Gametogenesis

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Gametogenesis

• Reproduction in vertebrates is by sexual means
  involving haploid (1N) germ cells
• Ovum female component
• Spermatozoa male component
• Both arise through meiosis cell division where
  each daughter cell receives ½ genetic material
  from original cell
• Primordial germ cells derived from extraembryonic
  endoderm (yolk sac) ? migrate to gonads

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Gametogenesis

• Oogenesis occurs in Ovary within a follicle of
  epithelial cells
• Spermatogenesis occurs in germinal epithelium
  lining seminiferous tubules of testis
• Oogenesis begins with oogonium Spermatogenesis
  begins with spermatogonium
• Both are normal 2N cells
• Reduction in chromosome number accomplished via
  two meiotic divisions

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Stages in Gametogenesis

• Pairing and doubling of chromosomes in ____gonia,
  followed by growth as primary ____ocyte (2 X 2N)
• 1st meiotic division produces two 2 X 1N cells (
  secondary ____ocytes)
• 2nd meiotic division produces four haploid cells
  (spermatids, ova)
• Spermatids mature and differentiate to form
  functional spermatozoa
• In spermatogenesis, all 4 sperm cells produced
  are viable
• In oogenesis, only 1 of 4 cells produced is
  viable.
• Others become abortive as polar bodies (only
  small amount of cytoplasm) that later degenerate

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Fig 14.22 - Oogenesis
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Fig 14.30 - Spermatogenesis
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Egg Membranes and Structure

• Cytoplasm enclosed within plasma membrane
• Vitelline membrane thin membrane closely
  attached to plasma membrane
• Zona pellucida glycoprotein layer (mammals)
• Corona radiata (mammals) layer of follicle
  cells that become sloughed off after fertilization

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Sperm Structure

• Spermatozoa from different animals have a wide
  variety of forms
• All have head and tail regions
• Head region serves two functions
• Contains nucleus (genetic function)
• Acrosomal cap contains enzymes that allow sperm
  to break down membranes around egg and fertilize
  egg
• Tail flagellum that provides motility
• Midpiece between head and tail contains
  mitochondria that provide ATP to fuel swimming

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Ovarian follicle
Spermatozoa
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Fertilization

• Several obstacles must be overcome for successful
  fertilization
• Sperm and egg must come into proximity
• Cell to cell contact must occur
• Sperm must penetrate egg cell

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Mechanisms for Proximity

• Transport occurs in liquid medium
• EXTERNAL FERTILIZATION
• Eggs and sperm simultaneously shed into water
• Occurs in fishes (except Chondrichthyes) and most
  anurans
• INTERNAL FERTILIZATION
• Sperm introduced directly into female tract
• Usually involves copulatory organs in males (none
  present in tuatara, birds, salamanders
  copulation by cloacal kiss)
• Occurs in animals with shelled eggs or viviparous
  habits as sperm must reach egg before shell is
  added (Chondrichthyes, most Amphibians, Amniotes)

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Mechanisms for Contact

• For internal fertilization, sperm travel within
  female tract by passive transport (dependent on
  muscular contractions and ciliary currents
  provided by female tract).
• Little active swimming by sperm for transport
  function
• Contact in external fertilization accomplished by
  random swimming movements of sperm in water

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Mechanisms for Egg Barrier Penetration

• Once contact with egg has been established, the
  next step is to penetrate the egg so that nuclear
  materials can unite to form the diploid zygote.
• Barrier penetration mechanisms are chemical in
  nature and involve acrosomal reaction
• Sperm Lysins enzymes that locally dissolve egg
  membranes
• Produced by acrosomal cap
• Sperm lysins differ among animal groups as
  membranes surrounding eggs differ (e.g., jelly
  coat in amphibians, follicle cells of corona
  radiata in mammals)

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Mechanisms for Egg Barrier Penetration

• Acrosome Reaction involves
• Release of sperm lysins
• Fusion of egg and sperm membranes
• In some animals, acrosomal reaction involves
  exposure of binding sites on plasma membrane of
  sperm, via acrosomal tubule or filament, which
  bind to receptors on p.m. of egg in
  species-specific manner
• This binding precedes fusion of sperm and egg
  plasma membranes

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rupture
Acrosomal Reaction in Hemichordates
?
sperm lysins
Binding sites exposed that bind to receptors on
egg plasma membrane
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Mechanisms for Egg Barrier Penetration

• In mammals, there is no development of acrosomal
  filaments
• Instead, fluids of female reproductive tract
  induce capacitation ? primes sperm for
  fertilization and includes removal of some
  components from sperm surface.
• After capacitation, hyaluronidase on the sperm
  head is exposed and breaks down the hyaluronic
  acid cementing the follicle cells of corona
  radiata (which surround the egg) together ?
  allows sperm passage through corona radiata to
  contact zona pellucida (a glycoprotein layer
  surrounding the egg)

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Mechanisms for Egg Barrier Penetration

• Zona pellucida has species-specific receptors for
  binding sperm
• Binding causes rupture of acrosome, which
  releases contents that break down zona pellucida
  and allow contact with egg plasma membrane
• Binding also exposes proteins on sperm surface
  that bind with receptors on egg plasma membrane
  to facilitate fusion of sperm and egg
• Fusion of plasma membranes releases sperm genetic
  material into egg as sperm pronucleus
• Male and female genetic material will soon
  combine forming a diploid zygote

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Post-fertilization Responses in Zygote

• Formation of Fertilization Cone outward bulge
  of egg cytoplasm that serves to engulf sperm
• Occurs upon fusion of sperm and egg plasma
  membranes
• Recession of cone brings sperm nucleus into egg
  cytoplasm
• Egg Activation
• Upon fusion (within 3 sec) get membrane
  depolarization or hyperpolarization
  (species-dependent) ? blocks entrance of gt 1
  sperm ( Fast block to polyspermy)

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Post-fertilization Responses in Zygote

• Next, get Ca2 release from internal stores
  within egg triggers cortical reaction ? release
  of cortical granules to perivitelline space
  around egg
• Cortical granule release causes development of
  fertilization membrane blocking further sperm
  entry ( Slow block to polyspermy)
• Slow block to polyspermy occurs about 25-30 sec
  post-fusion
• Seems to occur only for microlecithal eggs (e.g.,
  mammals) entrance of gt 1 sperm into eggs of
  birds, reptiles and some amphibians common, but
  only 1 sperm contributes to zygote (others
  somehow inactivated)

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Fast Block to Polyspermy
Slow Block to Polyspermy
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Post-fertilization Responses in Zygote

• Rearrangement of internal constituents within egg
• Sets up gradients of certain substances and plane
  of bilateral symmetry within zygote for some
  animals
• Fusion of Haploid Nuclei
• In most vertebrates, meiosis within egg arrested
  after 1st meiotic division. Sperm entry
  stimulates 2nd meiotic division to produce female
  pronucleus (and 2nd polar body)
• Once this 2nd division occurs, female pronucleus
  is ready for union with male pronucleus

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Post-fertilization Responses in Zygote

• Fusion of Haploid Nuclei (cont.)
• Male and female pronuclei next approach each
  other (mechanism by which this movement occurs is
  not known with certainty)
• Next get fusion of pronuclei
• In some animals (including most vertebrates),
  pronucleus membrane degenerates ? free
  chromosomes arrange themselves at spindle
  (metaphase of mitosis) ? completion of mitosis ?
  dipolid zygote

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Parthenogenesis

• Definition development of the egg in the
  absence of sperm
• Occurrence suggests that
• (1) egg activation and nuclear fusion are
  separate developmental processes
• (2) the ovum contains all the capacities
  necessary for embryo formation all that is
  necessary is some triggering agent
• Eggs can be activated by a number of chemical,
  thermal, electrical or mechanical means

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Parthenogenesis

• Parthenogenetic individuals are expected to be
  haploid (and many are), but these embryos are
  often diploid.
• Doubling of chromosomes accomplished in 3 ways
• Suppression of 2nd meiotic division occurs only
  in eggs completing this division after
  fertilization
• Refusion with second polar body
• Suppression of 1st mitotic division ( 1st
  cleavage division)

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Parthenogenesis

• Haploid embryos generally show premature
  developmental arrest
• Parthenogenetic diploid embryos also usually show
  premature developmental arrest
• However, in several invertebrates parthenogenesis
  is normal (e.g., male drones of bee colony) and
  there are several species of naturally occurring
  parthenogenetic lizards (the entire population is
  female)
• Artificial selection procedures have developed
  parthenogenetic strain of turkeys

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The asexual, all-female whiptail species
Cnemidophorus neomexicanus (center), which
reproduces via parthenogenesis, is shown flanked
by two sexual species having males, C. inornaus
(left) and C. tigris (right), which hybridized
naturally to form the C. neomexicanus species.
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Methods of Bearing Young

• Oviparous egg laying
• Primitive condition for vertebrates
• Occurs in most fishes, amphibians, reptiles, all
  birds, monotremes
• Viviparous live-bearing
• Advanced condition in vertebrates
• Some live-bearers occur in all vertebrate classes
  except cyclostomes and birds
• Evolved by retention of eggs within body to
  increase survival of young

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Placental Connections in Viviparous Vertebrates

• Anamniotes with connection between yolk sac and
  maternal tissues through which exchange of
  metabolites occurs (e.g., Chondrichthyes)
• Reptiles use yolk sac, chorion, allantois
  (extraembryonic membranes) or some combination
  for connection
• Mammals with a variety of connections

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Early Development/Placentation in Mammals

• After formation of zygote ? cleavage ? produces
  blastula
• Blastula forms before embryo reaches uterus
• Mammalian blastula consists of trophoblast and
  inner cell mass (ICM becomes embryo)
• Upon reaching uterus, trophoblast overlying ICM
  makes contact with uterine endometrium ?
  trophoblast cells rapidly multiply and insert
  among epithelial cells lining endometrium and
  endometrial cells degenerate ? implantation
• Continued trophoblast cell division ?
  placentation embryo becomes buried w/in
  endometrial lining

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Fig 5.32
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Mammalian Placenta Formation

• Structure produced by apposition and fusion of
  extraembryonic membranes of embryo with uterine
  endometrium of mother
• Extraembryonic membranes tissues external to
  embryo not participating in embryo formation, but
  functioning in maintenance of the embryo
• In Amniotes, four extraembryonic membranes exist

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Extraembryonic Membranes

• Yolk Sac forms from extraembryonic hypomere
  (splanchnopleure) that expands to enclose yolk
• This is the only extraembryonic membrane present
  in Anamniotes, so it occurs in all vertebrates
• Functions to derive nutrients from yolk in yolky
  eggs to nourish developing embryo
• In Amniotes, extraembryonic somatopleure grows
  over embryo by folding back on itself producing a
  double hood of somatopleure
• From this structure develop Amnion and Chorion

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Extraembryonic Membranes

• Amnion forms from inner somatopleure ectoderm
  (inside)
• Chorion forms from outer somatopleure
  ectoderm (outside)
• Amnion serves as fluid-filled sac for embryonic
  development
• Replicates aquatic developmental environment of
  primitive vertebrates.
• Allows complete conquest of terrestrial habitats
• Chorion functions in protection of embryo and in
  exchange of gases (and metabolites in placenta)

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Extraembryonic Membranes

• Outgrowth of splanchnopleure from posterior
  region of gut in Amniotes eventually expands to
  fill extraembryonic coelom ( space between
  amnion and chorion)
• This membrane is the Allantois composed of
  splanchnic mesoderm (outside) endoderm
• Mesoderm fuses with mesoderm of chorion to form
  Chorioallantoic Membrane main gas exchange
  organ for Amniote embryos
• Allantois also serves waste storage function

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Fig 5.29 Extraembryonic membrane formation in a
bird
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Fig 5.30
Fig 5.30 Extraembryonic membrane formation in a
bird
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Mammalian Placenta

• From chorion (outermost extraembryonic membrane),
  finger-like processes grow outward to interlock
  with uterine endometrium
• Blood streams of mother and fetus never mix
  always separated by epithelial membrane, so
  exchange of gases and nutrients occurs by
  diffusion across this membrane
• Chorion is not in direct contact with embryo so
  some means of blood supply from embryo to
  placenta (and back) must occur

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Mammalian Placenta

• Blood Supply to developing embryo differs between
  marsupial and placental mammals
• Marsupials mostly Choriovitelline fetal
  placenta
• Yolk sac associated with inner surface of chorion
• Blood vessels develop in mesoderm of yolk sac
• This situation also occurs to some extent in
  several placental groups (e.g., rodents)
• Placentals Chorioallantoic fetal placenta
• Dominant connection to chorion provided by
  allantois, yolk sac usually degenerates
• Allantoic mesoderm forms blood vessels that
  function in gas nutrient exchange, waste
  removal

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Fig 5.33 Fetal extraembryonic membranes in
various Amniotes
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Mammalian Placenta Types

• Primitive Condition apposition without fusion
  (non-deciduous)
• Advanced Condition fusion of maternal and fetal
  tissues (deciduous)
• Four Types occur
• Epitheliochorial most primitive
• Occurs in pig and some other mammals
• Maternal and fetal blood separated by 6 layers
  endothelium, CT, epithelium, epithelium, CT,
  endothelium

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Mammalian Placenta Types

• Syndesmochorial no uterine epithelium
• Occurs in ruminant mammals (cattle, sheep, etc.)
• Endotheliochorial no maternal epithelium or CT
• Occurs in carnivores
• Hemochorial advanced condition
• Chorionic epithelium bathed in maternal blood
• Occurs in primates and many rodents

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