
The second meiotic division of the germ cell results in four haploid spermatids, which develop into sperm cells as they move toward the lumen of the tubule.
The nucleus in what will become the head of the mammalian sperm becomes compact, and the surrounding cytoplasm is lost.
A flagellum develops, and the mitochondria become a condensed midpiece that will provide energy for motility.
A cap, or acrosome, forms over the nucleus in the head and contains digestive enzymes to penetrate the protective layers of the egg.
The Human Reproductive System
From the lumen of the tubules, sperm move to the epididymis, a storage sac where they mature.
The epididymis connects to the __________ via the vas deferens.
The urethra is the common duct for urinary and reproductive systems.
The components of semen come from several accessory glands.
The __________ glands produce a mucoid secretion that neutralizes acidity in the urethra and lubricates the tip of the penis.
The Human Reproductive System
Paired seminal vesicles produce about two-thirds of the volume of semen, consisting of mucus, protein, and fructose as an energy force for the sperm.
The prostate gland produces the thin, milky fluid that makes up the rest of the volume of semen.
Prostate fluid makes the uterine environment more hospitable to sperm and converts the semen, by enzyme action, into a gelatinous mass.
The Human Reproductive System
The penis and scrotum are the male genitalia.
The penis is a tubular shaft, the tip of which has sensitive skin called the glans penis that is very responsive to sexual stimulation.
A fold of skin called the foreskin covers the glans; the cultural practice of circumcision removes a portion of the foreskin.
Erections occur when the sexually aroused males autonomic nervous system causes penis blood vessel dilation.
This swells the spongy, erectile tissue and compresses the blood flow from the penis.
The Human Reproductive System
At the climax of copulation, semen is propelled through the vas deferentia and urethra in two steps.
During emission, contractions of smooth muscles in the vas deterentia and accessory glands move semen into the urethra.
During ejaculation (orgasm), contractions of the muscles at the base of the penis force semen through the urethra and out of the penis.
After ejaculation, the autonomic nervous system causes __________ of the vessels in the penis and thus a decrease in blood pressure in the erectile tissue; the compression of blood vessels leaving the penis is relieved and the erection declines.
The Human Reproductive System
Erectile dysfunction, or impotence, is an inability to achieve or sustain an erection.
The well-known drug Viagra treats erectile dysfunction by inhibiting the breakdown of the __________ that stimulates blood vessel dilation
The Human Reproductive System
The hormone testosterone, produced by the Leydig cells of the testes, controls spermatogenesis and maintains male secondary sexual characteristics.
At puberty, increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus stimulates cells in the anterior pituitary to to increase secretion of LH and FSH.
The Leydig cells are stimulated by LH to produce testosterone.
The rise in testosterone level promotes secondary sexual features, a growth spurt, increased muscle mass, and testes maturation.
The Human Reproductive System
Testosterone production after puberty is needed for the maintenance of secondary sexual characteristics and the production of sperm.
Spermatogenesis itself is under the control of FSH and testosterone on the Sertoli cells.
The Sertoli cells also produce the hormone inhibin, which exerts negative feedback on the production of FSH by the anterior pituitary.
The Human Reproductive System
In the female reproductive system, the mature egg is released into the body cavity and is swept into the end of the oviduct (Fallopian tube) by an undulating fringe of tissue.
Fertilization takes place in the oviduct.
Cilia lining the oviduct propel the fertilized or unfertilized egg toward the uterus, a muscular, thick-walled cavity.
The opening at the bottom of the uterus is the cervix, which leads into the vagina.
Sperm are deposited in the vagina, and the fetus must pass through it during birth.
The Human Reproductive System
The external opening of the vagina has two sets of folded skin, the labia majora and labia minora, which also surround the urethra.
At the tip of the labia minora is the clitoris, the anatomical analog of the male penis that is capable of erection and is highly sensitive to sexual stimulation.
Both the labia minora and clitoris become engorged with blood during sexual stimulation.
The opening to the vagina is initially covered by a thin membrane called the hymen, which ruptures with vigorous physical activity or first sexual intercourse.
The Human Reproductive System
To achieve fertilization, sperm swim up the vagina, assisted by contractions of the female reproductive tract.
The sperm then pass through the cervix and most of the oviduct to the egg (secondary oocyte) in the upper oviduct.
Fertilization stimulates completion of the second meiotic division, after which egg and sperm (both haploid) fuse to produce the diploid zygote.
Still in the oviduct, the zygote divides to become a blastocyst and continues down the oviduct.
In the uterus, the blastocyst attaches to the wall lining called the endometrium.
The Human Reproductive System
Stimulated by estrogen, the endometrium develops new blood vessels to cradle the blastocyst.
The blastocyst burrows in (implantation), interacting with the wall to form the placenta.
The placenta is the organ of exchange of nutrients and waste products between the embryos and the mothers blood.
If the blastocyst fails to arrive or embed, the endometrium regresses and is sloughed off in the subsequent monthly menstrual period.
The Human Reproductive System
The female reproductive cycle actually consists of two linked cycles: an ovarian cycle that produces eggs and hormones and a uterine cycle that prepares the endometrium for the arrival of a blastocyst.
The ovarian cycle repeats about every 28 days.
A womans fertile years total about 450 ovarian cycles. In each cycle, one oocyte matures and is released.
The end of fertility (menopause) occurs at about age 50, and only a few oocytes are left in each ovary.
The Human Reproductive System
Between puberty and menopause, six to twelve follicles (cells surrounding the primary oocyte) mature each month by proliferating beside the enlarging oocyte.
One of these follicles persists and continues to grow, while the others shrink and cease.
The enlarged follicle nourishes the growing egg with the nutrients and proteins it will use if fertilized.
After two weeks, ovulation occurs, a process in which the follicle ruptures and the egg is released.
The Human Reproductive System
After ovulation, the follicle continues to proliferate and forms an endocrine mass, called the corpus luteum, which produces estrogen and progesterone for another two weeks, then degenerates if a blastocyst is not implanted in the uterus.
The Human Reproductive System
The uterine cycle parallels the ovarian cycle and involves the buildup, then breakdown, of the endometrium.
About five days into the ovarian cycle, the endometrium builds in preparation for the blastocyst.
About five days after ovulation, the uterus is maximally prepared and stays that way for another nine days.
If the blastocyst does not arrive by then, the endometrium breaks down and sloughs off during menstruation.
The Human Reproductive System
Most other female mammals do not menstruate, but instead __________ the uterine lining.
These species demonstrate a seasonal state of sexual receptivity called __________ (heat) about the time of ovulation.
The human female is unusual in being __________ __________ __________ at all seasons of the year.
The Human Reproductive System
The ovarian and uterine cycles in human females are coordinated and timed by the same hormones that initiate sexual maturation.
At puberty, the hypothalamus signals an increase in gonadotropin-releasing hormone (GnRH), stimulating the anterior pituitary to secrete FSH and LH.
The ovarian tissue grows in response to these hormones and produces estrogen.
From puberty to menopause, interactions of GnRH, gonadotropins, and sex steroids control the ovarian and uterine cycles.
The Human Reproductive System
Menstruation marks the start of each uterine and ovarian cycle. FSH and LH levels increase and follicles begin maturing to produce estrogen.
All but one of the follicles wither away, but the remaining one continues to secrete estrogen causing the endometrium to grow.
Estrogen exerts negative feedback control on gonadotropin release during the first 12 days of the ovarian cycle.
On about day 12, estrogen exerts a positive rather than negative feedback control on the pituitary.
There is a surge of LH and some FSH, triggering the follicle to rupture and release the egg.
The Human Reproductive System
The follicle cells develop into the corpus luteum and secrete estrogen and progesterone to continue endometrial growth.
These hormones also provide negative feedback to the pituitary, inhibiting gonadotropin release to prevent maturation of new follicles.
If fertilization fails, the corpus luteum degenerates on about day 26 of the cycle, the endometrium sloughs off, and menstruation occurs.
Lowered levels of steroids in the blood cancel the negative feedback on the hypothalamus and pituitary, allowing GnRH, LH, and FSH to increase again.
The Human Reproductive System
After implantation in the uterus, a blastocyst begins to secrete human chorionic gonadotropin (hCG), which keeps the corpus luteum functional.
The presence of this hormone is the basis for pregnancy testing.
Tissues derived from the blastocyst also begin to produce estrogen and progesterone.
Continued high levels of estrogen and progesterone prevent the pituitary from secreting gonadotropins; thus, the ovarian cycle ceases for the duration of the pregnancy.
This same mechanism is the basis for birth control pills.
The Human Reproductive System
Throughout pregnancy, the muscles of the uterine wall periodically undergo weak Braxton-Hicks contractions (false labor contractions), which get gradually stronger by the third trimester.
Toward the end of the third trimester, the estrogenprogesterone ratio shifts in favor of estrogen; thus contractions become more stimulated.
Increased secretion of oxytocin by the pituitaries of mother and fetus stimulates muscle contraction and marks the onset of labor.
Mechanical stimulation comes from the stretching of the uterus by the fetus and the pressure of its head on the cervix.
The Human Reproductive System
A positive feedback loop develops (mechanical stimuli increase the release of oxytocin, and vice versa), and this converts Braxton-Hicks contractions into stronger labor contractions.
In the early stage of labor, contractions gradually become more frequent and intense until they have opened the cervix.
In the delivery stage, the babys head moves into the vagina and becomes visible from the outside.
The usual head-down position of the baby comes about during the seventh month of pregnancy.
The Human Reproductive System
Passage of the baby is assisted by the mothers bearing down with her abdominal muscles.
Once the baby is clear of the birth canal it can start breathing and become independent of the mothers circulation, so the umbilical cord is clamped and cut.
Finally, the placenta and fetal membranes are detached from the mother and expelled (several minutes1 hour).
Human Sexual Behavior
The hormonal and emotional biology of human reproductive behavior is complex and overlaid by social complexities and technological achievements.
Both men and women respond to sexual stimulation in four phases: excitement, plateau, orgasm, and resolution.
Human Sexual Behavior
In women:
Excitement: Heart rate and blood pressure increase, breasts swell and nipples become erect, external genitals and clitoris fill with blood, vaginal lubricating fluids are secreted.
Plateau: Breathing becomes rapid, clitoris retracts, entire genital area is sensitive.
Orgasm: A crescendo of physical pleasure and muscle tension with some loss of voluntary control; may last for minutes or repeat in rapid succession.
Resolution: Blood drains from genitals, body physiology returns to near normal.
Human Sexual Behavior
In men:
Excitement: Blood pressure, heart rate, and muscle tension increase, penis fills with blood and becomes erect.
Plateau: Breathing becomes rapid, glans of penis enlarges, lubricating fluid oozes from penis.
Orgasm: Pressure and friction on penis nerve endings trigger orgasm, muscles and accessory reproductive organs contract in spasms, resulting in ejaculation of semen out of body.
Resolution: Penis shrinks, body physiology returns to normal, a refractory period immediately begins during which a full erection or orgasm is impossible.
Human Sexual Behavior
The only sure methods of preventing conception and pregnancy are complete abstinence from sexual intercourse and surgical removal of gonads.
Since these methods are not acceptable to most people, various forms of birth control have been developed.
These birth control methods either block gametogenesis or block embryo development.
They vary enormously in effectiveness and acceptability.
Human Sexual Behavior
Nontechnological approaches:
The rhythm method requires a couple to avoid sex from day 10 to day 20 of the ovarian cycle, the females most fertile time. Body temperature is used to track the day of ovulation (temperature drops) and a sharp temperature rise the day after.
The annual failure rate is 1535 percent.
Coitus interruptus is the withdrawal of the penis before ejaculation.
Its annual failure rate may be as high as 40 percent.
Human Sexual Behavior
Barriers between the egg and sperm have been used for centuries.
A condom is a sheath of impermeable material (often latex) that is fitted over the erect penis, trapping the ejaculate inside so it does not enter the vagina.
Condoms also help prevent the spread of sexually transmitted diseases such as AIDS, gonorrhea, and syphilis.
If properly used, a condom can be highly effective, but the annual failure rate is about 15 percent.
Human Sexual Behavior
The diaphragm is a dome-shaped rubber device that fits over the womens cervix, blocking sperm from entering the uterus. A similar device, the cervical cap, fits snugly over the tip of the cervix.
Both are inserted before sexual intercourse and are first treated with a spermicidal jelly or cream, then set in place.
Annual failure rates of these devices are about 15 percent.
Human Sexual Behavior
Sperm-killing foams, jellies, and creams used alone have a 25 percent failure rate.
Douching (flushing the vagina with liquid after intercourse) is nearly useless as a birth control method.
Combinations of the above barrier methods usually improve the success rate.
Human Sexual Behavior
Oral contraceptives, or birth control pills, are low doses of synthetic estrogens and progestins, which, by negative feedback to the hypothalamus and pituitary, inhibit gonadotropin release.
Essentially, the ovarian (but not the uterine) cycle is suspended.
The risk of side effects is low; death risk from their use is less than that associated with a full-term pregnancy.
The pill is the most effective contraception method available today, with an annual failure rate of less than 1 percent.
Human Sexual Behavior
Long-lasting injectable or implantable steroids are also used to block ovulation.
They operate in the same manner as the pill but bypass the need for regular ingestion.
The mini-pill has very low doses of progestins and mainly acts on the female reproductive tract to make it inhospitable to sperm or to block sperm passage.
Human Sexual Behavior
Intrauterine devices (IUDs) are small pieces of plastic or copper inserted into the uterus to prevent implantation of a fertilized egg.
Their failure rate is low, about 17 percent.
Morning-after pills deliver high doses of steroids, which act on the oviduct and uterus, preventing implantation.
They can be effective up to several days after intercourse.
The drug RU-486 blocks progesterone receptors. It can be used even after the first missed menstrual period, at which point it causes the endometrial lining along with the embryo to be sloughed off.
Human Sexual Behavior
Sterilization is virtually foolproof as a contraceptive measure in men or women.
In male vasectomy, cutting of the vas deferentia prevents sperm from moving out of the testes.
It is minor surgery and does not affect hormone levels or the male sexual response.
The most common method of female sterilization is tubal ligation, in which the oviducts are cut and tied to block either sperm or egg transport.
Human Sexual Behavior
Abortion is the termination of a successfully implanted fertilized egg.
A spontaneous abortion, or miscarriage, is common in early pregnancy (about 10%) and may be due to fetal abnormality or aberrant implantation.
Abortion by medical intervention may be for therapeutic reasons (to protect the health of the mother or because prenatal testing has revealed a severe defect) or for fertility control.
Medical abortion is less risky in the first third of a pregnancy.
Human Sexual Behavior
Control of male fertility is more difficult than control of female fertility because sperm production is continuous rather than cyclical.
Since a single sperm out of millions produced may fertilize an egg, suppression of spermatogenesis must be total and requires continuous chemical intervention.
Human Sexual Behavior
The inability of a man and a woman to have children may be traced to a variety of problems ranging from gamete counts that are too low to chemical and structural incapacities.
The barriers to childbearing have been reduced by reproductive technologies.
Artificial insemination involves placing sperm in the female reproductive tract in a manner that promotes successful fertilization.
More recent advances called assisted reproductive technologies (ARTs) involve combining unfertilized eggs with sperm outside of the body.
Human Sexual Behavior
In in vitro fertilization (IVF), hormone stimulation of the follicles allows for the harvesting of many eggs, which are removed from the mother.
These eggs are combined with sperm collected from the father and cultured outside of the body (in vitro) where fertilization occurs.
The resulting embryos can be placed back inside the mother or kept frozen for later implantation. The success rate is 2025 percent.
Since 1978, thousands of babies have been produced using IVF.
Human Sexual Behavior
When the upper oviduct area is blocked, a technique called gamete intrafallopian transfer (GIFT) can be used.
Eggs and sperm are injected directly into the upper oviduct, where fertilization takes place and the blastocyst reaches the uterus via the normal route. The success rate is 30 percent.
Intracytoplasmic sperm injection (ICSI) is used when sperm cannot access the egg cell membrane. The success rate is 25 percent.
Human Sexual Behavior
Using IVF and genetic analysis, a procedure called preimplantation genetic diagnosis (PGD) can eliminate the risk that adults who are carriers of genetic diseases will produce affected children.
Blastocyst cells from embryos may be successfully removed at the 4- or 8-cell stage to determine if a harmful gene is carried.
The unremoved cells are not damaged, and only embryos free of the genetic defect are implanted in the mothers uterus.
Human Sexual Behavior
Copulation is one of the most intimate types of contact between two individuals.
It is not surprising that disease parasites have evolved using sexual contact between their hosts as their means of transmission.
These organisms cause sexually transmitted diseases (STDs) and include viruses, bacteria, yeasts, and protozoans.
Human STDs have been around since ancient times and are even today serious public health problems.
Human Sexual Behavior
Some, such as chlamydia and gonorrhea, are not fatal but may cause infertility and painful disease.
Syphilis, a spirochete, is fatal in about __________ percent of untreated cases.
AIDS, transmitted by a virus, is of recent origin and has a high rate of mortality.
The only birth control device that is also effective against STDs is the condom.
Animation 43.1 Fertilization in a Sea Urchin
Animation 43.2 The Ovarian and Uterine Cycles (Part 1)
Animation 43.2 The Ovarian and Uterine Cycles (Part 2)
Video 43.1 Asexual reproduction (budding) in hydra
Video 43.2 Spermatogenesis and the seminiferous tubules of the human male
Video 43.3 Gamete release and external fertilization in a sea star
Video 43.4 The amniote egg
Video 43.5 The female reproductive system and ovulation
Video 43.6 Human fetal development