An egg cell is the female sex cell. The term is often used to designate the ovules or gametes of plant species, although it can also be considered a synonym for female gametes in animals. In addition, some authors use it as a synonym for ovary.
In animals, ovules are usually large cells, without extensions for movement, rounded and rich in cytoplasm . In vegetables, the size and structure of the gametes are more variable. The new individual can originate from an embryo, from the zygote formed by the union of the oocell of the female gemetophyte with one of the nuclei from pollen.
In other plant species, the embryo can form without the need for the event of fertilization. In these cases, the egg cell can generate the embryo and this striking phenomenon is called apomixia. Let us remember that the propagation of plants is a quite variable and flexible phenomenon.
The counterpart is the male sex cell. These are generally smaller, exceptionally mobile, and produced in significant quantities. These haploid sex cells combine during fertilization to form a diploid zygote.
Before discussing the generalities of egg cells, we will describe the different types of gametes that exist among sexually reproducing organisms , to get an idea of how egg cells can vary in terms of size and structure.
Depending on the size and the relationship between the dimensions of the male and female gametes, sex cells are classified into:
-Isogamy : female and male gametes are identical in terms of their structure and size. This mode of reproduction is typical of sexual reproduction in plant species.
– Anisogamy : in this class of gametes, male and female cells differ in size and shape. Eggs are associated with females and sperm with males.
– Oogamy : oogamy is within the classification of anisogamy. Male gametes are small and very numerous. Females, for their part, are devoid of any structure that allows locomotion (flagellum), and are rich in organelles and reserve substances. These cells are immobile and few in number.
In mammals , the difference in size and production costs in gametes has been used by various authors to support the fact that females tend to be monogamous and more selective in seeking a mate, since their gametes are energetically expensive , in contrast to the “cheap” sperm of males.
Egg cell in animals
In animals, ovocells or ovules are large, haploid cells. They fall into the category of oogamy.
They are formed through a process called oogenesis or female gametogenesis. This process takes place in the female gonads: the ovaries. The egg generation process begins with a diploid germ cell that divides by mitosis on multiple occasions.
Following this increase in number, the cell grows for the accumulation of reserve substance. Finally, the cells undergo meiosis to reduce the number of chromosomes.
The end result of this process is a mature ovum that can be potentially fertilized and a series of polar bodies that degenerate. Meiotic divisions of the ovum do not end until fertilization occurs.
The ovum is covered by a series of layers. In the precise case of sea urchins, there is a gelatinous covering that surrounds an envelope of a protein nature.
The mammalian egg cell is characterized by having a series of proteins that participate in the recognition of sperm and in the fertilization process in general. This region is called the zona pellucida and is made up of various glycoproteins, grouped into four families.
The zona pellucida participates in the acrosome reaction, an event that involves the fusion of the sperm with the oocyte membrane. During fusion, the sperm releases a series of hydrolytic enzymes that were stored in a vesicle called the acrosome.
The objective of this phenomenon is the dissolution of the extracellular matrix that surrounds the female gamete and to achieve fertilization.
Egg cells in plants
In plants, the name of ovules is attributed to the seminal primordia, while the female gametes per se are called oospheres.
The oosphere is located inside the egg and is surrounded by two additional cells.
With the course of evolution, seeds have modified their location towards other organs of plants, since ancestrally the same isolated seed was the main organ of multiplication.
In gymnosperms, the seminal primordia are naked. In contrast, angiosperms have developed a structure that encloses the primordia, consisting of carpellar leaves and the ovary.
When the seeds have been formed, the fruit is formed. This organ can be formed from one or more parts of the flower. The fruits can be simple when they are single or compound, like strawberries, when they are made up of multiple units.
The process by which the oospheres originate is called megagametogenesis. This phenomenon begins with a haploid megaspore. This process varies in some of its steps depending on whether the group is a gymnosperm or an angiosperm .
When the haploid cells are obtained , they can be fused with the pollen grains. In plants, a phenomenon of double fertilization occurs.
In angiosperms, double fertilization is quite widespread. As its name indicates, it consists of the fusion of one of the nuclei of the pollen grain with the oosphere, and another nucleus of the pollen with one of the polar bodies of the cells of the embryo sac.
The first fusion gives rise to the formation of the diploid embryo. The fusion between the nucleus with the polar bodies gives rise to a triploid between that will give rise to the endosperm (a nutritional tissue of plants).
In various plants, fertilization is assisted by a process called pollination . The help can come mediated by wind, water or even by vertebrate or invertebrate animals that efficiently transfer pollen to the stigma.
- Agustí, M., & Fonfría, MA (2010). Fruit growing . Mundi-Press Books.
- Arnold, ML (2015). Divergence with genetic exchange . OUP Oxford.
- Campbell, NA (2001). Biology: Concepts and relationships . Pearson Education.
- Curtis, H., & Schnek, A. (2006). Invitation to Biology . Panamerican Medical Ed.
- Hall, BK (2012). Evolutionary developmental biology . Springer Science & Business Media.