Formation of ovum from primary germ cell in human female , Oogenesis

Oogenesis

The formation of egg or ovum or female gamete from primary germ cell or oogonium in ovary is called oogenesis.

Oogenesis differs from spermatogenesis in that oogenesis starts  in the foetus before birth. Primordial germ cells or oogonium  are   moved to  the cortex of the primordial gonad or foetal ovary and form primary oocyte by the meiosis I before the birth.

👌👌Remember - No oogonia are formed after the birth in the ovary of a female.

Development  of Follicle 

The germinal cell in the foetal ovary multiplies by mitosis and gives rise to many small cells known as follicles.  These follicles further form oogonium  after differentiation. All oogonia are used throughout the life of a woman , and are produced at the time of embryonic development in foetal ovary. These oogonia grow in size and attain maturity to form primary oocytes.  Primary oocytes are arranged in the gonads in a bulk. Primary oocytes are diploid.

Once puberty begins, a number of primary oocytes  begin to mature each month, although only one of these reaches full maturation to become an oocyte. The primary oocyte undergoes the first meiotic division  and grows to form a secondary oocyte in forthcoming puberty. 

The follicular cells that are present in the surrounding primary oocyte also grow and form a cuboidal epithelium. Actually  these are granulosa cells and  secrete glycoproteins. These chemicals form the zona pellucida around the primary oocyte. Primary oocyte is surrounded by a granulosa layer. This composite structure is called the primary follicle.

 Now surrounding connective tissue of primary oocyte differentiates and forms a theca, now structure is called secondary follicle. Theca is a specialised layer that is sensitive to LH and can secrete androgens under its influence.During this time, cells of zona pellucida are elongated and form corona radiata.

As development proceeds, A Fluid filled space is formed between granulosa cells, these eventually combine together to form a central fluid filled space called the antrum. Now the secondary follicle has theca and a fluid filled cavity, Antrum.

At the onset of puberty,  In each monthly cycle one of these secondary follicles becomes dominant and develops further under the influence of FSH, LH and oestrogen.

Formation of ovum and polar bodies

The peak level of LH,called LH surge, induces this stage.  Primary oocytes undergo meiosis first  Inside the follicle,  and give rise to unequally sized haploid cells. One of the haploid cells receives  less cytoplasm than the other and forms the first polar body or polocyte.  Another haploid cell is also formed, known as the secondary oocyte. Now both haploid  cells undergo  meiosis Second . The first  polar body replicates and  gives  rise to second  polar bodies but the secondary oocyte arrests in metaphase of meiosis second. Meantime , the secondary follicle has grown in size and matured. Now it is called a Graafian follicle. Secondary oocytes form ovum inside the follicle by meiosis second.

Ovulation

With the peak level of  LH, the activity of collagenase enzyme is increased.This is an enzyme that weakens the follicular wall of   graafian follicle This enzyme with  combined muscular contractions of the ovarian wall, results rupturing of graafian follicle and release   the ovum along with second polar body from the ovary. The ovum is then taken up into the fallopian tube via the fimbriae ,finger-like projections of the fallopian tube for fertilisation.

👌👌Remember- The function of polar body or polocyte is unknown so far.

If fertilization occurs , Ruptured graafian follicle is changed into yellowish corpus luteum. It release progesterone hormone for maintenane of pregnancy.

If fertlization does not occur, graaffian follicle is converted corpus albicans.

Next Article : The cyclical changes in female primate of mammal, Menstrual cycle