Gametogenesis
and fertilization
VBT 140
Gametes – germ cells
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Gametogenesis – differentiation of highly
specialized germ cells, which are able to form the
new being after fertilization
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Oocyte (ovum)
Sperm
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1. Origin of germ cells
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2. Their mitotic division within gonads
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3. Reduction of chromosome number – meiosis
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4. End stages – maturation and differentiation
Origin of primordial cells
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Germ cells can be demonstrated very early during
development (Drosophila – vegetative pole of the
cytoplasm in zygote)
Human - epiblast – transiently within extraembryonic tissue
–they are demonstrated within endoderm of yolk sac on
day 24 - dorsal wall of yolk sac near to allantois
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Migration to gonads throught mesenchyme of hindgut and
dorsal mesentery (4th - 6th week)
Extracellular matrix and chemotactic signals from
mesodern of future gonads. After migration, the germ cells
induce development of gonads
Number of cells increaces during migration
Gonads
Proliferation
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Oogonia and spermatogonia
Proliferative phase of development – from
thousands to 7 milions (in female) – mitosis
nd
Oogonia – proliferation during 2 - 5
monthes
th
7th fetal month – oogonia enter the prophase
of first meiotic division. The end of
proliferative phase
Spermatogonia enter meiosis after puberty,
ability to proliferate is preserved in men for
all reproductive period
Meiosis
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Reduction of normal number of
chromosomes from diploid to haploid
Two divisions without DNA synthesis
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Reduction division
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Second meiotic division
Recombination of genetic information
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Random distribution of maternal and paternal
chromosomes – exchange of homolog parts of
chromosomes during crossing over
First meiotic division
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Prophase I
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Leptoten
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Zygoten
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Pachyten
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Diploten
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Diakinesis
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Metaphase I
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Anaphase I
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Telophase I and Interphase
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Second meiotic division
Development of gamets
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In mammals – development of germ cells –
maintaince of pluripotency in germ cells
Activation of their differenciation – inductive
signals from trophoblast
Proliferation and maintaince – trophic factors
Extracellular matrix – regulates and directs
migration
Terminal differenciation
Spermatogenesis - 64 days
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Mitotic multiplication – spermatogonia : Type A –
population of stem cells, Type B – leaves mitotic
cycle – preleptoten spermatocytes)
Meiosa - Primary spermatocytes
Secondary spermatocytes
Spermiogenesis – Spermatids – transformation to
highly specialized cells – spermatozoa
(concentration of chromatin, decrease of size,
formation of acrosome, flagellum)
Spermatogenesis
Sertoli cells
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Sertoli cells – isolation of germ cells,
supportive cells, nutrition
Degradation of residual bodies
Synthesis of signal molecules (anti-Müllerian
factor)
Synchronization of development - waves
Spermatogonia A
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Stem cells, they
repetitive divide by
mitosis
Last division gives
arise to
spermatogonia B,
which divides to
primary spermatocyte
Primary spermatocyte and
synaptonemal complex
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Primary spermatocyte
enters prolongated
prophase
Synaptonemal
complex – homolog
chromosomes during
zygotene
Spermiogenesis
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Nucleus – concentration of chromatin – head
Golgi complex - proacrosomal granules acrosome
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Centrioles – anchorage of flagellum
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Axoneme– microtubules (9+2) and dynein
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Mitochondria – spiral arrangement within the
middle piece of flagellum – mitochondrial
helix (sheath)
Residual body
Spermiogenesis
Spermatid - flagellum
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Proximal centriole
remains
Distal centriole is
rebuild in axonemma
of flagellum
Spermatozoon
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Head (nucleus and acrosome)
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Flagellum
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Neck – connecting piece (proximal centriol)
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Middle piece (mitochondrial helix)
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Principal piece
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End piece
Spermatozoon
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Cross section through
principal piece:
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Axoneme
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Smooth chords
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Fibrous sheath
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Membrane
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Annulus
Sperm maturation
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New formed spermatozoa are unable to
fertilize. They maturate within genital system
–activation – increase of motility
Capacitation – end step of maturation –
changes in acrosome membrane, preparation
for enzyme release (it takes place in female
genital system)
Hyperactivation of sperms
Acrosome reaction – acrosome fuses with
plasma membrane and forms acrosomal
process
Oogenesis
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Oogonia – only one oocyte – first and
second pole body (DNA and a little
cytoplasm)
First meiotic division is not compleated till
puberty
Interuption of meoisis comes during
prophase I (diplotene) – oocyte
accumulates the reserve of yolk (under
control of OMI from follicular cells)
Second interuption during metaphase II –
division is completed just after fertilization
Lampbrush chromosomes
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Active transcription
during meiosis
Synthesis RNA – genes
form loops
Oogenesis
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At birth – 1 milion – 400 000 oocytes
Surrounded by one layer of follicular cells (granulosa
cells) – primordial follicle
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Only 400 (one per menstrual cycle) can mature
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Atresia (degeneration)
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Folliculogenesis
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Primordial
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Primary
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Secondary
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Graafian follicle - Ovulation
Oocyte
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Oocyte stores yolk as the reserve of nutrients
(and energy) for embryo
Proteins (amino acids, energy)
Ribosomes and t-RNA - proteosynthesis after
fertilization
M-RNA – early development - morphogenic
factors (transcriptional and growth factors)
Oocyte
Oocyte envelope
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Zona pellucida – glycoproteins ZP1, ZP2 and
ZP3, GAG, hyaluronic acid, sialic acid
Produced by oocyte
ZP-3 Sperm receptor and induction of
acrosome reaction
Corona radiata – follicular cells
First meiotic division just before
ovulation
Imprinting
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Genom arrising from oocyte is functionally
different from genom arrising from sperm
Imprinting is inactivation of genes, which is
dependent on sex – prevention of
parthenogenesis
Maternal genes are important for embryo
development (receptor for IGFII)
Paternal genes are important for placenta
development (IGFII – Beckwith-Wiederman
sy)
Fertilization
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Interaction between sperm and oocyte
Spermatozoon binds on specific sperm
receptor in zona pellucida (ZP3). Induces
enzyme release from acrosome
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Penetration of zona pellucida
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Fusion of sperm and oocyte
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Cortical reaction – cortical granules to perivitelline space (between oocyte and zona
pellucida) – alteration of receptors for sperms
– prevent polyspermy
Fertilization
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Fusion of sperm with oocyte induces competition of
meiosis – secondary pole body and definitive mature
oocyte
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Fertilized oocyte = zygote
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Content of sperm enters oocyte
– from sperm nucleus arrises
male pronucleus, centrioles
- mitotic spindle. All other
organeles are eliminated by
oocyte!!! Nuclear envelope disappears
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Replication od DNA – first mitotic division - 24 hours
Fertilization
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Centriole from sperm
is essential for mitosis
Centriole replicates
and forms mitotic
spindle for zygote
Oocyte after meiosis
has no centriole
It prevents
parthenogenesis
Fertilization
Prevention of polyspermy
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Fast blockade of polyspermy – changes in
electric potential (Ca++)
Slow blockade of polyspermy – cortical
granules contain enzymes – proteases –
liquidation of active sites on receptors (ZP2
and ZP3)
Fertilization envelope – space between zona
pellucida and oocyte - GAG, peroxidase and
hyaline – zonal reaction
Main result of fertilization
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Restoration of diploid number of chromosomes
(new combination of genetic information)
Determination of sex
Initiation of cleavage (without fertilization oocyte
usually degenerate in 24 hours after ovulation)
Cleavage
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Mitotic division without cell growth and
proteosynthesis
Doughter cells (blastomeres) decrease size –
embryo does not change size
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Mitotic division is equal and total
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4 cells – 40 hours
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3ED – 6-12 cells
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4ED – 16 -32 cells – morula