Abstract
Cortical granules are membrane bound organelles located in the cortex of unfertilized oocytes. Following fertilization, cortical granules undergo exocytosis to release their contents into the perivitelline space. This secretory process, which is calcium dependent and SNARE protein-mediated pathway, is known as the cortical reaction. After exocytosis, the released cortical granule proteins are responsible for blocking polyspermy by modifying the oocytes' extracellular matrices, such as the zona pellucida in mammals. Mammalian cortical granules range in size from 0.2 um to 0.6 um in diameter and different from most other regulatory secretory organelles in that they are not renewed once released. These granules are only synthesized in female germ cells and transform an egg upon sperm entry; therefore, this unique cellular structure has inherent interest for our understanding of the biology of fertilization. Cortical granules are long thought to be static and awaiting in the cortex of unfertilized oocytes to be stimulated undergoing exocytosis upon gamete fusion. Not till recently, the dynamic nature of cortical granules is appreciated and understood. The latest studies of mammalian cortical granules document that this organelle is not only biochemically heterogeneous, but also displays complex distribution during oocyte development. Interestingly, some cortical granules undergo exocytosis prior to fertilization; and a number of granule components function beyond the time of fertilization in regulating embryonic cleavage and preimplantation development, demonstrating their functional significance in fertilization as well as early embryonic development. The following review will present studies that investigate the biology of cortical granules and will also discuss new findings that uncover the dynamic aspect of this organelle in mammals.
Highlights
Mammalian fertilization is a sequence of coordinated events involving multiple steps of mutual recognitions between haploid male and female gametes
Fertilizable oocytes are ovulated along with the first polar body, as well as the extracellular matrix (ECM) zona pellucid (ZP) which is composed of three glycoproteins, ZP1, ZP2, and ZP3 [1,2] and the cumulus oophorus which is made of several layers of ovarian follicular granulose cells embedded in hyaluronic acid-containing ECM (Figure 1)
A sperm needs to swim through the perivitelline space (PVS) and fuses with the oocyte through a fertilin/integrin- mediated adhesion process [9,10,11,12,13,14,15]
Summary
Mammalian fertilization is a sequence of coordinated events involving multiple steps of mutual recognitions between haploid male and female gametes. Prior study of mouse oocytes reveals that the pre-fertilization exocytosis of cortical granules during metaphase I and II transition involves a specific population of granules [88]. This exocytotic event takes place during polar body extrusion and only in the. The pre-fertilization release of cortical granules during the metaphase I and II transition may be involved in modification of zona pellucida, perivitelline space, and/ or oolemma directly above the site of the release, thereby minimizing the likelihood for sperm to bind and penetrate oocytes through this area where maternal chromatin is located Establishment of this local block to sperm entry will reduce the possibility of damaging maternal genetic materials when paternal chromatin undergoes decondensation following gamete fusion. List of abbreviations used AAA: Aleuria aurantia agglutinin; AIA: Artocarpus integrifolia; CaMKII: Ca2 +/calmodulin-dependent protein kinase; CGFDs: cortical granule free domains; ConA: Canavalia ensiformis agglutinin; DAB: 3,3’-diaminobenzidine; DAG: diacylglycerol; DBA: Dolichos biflorus agglutinin; DSA: Datura stramonium; DSA: Datura stramonium agglutinin (DSA); ECM: extracellular matrix; FBP: Lotus tetragonolobus agglutinin; HPA: Helix pomatia agglutinin; IP3: Inositol trisphosphate; LCA: Lens culinaris agglutinin; LFA: Limax flavus; LPA: Limulus polyphemus agglutinin; MAA: Maackia amureusis agglutinin; MPA: Maclura pomifer; PAB: p-aminobenzamidine; PAD: peptidylarginine deiminase; PAS: periodic acid Schiff’s reagent; PHA-E: Phaseolus vulgaris; PIP2: phosphatidylinositol biphosphate; PKC: protein kinase C; PNA: peanut agglutinin; PVS: perivitelline space; RCA120: Ricinus communis agglutinin I; SBTI: soybean trypsin inhibitors; SNAP: synaptosome-associated protein; SNARE: soluble NSF-attachment protein receptors; tPA: tissue-type plasminogen activator; UEA: Ulex europaeus agglutinin; VAMP: vesicleassociated membrane protein; WGA: wheat germ agglutinin; ZP: zona pellucida
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