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#sperm

8 public questions tagged with this topic.

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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Sea urchin -l

Calcium channels activated by resact in sea urchin sperm are encoded by:

Resact peptide binding to receptor guanylate cyclase on sperm flagellum activates cGMP synthesis opening K+ channels and triggering Ca2+ oscillations critical for chemotactic steering. Electrophysiology and genomics identified cation channel of sperm, CatSper, as major Ca2+ entry route mediating oscillations. CatSper genes encode four homologous subunits forming pH-sensitive channel enriched in flagellar principal piece. Sea urchin genome contains CatSper orthologs; blocking pore abolishes Ca2+ influx and turning. Bindin, resact and hyalin encode adhesion proteins unrelated to conductance. Thus calcium channels activated by resact are encoded by CatSper genes.

Ref: Seifert et al., Nature 2015, CatSper channelosome in sperm; Kaupp Lab - resact/CatSper signaling.

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Fertilization in mammals-ll

Protamines primarily function to:

During spermiogenesis histones are sequentially replaced first by transition proteins then by small arginine-rich protamines PRM1 and PRM2. This exchange packages DNA into highly condensed toroidal structures, reducing nuclear volume, transcriptionally silencing genome and shielding paternal chromosomes from oxidative and nucleolytic damage during transit through male and female tracts. Disulfide crosslinking between protamines stabilizes compaction giving characteristic hydrodynamic sperm head shape. Balanced PRM1 to PRM2 ratio is critical; imbalance correlates with DNA fragmentation, male infertility, poor embryo development due to faulty chromatin packaging, genome instability and epigenetic dysregulation.

Ref: Gilbert, Developmental Biology, 11th ed., Chapter 19: Protamines and sperm chromatin compaction during spermiogenesis.

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Fertilization in mammals-ll

Izumo protein on sperm binds to egg membrane protein:

Izumo1 is sperm-specific immunoglobulin superfamily transmembrane protein that becomes exposed on equatorial segment after acrosome reaction. Its cognate receptor on egg plasma membrane is Juno, also known as folate receptor 4, enriched on microvillar oolemma. Structural studies show Izumo1 helical bundle binds Juno with nanomolar affinity, providing essential adhesion preceding fusion. CD9 organizes membrane microdomains facilitating clustering. After fertilization Juno is rapidly shed in extracellular vesicles, preventing additional sperm attachment. Izumo-Juno interaction is conserved, indispensable for mammalian fertilization success, monospermy enforcement and initiation of egg activation cascade.

Ref: Bianchi et al., Nature 2014, Izumo1-Juno interaction: essential sperm-egg adhesion pair in mammalian fertilization.

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Fertilization in mammals-iv

Sperm centriole contributes to:

Mammalian zygote requires centrosome to assemble mitotic spindle for first cleavage. Human oocyte lacks functional centrioles after pachytene elimination, so paternal contribution is essential. Proximal centriole introduced by sperm at fertilization recruits maternal pericentriolar material including gamma-tubulin, pericentrin and centrin to regenerate functional centrosome. This centrosome duplicates and nucleates microtubules organizing syngamy and first bipolar spindle aligning parental genomes. Without sperm centriole, parthenogenetic embryos often display abnormal spindles. It does not mediate zona digestion or mitochondrial energy production, but provides microtubule organizing center initiating embryonic cell divisions and polarity establishment.

Ref: Palermo et al., Human Reproduction 1994 & Schatten, Cell Motil 1994: Paternal centriole reconstituting centrosome for first spindle.

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Fertilization in mammals-iv

Sperm hyperactivation depends on:

Hyperactivation is whiplash-like, high-amplitude flagellar beating enabling sperm to detach from oviductal epithelium, traverse viscous mucus and penetrate cumulus and zona matrix. It is initiated during capacitation in female tract by alkalinization and progesterone from cumulus cells opening CatSper, sperm-specific pH-sensitive calcium channel complex located in principal piece of flagellum. Calcium entry through CatSper raises intracellular calcium, alters dynein sliding, producing asymmetric bends. Knockout of CatSper subunits in mice abolishes hyperactivation and fertility despite normal motility, while potassium, chloride or sodium channels alone are insufficient to drive this capacitation-dependent motility switch.

Ref: Qi et al., eLife 2007 & Lishko et al., Cell 2012: CatSper calcium channels driving sperm hyperactivation and chemotaxis.

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Fertilization in mammals-iv

Sperm exclusively contributes:

Human oocyte centrioles degenerate during oogenesis, leaving mature egg lacking functional centrosome. Fertilizing sperm introduces proximal centriole, often with atypical structure, which recruits maternal pericentriolar material to reconstitute active centrosome in zygote. This centrosome nucleates microtubules and organizes first mitotic spindle, ensuring accurate segregation of parental genomes during cleavage. Mitochondria, ribosomes, Golgi and endoplasmic reticulum are predominantly maternal due to oocyte cytoplasmic abundance and active degradation of paternal mitochondria via ubiquitin-proteasome and autophagy, establishing maternal inheritance pattern for mitochondrial DNA and ooplasm.

Ref: Albert et al., Molecular Biology of the Cell, Chapter 20: Centrosome inheritance - paternal centriole contribution and mitochondrial degradation.

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