Egg Cell Coating Blocks Extra Sperm

Biological Strategy

Mammals

Eleanor Banwell

Image: koya79 / iStock / Some rights reserved

Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Chemically Break Down Polymers

The vast majority of biochemical assembly and breakdown processes–even by the most complex organisms–occur within cells. In fact, cells are able to perform hundreds, even thousands of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). For example, one of the biochemical pathways that break down proteins into their constituent amino acids is the protease enzyme-catalyzed processes that occur within the confined space of the lysosome–a membrane bound organelle in eukaryotic cells.

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Catalyze Chemical Breakdown

Life depends upon the building up and breaking down of biological molecules. Catalysts, in the form of proteins or RNA, play an important role by dramatically increasing the rate of a chemical transformation––without being consumed in the reaction. The regulatory role that catalysts play in complex biochemical cascades is one reason so many simultaneous chemical transformations can occur inside living cells in water at ambient conditions. For example, consider the 10-enzyme catalytic breakdown and transformation of glucose to pyruvate in the glycolysis metabolic pathway.

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Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Coat surrounding egg cell of mammals rapidly hardens to block extra sperm.

In mammals, fertilization of the ovum (egg cell) by sperm usually occurs within the fallopian tube (the duct connecting the ovary to the uterus) of the female. The ovum and the sperm each contain half the DNA required to produce a viable embryo. When a sperm and ovum fuse, the resulting cell contains two copies of each chromosome. Polyspermy is the fusion of more than one sperm with a single ovum. When this happens, there are extra copies of all the chromosomes in the cell and the fertilized egg is usually not viable. In order to prevent this, mammalian egg cells undergo a rapid surface change called a cortical reaction following fertilization. The cortical reaction prevents other sperm from fusing.

Mammalian ova are covered in a thick coating that sperm must burrow through to reach the cell membrane. This, together with other large-scale anatomical features, slow sperm down, meaning only the strongest reach the egg, and in relatively small numbers. The first sperm to reach the membrane of the egg cell fuses with it, releasing its DNA payload along with signalling factors. One factor, an enzyme called PLCζ, is thought to be important for triggering the cortical reaction.

PLCζ triggers the release of calcium ions from stores within the egg cell. The calcium washes through the cell to the membrane, where it triggers the release of cortical vesicles. Cortical vesicles are membrane-bound sacs containing a cocktail of reactive chemicals and enzymes. Once released into the space between the membrane and the coating, they simultaneously clip the tips of sperm receptors on the cell surface, preventing more sperm from binding, and crosslink the thick coating of the egg, setting it like concrete and making it impossible for late arrivals to burrow through.

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Last Updated April 20, 2018

References

“The cortical reaction is a process through which cortical granules from the oocyte are released preventing polyspermy. The fast block of polyspermy immediately prevents additional sperm getting attached to the oocyte. On the other hand, the cortical reaction establishes a permanent barrier to sperm entry and functions as the main part of the slow block of polyspermy in many animals. The cortical reaction is propagated over the surface of the egg by a wave of Ca++.” (Georgadaki et al. 2016:982)

“In this process, secretory vesicles located in the region below the plasma membrane of the oocyte, are fused with the oocyte plasma membrane. This results in the release of contents of the cortical granules, modifying the extracellular matrix so as to be impenetrable to other sperm. The cortical granules contain hydrolytic enzymes, such as proteases that clip perivitelline proteins, peroxidases that harden the vitelline envelope and glycosaminoglycans that attract water into the perivitelline space, causing it to expand and form the hyaline layer. ” (Georgadaki et al. 2016:981)

“In mammals, the cortical reaction modifies the zona pellucida, leading to the block of polyspermy. Several enzymes are released by the cortical granules, leading to the digestion of the sperm receptor glycoproteins ZP2 and ZP3, so that they can no longer bind spermatozoon” (Georgadaki et al. 2016:983)

Journal article

The molecular basis of fertilization

International Journal of Molecular Medicine, 38: 979-986 | 01/10/2016 | Georgadaki K; Khoury N; Spandidos DA; Zoumpourlis V

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