Updated by: Arabinda Naik
it occurs in the ovary of female animals. the process of formation of a mature female gamete is called oogenesis. it is the process of formation of haploid gamete called ova from the diploid cell called oogonia in ovary of female. Oogenesis is a discontinuous process.
it is divisible into following three phases:
1. Multiplication phase:
Oogenesis starts during embryonic development stage. in the foetal development, certain cell in the germinal epithelium of the ovary of the foetus are larger than others. these cells divide by mitosis, producing millions of oogonia in each ovary of foetus. no more oogonia are formed or added after birth. The oogonia multiplies by mitotic divisions forming the primary oocytes. one cell of oogonia enters prophage-I of meiosis and gets arrested at the diplotene stage becoming the primary oocyte. The structure thus formed is known as the primary follicle.
2. Growth Phase: This phase of the primary oocyte is very long. It may extend over many years. the oogonium does not divide but increases in size enormously to form a primary oocyte. each primary oocyte then gets surrounded by a layer of granulosa cells to form a primary follicle. The primary follicles get surrounded by more layers of granulosa cells and form secondary follicles. The growth is associated with both nuclear and cytoplasmic growth. The nuclear growth is due to accumulation of large amount of nuclear sap and is termed as germinal vesicle. Cytoplasmic growth is associated with an increase in the number of mitochondria, endoplasmic reticulum and Golgi complex and accumulation of reserve food material called yolk or vitellin. Many of these follicles degenerate from birth to puberty. so, at puberty only 60,000 to 80,000 primary follicles are left in the ovary.
3. Maturation phase:
After attainment of puberty, follicles and their primary oocyte grow further and complete meiosis I, and it produces two unequal haploid cells. The large one is called secondary oocyte and has almost whole of cytoplasm and smaller one is called first polar body and has a small amount of cytoplasm. Both these cells are haploid(n) with 23 chromosomes. The secondary oocyte proceeds to metaphase stage of meiosis-II and again it is arrested there and waits for arrival of sperm for completion of meiosis-II and again it arrested there and waits for arrival of spermatozoa for competition of meiosis-II. Entry of sperm completes the meiosis-II resulting in a fertilized ovum (2n) and a second polar body. Hence from primary oocyte(2n) to one ovum (n) and two polar bodies(n) are produced. Thus, only one functional ovum is formed which is the actual female gamete and two polar bodies take no part in reproduction and hence, soon degenerates.
one oogonium produces one ovum and two polar bodies.
polar bodies have small amount of cytoplasm. formation of polar bodies maintains half the number of chromosomes in the ovum.
during meiosis-I crossing overtake place which brings about variations.
Ovulation:
In human ovulation is the release of secondary oocyte from Graafian follicle of ovary. the wall of ovary gets ruptures to release the secondary oocyte which occurs on 14th day of menstrual cycle.
Ovum:
The mature ovum is spherical in shape. The ovum is without yolk; hence it is called acleithral. Its cytoplasm is called ooplasm containing a large nucleus called germinal vesicle. The nucleus contains a prominent nucleolus. The cytoplasm is enclosed by a plasma membrane. The membrane forming the surface layer of the ovum is called vitelline membrane. The plasma membrane of the ovum is surrounded by a thick and non-cellular zona pellucida.is surrounded by a layer called corona radiate. A narrow perivitelline space is present between the zona pellucida and the plasma membrane. This side of the ovum forming polar bodies is called an animal pole, and the opposite is the vegetal pole. The human ovum loses its ability to be fertilized 24 hours after ovulation.