Oogenesis is the production of an ovum or egg cell, the female gamete or sex cell. It is one type of gametogenesis, or sex cell production, the other being the male process of spermatogenesis. Oogenesis happens in all sexually reproductive species, and it includes all of the immature stages of the ovum. As it matures, the ovum passes through five stages in mammals: the oogonium, the primary oocyte, the secondary oocyte, the ootid, and the ovum.
In most sexually reproductive species, the ovum contains half of the genetic material of a mature individual. Reproduction takes place when the egg cell is fertilized by the male gamete, or sperm. The sperm also contains half the genetic material of a mature individual, so the embryo formed by fertilization will contain a full set of genetic material, half from the ovum and half from the sperm.
The first stage of the immature ovum is the oogonium, formed by mitosis in the very early 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 only formed by meiosis, in which a cell divides without replication, resulting in daughter cells with only half the number of chromosomes of the parent cell. All other body cells are formed by mitosis.
In the first stage of oogenesis, the 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 halted halfway through, 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 mature egg cell. In humans and other mammals, the secondary oocyte does not become an ootid until is is ready to be released during the menstrual cycle.
In protists, such as algae, and gymnosperms, the non-flowering seed-bearing land plants, oogenesis begins not in the oogonium, but in a specialized structure called the archigonium. In flowering plants, it takes place within the megagametophyte, or embryo sac, contained in the ovule in the flower's ovary. When the egg cell is mature, the ovule will become the seed, which protects and nourishes the egg cell. In some organisms, notably the parasitic roundworm ascaris, the meiosis period only begins if the sperm comes into contact with the primary oocyte.