Sexually reproducing organisms require an egg and a sperm to create new life. This is the story of the egg (or ovum), the female sex cell and one of the most crucial elements of biological diversity.
All sexually reproducing organisms make sex cells called gametes. The gamete, produced by the female is called the egg or ovum (plural = ova).
It joins with the sperm, the male gamete, during fertilization to form the embryo, which will eventually grow into a new organism.Ova are produced by sexually reproducing animals, protists, fungi and flowering plants and ferns. In animals, they are produced by follicle cells in the ovaries of the female. In plants, egg cells are produced by ovules found inside the ovary (the part which then becomes the fruit).
Most of the inner structures of the egg cell are the same as those in any other animal cell, but they are given special names. For example, the nucleus is referred to as the ‘germinal vesicle‘ and the nucleolus as the ‘germinal spot.’The cytoplasm of the ovum is called the ‘ooplasm‘ (meaning ‘egg material’) or ‘vitellus.’ As if two names were not enough, it is also known as the ‘yolk‘ of the egg.
This can be a bit confusing when you think of one of the most common, visible and edible ovum around the chicken egg, in which the yolk looks like the nucleus of the cell but actually contains most of the egg cell. The yolk supplies nutrients to the growing embryo, a smaller amount in mammals compared to that of egg-laying animals.
The plasma membrane of the ovum is called the ‘vitelline membrane,’ and it has the same functions as in other cells, mainly to control what goes in and out of them.
The zona pellucida, more commonly known as ‘jelly coat,’ is a thick, protein-based layer covering the outside of the vitelline membrane that helps protect the egg. It is also involved in the binding of sperm during fertilization and prevents more than one sperm from entering the egg.The outermost layer is called the corona radiata. It is made up of several rows of granulosa cells that were left sticking to the egg after it was expelled from the follicle.
The corona radiata provides the ovum with essential proteins and acts like bubble wrap, protecting the ovum as it travels down the fallopian tubes.
It was once believed that new life was carried in the male sperm. The female egg was seen as a vessel in which this new life would grow, a flower pot for the seedling. This view was held by the great Aristotle and by other philosophers and scientists for centuries.
Today we know that the ovum, the gamete produced by females, and sperm, the male gamete, contribute equally to the creation of offspring, providing one set of chromosomes each, to be exact. On top of that, the ovum also provides a perfect environment and nutrients for the growing embryo.
Finish this lesson so that you can later do the following:
- Describe the production of the female gamete
- Emphasize the function of the ovum
- Point out the different parts of the ovum and understand its purpose