Gametogenesis Developmental Biology Gilbert pp. 612- 617 *

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Gametogenesis

• Developmental Biology
• Gilbert pp. 612- 617

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Todays Objectives

• Identify the process by which meiotic divisions
  lead to both male and female mammalian games
• Identify the following important components of
  the process of fertilization gametes,
  spermatogonia, flagellum, tubulin, oocyte,
  pronuclei, vitelline membrane, zona pellucida

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Gametogenesis

• Process of creating gametes
• Germ cells are set aside early in development
• Migrate to the gonad
• Undergo meiotic divisions to make haploid germ
  cells

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Meiosis - A review

• What is the ploidy of the somatic cell that will
  undergo meiosis to form gametes?
• How many cell divisions take place?
• What happens to the genome before the first
  division?
• What are the phases in each meiotic division?
• How many daughter cells are made?
• What is the ploidy of those daughter cells?

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Gametogenesis in Mammals

• Spermatogenesis
• Process of producing sperm
• Occurs in seminiferous tubules of the testes
• Oogenesis
• Process of producing oocytes
• Occurs in ovaries

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Spermatogenesis

• Spermatogonia are the germ cells that will
  eventually develop into the mature sperm the
  first step in this development is the duplication
  of homologous chromosomes to get ready for
  meiosis
• 2) Primary spermatocyte the first meiotic
  division separates the homologous chromosomes
  from each parent
• 3) Secondary spermatocyte the second meiotic
  division separates the 2 chromatids and creates 4
  haploid cells
• 4) Spermatids Will produce 4 sperm cells by the
  process of spermiogenesis.
• Sperm cells differentiate into the shape we
  commonly know(will talk more about structure next
  time)

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• Simplified view
• of
• spermatogenesis

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Spermatogenesis (more detail - dont memorize!)
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When are sperm made in mammals?

• In males, the spermatogonia enter meiosis and
  produce sperm from puberty until death.
• The process of sperm production takes only a few
  weeks.
• 100 to 500 million sperm can be released at once.

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Oogenesis

• 1) Oogonia are the germ cells that will
  eventually develop into the mature oocytes
• 2) Primary oocyte the first step in this
  development is the duplication of homologous
  chromosomes to get ready for meiosis
• 3) Secondary oocyte the first meiotic division
  separates the homologous chromosomes from each
  parent
• 4) Egg the second meiotic division separates the
  2 chromatids and creates 4 haploid cells
• In females, it produces 1 egg and 3 polar bodies.
  This allows the egg to retain more cytoplasm to
  support early stages of development

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• Simplified view
• of Oogenesis

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When does Oogenesis occur in mammals?

• In females, this process is more complex than in
  males
• The first meiotic division starts before birth
  but fails to proceed.
• It is eventually completed about one month before
  ovulation in humans.
• In humans, the second meiotic division occurs
  just before the actual process of fertilization
  occurs.

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• Thus, in females, the completion of meiosis can
  be delayed for over 50 years.
• This is not always good.
• Why not? What could happen?
• Only I egg produced per month (usually)
• What event provides an example of a human the
  exception to this?
• In addition, all meiosis is ended in females at
  menopause.

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In older women, failure of the homologous
chromosomes to separate properly can cause
genetic disease
Down syndrome is trisomy 21. It results in short
stature, round face and mild to severe mental
retardation. This is the failure of the 2
chromatids to separate during meiosis 2. It
results in one oocyte receiving 2 instead of 1
chromatid. In older women, long term association
of chromatids (i.e., over 50 years) results in
the axial proteins failure to separate. Down
syndrome occurs with a frequency of 0.2 in women
under 30 but at 3 in those over 45 years of age.
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of female germ cells over time
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Structure of the Gametes

• Gilbert Ch. 7 pp. 175-180

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Structure of the Gametes Sperm

• Parts of mature sperm
• Head
• Haploid nucleus
• Little cytoplasm
• Acrosome
• Neck/Midpiece
• Mitochondria
• Centriole
• Tail (or propulsion system)
• Some species - ameboid motion
• Most sperm are propelled by flagella
• Formed by microtubles

Highly Specialized Cell Type!
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Figure 7.2(1) The Modification of a Germ Cell to
Form a Mammalian Sperm
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Flagella structure

• Must allow sperm to travel long distances, using
  plenty of energy
• Axoneme motor portion
• Microtubules in a 92 configuration
• 2 central microtubules, 9 doublets
• Made up of the protein tubulin
• Dyenin molecules attach to microtubules and
  provide motor activity by hydrolysis of ATP
• Allows filaments to slide and flagellum to bend

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Sperm Capacitation

• Upon release, mammalian sperm are able to move,
  but do not yet have the capacity to bind an egg
• Must enter the female reproductive tract to
  complete the last step of the maturation process
  (Capacitation) and acquire the ability to bind
  the egg

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Structure of Gametes The egg

• Ovum (mature egg) stores all material for
  beginning of growth and development
• Unlike sperm, the egg conserves and acquires more
  cytoplasm as it matures
• Synthesizes and stores proteins (like yolk) as
  reservoirs for the developing embryo
• The components of the egg vary from species to
  species

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Structure of the gametes The egg

• PARTS OF THE EGG
• Cytoplasm - many components
• Haploid nucleus
• Cell membrane
• will fuse with sperm plasma membrane
• Vitelline envelope
• Contains glycoproteins essential for species
  specificity sperm binding
• Zona pellucida (mammals) extra coating made of
  Extracellular matrix

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Structure of the Gametes The egg (contd)

• Cumulus (mammals) layer of cells that nurture
  the egg
• Innermost layer is called Corona Radiata
• Cortex
• Beneath the cell membrane
• Gel-like cytoplasm - may help sperm entry into
  the cell
• Cortical granules
• Inside cortex
• Membrane bound vesicles (like the acrosome in
  sperm)
• Help prevent polyspermy
• Egg jelly (some species)
• Attract/activate sperm

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Sea urchin egg at fertilization
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Egg Cytoplasm

• Proteins energy, amino acids
• mRNA
• To provide early instructions for development
• Ribosomes and tRNA
• To aid in protein synthesis early in development
• Morphogenetic factors
• Molecules that effect differentiation of various
  cell types (can be localized to specific areas of
  the cell)
• Protective Chemicals
• UV filters, DNA repair enzymes, antibodies (birds)

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Egg maturation at the time of fertilization in
various species
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HUMANS
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Hamster Eggs Before Fert.