Human Development

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Human Development
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How do we develop?

• On ovulation day, egg and sperm fuse to form
  zygote.
• Zygote divides, implants onto uterus and grows
  into Embryo and hangs out for about 9 months.
• Embryo decides it is time to breathe air, fetal
  adrenal glands trigger contractions and out comes
  baby.
• Baby grows grows grows into child, child
  undergoes puberty and becomes adult.
• Adult lives, works, reproduces (perhaps), gets
  gray hair and croaks.

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Fertilization

• Egg must develop and be released on ovulation
  day.
• Egg must be correctly positioned in the oviduct
  and attract sperm.
• Vaginal tract must activate sperm.
• Hormonal levels must be exact.
• Ensure only one sperm joins with egg.

• Sperm must undergo capacitation--process of
  activation by substances in female vaginal tract
  fluids.
• Sperm motor from vagina up through cervix,
  uterus, to the oviduct.
• Many sperm attempt fertilization, only one
  succeeds (except for twins).

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Egg and Sperm

• Egg--
• Corona Radiata Cells form follicle that
  nourished egg in ovary.
• Zona Pellucida layer that covers plasma
  membrane--will form fertilization envelope
• Plasma Membrane cell membrane around egg
• Egg Nucleus contains DNA

• Sperm--
• Head Contains sperm nucleus and acrosome.
• Acrosome Contains enzymes.
• Mid Piece Contains Mitochondria
• Tail Flagella made from Microtubules

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REMEMBER!!!!!!!!!

• If viable sperm contact an egg at the time of
  ovulation fertilization will occur.
• This typically occurs on day 14. Remember Day
  1 is first day of menstruation.
• The fertilized egg will implant on day 6.
• The new embryo will begin to produce HCG--Human
  Chorionic Gonadotripin.
• HCG maintains the corpus luteum and allows the
  production of progesterone and estrogen until the
  placenta takes over this task.

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Dev before Implantation

• Fertilization
• Cleavage successive rounds of cell division. A
  one cell zygote--2 cell--4 cell--8 cell--.
• Cleavage occurs in the oviduct.
• Morula 16 cell stage--enters the uterus.
• Key cell differentiation step
• Trophoblast
• Inner Cell Mass

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Blastocyst

• Hollow ball of cells.
• Each cell is called a blastomere.
• Inner cell mass--become the embryo.
• Trophoblast--Incredible Altruistic Cells!
• Escape from the Zona Pellucida
• Digest through Endometrium
• Initiate HCG secretion
• Form the Placenta

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Dev before Birth

• Cleavage--process of cell division.
• Morphogenesis--shaping the embryo.
• Differentiation--how do different cells get their
  identity?
• Growth--increase in size--cell divisions continue
  and cells themselves grow.

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Gastrulation

• Truly the most important day of your life!
• Process of forming 3 germ layers--this process
  requires cell movement.
• Each germ layer forms specific tissues and organs
• Ectoderm--(blue)--will form skin and nervous
  system.
• Mesoderm--(red)--will form muscles, kidneys,
  connective tissue, and reproductive organs.
• Endoderm--(yellow)--will form digestive tract,
  lungs, liver and bladder.

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Extraembryonic Membranes

• Establishing extraembryonic membranes is
  critical. These membranes protect the embryo and
  link embryo to mother
• Amnion--provides fluid environment for fetus.
• Chorion--becomes the placenta--site of gas and
  nutrient exchange with mother.
• Allantois--becomes unbilical blood vessels

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The Placenta

• Nutrient and Gas Exchange between fetus and
  mother.
• Fetal side--from chorion.
• Maternal side--from uterine tissue
• Blood of fetus and mother do not mix.
• Fetal chorionic villi project into maternal
  blood.
• Exchange occurs across membranes.
• Umbilical cord stretches between placenta and
  fetus.

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Birth Defects

• 1 in 16 newborns (6.25 out of 100) born with
  birth defect. Many minor, but some serious or
  fatal.
• 20 of defects (3.125 out of 1000) are genetic.
• Causes
• neural tube closure problems--folic acid.
• drugs--aspirin, caffeine, alcohol, vitamin A
  creams, cigarette smoke, cocaine, heroine.
• pathogens--rubella, HIV, STDs, listeria, others.

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Genetic screening

• Amniocentesis--remove fluid from amniotic cavity,
  analyze cells for genetic abnormalities.
  Performed 15th -17th week of pregnancy.
• Chorionic villi sampling--remove villi by
  suction, test for genetic abnormalities.
  Performed 5th to 12th week of pregnancy, chance
  of risk for fetus.
• Screening eggs--obtain eggs and test a polar body
  (eggs clone). If polar body is normal,
  fertilize and implant the egg.

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Parturition--Stages

• Stage I water breaks, cervix dilates.
• Stage II contractions increase to every 1-2
  min, baby emerges. Episiotomy (cut vaginal
  orifice) can prevent ripping. Baby emerges,
  umbilical cord cut.
• Stage III Placenta is delivered about 15min
  after birth.

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Sex determinationin Humans
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Embryonic Indifference

• Both male and female embryos are indifferent.
• That means they have the ability to be both male
  and female.
• Wolfian--will form male genetalia.
• Mullarian--will form female genitalia.

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Sex Determination

• How does this work? All embryos start on a
  neutral or "indifferent" path. The 4 week old
  embryo is indifferent
• By 7 weeks, the SRY gene encoded by the short arm
  of the Y chromosome begins to roar.
• Indifferent embryos have two sets of ducts
  Müllerian ducts--will be come the future
  oviducts--thus female.
• Wolfian ducts--will become the future vas
  deferens--thus male.

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Sex Determination

• How does SRY determine which duct will develop?
  SRY causes testes to develop.
• The Wolfian duct is dependent on testosterone for
  its continued development. It needs testosterone
  produced from the testes to grow.
• The testes also produce an anti-Müllerian hormone
  that promotes regression of the Müllerian ducts.
  
• So without SRY, the indifferent embryo will
  naturally develop into a female.

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Embryonic Indifference

• Both male and female embryos are indifferent.
• That means they have the ability to be both male
  and female.
• Wolfian--will form male genetalia.
• Mullarian--will form female genitalia.

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Sex Chromosomal Disorders

• Turner Syndrome XO only one sex chromosome
• Short, thick neck and stature
• Do not undergo puberty, or menstruate,
• no breast development
• Kleinfelter Syndrome XXY
• Testis and prostate underdeveloped
• No facial hair
• Brest development
• Long arms and legs big hands and feet
• Can be mentally retarded

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Two copies of DAX (double X) inactivate SRY, thus
this individual would be genetically male, but
look female.
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An XY Individual with Androgen Insensitivity
Syndrome
Androgen Insensitivity Syndrome is a sex reversal
condition where XY individuals look female.
These individuals have the Y chromosome and
functional SRY. These individuals have testis
which generate AMH and testosterone. However,
the genetic mutation results in a lack of the
testosterone receptor. Estrogens are made in the
adrenal gland which drive phenotypic development.
As adults, these individuals have testes in the
abdomen and lack a uterus and oviducts.
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Chromosomal Inheritance is ultimately the only
inheritance we need. Some believe that our
genes desire to be passed on ultimately drives
us. Richard Dawkins argues this in his book the
selfish gene. Paraphrased Individuals are
not stable things, they are fleeting. Genes,
like diamonds, are forever! p35.
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What do other animals do?

• Temperature controls the aromatase gene.
• Aromatase regulates estrogen/testosterone levels.
  It converts testosterone into estrogen.
• Both estrogen and testosterone are built from
  cholesterol and are almost identical!

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Fish Development

• In northern Atlantic ocean regions, fish use
  temperature to help determine sex. More females
  are born during early spring, more males during
  summer.
• Why? Females need to grow larger to produce
  eggs--eggs require more nutrients. Males do not
  need to grow as large and sperm production
  requires less time and energy.

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Figure 17.24 Demasculinization of Frogs by Low
Amounts of Atrazine
Atrazine is a very widely used herbicide. In the
US, 60 million pounds are used annually.
Atrazine has been demonstrated to have a profound
effect on male amphibian development as Atrazine
is thought to induce aromatase--thus estrogen
will be converted from testosterone. Panel A
shows a testis of a frog from a site with 0.5 ppb
Atrazine--lobules are developing both sperm and
oocytes. Panel B--testis from a site with 0.8
ppb atrazine--note the sever testicular
dysgenisis. Panel C shows an experiment where
frogs were exposed to Atrazine over a 46 day time
course.
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The End.