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Ovum: Definition, Function & Structure

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  • 0:02 Definition
  • 0:39 Function
  • 1:09 Structure
  • 2:55 Lesson Summary
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Instructor: Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

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.

Definition

Egg and Sperm during Fertilization

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).

Plant and Animal Ovaries

Function

Gametes are the only type of cells that are haploid. They contain only one set of chromosomes, which is half of the genetic material required to make an organism they are found in. In humans, this means gametes have 23 chromosomes. The function of the ovum is to carry the set of chromosomes contributed by the female and create the right environment to enable fertilization by the sperm. Ova also provide nutrients for the growing embryo until it sinks into the uterus and the placenta takes over.

Structure

Despite its large size - it's the only animal cell you can see with the naked eye and is as big as the period at the end of this sentence - most of the egg cell is padding, layers of which protect the valuable information in its nucleus.

Structure of an Ovum

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.

Egg Cell inside Chicken Egg

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.

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