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Fig 20-30
Human sperm contacting a hamster egg.
The ZP of the egg has been removed, exposing the
plasma membrane.
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Spermatozoa sperm animals

• activates the egg supplies haploid nucleus
• No ribosomes, ER and Golgi
• A lot of mitochondria power plants
• Condensed haploid nucleus tightly packed with
  protamines
• Acrosome vesicles contains hydrolytic enzymes
  that digest proteins and sugars.

Fig 20-25
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Axoneme consist of two singlet MTs surrounded by
nine MT doublets. Flagellar movement is driven
by dynein motor proteins
Fig 20-26
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Fig 20-28

• proliferating spermatogonia (self-renewing stem
  cell) are located at the outer surface of the
  tubules
• 24 days for a spermatocyte to become a spermatid.
• 5 additional weeks for a spermatid to develop
  into a sperm.

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Stages of Spermatogenesis
Spermatogonia proliferate continuously by
mitosis.
Fig 20-27
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• meiotic division II
• forms 4 haploid spermatids not yet functional
  gametes
• further specialization required to become
  mature spermatozoa

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formation of a syncytium
Fig 20-29
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• Spermatogenesis differs from oogenesis
• New cells enter meiosis continuously from time
  of puberty
• Each cell that begins meiosis gives rise to four
  mature gametes
• Sperm maturation occurs after meiosis is
  complete
• About twice as many cell divisions occur in the
  production of a sperm as in the production of a
  egg. (in mouse, 56 divisions for sperm vs. 27
  divisions for eggs).

• oogonia proliferate only in the fetus, enter
  meiosis before birth, and become arrested as
  oocytes in the meiotic phase I.
• Individual oocytes mature from strictly limited
  stock, and are ovulated at intervals, generally
  one at a time. beginning at puberty.

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• Compactiion sperm must go through this process
  in the female reproduction tract before they
  become competent for fertilization.
• Bicarbonate ions (HCO3-) enter the sperm and
  directly activate a soluble adenylase cyclase to
  produce cyclic AMP.
• This alters the lipid and glycoprotein
  composition of the sperm plasma membrane,
  increases sperm metabolism and motility, and
  reduces the membrane potential (hyperpolarized).

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Release proteases hyaluronidase

• ZP3 is responsible for the species-specific
  binding of sperm to the zona.
• ZP3 in the zona triggers the acrosome reaction.
  Ca2 enters into the sperm cytoplasm, which in
  turn initiates exocytosis.

Fig 20-31
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Release proteases hyaluronidase

• Fusion of sperm with the egg plasma membrane
  causes a local increase in cytosolic Ca2.
• Ca2 wave is induced and cortical granules
  releases their contents by exocytosis.
• Injection of Ca2 or use of Ca2 ionophores -
  eggs become activated

Fig 20-31
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B
A
C
D
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One sperm fertilizes one egg
If polyspermy occurs,
extra spindles are formed, resulting in faulty
segregation of chromosomes during cell division.
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Prevention of polyspermy

• Fast reaction (primary block)
• rapid depolarization of egg plasma membrane after
  Na influx
• Slow reaction (secondary block)
• phospholipase C activation
• inositol phospholipid signaling
• Ca2 increase
• cortical reaction
• hardening of zona pellucida

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Cortical reaction
Fig 20-32
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Fertilin protein in the sperm plasma membrane

• composed of two glycosylated transmembrane
  subunits, a and b.
• helps binding of sperm to the egg plasma membrane
  via ZP
• plays a role in the fusion of two plasma
  membranes.
• required to migrate out of the uterus into the
  oviduct.
• N-terminal region interacts with integrin (in the
  egg plasma membrane)
• Fusogenic region resembles the fusogenic region
  of viral fusion proteins.

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Immunofluorescent staining of a fertilized human
egg
B) Fertilized egg. Note the extrusion of a second
polar body and an array of microtubules by a
sperm head
A) meiotic spindle in mature, unfertilized egg
D) Centrosomes have duplicated. Daughter
centrosomes have organized a bipolar mitotic
spindle.
C) Pronuclei fusion
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• In fertilized mammalian eggs, pronuclei migrate
  toward the center of the egg
• Sperm donates not only DNA but also a centriole
• The egg has a centrosome, but no centriole
• Centrosome helps organize the assembly of the
  first mitotic spindle

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