Human Reproduction of Class 12
The process of formation of haploid gametes in gonads for sexual reproduction by meiotic division.
Formation of female gamete is oogenesis and male gamete is spermatogenesis. Both undergo similar basic process with some differences as
influenced by FSH and other hormones from anterior pituitary and the gonads. Both sperm and ova originate from primordial germ cell (PGC)
extra gonadal in origin.
Takes place in seminiferous tubules in 3 stages : (i) spermatocytogenesis (ii) Meiosis I and Meiosis II and (iii) Spermiogenesis (spermateleosis).
First two stages are continuous involving divisions in the primordial germ cells spermatogonia (12 µm size), while the last stage involves no
division but only morphological changes.
The primordial germ cells along the wall of seminiferous tubules (germinal epithelium) are called the spermatogonia. These diploid (2n) cell
divide mitotically many times giving rise to type A spermatogonia and type B spermatogonia, the former serve as stem cells while later
differentiate into primary spermatocyte (2n) becoming double in size;
1st meiotic division results in two secondary spermatocytes (n). In some, both remain interconnected cytoplasmically.
By 2nd meiotic division each produces two haploid spermatids (interconnected).
Thus altogether 4 haploid spermatids are produced from each primary spermatocyte.
Fig. Spermatogenesis Fig. Hormonal control of male reproductive system
The spermatids separate from each other; flagellum is formed and the organelles get rearranged forming the parts like head, with cap (acrosome)
Sperms achieve its motility after staying in epididymis.
The process of spermatogenesis in mammal requires low temperature hence testis descends into scrotum.
The formation of ovum takes place in the follicles which are initially formed as cluster of oogonia as primary follicles. All such follicles are
formed only once when the foetus is 25 weeks old in her mother’s womb. The primary oocytes already remain halted in the diplotene stage its first meiotic division. After puberty these follicles start maturing one by one completing meiosis I to form one secondary oocyte.
The matured Graafian follicle contains haploid (n) secondary oocyte which is released through ruptured ovarian surface; enters ostia fimbriated funnel passes down the fallopian tube, undergoes second meiotic division and remains suspended in the second metaphase until
fertilization takes place.
During growth phase of egg, golgi bodies from cortical granules.
Entry of sperm restarts the cycle by activating the APC (anaphase promoting factor) and MPF (maturation promoting factor) completion of
meiosis results in forming ootid and a second polar body.
Thus, finally, from one primary oocyte only one ovum (ootid) is formed.
Fig. Oogenesis Fig. Hormonal control of female reproductive system