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#acrosomal reaction

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Regeneration -ll

In sea urchins, the sperm acrosomal reaction begins when sperm contacts:

The acrosome reaction is triggered before sperm contacts vitelline envelope or egg plasma membrane, ensuring bindin exposure at correct time. Initial stimulus comes from fucose-sulfate-rich glycoconjugates and low molecular weight peptides within egg jelly surrounding egg. Jelly components induce calcium influx via voltage-gated channels and activation of Na+/H+ antiport raising intracellular pH, causing outer acrosomal membrane fusion with plasma membrane releasing acrosomal enzymes and polymerizing actin to project acrosomal process. Vitelline envelope interaction follows jelly-induced priming; egg membrane fusion occurs afterwards, and nuclear contact never directly triggers reaction.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Acrosome reaction triggered by egg jelly polysaccharides.

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Sea urchin -l

In sea urchins, the sperm acrosomal reaction begins when sperm contacts:

Arriving fertilization-competent spermatozoa first encounter outermost gelatinous egg jelly layer surrounding vitelline envelope, rich in sulfated fucan polysaccharides. Physical binding of jelly sulfated polysaccharides to sperm surface receptors triggers Ca2+-dependent signal transduction leading to acrosomal vesicle exocytosis. Acrosomal membrane fuses releasing proteases and exposing filamentous bindin on elongated acrosomal process, enabling penetration through jelly remnants and firm adhesion to vitelline envelope. Direct contact with egg plasma membrane or vitelline envelope occurs only after acrosomal reaction completion and jelly traversal. Therefore initial molecular trigger for acrosome reaction is contact with egg jelly, not egg cell membrane apposition itself.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Acrosome reaction triggered by egg jelly sulfated polysaccharides.

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Fertilization in mammals-lll

Acrosomal reaction primarily involves:

Acrosomal reaction is exocytotic fusion of sperm plasma membrane with outer acrosomal membrane releasing enzymatic cargo after binding zona pellucida. Key event is secretion of serine protease acrosin, hyaluronidase and esterases that locally hydrolyze ZP glycoprotein filaments, particularly ZP2 and ZP3, creating penetration tract. Calcium influx mediated via ZP3-triggered signaling and CatSper amplifies fusion. Repulsive interaction or simple plasma membrane fusion does not describe enzymatic step; fusion of gamete membranes occurs later in perivitelline space after zona traversal. Thus reaction functionally corresponds to zona digestion enabling sperm transit to egg surface for subsequent Izumo-Juno mediated fusion.

Ref: Gilbert, Developmental Biology, Chapter 7: Acrosomal reaction mechanism - zona digestion and acrosomal exocytosis enzymes.

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Fertilization in mammals-iv

Acrosomal reaction primarily involves:

Acrosomal reaction is calcium-dependent exocytosis of acrosomal vesicle at anterior sperm head occurring after zona pellucida binding. Fusion of outer acrosomal membrane with plasma membrane releases soluble enzymes including acrosin, hyaluronidase and other proteases. These lysins locally digest zona pellucida glycoprotein matrix, creating penetration slit allowing sperm to reach perivitelline space. Reaction also exposes inner acrosomal membrane proteins like Izumo1 essential for egg fusion. Without zona digestion sperm cannot traverse thick extracellular coat, making this enzymatic breaching crucial for successful fertilization in mammals.

Ref: NCBI Bookshelf, Gilbert Developmental Biology, Chapter 7: Fertilization - acrosome reaction, acrosin and zona pellucida penetration.

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