Germ cells, fertilization, and sex Flashcards
germ cells cycle
- Primordial germ cell precursors specification
- Primordial germ cell migration during embryonic development
to end up in the genital ridge - Meiosis
- Genomic imprinting
- Fertilization
- Sex determination
- X chromosome inactivation
Key functions of germ cells:
- Preservation of genetic integrity (prevention of aging)
- Generation of genetic diversity
- Transmission of genetic information to the next generation
Early germ cell in drosophilla
Cytoplasm is not equally distributed over the whole oocyte, there is a gradient of certain molecules:
- Pole plasm or germ plasm
o Specialized cytoplasm (needed for germ cell specification)
o Large organelles: polar granules
o Involved in specifying germ cells
o Activity destroyed by UV (no germ cells will be formed then)
o Located a posterior end
- Primordial germ cells (pole cells)
o Formed before cellularization of the rest of the embryo
Specification of germ cells in drosophila
- Maternal genes are involved in pole plasm formation
- Oskar is expressed at the posterior pole of the oocyte
- Oskar organizes the germ plasm and directs localization of the posterior determinant nanos
These gradients in the oocyte are set up by the mother. This is done by:
- Bicoid and oskar mRNAs are delivered into oocyte by nurse cells
- The motor protein dynein transports bicoid mRNA towards minus ends of
microtubules → anterior end of the oocyte
- The motor protein kinesins transports oskar mRNA towards the plus end of
microtubules → posterior end of the oocyte
o The localization of oskar mRNA and assembly of germ plasm is required for
localization of nanos mRNA to posterior end
o Nanos is a well preserved protein over the animal kingdom, to make sure that the
germ cells will develop and nanos itself is also a marker of germ cells.
Early germ cell development in zebrafish
- Germ plasm present at cleavage furrows, consists maternally encoded proteins & mRNA
- 32-cell stage: the germ plasm is found in distinct primordial germ cells
- sphere stage: equal divisions take place
- Vasa and nanos1 mRNA are specifically expressed in germ cells.
*Removal of the germ plasm results in absence of the germ cell lineage
Germ cell migration in zebrafish
Primordial germ cell (PGCs) sense their own environment and migrate to the gonad(s) guided by SDF1
germ cell development in mouse
- No evidence for germ plasm in mammals
- Germ cell specification involves cell-cell interactions
- Cultured embryonic stem cells can give rise to germ cells and somatic cells
Normally cells would like to develop into a somatic cell. Organisms need to repress genes associated with the somatic development to create germ cells. Germ cells always repress some genes to make sure their fate is not changed. E.g. blimp 1
Germ cell migration in mouse
- PGC migrate anteriorly and incorporate in hindgut endoderm
- PGC leave hindgut and migrate into genital ridge through the dorsal mesentery
(SDF1 acts as guidance cue for final part migration from hindgut to genital ridge) - Gonad (ovary or testis) develops from the mesoderm lining the abdominal cavity
- 8000 primordial germ cells in genital ridge
Spermatogenesis in mammals
- PGCs multiply during migration. They’re arrested in mitotic cell cycle in embryonic testis (G1)
- Sexually mature: spermatogonial stem cells → differentiating spermatocytes
- After birth
a. Diploid spermatogonia multiply by mitosis in the testes (stem cells)
b. Start meiosis and form 4 haploid spermatids from 1 spermatocyte - Lifelong production
- Meiosis in gonad
- Arrested in prophase I (recombination) & arrest depends on cyclic AMP
- Transcription and oocyte growth (meiosis I) continues in adult (ovulation)
a. 1 mature oocyte and polar body formation - After ovulation the oocyte is arrested in metaphase II until fertilization
- No proliferation after entry meiosis → fixed number of oocytes produced
- Decline in human oocytes by age
corona radiata and zona pellucida
The corona radiata is surrounding and supporting the oocyte
The zona pellucida is a protein barrier around the oocyte
Sperm has to penetrate several physical barriers:
- Cumulus cells: follicle cells + hyaluronic acid
- Zona pellucida: layer of fibrous glycoproteins
- Penetrations through zona pellucida
- Fusion of plasma membranes
a. Proteins on cell surface involved in fusion sperm and egg membranes
i. Sperm: Izumo, ligand on egg: Juno - Sperm nucleus enters egg cytoplasm: Egg activation
a. Release of cortical granules
b. Sperm nucleus enters egg cytoplasm
block to polyspermy in sea urchin (2)
o Rapid block to polyspermy, depolarization of egg plasma membrane
-Triggered within seconds (10 seconds after) by a transient depolarization across plasma
membrane, occurs on sperm-egg fusion
-Electrical membrane potential goes from -70mV to +20mV
o Slow block to polyspermy, calcium release (also present in mammals)
-Triggered by wave of calcium
release in the egg due to sperm
entry
-Cortical granules cross-link molecules in vitelline membrane → ‘hardened’ fertilization membrane produced
slow polyspermy block in mammals
- After fusion first sperm, granules egg cortex are released by exocytosis
- Contents form layer outside egg plasma membrane
- Protease ovastacin is a component of the cortical granules
o Cuts ZP2 protein in zona pellucida, sperm can no longer bind and have acrosomal reaction
IUI (intrauterine insemination)
Also known as: artificial insemination. The lab assess sperm motility, number, and viability & the female’s ovulation can also be assessed. Drugs to stimulate the cycle might be used
IVF (in vitro fertilization)
The process by which an egg is fertilized by sperm outside the body
