Question

Oogenesis takes place in?

Answer 1

Hint: The process of the formation of gametes is gametogenesis. It occurs in the primary reproductive organ. The process of formation of male gamete sperm in primary male reproductive organ testes is called spermatogenesis. Similarly, the process of formation of female gamete ovum in primary female reproductive organ ovary is called oogenesis. These processes differ from each other in many ways.

Complete answer:

Pair of ovaries are present in the pelvic, each connected to the pelvic wall of the uterus by ovarian ligament. The process of oogenesis in females starts in the embryonic stage itself. 

In fetal life the germinal epithelium (germ cells (2n)) forms gamete mother cells oogonia. In every ovary there are several million oogonia and no longer after birth oogonia are added.

 After birth the oogonia undergoes mitosis and differentiation to form primary oocytes \[\left( {2n} \right)\]. The meiotic division starts in the primary oocyte where the meiotic-I division pauses and it gets arrested at the diplotene stage of prophase-I of meiosis-I. These primary oocytes are lined by a layer of granulosa cells called primary follicles.

By the time a female reaches her stage of puberty, a very large number of follicles degenerate by process known as ‘Atresia’ and till puberty only around \[60,000 - 80,000\] primary follicles are left in each ovary and the others degenerate called as Atretic follicles.

When puberty is reached, the follicles start maturing. More layers of granulosa cells surround the primary oocyte along with an undifferentiated theca layer and the primary follicle is now called a secondary follicle.

The secondary follicle grows further to form a tertiary follicle which now has a differentiated theca layer theca externa (outer) and inner theca interna. Also, a fluid filled cavity called an antrum is formed. Note that this is the stage where the primary oocyte completes its first meiotic division (unequal) and forms a secondary oocyte (n) and a polar body (n). The secondary oocyte is larger and retains the bulk of the nutrient rich cytoplasm of the primary oocyte.

The tertiary follicle grows further and forms a mature graafian follicle characterized by a layer ‘zona pellucida’ formed by the primary oocyte. Also, the granulosa cells close to zona pellucida elongate to form ‘corona radiata’.

Two very important points need to be remembered here; first, the secondary oocyte does not complete meiosis -II and is arrested at Metaphase-II stage of Meiosis-II and second, at the \[{14^{th}}\] day of menstrual cycle, due to LH surge the graafian follicle releases ‘secondary oocyte’ (and not ovum) by the process called as ovulation.

After ovulation, only if fertilization occurs, meiosis-II is completed to form ovum (n) and another polar body is released. This is also an unequal division since the ovum is larger and retains most of the content of secondary oocytes. The graafian follicle further forms the corpus luteum.

If the fertilization does not occur, the sec. oocyte and corpus luteum degenerates. 

Hence, Oogenesis takes place in follicles of the ovary.

Additional Information:

Spermatogenesis differs in a lot of ways from oogenesis. Unlike oogenesis, it starts at puberty, and it has no end while menopause is the stage when the menstrual cycle stops in women and no further release of oocyte occurs. Also, in spermatogenesis one spermatogonia results in formation of \[4\] sperms whereas only one ovum/egg is formed at the end of oogenesis. The phases in spermatogenesis are shorter and there are no arresting stages to be found.

Note:

After ovulation, the ruptured graafian follicle first changes to corpus hemorrhagium (red colored) and then to corpus luteum (yellow body). In case of pregnancy, the corpus luteum is stable till the formation of placenta and acts as temporary endocrine gland by releasing hormones like progesterone to support pregnancy. No menstrual cycle occurs during pregnancy. If fertilization does not occur corpus luteum degenerates and forms corpus albicans (white).