Oogenesis is the process of producing an egg cell, which goes through five stages of development. The first stage is the oogonium, which undergoes mitosis to create the primary oocyte. Meiosis then produces the haploid secondary oocyte, which becomes an ootid and then matures into the ovum. Oogenesis occurs in all sexually reproductive species and is necessary for fertilization to occur.
Oogenesis is the production of an ovum or egg cell, the female gamete or sex cell. One type of gametogenesis, or sex cell production, the other is the male process of spermatogenesis. Oogenesis occurs in all sexually reproductive species and includes all immature stages of the ovule. As it matures, the ovum goes through five stages in mammals: the oogonium, primary oocyte, secondary oocyte, ootid, and ovum.
In most sexually reproducing species, the ovule contains half of the genetic material of a mature individual. Reproduction occurs when the egg cell is fertilized by the male gamete, or sperm. Sperm also contains half of the genetic material of a mature individual, so the embryo formed by fertilization will contain a complete set of genetic material, half from the egg and half from the sperm.
The first stage of the immature ovule is the oogonium, formed by mitosis in the very first years of life of the organism. In mitosis, a cell replicates its DNA, its genetic material, before dividing into two identical daughter cells. Mitosis is also a method of asexual reproduction. In animals, sex cells or gametes, including egg cells, are formed only by meiosis, in which a cell divides without replicating, resulting in daughter cells with only half the number of chromosomes as the parent cell. All other cells in the body are formed by mitosis.
In the first stage of oogenesis, oogonium undergoes oocytogenesis, creating the primary oocyte through mitosis. Like the oogonium, the primary oocyte is a diploid cell, containing two complete sets of chromosomes. Sex cells are haploid cells, containing only half the amount of chromosomes in a diploid cell. Haploid cells are formed from diploid cells by meiosis.
Through ootidogenesis, a form of meiosis, the primary oocyte produces the haploid secondary oocyte. The process of ootidogenesis is stopped midway, which is called dictyate, until ovulation, when it is completed to produce the released egg or ootid. In the final stage, the ootid develops into the ovum, the egg cell matures. In humans and other mammals, the secondary oocyte does not become an ootid until it is ready to be released during the menstrual cycle.
In protists, such as algae and gymnosperms, non-flowering land plants, oogenesis begins not in the oogonium, but in a specialized structure called an archigonium. In flowering plants, it occurs within the megagametophyte, or embryo sac, contained within the ovule in the ovary of the flower. When the egg cell is mature, the egg cell will become the seed, which protects and nourishes the egg cell. In some organisms, especially the parasitic roundworm, the meiosis period only begins if the sperm comes into contact with the primary oocyte.
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