Abstract
Fertilization by multiple sperm leads to lethal chromosomal number abnormalities, failed embryo development, and miscarriage. In some vertebrate and invertebrate eggs, the so-called cortical reaction contributes to their activation and prevents polyspermy during fertilization. This process involves biogenesis, redistribution, and subsequent accumulation of cortical granules (CGs) at the female gamete cortex during oogenesis. CGs are oocyte- and egg-specific secretory vesicles whose content is discharged during fertilization to block polyspermy. Here, we summarize the molecular mechanisms controlling critical aspects of CG biology prior to and after the gametes interaction. This allows to block polyspermy and provide protection to the developing embryo. We also examine how CGs form and are spatially redistributed during oogenesis. During egg activation, CG exocytosis (CGE) and content release are triggered by increases in intracellular calcium and relies on the function of maternally-loaded proteins. We also discuss how mutations in these factors impact CG dynamics, providing unprecedented models to investigate the genetic program executing fertilization. We further explore the phylogenetic distribution of maternal proteins and signaling pathways contributing to CGE and egg activation. We conclude that many important biological questions and genotype–phenotype relationships during fertilization remain unresolved, and therefore, novel molecular players of CG biology need to be discovered. Future functional and image-based studies are expected to elucidate the identity of genetic candidates and components of the molecular machinery involved in the egg activation. This, will open new therapeutic avenues for treating infertility in humans.
Highlights
Sexual reproduction requires the interaction of gametes, cells highly specialized for fertilization
The slow or mechanical blockade of the polyspermy has been related to the cortical reaction, which includes CG exocytosis (CGE) and subsequently, the extracellular coat remodeling
Increases in [Ca2+]i at fertilization are required for CGE and actin remodeling. These findings suggest an additional role for brb/heterogeneous nuclear ribonucleoprotein I (hnRNP I), as a regulator of the actin cytoskeletonbased kinetics of CGE in a Ca2+-dependent manner (Mei et al, 2009)
Summary
Sexual reproduction requires the interaction of gametes, cells highly specialized for fertilization. The slow or mechanical block to polyspermy is a key event and involves the exocytosis of cortical granules (CGs) After it is initiated, the subsequent elevation of the extracellular coat or the modification of the ZP becomes unreceptive to the sperm (Wessel et al, 2001). The release of calcium (Ca2+) at fertilization results in a cascade of events that includes exocytosis of CGs (Figure 1) These secretory vesicles are egg-specific membrane-bound organelles that, upon egg activation, fuse with the PM and release their content into the extracellular space. CG exocytosis (CGE) is executed after fertilization and functions to prevent polyspermy, and regulate the early embryo’s developmental progression To facilitate this immediate response, CGs become localized at the PM during oocyte maturation. We discuss the significant progress made in linking animal phenotypes and genetics (phenogenetics) to elucidate the molecular identity and functionality of factors regulating CGE and fertilization
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