Oogenesis: The Journey Of Ovum

Oogenesis is a growth process occurring in the ovary of the human female reproductive system in which primordial germ cells form a mature egg by mitos

Hi there! Welcome to yet another educative post. Let's accelerate your mind and get started. First of all, let we talk about what is Oogenesis?

Oogenesis

Oogenesis is a growth process occurring in the ovary of the human female reproductive system in which primordial germ cells form a mature egg by mitosis, differentiation, and meiosis. This mature egg is called ovum (plural ova) which is a female gamete.

So now, let's talk about how is the process of oogenesis? id est, how is mature ovum formed in a female?

Oogenesis consists of several sub-processes:

Oocytogenesis: In this process, primordial germ cells are transformed into primary oocyte. Oocytogenesis is complete either before or shortly after birth.
Ootidogenesis: In this process, primary oocytes are transformed into ootids.
Maturation of Ovum: In this process, ootids are matured into ovum or egg.

Folliculogenesis is a separate sub-process that accompanies and support oogenesis.

Primordial Germ Cells

After their migration into the genital ridges, the germ cells (PGCs) proliferate, are surrounded by the coelomic epithelium and form sex cords, which retain their contact with the coelomic epithelium. We now distinguish a cortical zone (ovarian cortex) and a medullary zone (ovarian medullary) it should be noted that in the female sex the sexual cords probably never reach the medullary zone.

The “seed” cell of every oocyte is the primordial germ cell. These are embryonic cells that will eventually become either sperm or oocyte cells. In the developing embryo, these cells move into the area that will eventually become either the testis or ovaries (also known as the gonads).

Oogonia

In a human female, the process of oogenesis begins before her birth, at 8-9 weeks of gestational age. Primordial germ cells grow through mitosis proliferation, i.e. increase its number and at 14-16 weeks, primordial germ cells are transformed into oogonia (singular oogonium) with differentiation.

Primary Oocyte

Oogonia proliferates extensively from mitotic division and increases its number to 5-7 million, but before birth, a lot of oogonia destroyed due to programmed cell death mechanism (apoptosis), it is called atresia and some oogonia convert from differentiation to primary oocytes and enter into prophase I stage of meiosis I (diplotene stage or dictyate) of the first meiotic division and remain halted (blocked) until puberty.

Just before birth, all the oocytes in the fetus have reached the diplotene stage. The resting phase of meiosis that follows is called dictyotene and persists until puberty.

Thus, only about 2,00,000 primary oocytes remain in the ovary at the time of birth. Out of this, most of the primary oocytes are apoptotic demise till puberty and 40,000 primary oocytes are left every ovary. Of these, only 400 primary oocytes mature and ovulate during the complete reproductive life cycle of a human female.

Primary oocytes are isolated and each surrounded by a unistratified layer of flattened follicular epithelial cells (originating from the coelomic epithelium), thus forming the primordial follicle (primary oocyte + follicular epithelium = primordial follicle)

In primary oocyte, the amount of DNA is 4N (replicated) and number of chromosome is 2n (Diploid), that is 2 × 23 = 46, out of which 23 come from maternal side and 23 come from paternal side.

Secondary Oocyte

Immediately after meiosis I is completed, primary oocyte divides into two daughter cells. The smaller cell is called the first polar body, and the larger cell is referred to as the secondary oocyte. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg or ovum, while the smaller polar body only get a small amount of cytoplasm.

Secondary oocyte halted at the metaphase II stage of meiosis II until fertilization, if such should ever occur. After ovulation, If the egg is not fertilized in oviduct, it is disintegrated within about 24 hours and released (menstruation) and the secondary oocyte does not complete meiosis II. Unfortunately it doesn't become an ovum. When meiosis II has completed, an ootid and second polar body have now been created.

Now both in the secondary oocyte and the first polar body, the amount of DNA is 2N (replicated) and number of chromosome is 1n (Haploid), that is 1 × 23 = 23, which is due to meiosis.

Only the secondary oocyte consisting of sperm can complete the process of fertilization and only during the fertilization, meiosis II is completed.

The secondary oocyte is the largest cell in the body, and in humans is just visible to the naked eye.

Ootid

And again immediately after meiosis lI is completed during the fertilization, secondary oocyte also divides into two daughter cells, Just like primary oocyte. Now the smaller cell is called the second polar body, and the larger cell is referred to as the ootid. Most of the cytoplasm is segregated into ootid, which becomes the mature egg or ovum, while the smaller polar body only get a small amount of cytoplasm.

Ootid doesn't really have any significance in itself, since it is very similar to the ovum. However, it fills the purpose as the female counterpart of the male spermatid in spermatogenesis. In other words, the ootid is the immature ovum formed shortly after fertilization, but before complete maturation into an ovum. Thus, the time spent as an ootid is measured in minutes.

Now both in the ootid and the second polar body, the amount of DNA is 1N (non-replicated) and number of chromosome is 1n (Haploid), that is 1 × 23 = 23, which is due to meiosis.

Polar Bodies

4 polar bodies are formed in during the entire oogenesis. First polar body originates from primary oocyte. Second polar body originates from secondary oocyte. Third and fourth polar bodies originate from first polar body. All these polar bodies disintegrate at the end of meiosis II.

Mature Ovum or Egg

Eventually the auspicious moment comes when the secondary oocyte after fertilization is formed into the mature ovum after halted a few minutes as the ootid and the process of oogenesis is completed.

Thus, a mature ovum or egg in human female is formed by the process of oogenesis. Now this egg is fully ready to start life as a new generation of human.

Now the mature ovum or egg will develop into an zygote, embryo and, about nine months later, a newborn baby.

In fertilized egg, the amount of DNA is 2N (non-replicated) and number of chromosome is 2n (Diploid), that is 2 × 23 = 46, out of which 23 come from ovum and 23 come from sperm.

Suggestion: Now that we are learning about Oogenesis, it is also very important to know Folliculogenesis. If you don't know folliculogenesis, then I would like to recommend that you must read Folliculogenesis.

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