Cortical granule

Cortical granules are membrane‑bound secretory organelles located just beneath the plasma membrane (cortex) of unfertilized oocytes (egg cells) in many animal species, including mammals, amphibians, and birds. They are synthesized during oogenesis and stored in the oocyte cortex until the moment of fertilization.

Structure
Each cortical granule consists of a lipid bilayer membrane that encloses a heterogeneous matrix of proteins, enzymes, glycoproteins, and polysaccharides. The exact composition varies among species, but typical components include proteases (e.g., ovastacin), glycosidases, lectins, and zona pellucida‑modifying enzymes.

Distribution
In mature oocytes, cortical granules are densely packed in the cortical region, forming a continuous band that parallels the plasma membrane. Their distribution is often polarized, with higher concentrations near the animal pole in species that exhibit polarity during embryo development.

Physiological role
The primary function of cortical granules is to mediate the cortical reaction, a rapid exocytotic event triggered by the rise in intracellular calcium that occurs upon sperm entry. The steps are:

1. Calcium wave: Sperm binding induces a series of calcium oscillations in the oocyte cytoplasm.
2. Exocytosis: Elevated calcium concentrations cause cortical granules to fuse with the oolemma (egg plasma membrane) and release their contents into the perivitelline space.
3. Zona pellucida modification: Released enzymes modify the extracellular matrix surrounding the oocyte (the zona pellucida in mammals). Modifications include proteolytic cleavage of zona proteins, cross‑linking of zona glycoproteins, and addition of glycosylated residues.
4. Block to polyspermy: These biochemical changes create a physical and enzymatic barrier that prevents additional sperm from binding or penetrating the zona pellucida, thereby ensuring monospermic fertilization.

Molecular components
Key molecules identified in cortical granules include:

• Ovastacin (ASTL) – a metalloprotease that cleaves the zona pellucida protein ZP2 after fertilization, a critical step in hardening the zona.
• Cortical granule exocytosis proteins – such as synaptotagmins and SNARE complex proteins that mediate membrane fusion.
• Glycosidases and lectins – which alter carbohydrate structures in the zona pellucida.

Regulation of release
Cortical granule exocytosis is tightly regulated by calcium signaling pathways. In mammals, the sperm‑derived phospholipase Cζ (PLCζ) initiates calcium oscillations, while downstream effectors such as calmodulin‑dependent protein kinase II (CaMKII) and protein kinase C (PKC) contribute to granule release.

Developmental timing
Cortical granules are generated during the growth phase of oogenesis, particularly in the late germinal vesicle and metaphase I/II stages. After ovulation, the oocyte arrests at metaphase II, retaining the granules until fertilization.

Clinical and research relevance

• Assisted reproductive technology (ART): Abnormalities in cortical granule formation or release can contribute to fertilization failure or polyspermy in in vitro fertilization (IVF) cycles. Assessment of cortical granule status is sometimes performed on oocytes retrieved for research purposes.
• Genetic disorders: Mutations in genes encoding cortical granule proteins, such as ASTL, have been linked to infertility phenotypes characterized by failure to establish an effective block to polyspermy.
• Model systems: Studies in mouse, zebrafish, and Xenopus provide insight into the conserved mechanisms of cortical granule exocytosis and its role in early developmental competence.

Historical note
The existence of cortical granules was first reported in the early 20th century through electron microscopy of amphibian oocytes. Subsequent biochemical and molecular investigations clarified their function in the cortical reaction and polyspermy block.

References (representative)

1. Swann, K. & Ozil, J. (2005). "The cortical reaction: a review of the biochemistry and physiology of egg activation." Developmental Biology, 283(1), 1‑16.
2. Burkart, A. D. et al. (2012). "Ovastacin cleaves ZP2 in the zona pellucida to prevent polyspermy." Science, 335(6075), 1447‑1450.
3. Whittington, R. J. & Suarez, S. S. (2020). "Calcium signaling in mammalian fertilization." Physiological Reviews, 100(2), 527‑570.

This entry reflects current consensus in the scientific literature as of 2024.