Chapter 22: Development and Aging

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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 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.

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Fertilization
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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.

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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 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.

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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 frogs 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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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.

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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.

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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 39).
• 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 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.

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Human development before implantation
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• 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.

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Human embryonic development
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• 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
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• 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.

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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.

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The three-to four-month-old fetus looks human
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• 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).

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• 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.

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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 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.

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• Stage 2
• Uterine contractions occur each 12 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.

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Three stages of parturition (birth)
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• Female Breasts and Lactation
• The breast contains 1525 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 babys 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 Aging

• 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.

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• 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.

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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 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.

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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.

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• 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.

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• 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.

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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 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
  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.