Explain the development of a secondary oocyte (ovum) in a human female from the embryonic stage upto its ovulation. Name the hormones involved in this process.

Explanation

Oogenesis is the formation of a female gamete or an ovum in the ovary.
(i) In human females, primary oocytes are formed during the embryonic developmental stages in the foetal ovaries.
(ii) (a) Primary oocytes start dividing and enter prophase-! of meiosis to remain suspended at this stage.
(b) Each primary oocyte is surrounded by a layer of granulosa cells and becomes the primary follicle.
(c) The primary follicle when surrounded by more layers of granulosa cells, is called a secondary follicle.
(d) Secondary follicle transforms into a tertiary follicle, with the development of a fluid-filled cavity (antrum) around the primary oocyte.
(e) Granulosa cells become organised into an outer theca externa and an inner theca interna.
(f) Now, primary oocyte completes meiosis-I and forms a larger haploid secondary oocyte and a tiny first polar body.
(g) Tertiary follicle grows and becomes a mature follicle called Graafian follicle.
(h) Secondary oocyte secretes a new membrane called zona pellucida around it.
(i) At this stage, follicle ruptures to release the secondary oocyte, which moves into the cytoplasm.
(j) Secondary oocyte completes meiosis-llonly when a sperm enters its cytoplasm. It forms a larger cell, the ootid and a small cell, the second polar body, (iii) Influence of Gonadotropins on Oogenesis
(a) Gonadotropins, i.e. LH and FSH stimulate follicular development and secretion of oestrogens by the growing follicles.
(b) Both LH and FSH attain a peak level in the middle of the cycle (14th day).
(c) Rapid release of LH during mid-cycle causes ovulation.
(d) LH also stimulates the formation of corpus luteum from the ruptured follicle and secretion of progesterone from corpus luteum.