Development and Aging

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Chapter 16

• Development and Aging

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Outline

• Fertilization
• Development before Birth
• Fetal Circulation
• Embryonic Development
• Fetal Development
• Birth
• Development after Birth
• Aging

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Fertilization

• requires the collective action of many sperm
• Several sperm must work to penetrate the corona
  radiata
• Several sperm must work to penetrate zona
  pellucida

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Fertilization

• enzymes in acrosome of sperm dissolve away the
  corona and zona

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Fertilization membrane

• As soon as sperm penetrates, the eggs plasma
  membrane and zona pellucida change to prevent
  polyspermy

before
after
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Fertilization

• One sperm enters egg and nuclei fuse, producing a
  zygote

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Zygote
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Processes of pre-natal development

• Cleavage - Cell division without growth
• begins with zygote formation
• Morphogenesis - Shaping of embryo
• begins shortly before implantation
• Differentiation - Cells take on specific
  structure and function
• immediately follows morphogenesis
• Growth - Increase in number and size of cells

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Embryonic Development

• When a zygote begins dividing, it is termed an
  embryo.
• Developing embryo travels down oviduct and
  eventually implants in endometrium
• human chorionic gonadotropic hormone (hCG)
  produced by embryo confirms pregnancy
• If implantation does not occur, a woman never
  knows fertilization took place

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Development before Birth

• Stages of development.
• Morula - Solid mass of cells resulting from
  cleavage.
• Blastocyst - Ball of cells formed from morula.
• Embryonic disk - Inner mass of cells of
  blastocyst.
• Gastrula - Embryo composed of three tissues.
• Ectoderm, mesoderm, endoderm.

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Fertilization - Implantation

• approximately 7 days
• cleavage leads to formation of morula (mulberry)
• beginning of morphogenesis causes morula to
  become a blastocyst
• inner cell mass embryo
• trophoblast secretes hCG

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Pre-implantation Development
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Post-Implantation Development

• inner cell mass becomes two layers of
  cellsembryonic disk
• cavity above disk becomes amniotic cavity
• cavity below becomes yolk sac
• gastrulation third layer of cells forms
  (mesoderm)

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Primitive Node and Primitive Streak

• Marks extent of gastrulation
• resulting mesoderm will
• line future coelom
• form somites

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Neurula Stage

• Mesoderm becomes notochord
• ectoderm located just above the notochord
  develops into beginning of nervous system by
  induction
• induction the process where one tissue induces
  another to differentiate

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Neural Folds

• mark differentiation of ectoderm into neural
  tissue

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Neural Tube

• Neural folds fuse to form neural tube

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Primitive Streak and Neurula
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Extraembryonic Membranes

• Membranes external to the embryo
• Amnion - Provides fluid environment for
  developing embryo and fetus
• Yolk sac - First site of red blood cell formation
• Allantois - Contributes to cardiovascular system

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Extraembryonic Membranes
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Placenta

• fetal contribution from the chorion
• complete at 2 months

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Fetal Circulation

• The umbilical cord stretches between the placenta
  and the fetus and contains the umbilical arteries
  and veins
• Exchange of gases and nutrients between maternal
  and fetal blood takes place in the umbilical
  arteries
• Umbilical vein carries blood and oxygen away from
  the placenta to the fetus

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fetal circulation
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Embryonic Development

• Embryonic development occurs from the second week
  to the eighth week.
• Fetal development occurs from the third month
  through the ninth month.

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Summary of Embryonic Development

• blastocyst first week
• Implantation by end of the second week
• gastrulation by end of the third week
• Placenta forming by end of fourth week, complete
  by end of second month

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Day 14 Implantation Complete
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Day 21 Gastrulation
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Day 35 Placenta Forming
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Five Week Embryo
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End of the Second Month

• all organs have appeared and the placenta is
  fully functioning
• Embryonic development complete

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Fetal Development

• At the beginning of the third month, head growth
  begins to slow and the body increases in length
• Ossification centers appear in bones
• Sex can be determined sometime in the third month

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Three-Four Month Fetus
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Fifth through Seventh Months

• Mother begins to feel fetal movement
• Wrinkled skin covered by fine hair, lanugo, is
  covered by a greasy substance vernix caseosa
• Lungs lack surfactant

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Six Month Fetus
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Eighth and Ninth Months

• Fetus usually rotates so head is pointed down
  toward cervix
• Fetus is now about 530 mm in length and weighs
  about 3,400 g
• Full-term babies have the best chance of survival

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Development of Male and Female Sex Organs

• Sex of an individual is determined at the moment
  of fertilization?
• Gonads arise form indifferent tissue that can
  develop into ovaries or testes, depending on the
  action of hormones
• In the absence of a Y chromosome and in the
  presence of two X chromosomes, ovaries develop
  instead of testes

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At 6 Weeks Indifferent Gonads

• Y chromosome causes gonads to become testes
• testosterone secreted by testes causes male
  pattern of development
• without Y chromosome and/or testosterone, female
  (default) pattern of development occurs

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Development of Male Duct Systems

• mesonephric ducts develop into the male duct
  system (epididymis and vas) because of
  Testosterone (androgens)
• androgens (anti Mullerian hormone) cause
  paramesonephric ducts to degenerate

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Development of Female Duct Systems

• paramesonephric ducts persist and become uterine
  tubes and uterus in the absence of testosterone

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Male versus Female Duct Development
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At 6 Weeks -- Indifferent Genitalia

• glans and urogenital groove can develop into
  either sex

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Development of Male Genitalia

• by 14 weeks testosterone causes urogenital groove
  to close
• glans becomes glans and shaft of penis
• labioscrotal folds become scrotum

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Development of Male Genitalia

• without testosterone urogenital groove does not
  close
• glans becomes shaft and glans of clitoris
• labioscrotal folds become labia majora

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Male versus Female Genital Development
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Birth

• True labor is marked by uterine contractions that
  occur regularly every 15-20 minutes and last for
  40 seconds or more.
• Positive feedback control.
• Parturition.
• Stage 1.
• Mucous plug may be expelled from cervical canal.
• Cervix dilates completely.

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Birth

• Stage 2.
• Babys head descends into the vagina.
• Baby is delivered.
• Stage 3.
• Placenta delivered.

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Stages of Parturition
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Female Breast and Lactation

• Female breast contains 15-20 lobules, each with a
  milk duct beginning at the nipple and ending in
  alveoli.
• In pregnancy, breasts enlarge as ducts and
  alveoli increase in number and size.
• Milk usually not produced during pregnancy.
• Prolactin suppressed due to increase in estrogen
  and progesterone.
• Suckling stimulates release of oxytocin.

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Female Breast Anatomy
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Development after Birth

• Aging encompasses progressive changes that
  contribute to an increased risk of infirmity,
  disease, and death.
• Theories.
• Genetic in Origin.
• Whole-Body Process.
• Extrinsic Factors.

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Effect of Age on Body Systems

• Skin.
• Skin becomes less elastic due to changes in
  elastic fibers.
• Processing and transporting.
• Heart shrinks due to a reduction in cardiac
  muscle.
• Blood pressure gradually increases.
• Liver not as efficient in metabolizing drugs.
• Blood supply to kidneys reduced.

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Effect of Age on Body Systems

• Integration and coordination.
• Few neural cells of the cerebral cortex are lost
  during the aging process.
• Reaction time slows.
• Loss of skeletal muscle mass not uncommon.
• Reproductive system.
• Females undergo menopause.
• Male androgen levels fall between ages 50-90, but
  sperm produced until death.

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Review

• Fertilization
• Development before Birth
• Fetal Circulation
• Embryonic Development
• Fetal Development
• Birth
• Development after Birth
• Aging

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