Gametogenesis

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Gametogenesis

• 2009

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Gametes reproductive cells

• Ovum
• Spermatozoon
• Gametogenesis differentiation of highly
  specialized sex cells capable of uniting at
  fertilization
• 1. Origin of the germ cell
• 2. Multiplication in the gonads by mitosis
• 3. Reduction of chromosomes meiosis
• 4. Final stages of maturation and differentiation

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Origin of primordial cells

• Germ cell can be recognized very early vegetal
  pole cytoplasm in the zygote
• Epiblast temporary residence in extraembryonic
  tissues recognizable at 24 ED in the endoderm
  of yolk sac
• Migration within mesenchyme of posterior wall of
  yolk sac (near the allantois), gut, and dorsal
  mesentery (4 -6 week) to the gonads
• Extracellular matrix and chemotactic influence
  from gonad resident germ cells induce formation
  of gonads
• Number of cells increases during migration

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Proliferation

• Oogonia and spermatogonia
• Proliferative phase of development from
  thousands to about 7 million (in female)
  mitosis
• Oogonia division during 2.-5. months
• By the seventh month oogonia entere the prophase
  of first meiotic division and end proliferative
  phase
• Spermatogonia enter meiosis after puberty,
  mitotic capability continues as long as the male
  is capable of reproduction

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Meiosis

• Reduction of normal number of chromosomes
• From diploid to haploid
• Two maturation divisions without new DNA
  synthesis
• Reductional division
• Equational meiotic division
• Recombination of genetic information
• Random distribution of maternal and paternal
  chromosomes
• Exchanging of portions of homologous chromosomes
  by crossing over

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First Meiotic Division

• Prophase I
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I and Interphase
• Second Meiotic Division

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• In Mammals - initiation of germ line development-
  maintain pluripotency within germ cells
• Activation of differentiation inductive signal
  from trophoblast
• Proliferation and survival trophic factors
• Extracellular matrix direct the migration
• Final differentiation

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Spermatogenesis - 64 days

• Mitotic multiplication spermatogonia (Type A
  stem cell population, Type B leave mitotic
  cycle - preleptotene spermatocytes)
• Meiosis - Primary spermatocytes
• Secondary spermatocytes
• Spermiogenesis Spermatides transformation
  into extremely specialized cells spermatozoa
  (concentration of chromatin, decrease of size,
  formation of acrosome, flagellum)

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Male germ cells

• Sertoli cells isolation of germ cells, support
  and nutrition
• Degradation of residual bodies
• Synthesis of signal molecules (Anti-Müllerian
  factor)
• Synchronization of development- waves

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Spermiogenesis

• Nucleus concentration of chromatin head
• Golgi complex- proacrosomal granules - acrosome
• Centrioles achorage of flagellum
• Axoneme microtubules (92) and dynein
• Mitochondria spiral investment around proximal
  part of flagellum mitochondrial helix
• Residual body

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Spermatozoon

• Head (nucleus and acrosome)
• Neck (proximal centriole)
• Middle piece (flagellum, centriole, mitochondrial
  helix)
• Tail - flagellum

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Sperm maturation

• Newly formed spermatozoa are not capable of
  fertilization. Maturation in genital tract
  activation increase of motility
• Capacitation final step of sperm maturation-
  changes in acrosome, preparing the enzyme release
  (in female genital tract), changes in sperm
  membrane
• Sperm attraction and hyperactivation
• Acrosome reaction fusion of the acrosome with
  plasma membrane, extension of the acrosomal
  process

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Oogenesis

• Oogonium gives arise to only one ovum first and
  second polar body (DNA and only little
  cytoplasma)
• First meiotic division is not completed untill
  puberty
• Meoitic arrest occurs during prophase I
  (diplotene) egg builds up its stores of yolk
• Second arrest during metaphase II mitosis is
  finished after fertilization

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Lampbrush chromosomes

• Active transcription during meiosis
• Synthesis of RNA genes loop out

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Oogenesis

• At birth 1 milion oocytes
• Surrounded by a layer of follicular cells
  (granulosa cells) follicle
• Only 400 (one per menstrual cycle) reach maturity
• Atresia (degeneration)
• Folliculogenesis
• Primordial
• Primary
• Secondary
• Graafian follicle - Ovulation

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Egg

• Egg accumulates yolk as reservoir of food
  (energy) for embryo
• Proteins (Amino acids, Energy)
• Ribosomes and tRNA- proteosynthesis after
  fertilization
• mRNA early development - morphogenic factors

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Coverings of eggs

• Zona pellucida Glycoproteins, GAG, Hyaluronic
  acid, Sialic adid. It is produced by oocyte
• ZP-3 Sperm receptor and induction of acrosome
  reaction
• Corona radiata follicular cells

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Fertilization

• It is an interaction between sperm and oocyte
• Spermatozoon binds to specific sperm receptor in
  the zona pellucida (ZP3). It induces release of
  enzymes from acrosome
• Penetration the zona pellucida
• Sperm and oocyte fuse
• Cortical reaction cortical granules release to
  perivitelline space (between oocyte and zona
  pellucida) alteration of receptors for sperms
  prevent polyspermy

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Prevention of polyspermy

• Fast block of polyspermy change the electrical
  potential
• Slow block of polyspermy - cortical granules
  -enzymes proteases clip off binding receptor
• Fertilization envelope space between zona
  pellucida and egg - GAG, peroxidase, and hyalin
  zona reaction

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Fertilization

• Fusion with sperm induces oocyte to resume
  meiosis second polar body and definitive oocyte
• Fertilized oocyte zygote
• Female and male pronuclei
• Membrane disapears
• Replication
• First mitotic division
• 24 hours

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Imprinting

• Egg-derived genome is functionally different from
  sperm-derived
• Imprinting is inactivation of gene depending on
  gender - prevent parthenogenesis
• Maternal genes are important for embryo
  development (receptor for IGFII)
• Paternal genes are important for placenta
  development (IGFII Beckwith-Wiederman sy)

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Cleavage

• Mitotic division without cell growth
• Daughter cells (Blastomeres) get smaller -
  embryo does not change in size
• Mitotic division is equal and total
• 4 cells 40 hours
• 3ED 6-12 cells
• 4ED 16 -32 cells morula (mulberry)

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Segregation of blastomeres into embryoblast and
trophoblast

• Starting at 8 cell stage changes in
  intercellular juctions compaction
  polarization of cells
• Tight junction and gap junctions outer cell
  mass
• Cells in centre inner cell mass (embryoblast)
  and outer cell mass (trophoblast).
• Fluid is collected blastocyst cavity
• Blastocyst Embryonic pole
• Abembryonic (vegetative)
  pole

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Genetic regulation of germ cell formation,
proliferattion, migration, and development

• Regulatory gene cascade sequential activation
  of genes that direct the initial induction and
  development, proliferation, survival, migration
  and differentiation of the germ cells
• Maternal effect genes germ plasm in zygote

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Twins and embryonic stem cells

• Monozygotic twins - before hatching at 5.ED
  dichorionic
• Later monochorionic,diamniotic
• Monochorionic monoamniotic
• Conjoined twins (after ED9)
• Inner cell mass embryonic stem cells