Gametogenesis and fertilisation Flashcards
What are primordial germ cells (PGCs) and their role?
PGCs are the precursors of gametes.
How does meiosis contribute to gametogenesis?
Meiosis reduces chromosome number by half and introduces genetic variability. MI separates homologous chromosomes; MII separates chromatids.
What is the main difference in the timing of meiosis in spermatogenesis vs. oogenesis?
In spermatogenesis, meiosis occurs before differentiation; in oogenesis, it occurs after differentiation.
What is the outcome of spermatogenesis from one primary spermatocyte?
Four haploid spermatozoa.
What is spermiogenesis?
The process by which spermatids differentiate into mature spermatozoa, involving acrosome formation, flagellum development, cytoplasm removal, and nuclear condensation.
What structures are in the sperm tail that aid motility?
The axoneme with a 9+2 microtubule structure powered by dynein and ATP hydrolysis.
What is the acrosome and what does it contain?
A cap-like structure derived from the Golgi that contains enzymes like acrosin, β-N-acetylglucosaminidase, and phospholipase C to digest egg coverings.
How many gametes are produced from one primary oocyte?
One ovum and two polar bodies.
When is meiosis I completed in oogenesis?
At ovulation.
When is meiosis II completed in oogenesis?
After fertilisation.
What features make the egg a specialized cell?
Contains yolk, ribosomes, mRNAs, morphogenetic factors, UV-protective enzymes, and an extracellular coat (zona pellucida).
What glycoproteins make up the zona pellucida?
ZP1, ZP2, and ZP3.
What is the function of ZP3?
It acts as the sperm receptor, determining species specificity via its O-linked polysaccharide.
What is the cortical reaction?
Exocytosis of cortical granules triggered by Ca²⁺, leading to modification of the zona pellucida to block polyspermy.
What molecules mediate sperm-oocyte membrane fusion?
Sperm protein Izumo binds to oocyte protein Juno, recruiting CD9 for membrane fusion.
What triggers calcium release in the oocyte during fertilisation?
Sperm entry triggers calcium release from the egg’s ER, initiating egg activation and development.
What happens after the sperm enters the oocyte?
The oocyte completes meiosis II, forming the definitive oocyte and second polar body.
What is the role of the sperm centriole after fertilisation?
It forms a centrosome that organizes the sperm aster for the first mitotic spindle.
What are the 10 steps of fertilisation in order?
1) Penetration of cumulus cells,
2) Binding to zona pellucida (ZP3),
3) Acrosomal exocytosis,
4) Passage through zona,
5) Membrane fusion (Izumo-Juno),
6) Ca²⁺ spiking,
7) Cortical granule release,
8) ZP modification,
9) Pronuclear DNA replication,
10) Mitotic spindle formation.
What do the sperm and egg contribute to the zygote?
Sperm: haploid genome + centriole. Egg: haploid genome + mitochondria + mRNAs + proteins for early development.
What are the three key outcomes of fertilisation?
Restoration of diploidy, sex determination (XX or XY), and initiation of cleavage.
Where do PGCs originate?
They originate in the extra-embryonic tissue, migrate via the gut into the genital ridge, multiply by mitosis, and later enter meiosis to become gametes.
Where does spermatogenesis occur?
Seminiferous tubules
How does the sperm flagella bend?
Dynein uses the energy of ATP hydrolysis to slide the MTs past one another
Briefly state the 5 spermatogenesis steps
PGCs → Spermatogonia: Divide by mitosis.
Spermatogonia → Primary spermatocytes (diploid).
Primary spermatocytes → 2 Secondary spermatocytes (Meiosis I).
→ 4 Spermatids (Meiosis II, haploid).
→ Spermatozoa (mature sperm through spermiogenesis).
What is the acrosomal reaction? Describe how it happens
GalT-ZP3 crosslinking causes GalT
proteins to cluster, triggering G protein
activation. The change in membrane
potential opens voltage-gated calcium
channels, increasing intracellular Ca2+.
Calcium-mediated exocytosis of the
acrosomal vesicle is initiated: the
acrosomal reaction
What allows the sperm and oocyte plasma membranes to fuse?
Sperm Izumo binds oocyte Juno, recruiting oocyte CD9, causing the plasma membranes to fuse, and the sperm enters the oocyte
What does the calcium wave in the oocyte trigger?
- The cortical reaction
- Completion of meiosis II
What is the cortical reaction?
The egg releases enzymes from cortical granules.
These enzymes harden the zona pellucida, so no other sperm can enter. This blocks polyspermy
