Cell Fertilization

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 (learn stages of 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 for the egg to be motile and the sperm to not be, in some water-dwelling invertebrates both gametes are motile. Only one type of cell contains the all too vital sex determining allele. Which gamete this is depends on the species and its sex determining system.

The method by which the sperm and egg come together is wildly different between species, but can be divided into that which is external and that which is internal. As the name suggest, in external reproduction the female will produce a clutch of unfertilized ova that the male will then add his sperm to. In some species, both the male and female will release their motile gametes (usually into ocean water), and fertilization occurs without either parent being present. Internal fertilization requires that the male insert his copulatory apparatus into the orifice or organ on the female of the species that accepts the sperm. Fertilization will then take place inside the female resulting in either the laying of an egg or, after gestation, a live birth.

Outside the male body, various chemical and physical barriers ensure that only the fittest sperm reach their goal. Chemotaxis describes certain chemical reactions that occur within the body to direct cellular movement in the right direction. The female egg sends off a specific kind of hormone that the sperm cell is attracted to. This is a species specific chemical reaction, so a sperm will only seek out and try to fertilize an egg of the same species. After taking the brutal journey through water, air, or the female reproductive organs, the sperm will bond with the outer layers of the egg cell. At this point, the acrosome will meld with the zona pellucida in an event known as the acrosome reaction.

The enzymes stored within the acrosome will be released and start to eat away at the zona pellucida, allowing the sperm to more easily dig its way through to the egg’s plasma membrane. The sperm then fuses with the plasma membrane and releases its flagella and nucleus into the cell proper. The two nuclei then merge to form a new nucleus and the flagella is usually destroyed. This new diploid cell, known as the zygote, contains equal amounts of information from mother and father and the full chromosomal complement of its species. The single-celled zygote then undergoes repeated rounds of mitosis, eventually giving rise to a new, viable organism.