Oogenesis Definition and Stages

Define oogenesis

Oogenesis is the process by which female gametes are formed. This cycle begins inside the embryo before fertilization. The stages of oogenesis lead up to primary oocyte formation that takes place before birth. The primary oocytes are not split further. Instead, they either develop into secondary oocytes or degenerate into them. Oogenesis occurs in the ovary's outermost layers.

Oogenesis is how an ovum develops into a mature ovum in the human female reproductive system. The egg development begins before the female is even born in any generation. It occurs 8 to 21 weeks after the fetus begins to grow. Cells that become mature ova multiply. By the time the female is born, all of the ovaries' eggs will release during the female's active reproductive years are already present in the ovaries. These cells, known as primary ova, are around 450,000 in number. The primary ova remains dormant before ovulation. After ovulation starts, an egg is released from the ovary.

Some egg cells do not develop for 40 years. Others decline and never mature. The egg cell remains the chief ovum until it is prepared to be unconfined from the ovary. The egg then hatches and enters a cell. The nucleus splits such that half of the chromosomes go into one cell and the other half go into another. One of the two new cells is usually more significant than the other. It is referred to as the secondary ovum. The secondary ovum develops in the ovary until it matures. At this point, it breaks out and is moved into the fallopian tubes. Once in the fallopian tubes, the secondary eggs are ready for fertilization by sperm cells.

The Oogenesis Process

The next three steps complete the oogenesis process:

1. Prenatal stage

The primary oocyte develops when meiosis-I is at a standstill. They multiply and segregate into a stratified cuboidal epithelium. To create the zone pellucid, these cells discharge glycoproteins around the primary oocyte.

2. Antral Stage

The gaps filled with fluid between granulosa cells combine to produce the antrum. The antrum is a central fluid-filled area. These are secondary follicles. During each monthly cycle, secondary hairs grow under the influence of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

3. Pre-ovulation stage

An LH gush initiates this stage during the pre-ovulation phase. It is completed by meiosis-I. Inside the follicle, two haploid cells of different sizes develop. The offspring cells that receive less cytoplasm becomes a polar body. This cell does not contribute to the formation of an ovum. The other daughter cell becomes the secondary oocyte. Meiosis-II takes place in the two daughter cells. The polar body duplicates to create two polar bodies, whereas the secondary oocyte halts in the meiosis-II metaphase.

Process of ovulation

Oocytes are formed in the ovaries. Follicle cells surround each oocyte to create a follicle. As the menstrual cycle begins, primary oocytes become more extensive, and the number of follicle cells rises, leading the follicle to become more prominent. Some nursing oocytes deteriorate and leave just one follicle to develop. Fraternal twins with genetically different parents can be born here. When a follicle matures and becomes a secondary oocyte, the primary oocyte begins its primary meiotic division. Even if the second meiotic division has not happened, the follicle divides, and secondary oocytes are discharged shortly after in the Fallopian tube. Ovulation is defined as the discharge of a secondary oocyte from the ovaries

The process of fertilization

In humans, all of a female's oogonia are formed when she is still a fetus and has not been born. Around one or two months before the birth of a baby girl, the majority of her approximately seven million oogons die. Then, every month for 30 to 45 years, primary oocytes restart meiosis and finish the first meiotic division. This division, like the first, is uneven, with half of the chromosomes going to another degenerate polar body and the other half retained by the ovum.