A woman is born with all the oocytes which are required in her complete life before her birth. On this page let us know in brief the formation of an ovum through the process of oogenesis.
Oogenesis is the process through which ova or the female gametes are formed and this female gamete is known as an ovum. Sometimes we refer female gametes to eggs but the word egg can involve different stages of development and the significance of egg varies based on types of organism.
For instance, the complete development of birds takes place inside the eggs while in placental mammals once the egg is fertilized and begins to divide, you don’t call it as egg anymore. You need to recall that every ovum has to be haploid and consist of a single copy of every chromosome.
You must have recalled that to form haploid gametes, the cell has to go through meiosis process that involves replication of its genome and separation twice. This theory creates four haploid gametes from one diploid cell.
However, the case of female human gametes is slightly different. Let’s get to know the oogenesis process to describe how it works and the number of haploid gametes formed by a diploid cell.
The diploid cells which are capable of developing into ova are known as oogonia. Every female’s oogonia are formed when she’s still a fetus and has not even born. Moreover, around one or two months before a girl baby is born, over seven million oogonia vanish and the survived oogonia move towards meiosis I and become primary oocytes. These oocytes pause their advancement in prophase I soon after replicating their genomes. They stay paused for a decade until a girl starts her first menstrual cycle. For the next 30-40 years, primary oocytes retrieve meiosis from where they left and finish the first meiotic division on monthly basis.
When primary oocyte does complete its first meiotic division, it separates the chromosomes evenly but does not separate cytoplasm evenly. Almost whole cytoplasm stays in one of the two daughter cells that become a secondary oocyte. The development of a polar body enables the primary oocyte to minimize its genome by half and save most of its cytoplasm in the secondary oocyte.
Secondary oocyte still holds two copies of the chromosome and has to undergo second meiotic division. This is also not even similar to first with half of the chromosomes moving to another small degenerate polar body and the other half get retrieved by the ovum with the whole cytoplasm.
Development of oocyte takes place in ovaries. Every oocyte is neighboured by follicle cells to form a follicle. As the menstrual cycle starts, primary oocytes initiate to grow bigger and follicle cells rise in number causing follicle to go larger too. Normally, some nurturing oocytes degenerate and leave just one follicle to mature. Here, fraternal twins may be born which distinct genetically. When a follicle attains maturity, the primary oocyte finishes its primary meiotic division and becomes secondary oocyte. Soon after, the follicle breaks and secondary oocyte is liberated in the fallopian tube even when the second meiotic division has not happened. This release of a secondary oocyte from ovaries is known as ovulation.
Stay tuned with Byju’s to learn more about oogenesis.