Germ Cell Development

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Germ Cell Development
- Germ cells provide continuity of live between
generations
- mitotic ancestors of our germ cells once
resided in the gonads of reptiles, amphibians,
fish, invertebrates,.... gtgtimmortal cell
type
- separation of germ cells from somatic cells
early in development in many species except
mammals and salamanders
- germ cells do not arise in gonad, rather
primordial germ cells (PGCs) migrate into gonads
during development
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Germ cell determination by germ plasm
Parascaris
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(No Transcript)
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Pole plasm rescues UV-induced pole cell ablation
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Germ Plasm determines fate of pole
cells (Illmensee Mahowald, 1974)
Post
ant
ant
Post
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Genetics of the determination of PGC fate
Pole plasm fibrills, mitochondria, polar granules
Genetic screen for mutants with no
grandoffspring
gcl germ cell-less, maternal mRNA and protein
located at posterior pole
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2nd set of pole plasm components
Oskar ectopic expression gtgt ectopic germ
cells Required for recruiting germ cell
determinants to the posterior pole
Nanos posterior axis specification required
for pole cell formation prevents mitosis and
transcription in germ cells
Vasa RNA-binding protein, in pole cells, also
found associated with germ cells of higher
animals
Mitochondrial ribosomal RNA (mtrRNA) located
outside the mitochondria (by tudor) restores
pole cell formation after UV-irradiation compon
ent of polar granules
Polar granule component non-translatable RNA
required for pole cell formation,
anti-sense blocks migration into gonads
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Xenopus germ plasm localized at the vegetal pole
Xcat2 (Xenopus nanos homologue)
PGC in presumptive endoderm germ plasm contains
Dros. Homologues such as nanos and mtrRNA
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Germ plasm in vertebrates Xenopus
- PGC in post. Larval gut - along dorsal gut gt
dorsal mesentery, - then along abdominal wall
into gonads 30 PGC colonize gonads gt
proliferate to give rise to germ cells
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Primordial germ cell migration in birds
PGC derive from epiblast migrate to germinal
crescent where they multiply populate gonad by
transport through blood vessels
Vasa staining
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In capillary
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PGC development in vertebrates
- in mammals no evidence for maternal cytoplamic
determinants (germ plasm) - PGC induced during
gastrulation by cell-cell interactions - in
mammals BMP-signalling involved - new insights
into PGC development and migration by genetic
analysis of PGC development in zebrafish (does
involve germ plasm)
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Migration of Primordial Germ Cells in Mammalians
No obvious germ plasm in mammals PGC form at
post. of epiblast at junction of extra-embryonic
tissue, primitve streak and allantois BMP4 and
BMP8b from extraembryonic ectoderm induce
PGCs PGC express fragilis (TM protein AND stella
(unknown function) PGC migrate into post
hindgut PGC migrate toward genital ridge at
about E9 and enter the gonad at 11.5. expansion
of PGCs from initially 10-100 to 2500-5000 PGCs
at E12
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Maintaining totipotency
Oct-4 expressed in totipotent cells expression in
duced by stem cell factor
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Zebrafish PGC development
Isolation of vasa homologue allowed tracking of
PGCs
Reviewed in E Raz (2003). Nature Rev Genet 4, 690.
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Positional cues directing PGC migration in
zebrafish
Reviewed in E Raz (2003). Nature Rev Genet 4, 690.
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Screen for molecules directing PGC migration
Reviewed in E Raz (2003). Nature Rev Genet 4, 690.
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Cxcr4b and Sdf1a control migration of PGCs in
zebrafish
Brown staining sdf1a mRNA
Sdf1a expressed in gonad Cxcr4b (maternal mRNA in
PGCs
Cxcr4b-MO
Sdf1 inj.
WT
Reviewed in E Raz (2003). Nature Rev Genet 4, 690.
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SDF-1 and CXCR4b expression during PGC migration
Doitsidou et al., Cell 2002
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SDF-1 and CXCR4 are involved in PGC guidance
Doitsidou et al., Cell 2002
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SDF-1 - CXCR4b interaction guide PGC towards
gonade
Doitsidou et al., Cell 2002
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Snapshot Summary The Germ Line1. The
precursors of the gametesthe sperm and eggsare
the primordial germ cells. They form outside the
gonads and migrate into the gonads during
development.2. In many species, a distinctive
germ plasm exists. It often contains the Oskar,
Vasa, and Nanos proteins or the mRNAs encoding
them.3. In Drosophila, the germ plasm becomes
localized in the posterior of the embryo and
forms pole cells, the precursors of the gametes.
In frogs, the germ plasm originates in the
vegetal portion of the oocyte.4. In amphibians,
the germ cells migrate on fibronectin matrices
from the posterior larval gut to the gonads. In
mammals, a similar migration is seen, and
fibronectin pathways may also be used. Stem cell
factor is critical in this migration, and the
germ cells proliferate as they travel.5. In
birds, the germ plasm is first seen in the
germinal crescent. The germ cells migrate through
the blood, then leave the blood vessels and
migrate into the genital ridges.6. Germ cell
migration in Drosophila occurs in several steps
involving passive translocation, repulsion from
the endoderm, and attraction to the gonads.7.
Before meiosis, the DNA is replicated and remains
bound at the kinetochore. Homologous chromosomes
are connected through the synaptonemal complex.
The configuration of the four chromatids is
called a tetrad.8. The first division of meiosis
separates the homologous chromosomes. The second
division of meiosis splits the kinetochore and
separates the chromatids.9. The meiosis/mitosis
decision in nematodes is regulated by the Delta
proteins, which bind to the Notch proteins on the
PGCs. The decision for a germ cell to become
either a sperm or an egg is regulated at the
level of translation of the fem-3 message.10.
Spermatogenic meiosis in mammals is characterized
by the production of four gametes per meiosis and
by the absence of meiotic arrest. Oogenic meiosis
is characterized by the production of one gamete
per meiosis and by an arrest at first meiotic
prophase to allow the egg to grow.11. In some
species, meiosis is modified such that a diploid
egg is formed. These species can produce a new
generation parthenogenetically, without
fertilization.12. The egg not only synthesizes
numerous compounds, but also absorbs material
produced by other cells. Moreover, it localizes
many proteins and messages to specific regions of
the cytoplasm, often tethering them to the
cytoskeleton.13. The Xenopus oocyte transcribes
actively from lampbrush chromosomes during the
first meiotic prophase.14. In Drosophila, the
oocyte is relatively dormant in terms of
transcription. Rather, nurse cells make mRNAs
that enter the developing oocyte. Which of the
cells derived from the primordial germ cell
becomes the oocyte and which become nurse cells
is determined by the fusome and the pattern of
divisions.15. In mammals, the hormones of the
menstrual cycle integrate the ovarian cycle, the
uterine cycle, and the cervical cycle. This
integration allows the uterus to be ready to
receive an embryo shortly after ovulation occurs.