Download Gametes – reproductive cells

Gametogenesis
2009
Gametes – reproductive cells
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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
Origin of primordial cells
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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
Proliferation
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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
Meiosis
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Reduction of normal number of chromosomes
From diploid to haploid
Two maturation divisions without new DNA
synthesis
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Reductional division
Equational meiotic division
Recombination of genetic information
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Random distribution of maternal and paternal
chromosomes
Exchanging of portions of homologous
chromosomes by crossing over
First Meiotic Division
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Prophase I
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Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
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Metaphase I
Anaphase I
Telophase I and Interphase
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Second Meiotic Division
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In Mammals - initiation of germ line developmentmaintain pluripotency within germ cells
Activation of differentiation – inductive signal
from trophoblast
Proliferation and survival – trophic factors
Extracellular matrix – direct the migration
Final differentiation
Spermatogenesis - 64 days
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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)
Male germ cells
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Sertoli cells – isolation of germ cells, support and
nutrition
Degradation of residual bodies
Synthesis of signal molecules (Anti-Müllerian
factor)
Synchronization of development- waves
Spermiogenesis
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Nucleus – concentration of chromatin – head
Golgi complex- proacrosomal granules - acrosome
Centrioles – achorage of flagellum
Axoneme – microtubules (9+2) and dynein
Mitochondria – spiral investment around proximal
part of flagellum – mitochondrial helix
Residual body
Spermatozoon
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Head (nucleus and acrosome)
Neck (proximal centriole)
Middle piece (flagellum, centriole, mitochondrial
helix)
Tail - flagellum
Sperm maturation
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Newly formed spermatozoa are not capable of
fertilization. Maturation in genital tract –
activation – increase of motility
Capacitation – final step of sperm maturationchanges 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
Oogenesis
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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
Lampbrush chromosomes
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Active transcription during
meiosis
Synthesis of RNA – genes
loop out
Oogenesis
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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
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Primordial
Primary
Secondary
Graafian follicle - Ovulation
Egg
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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
Coverings of eggs
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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
Fertilization
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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
Prevention of polyspermy
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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
Fertilization
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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
Imprinting
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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)
Cleavage
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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)
Segregation of blastomeres into
embryoblast and trophoblast
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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
Genetic regulation of germ cell
formation, proliferattion, migration,
and development
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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
Twins and embryonic stem cells
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Monozygotic twins - before hatching – at 5.ED –
dichorionic
Later monochorionic,diamniotic
Monochorionic monoamniotic
Conjoined twins (after ED9)
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Inner cell mass – embryonic stem cells
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