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Chapter 22: Development and Aging


Chapter 22: Development and Aging Early Developmental Stages Fertilization occurs when the sperm and egg interact to produce a zygote. During fertilization, the ... – PowerPoint PPT presentation

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Title: Chapter 22: Development and Aging

Chapter 22 Development and Aging
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
  • Several sperm penetrate the corona radiata,
    several attempt to penetrate the zona pellucida,
    but only one sperm enters the egg.

  • Depolarization of the eggs 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
  • Following fertilization, the zygote undergoes
    cleavage, a period of cell division without
  • Cleavage leads to a ball of cells called the

  • 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
  • The presence of yolk causes cells to cleave more
  • 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
  • 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
  • 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
  • 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
Chordate embryo, cross section
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.

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 frogs egg is required for
    an embryo to develop.

Cytoplasmic segregation
Cytoplasmic influence on development
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  • 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
  • The presumptive (potential) notochord tissue
    induces the formation of the nervous system.
  • The vertebrate eye likewise forms by a series of

Control of nervous system development
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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
  • 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
  • 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
  • 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
Human Embryonic and Fetal Development
  • Nine months of human development is divided into
    the embryonic period (first 2 months) and the
    fetal period (months 39).
  • A human embryo is surrounded by four
    extraembryonic membranes
  • 1) The amnion envelops the embryo/fetus in a
    protective amniotic fluid.

  • 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.

Extraembryonic membranes
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.

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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
  • 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

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
  • Stage 1
  • Prior to parturition (giving birth), contractions
    of labor move the babys 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 12 minutes and
    the mother experiences a desire to push.
  • An episiotomy may be performed to prevent
  • 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 1525 lobules with milk
  • 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 babys immature immune system.

Female breast anatomy
Human Development After Birth
  • Development continues throughout all the stages
    of life infancy, childhood, adolescence, and
  • 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

Theories of Aging
  • Genetic in Origin
  • Evidence suggests aging has a genetic basis
  • 1) Cells of a species divide only a set number of
  • 2) As we grow older, it may be that more cells
    age, become non-functional, or die due to
  • 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

  • Extrinsic Factors
  • 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
  • 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
Chapter Summary
  • The first stages of human embryonic development
    lead to establishment of embryonic germ layers.
  • Differentiation and morphogenesis are two
    processes that occur when specialized organs
  • The roundworm and fruit fly have served as model
    organisms to determine the function of certain
    genes during development.

  • Extraembryonic membranes make life on land
  • 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.