#fertilization process

Sea urchin -l

Fertilization cone formation involves:

Immediately after sperm-egg plasma membrane fusion, egg cortex locally remodels beneath bound spermatozoon to form conspicuous fertilization cone. Process begins with transient depolymerization of cortical actin followed by rapid GTPase-dependent actin polymerization involving Arp2/3 complex and formins, pushing plasma membrane outward into blunt protrusion engulfing sperm head and midpiece while drawing sperm nucleus into ooplasm. Myosin II contractility and microtubule-based transport assist later incorporation and pronuclear migration but initial protrusion itself driven by localized actin polymerization triggered by calcium and small GTPases. Cytochalasin B inhibition blocks cone formation preventing sperm internalization, demonstrating dependence on actin dynamics.

Ref: Gilbert, Developmental Biology, Chapter 7: Fertilization cone - actin-based engulfment of sperm.

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Sea urchin -l

Cortical granules are homologous to:

Cortical granules are dense membrane-bound vesicles arranged in monolayer underlying egg plasma membrane, stockpiled with proteases, peroxidase, glycosaminoglycans and structural proteins destined for rapid Ca2+-triggered exocytosis at fertilization. Detailed ultrastructural and biochemical comparison demonstrates striking homology with acrosomal vesicles of spermatozoa – both derive from Golgi apparatus in their respective gametes, contain hydrolytic enzymes, and undergo regulated SNARE-mediated exocytosis releasing contents to modify extracellular coats. Functionally, cortical reaction transforms vitelline layer similarly to how acrosomal reaction exposes bindin. They are not ribosomes, nuclei or lysosomes although enzymatically similar. Homology reflects shared evolutionary origin of secretory vesicles.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical granules and acrosome vesicle homology.

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Fertilization in mammals-l

Ions primarily increasing during capacitation:

Biochemical hallmarks of capacitation include elevated intracellular bicarbonate and calcium concentrations. Bicarbonate enters via Na/HCO3 cotransporters activating soluble adenylyl cyclase producing cAMP which stimulates PKA tyrosine phosphorylation cascade promoting membrane fluidity and CatSper priming. Concurrent increase in intracellular pH and calcium influx prepares sperm for hyperactivation and acrosome exocytosis. Removal of cholesterol by albumin accelerates ion permeability. These ionic shifts are measurable in vitro and are required for fertilizing ability, distinguishing capacitated from non-capacitated populations experimentally for assisted reproduction technologies, diagnostics, semen analysis in clinical laboratories and fertility assessment protocols.

Ref: Austin & Bavister, Exp Cell Res: Bicarbonate, calcium rise and tyrosine phosphorylation during sperm capacitation events.

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Fertilization in mammals-l

Protamines in sperm primarily:

Sperm DNA packaging diverges from somatic chromatin organization to ensure hydrodynamic shape and protection during transit. Histones replaced by protamines PRM1 and PRM2 small basic proteins rich in arginine and cysteine forming disulfide bonds. Protamines coil DNA into donut-shaped toroids, reducing nuclear volume tenfold, silencing transcription and conferring resistance to nucleases and oxidative stress. Correct protamine ratio critical; excess PRM1 or PRM2 leads to incomplete condensation, DNA fragmentation and infertility. Compact protamine-based chromatin safeguards paternal genome en route to oocyte and ensures epigenetic delivery, embryonic integrity and genome stability post-fertilization.

Ref: Balhorn, Genome Biology 2007: Protamine structure, DNA toroids and paternal chromatin condensation in sperm.

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Fertilization in mammals-ll

Mammalian sperm capacitation occurs primarily in:

Capacitation is a final functional maturation occurring only within female reproductive tract fluids of uterus and oviduct after epididymal transit. Albumin acts as cholesterol acceptor, extracting cholesterol from sperm plasma membrane, increasing fluidity and permeability. Bicarbonate influx activates soluble adenylyl cyclase, elevating cAMP, stimulating PKA-dependent tyrosine phosphorylation cascades. Membrane remodeling opens CatSper calcium channels, promotes hyperactivated motility and primes acrosomal responsiveness, conferring true fertilizing ability solely after exposure to female environment essential for successful conception, implantation competence and embryonic development initiation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Fertilization – capacitation and sperm activation in female tract.

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