by A. C. Bowie

Created on: April 30, 2010   Last Updated: May 04, 2010

Cellular Fertilization in the Kingdom Animalia

Cellular fertilization is the mechanism by which sexual reproduction yields a new generation of organism. Between kingdoms, the process varies far and wide encompassing everything from the simplicity of transferring genetic information between cells via a biological tube to the emotional and physical complexities of human intercourse. Even among species in the animal kingdom, there’s a list of exceptions and discrepancies almost as long as the list of steadfast biological rules. However, the intricate method that has the capability to bring about both a brine shrimp and a blue whale always begins with two microscopic cells.

It all starts within the reproductive organs of each parent organism often before even coming in contact with each other. The cells of any offspring carry half of its genetic information from the mother and half from the father. Therefore, it is necessary for what will become the parent cell to undergo meiosis. Through this cell splitting process, a sex cell is created carrying only half the chromosomal content of the parent. These haploid cells are called gametes. The female produced gamete is known as the ovum (egg in mammals specifically) while the male produced gamete is a sperm.

In most animal species, sperm cells are generally motile, or able to move. This movement is typically accomplished by one or more flagella, or tails, connected to the midpiece of the sperm.  The nucleus of the sperm cell, the home of the chromatin fibers containing the DNA, is located within the head of the sperm. Over the anterior half of the head is the acrosome, a cap filled with enzymes that will later aid the sperm in interacting with the egg cell. Attached to the head with a slender neck is the midpiece.

Tightly coiled around the core of the midpiece are mitochondria, the ATP power source of the cell. Sperm are created in the male testes. The egg cell, created in the ovaries, typically takes on a more recognizable cell structure. In addition to the nucleus, it retains the rest of its organelles after meiosis and is enclosed within a glycoprotein layer known as the zona pellucida. In mammals,  there is an additional outer layer known as the corona radiata that consists of two or three layers of follicle cells that provide additional protection and nourishment to the egg.

The egg cell is usually massively larger than the sperm cell and is non-motile.  While it’s rare

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