a. Prior to ejaculation, the membrane of the spermatozoon head contains a substantial amount of strengthening cholesterol.

b. After ejaculation, however, fluids of the female reproductive tract wash away cholesterol and other inhibitory factors in the semen. The membrane of the spermatozoon head becomes more fragile.

a. To fertilize an egg proper, a spermatozoon must first penetrate the granulosa cells and zona pellucida, which may actually require hundreds of spermatozoa to clear the path for the one that penetrates the egg.

b. When a path has been cleared, a spermatozoon binds to the zona pellucida and releases its enzymes by exocytosis. It digests a pathway through the zona pellucida until it contacts the egg itself. The membranes of the two gametes then fuse, and the sperm nucleus enters the cytoplasm.

a. Cleavage refers to the mitotic divisions that occur in the first three days after fertilization and produce daughter cells called blastomeres.

b. The first cleavage occurs about 30 hours after fertilization. The next divisions occur at shorter intervals until, 72 hours after ovulation, a ball of 16 cells, called a morula, is formed.

c. The morula reorganizes, and becomes a blastocyst. Inside is the inner cell mass that will develop into the embryo. An outer layer of cells, called the trophoblast, will implant into the uterus.

a. About 6 days after ovulation, the blastocyst attaches to the endometrium.

b. The trophoblast cells secrete enzymes that stimulate thickening of the adjacent endometrium.

c. Implantation takes about a week and is completed about the time the next menstrual period would have occurred if the woman had not become pregnant.

d. The trophoblast secretes HCG.

a. During implantation, the embryoblast undergoes embryogenesis, in which the blastomeres becomes organized into the three primary germ layers: ectoderm, mesoderm, and endoderm.

b. The amniotic cavity becomes evident, along with an embryonic disc. As the disc elongates, a primitive streak forms along the midline of the ectoderm.

a. In trophoblastic nutrition, the conceptus is nourished by digesting cells of the mother's endometrium.

b. Gradually, placental nutrition follows, in which the conceptus is nourished by diffusion of nutrients from the mother's bloodstream through the placenta.

c. The placenta begins to develop about 11 days after conception. It starts with development of chorionic villi from the former trophoblast.

d. The smooth surface of the placenta facing the fetus gives rise to the umbilical cord, which contains three blood vessels that connect the fetus to the placenta.

a. As the villi grow and branch, their surface area increases and the membranes become thinner and more permeable. Thus there is a dramatic increase in placental conductivity, the rate at which substances diffuse through the membrane.

a. The amnion is a transparent sac that develops from cells of the embryonic disc. It grows to completely enclose the embryo and becomes filled with protective amniotic fluid.

b. The yolk sac arises partly from cells of the embryonic disc, opposite the amnion. It contributes to the formation of the digestive tract and produces the first blood cells and germ cells.

c. The allantois is an outpocketing of the posterior end of the yolk sac. It forms the foundation for the umbilical cord.

d. The chorion is the outermost membrane, enclosing all the rest of the membranes and the embryo. It develops into the placenta.

a. Infants born before this have a deficiency of pulmonary surfactant, causing respiratory distress syndrome (RDS). The infant becomes very fatigued by the high energy demand of breathing. RDS remains the most common cause of neonatal death.

a. Common viral infections of the fetus and newborn include herpes simplex, rubella, cytomegalovirus, and HIV.

b. Congenital bacterial infections include gonorrhea and syphilis. Toxoplasma, a protozoan contracted from meat, unpasteurized milk, and cats, is another common cause of fetal deformity.

c. Some of these pathogens have only mild effects of adults, but the fetus is vulnerable to devastating effects such as blindness, hydrocephalus, cerebral palsy, seizures, and profound mental and physical retardation.

a. Teratogens are viruses, chemicals, and other agents that cause anatomical deformities in the fetus.

b. A general rule of thumb is that all pregnant women should avoid sedatives, barbituates, and opiates.

c. Alcohol causes more birth defects than any other drug.

d. Cigarette smoking also contributes to fetal and infant mortality, ectopic pregnancy, anencephaly, cleft lip and palate, and cardiac abnormalities.

a. A mutagen is any agent that alters DNA or chromosome structure. Ionizing radiation and some chemicals have mutagenic, teratogenic, and carcinogenic effects, with extremely diverse results.

b. Some of the most common genetic disorders result not from mutagens, but from nondisjunction during meiosis.

c. Aneuploidy is the presence of an extra chromosome or a lack of one, accounting for 50% of spontaneous abortions.

d. Examples of aneuploidy include triplo-X syndrome, Klinefelter syndrome, Turner syndrome, and Down syndrome.

e. In Down syndrome, mental retardation is common and sometimes severe, but not inevitable. Nearly 50% of victims die within the first year of life due to immune deficiency or abnormalities of the heart and kidneys. Incidence of having an infant with Down syndrome increases with increasing maternal age.

a. This includes a gradual loss of reserve capacity, reduced ability to repair damage and compensate for stress, and increased susceptibility to disease.

b. Organ systems do not all degenerate at the same rate. Some functions decline slowly, while others, like female menopause, mark a rather abrupt decline in function.

a. Many changes occur to the skin and hair with age, including thinning and graying of hair, loss of elastic fibers and drying of the skin, fewer blood vessels, and problems with thermoregulation.

b. Normal changes of the skin can be contrasted with photoaging, or the changes in the skin that occur as a result of years of prolonged exposure to the sun. Skin cancer, age spots, and wrinkling are but a few of the symptoms of photoaging.

a. After age 30, osteoblasts become less active than osteoclasts, resulting in osteopenia, or bone loss. When this loss becomes severe, osteoporosis is the result.

b. Bones also become more brittle with age.

c. Osteoarthritis is a common complaint as the cartilage within joints degenerates after a lifetime of wear.

a. One of the most noticeable age-related changes is the replacement of lean muscle with fat, and accompanying loss of strength.

b. Aged muscle fibers have fewer myofibrils, so they are smaller and weaker. Muscle mitochondria are smaller with reduced quantities of oxidative enzymes. Muscles fatigue quickly.

a. The nervous system reaches its peak development at age 30. From age 35 on, probably 100,000 brain cells die yearly. The remaining cortical neurons have fewer synapses and synaptic transmission is less efficient. Degeneration of myelin sheaths slows down axonal transmission.

b. Not all functions of the nervous system are equally affected by senescence.

a. Visual acuity declines and requires corrective lenses with age. Cataracts are more common, and night vision diminishes as more light is needed to stimulate the retina.

b. Auditory sensitivity peaks in adolescence and declines afterward. Vibrations are transferred less effectively to the inner ear, creating a degree of conduction deafness. Sensitivity to high-frequency sounds is most diminished.

c. The senses of taste and smell are blunted as the number of taste buds, olfactory cells, and second-order neurons in the olfactory bulbs decline.

a. The endocrine system degenerates less than any other organ system. Reproductive hormones drop sharply, and growth hormone and thyroid hormone secretion decline steadily after adolescence, but other hormones continue to be secreted at fairly stable levels even into old age.

b. Target cell sensitivity declines, however, so some hormones have less effect.

a. Cardiovascular disease is a leading cause of death in old age. Senescence has multiple effects on the blood, heart, arteries, and veins. Anemia is common, although erythropoiesis remains steady.

b. Everyone exhibits coronary atherosclerosis with age. The heart wall becomes thinner and weaker, and cardiac output declines. Degenerative changes in the nodes and conduction pathways of the heart lead to a higher incidence of cardiac arrhythmia and heart block.

c. Atherosclerosis narrows the arteries and reduces perfusion in most organs. Vein valves degenerate and blood pools in the extremities.

a. The amounts of lymphatic tissue and red bone marrow decline with age; consequently there are fewer hemopoietic stem cells, and leukocytes to fight disease.

b. An older person has less protection against cancer and infectious diseases.

a. Pulmonary ventilation declines steadily after the 20s and results in loss of stamina. Chronic lung diseases are more common in old age since they represent the cumulative effects of a lifetime of degenerative change.

b. As lung capacity declines, the elderly are less able to clear their lungs of irritants and are thus more prone to respiratory infections.

a. The kidneys exhibit a striking degree of atrophy with age. The number of nephrons declines by 40% and up to a third of the remaining glomeruli become ineffective. Glomerular filtration rate is slower and there is little reserve capacity.

b. Drug doses often need to be reduced in old age because the kidneys cannot clear drugs from the blood as rapidly.

c. Water balance becomes more precarious in old age because the kidneys are less responsive to antidiuretic hormone and because the sense of thirst is sharply reduced. Dehydration is therefore more common.

a. Less saliva is secreted with old age, making food less flavorful, swallowing more difficult, and teeth more prone to caries.

b. Heartburn is more common as the weakening gastroesophageal sphincter fails to prevent reflux into the esophagus.

c. The most common digestive complaint of older people is constipation, which results from reduced muscle tone and weaker peristalsis of the colon.

d. Older people tend to reduce their food intake due to reduced appetite and lower energy demands. In eating less, however, nutrition make become inadequate. Vitamin and mineral supplements may be needed.

a. In men, the senescent changes in the reproductive system occur gradually, and include declining sperm count, less testosterone secretion, and waning libido. Men remain fertile well into old age but impotence can occur.

b. In women, the changes are more pronounced and develop more rapidly over the course of menopause. Gametogenesis ceases and the ovaries stop producing sex steroids. Various physical changes occur from lack of ovarian hormones.

a. Exercise maintains endurance, strength, and joint mobility while reducing the incidence and severity of hypertension, osteoporosis, obesity, and diabetes mellitus.

b. Resistance training may be the most effective way to reduce accidental injuries, while endurance training reduces body fat and increases muscle mass, cardiac output, and maximum rate of oxygen uptake.

a. Many hypotheses have been proposed to explain why organ function degenerates with age. Some authorities argue that senescence is governed by preprogrammed changes in cell function. Others attribute senescence to extrinsic (environmental) factors that damage cells over a lifetime.

b. A number of hypotheses are currently under study. These include the programmed cell death hypothesis, the Hayflick phenomenon of limited numbers of cell divisions, the telomere hypothesis, the cross-linking hypothesis, protein abnormalities, the free radical hypothesis, and the autoimmune hypothesis.

c. In all likelihood, a combination of one or more of these hypotheses gives the explanation for senescence.

a. If certain genes contribute to senescence, then why doesn't natural selection eliminate them? Death was once interpreted as being good for the species. However, natural selection works only through the effects of genes on the reproductive rates of individuals. These days, senescence occurs long after most individuals have reproduced.

CHAPTER ESSAY: Reproductive Technology - Making Babies in the Laboratory (p. 1055; Fig. E.1)