What is oogenesis? Give a brief account of oogenesis.
What is oogenesis? Give a brief account of oogenesis.
Oogenesis is the process of formation of mature female gametes from primordial germ cells.
(i) This process is initiated during embryonic developmental stage when about two million gamete mother cells (oogonia) are formed in each foetal ovary.
(ii) Oogonia start meiotic division which gets arrested at prophase -I stage. They are referred to as primary oocytes.
(iii) Each of these gets covered with layers of granulosa cells and are then called primary follicle.
(iv) Many of the primary follicles degenerate from birth to puberty,leaving about 60000-80000 in each ovary at puberty.
(v) More layers of granulosa cells and another theca layer surround it and now it is called secondary follicle. Theca layer is arranged as inner theca-interna and outer theca-externa.
(vi) Secondary follicle transforms into tertiary follicle that is characterised by a fluid filled cavity called antrum.
(vii) The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division now.
(viii) This is an unequal division resulting in the formation of
(a) a large haploid cell ( that keeps the majority of nutrient rich cytoplasm) called secondary oocyte.
(b) a tiny cell, with haploid nucleus and almost no cytoplasm called first polar body.
(ix) The tertiary follicle undergoes certain changes to mature into Graafian follicle.
(x) Secondary oocyte forms a new membrance around it, called zona pellucida.
(xi) Under the influence of LH, the Graaffian follicle now ruptures to release secondary oocyte from the ovary by a process called ovulation.
Summary of oogenesis
Oogonia → Meiosis initiated → Primary oocyte (arrested at prophase -I) → Granulosa layer builds → Primary follicle → More granulosa and theca layer added → Secondary follicle → Fluid filled cavity develops → Tertiary follicle → Primary oocyte completes meiosis-I → Secondary oocyte (haploid) + polar body → Tertiary follicle transforms to Graafian follicle → Zona pellucida builds around secondary oocyte → Graafian follicle ruptures → Secondary oocyte (ovum) released.
Meiosis-II will occur only at the time of penetration of sperm into the ovum.
Oogenesis is the process of formation of mature female gametes from primordial germ cells.
(i) This process is initiated during embryonic developmental stage when about two million gamete mother cells (oogonia) are formed in each foetal ovary.
(ii) Oogonia start meiotic division which gets arrested at prophase -I stage. They are referred to as primary oocytes.
(iii) Each of these gets covered with layers of granulosa cells and are then called primary follicle.
(iv) Many of the primary follicles degenerate from birth to puberty,leaving about 60000-80000 in each ovary at puberty.
(v) More layers of granulosa cells and another theca layer surround it and now it is called secondary follicle. Theca layer is arranged as inner theca-interna and outer theca-externa.
(vi) Secondary follicle transforms into tertiary follicle that is characterised by a fluid filled cavity called antrum.
(vii) The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division now.
(viii) This is an unequal division resulting in the formation of
(a) a large haploid cell ( that keeps the majority of nutrient rich cytoplasm) called secondary oocyte.
(b) a tiny cell, with haploid nucleus and almost no cytoplasm called first polar body.
(ix) The tertiary follicle undergoes certain changes to mature into Graafian follicle.
(x) Secondary oocyte forms a new membrance around it, called zona pellucida.
(xi) Under the influence of LH, the Graaffian follicle now ruptures to release secondary oocyte from the ovary by a process called ovulation.
Summary of oogenesis
Oogonia → Meiosis initiated → Primary oocyte (arrested at prophase -I) → Granulosa layer builds → Primary follicle → More granulosa and theca layer added → Secondary follicle → Fluid filled cavity develops → Tertiary follicle → Primary oocyte completes meiosis-I → Secondary oocyte (haploid) + polar body → Tertiary follicle transforms to Graafian follicle → Zona pellucida builds around secondary oocyte → Graafian follicle ruptures → Secondary oocyte (ovum) released.
Meiosis-II will occur only at the time of penetration of sperm into the ovum.
