The cortical reaction is a biological event occurring during fertilization. This process unfolds within the egg cell shortly after contact with a sperm. It represents a coordinated cellular response that ensures the proper initiation of embryonic development.
The Purpose of the Cortical Reaction
The cortical reaction serves as a defense mechanism against polyspermy, which is the fertilization of an egg by more than one sperm. This can lead to severe genetic abnormalities. If multiple sperm contribute genetic material, the embryo would have an incorrect number of chromosomes, incompatible with normal development.
Such chromosomal imbalances result in abnormal embryonic development and can lead to early embryonic failure. The cortical reaction acts as a safeguard, ensuring only a single sperm fertilizes the egg. This mechanism is the main component of the “slow block to polyspermy,” establishing a permanent barrier to additional sperm entry.
How the Cortical Reaction Occurs
The initiation of the cortical reaction begins immediately after a single sperm successfully fuses with the egg’s plasma membrane. This fusion triggers a rapid and widespread increase in the concentration of calcium ions (Ca2+) within the egg’s cytoplasm. The calcium surge originates at the point of sperm entry and propagates across the entire egg as a wave, similar to a ripple effect.
This calcium wave acts as a powerful intracellular signal, prompting specialized secretory vesicles called cortical granules to undergo exocytosis. These cortical granules are small, membrane-bound sacs located just beneath the egg’s plasma membrane, in the region known as the cortex. The elevated calcium levels cause these granules to fuse with the egg’s outer membrane, releasing their contents into the perivitelline space, which is the narrow gap between the egg membrane and its outer protective layer, the zona pellucida.
The contents of these cortical granules are enzymes and other molecules. In mammals, the sperm introduces a specific enzyme that triggers the release of stored calcium ions, generating the calcium wave.
What Happens After the Cortical Reaction
Following the release of their contents, enzymes from the cortical granules modify the structure of the zona pellucida. This modification, known as “zona hardening” or the “zona reaction,” renders the zona pellucida impenetrable to subsequent sperm. These enzymes alter sperm receptor glycoproteins on the zona pellucida, preventing other sperm from binding or penetrating.
The released contents also include proteases that clip perivitelline tether proteins, peroxidases that contribute to hardening the extracellular matrix, and glycosaminoglycans that draw water into the perivitelline space, causing it to expand. This expansion and hardening of the zona pellucida serve as an irreversible barrier, preventing additional sperm from reaching and fusing with the egg. This ensures that only one sperm fertilizes the egg, establishing a diploid zygote with the correct number of chromosomes for healthy embryonic development.