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#egg biology

5 public questions tagged with this topic.

Regeneration -ll

Egg cytoplasmic stores do NOT contain:

Mature sea urchin oocytes accumulate maternal provisions supporting early embryogenesis before zygotic genome activation. Cytoplasm holds abundant ribosomes, tRNAs, histone variants, metabolic enzymes, lipid platelets, yolk proteins providing amino acids, and localized morphogenetic determinants such as Dishevelled and beta-catenin patterning animal-vegetal axis. These reserves sustain protein synthesis and axis specification. In contrast, sperm-activating peptides like resact and speract are secreted outward into jelly coat during oogenesis to act extracellularly as chemoattractants. They function outside egg, binding sperm guanylate cyclase receptors, thus not stored intracellularly as nutritive or patterning reserves within egg cortex.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Oocyte cytoplasmic composition - maternal stores versus jelly chemoattractants.

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Regeneration -ll

Slow block to polyspermy involves:

Slow block provides long-lasting barrier after momentary electrical block decays. Fertilization triggers phospholipase C activation generating IP3 that opens endoplasmic reticulum calcium channels, creating traveling calcium wave across egg cortex. Elevated calcium drives SNARE-dependent exocytosis of cortical granules releasing proteases that sever bindin receptor EBR1 connections, glycosaminoglycans that osmotically lift envelope, and ovoperoxidase producing dityrosine crosslinks hardening envelope. Resulting fertilization envelope is physically elevated and biochemically unreceptive, preventing late sperm entry. Membrane potential change mediates fast block, seawater calcium influx insufficient, and acrosomal vesicle fusion belongs to sperm not egg.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Slow block - cortical reaction and fertilization envelope formation.

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Regeneration -ll

Fast block to polyspermy is mainly due to:

Fast block to polyspermy in sea urchin eggs is achieved electrically rather than structurally. Within one to three seconds of sperm-egg fusion, sodium channels open producing rapid depolarization from resting -70 mV to positive +20 mV, known as fertilization potential. This voltage shift renders plasma membrane refractory to additional sperm fusions because sperm-egg fusion is voltage-dependent and favored only at negative potentials. Depolarization is transient lasting about a minute until slow block secures envelope elevation. Cortical granule discharge, protease activation, and bindin binding correspond to slow permanent block, not immediate electrical mechanism preventing supernumerary entries.

Ref: NCBI Bookshelf, Developmental Biology: Fast block to polyspermy - Jaffe's electrophysiological studies of membrane depolarization.

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Regeneration -ll

The vitelline envelope contains:

The vitelline envelope is an extracellular coat deposited around growing oocyte by follicle cells and oocyte itself, composed primarily of sulfated and non-sulfated glycoproteins homologous to mammalian zona pellucida proteins ZP1, ZP2, ZP3 equivalents. In sea urchins it includes large molecular weight glycoproteins bearing EBR1 receptor with oligosaccharide chains conferring species-specificity for bindin recognition. Matrix lacks bulk lipids, genomic nucleic acids, or regulatory microRNAs; integrity depends on disulfide crosslinks and carbohydrate-carbohydrate interactions. During slow block, serine protease clips receptors and ovoperoxidase crosslinks glycoproteins to form hardened fertilization envelope.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 21: Egg coats - vitelline envelope glycoproteins and fertilization.

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Fertilization in mammals-lll

Polyspermy prevention primarily relies on:

Prevention of polyspermy in mammals employs fast electrical depolarization and slow zona block. Upon gamete fusion, cortical zinc stores release massive burst called zinc spark into extracellular milieu. Zinc binds zona pellucida, increasing stiffness and inducing structural remodeling that blocks additional sperm binding, complementing ovastacin-mediated ZP2 cleavage. Calcium influx triggers exocytosis but zinc directly hardens matrix and reduces sperm motility in perivitelline space. Sodium influx mediates fast block in some non-mammalian species, potassium efflux not implicated. Thus zinc spark is recognized as essential chemical signal coupling egg activation to physicochemical hardening of zona ensuring monospermy and viability.

Ref: Fissore et al., PNAS 2016: Zinc spark contribution to zona hardening and block to polyspermy in mammalian eggs.

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