Chapter 22: Aging

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Fertilization

• Depolarization of the egg’s plasma membrane after the sperm touches the egg and separation of the zona pellucida prevent a second sperm from fertilizing the egg.

• The sperm enters the egg and the sperm nucleus fuses with the egg nucleus.

Embryonic Development• Development is all the changes that occur

during the life cycle of an organism.• The embryo is the first stage in human

development. • Following fertilization, the zygote

undergoes cleavage, a period of cell division without growth.

• Cleavage leads to a ball of cells called the morula.

• The morula becomes a blastula when an internal cavity, the blastocoel, appears.

• At the gastrula stage, invagination of cells into the blastocoel results in formation of the germ layers: ectoderm, mesoderm, and endoderm; mesoderm arises from pouches in endoderm.

• Two layers of mesoderm form, and the space between them becomes the coelom.

• The three germ layers will have different developmental fates.

Lancelet early development

The Effect of Yolk• Yolk is a dense nutrient material found in

various amounts in the eggs of animals.• The amount of yolk affects the process of

gastrulation, the formation of the three germ layers.

• The presence of yolk causes cells to cleave more slowly.

• The aquatic frog and lancelet need less yolk as they develop rapidly.

• A chick egg has so much yolk that the embryo lies flat and endoderm formation does not occur by invagination.

• Instead an upper layer of cells becomes ectoderm, and a lower layer becomes endoderm; mesoderm invaginates between the two layers, and the furrow that develops is called a primitive streak.

• Because of a shared evolutionary history, gastrulation in humans is like that of the chick even though the human egg has little yolk.

Comparative animal development

Neurulation and the Nervous System

• The notochord forms from mesoderm.• During neurulation, the nervous system

develops from midline ectoderm, just above the notochord; the notochord induces formation of the nervous system.

• A neural plate is seen first, then a neural tube; the anterior neural tube becomes the brain.

• At the neurula stage, cross sections of all chordate embryos appear similar.

Development of neural tube and coelom in a frog embryo

Chordate embryo, cross section

Cytoplasmic Segregation

• The cytoplasm of an egg is not uniform but contains maternal determinants that are parceled out during mitosis.

• Cytoplasmic segregation helps determine how the various cells of the morula will develop.

• The gray crescent in a frog’s egg is required for an embryo to develop.

Cytoplasmic segregation

Cytoplasmic influence on development

Induction• Induction occurs when embryonic cells

influence one another to develop in a particular way.

• A molecular concentration gradient may act as a chemical signal to induce germ layer differentiation.

• The presumptive (potential) notochord tissue induces the formation of the nervous system.

• The vertebrate eye likewise forms by a series of inductions.

Control of nervous system development

Model Organisms• The Roundworm Experiments

• The roundworm Caenorhabditis elegans develops into an adult of 959 cells; researchers have traced every cell division from the first fertilized egg and developed a fate map.

• Work with the roundworm shows that induction involves signals that activate new genes that provide new signals, and that induction requires the regulation of genes in a particular sequence.

• Programmed cell death (apoptosis) plays a role during development.

• A good example is the development of fingers and toes in humans due to death of cells between the digits.

• The fate maps of C. elegans indicate that apoptosis occurs in 131 cells as development takes place.

Development of C. elegans, a small worm

The Fruit Fly Experiments

• Research with fruit flies has shown how morphogenesis comes about; that morphogen genes determine the pattern of an animals and its parts.

• Each morphogen gene codes for a protein that is present in a gradient.

• Homeotic genes control the organization of differentiated cells into specific three-dimensional structures.

Pattern formation in the fruit fly

• Homeotic genes form protein gradients that determine if a segment will bear antennae or legs or wings.

• The same sequence of genes is found in many organisms; the same sequence of nucleotides is a homeobox.

• A homeobox codes for a sequence of 60 amino acids called a homeodomain.

• Homeodomain proteins bind to DNA and determine which genes are turned on.

Homeotic mutations

Embryonic Development• The First Week

• Fertilization occurs in the upper third of the oviduct; a zygote is produced.

• The embryo is ball of cells called a morula when it reaches the uterus on the third day.

• By the fifth day, the morula is transformed into a blastocyst which consists of an outer trophoblast and an inner cell mass.

Human development before implantation

• The Second Week• The embryo begins to implant in the uterine

lining at end of first week.

• The trophoblast secretes human chorionic gonadotropin (HCG), a hormone that maintains the corpus luteum.

• The yolk sac and amnion form.

• Gastrulation occurs and the inner cell mass becomes the embryonic disk while the trophoblast becomes the chorion.

Human embryonic development

• The Third Week• Neurulation occurs and the nervous

system is the first visible organ system.

• The heart begins to form and pump blood when right and left heart tubes fuse.

• The Fourth and Fifth Weeks• The allantois forms and is contained within

the umbilical cord.

• Limb buds appear and sense organs develop.

Human embryo at beginning of fifth week

• The Sixth Through Eighth Weeks• By end of eight weeks, the embryo is only

38 mm (1.5 inches) long but is easily recognized as human.

• All organ systems are established, even though the embryo weighs no more than an aspirin tablet at this point.

Fetal Development and Birth

• The Third and Fourth Months• During the third and fourth months, the

body increases in size, and epidermal refinements (eyelashes, nipples) become apparent.

• Bone is replacing cartilage. • It is now possible to distinguish males from

females, and the heartbeat is audible with a stethoscope.

The three-to four-month-old fetus looks human

• The Fifth Through Seventh Months• The thin skin is covered with lanugo and

coated with a vernix caseosa.

• The eyelids open fully.

• At the end of seven months, the fetus can possibly survive if born prematurely.

• The fetus is now 300 mm (12 inches) in length and weighs 1,380 grams (3 lb).

• Fetal Circulation• Blood passes from the right to the left

atrium through an oval opening, the foramen ovale, and an arterial duct, the ductus arteriosus, shunts blood between the pulmonary trunk and aorta.

• These features enable blood to bypass the non-funtioning lungs.

• Two umbilical arteries that branch off the iliac arteries lead to the placenta.

• One umbilical vein takes nutrients to the systemic system when the umbilical vein joins the vena cava by a venous duct.

• If the oval opening fails to close, it causes a “blue baby” that receives a mixture of oxygenated and unoxygenated blood.

Fetal circulation and the placenta

• The Structure and Function of the Placenta

• Chorionic villi project into maternal tissue as the placenta develops.

• By the tenth week, the placenta is fully formed and secretes estrogen and progesterone that maintains the lining and prevents further menstrual cycling and ovulation.

• Fetal and maternal blood cells do not mix within the placenta.

• Carbon dioxide and wastes diffuse from the fetal to the maternal side, and oxygen and nutrients diffuse from the maternal to the fetal side.

• Harmful chemicals can cross the placenta and some alter normal fetal development.

Anatomy of the placenta in a fetus at six to seven months

Birth

• Stage 1• Prior to parturition (giving birth),

contractions of labor move the baby’s head downward, causing effacement and dilation of the cervix.

• The amnion (bag of waters) breaks, releasing amniotic fluid.

• The cervix is dilated completely at the end of this stage.

• Stage 2• Uterine contractions occur each 1–2

minutes and the mother experiences a desire to push.

• An episiotomy may be performed to prevent tearing.

• The baby is pushed out during this stage, and the umbilical cord is cut and tied.

• Stage 3• The afterbirth (placenta) is delivered.

Three stages of parturition (birth)

• Female Breasts and Lactation

• The breast contains 15–25 lobules with milk ducts.

• No milk is produced during pregnancy, but milk ducts and alveoli proliferate during that time, and breasts enlarge.

• Once the baby is delivered, the pituitary secretes prolactin, and milk is produced.

• Suckling of the baby at the breast stimulates the hypothalamus to direct the pituitary to secrete oxytocin that, in turn, causes milk to flow.

• Breast milk contains antibodies that supplements the baby’s immature immune system.

Female breast anatomy

Aging

Theories of Aging: WHY oh WHY?• Genetic in Origin

• Evidence suggests aging has a genetic basis:

• 1) Cells of a species divide only a set number of times.

• 2) As we grow older, it may be that more cells age, become non-functional, or die due to mutations.

• 3) In addition, offspring of long-lived people also tend to be long-lived.

• Whole-Body Process• A second theory of aging suggests that a

hormonal decline can affect many different organ systems.

• Type II diabetes is due to cells lacking receptors to take up insulin; menopause is a similar failure by ovaries to take up the follicle-stimulating hormone.

• The thymus gradually gets smaller with age.

• The immune system no longer performs as well, which is perhaps why cancer and autoimmune diseases are more prevalent in the elderly.

• Aging may also be due to a tissue change that affects all organs throughout the body.

• Collagen fibers become cross-linked which leads to loss of elasticity throughout many body organs.

Extrinsic Factors: chasing youth!• A third theory on aging suggests that years of

poor health habits contribute most to aging.

• Insufficient calcium intake and smoking increase osteoporosis, for example.

• Exercise and adequate servings of fruits and vegetables help eliminate cardiovascular disease.

Effect of Age on Body Systems• Skin

• Skin loses elasticity and becomes thinner with age, resulting in sagging and wrinkling.

• Fewer sweat glands are present, so temperature regulation is less efficient.

• The number of oil glands is reduced, so skin tends to crack.

• Pigmented blotches appear on the skin.

• Processing and Transporting

• Cardiovascular disorders are the leading cause of death among the elderly; the heart shrinks with age, and fatty deposits clog arteries.

• Lungs lose elasticity, so ventilation is reduced.

• A reduced blood supply to the kidneys results in the kidneys becoming smaller and less efficient.

• The digestive tract may lose muscle tone but still absorbs nutrients efficiently.

• Integration and Coordination• Normal aging results in the loss of few nerve

cells; short-term memory may decline but overall cognitive skills remain.

• After age 50, there is a decline in the ability to hear higher frequencies, and the lens of the eye does not accommodate as well.

• Loss of skeletal muscle mass and bone density is common but can be controlled through exercise and adequate calcium intake.

• The Reproductive System

• Females undergo menopause and are no longer reproductive.

• In males, sperm production declines after age 50 but continues until death.

• Women as a group outlive men.

• Conclusion

• Good health habits, started when young, slow the aging process and contribute to a long, healthy life span.

Remaining active