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Chapter 22: Development and Aging
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Early Developmental Stages
Fertilization occurs when the sperm and egg interact to produce a zygote.
During fertilization, the acrosome of a sperm releases enzymes that digest a hole in the corona radiata, then in the zona pellucida around the egg.
Several sperm penetrate the corona radiata, several attempt to penetrate the zona pellucida, but only one sperm enters the egg.
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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.
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Fertilization
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Embryonic DevelopmentDevelopment 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.
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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.
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Lancelet early development
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The Effect of YolkYolk 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.
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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.
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Comparative animal development
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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.
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Development of neural tube and coelom in a frog embryo
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Chordate embryo, cross section
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Developmental Processes
Development requires growth, cellular differentiation, and morphogenesis.
Cells differentiate when they become specialized in structure an function.
Morphogenesis produces shape and form. There is no loss of genes during
development; each cell remains totipotent, containing all instructions for any other specialization.
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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.
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Cytoplasmic segregation
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Cytoplasmic influence on development
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InductionInduction 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.
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Control of nervous system development
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Model OrganismsThe 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.
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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.
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Development of C. elegans, a small worm
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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.
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Pattern formation in the fruit fly
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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.
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Homeotic mutations
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Human Embryonic and Fetal Development
Nine months of human development is divided into the embryonic period (first 2 months) and the fetal period (months 3–9).
A human embryo is surrounded by four extraembryonic membranes:
1) The amnion envelops the embryo/fetus in a protective amniotic fluid.
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2) The yolk sac is the first site of red blood cell formation.
3) The blood vessels of the allantois become the umbilical blood vessels.
4) The chorion contributes to the placenta.
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Extraembryonic membranes
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Embryonic DevelopmentThe 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.
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Human development before implantation
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The Second WeekThe 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.
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Human embryonic development
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The Third WeekNeurulation 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 WeeksThe allantois forms and is contained
within the umbilical cord.
Limb buds appear and sense organs develop.
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Human embryo at beginning of fifth week
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The Sixth Through Eighth WeeksBy 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.
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Fetal Development and Birth
The Third and Fourth MonthsDuring 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.
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The three-to four-month-old fetus looks human
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The Fifth Through Seventh MonthsThe 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).
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Fetal CirculationBlood 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.
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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.
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Fetal circulation and the placenta
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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.
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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.
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Anatomy of the placenta in a fetus at six to seven months
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Birth
Stage 1Prior 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.
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Stage 2Uterine 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 3The afterbirth (placenta) is delivered.
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Three stages of parturition (birth)
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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.
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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.
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Female breast anatomy
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Human Development After Birth
Development continues throughout all the stages of life: infancy, childhood, adolescence, and adulthood.
Aging encompasses these progressive changes.
The study of aging is gerontology; its goal is to increase the health span.
The human life span extends a maximum of 120-125 years.
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Aging
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Theories of AgingGenetic 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.
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Whole-Body ProcessA 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.
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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.
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Extrinsic FactorsA 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.
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Effect of Age on Body Systems
SkinSkin 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.
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Processing and TransportingCardiovascular 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.
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Integration and CoordinationNormal 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.
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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.
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Remaining active
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Chapter SummaryThe first stages of human embryonic
development lead to establishment of embryonic germ layers.
Differentiation and morphogenesis are two processes that occur when specialized organs develop.
The roundworm and fruit fly have served as model organisms to determine the function of certain genes during development.
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Extraembryonic membranes make life on land possible.
Human embryonic and fetal development is a series of steps that end at childbirth.
Breasts develop mammary glands to supply nutrients to the infant.
Research on the processes of aging may identify underlying causes of degeneration and prolong the health span of individuals.
