(a) Explain the process of microsporogenesis in angiosperms.

(b) Draw a labelled diagram of a mature pollen grain.

OR

When does oogenesis start in humans? Name the three hormones and their source linked with oogenesis. How do they influence the processes of oogenesis? Explain.

a) As the anther develops, the cells of the sporogenous tissue would undergo meiotic divisions forming microspore tetrads. Each one of the microspore tetrad is a potential pollen or microspore mother cell. The process of formation of microspores from a pollen mother cell (PMC) by the process of meiosis is called microsporogenesis. The microspores, are arranged in a cluster of four cells which is called the microspore tetrad. As the anthers mature and dehydrate, the microspores would eventually dissociate to form pollen grains.

b) A labelled diagram of a mature pollen grain is as follows:

Pollen grains are spherical and have a prominent two-layered wall. The hard outer layer is called the exine made up of sporopollenin and has prominent apertures called germ pores where sporopollenin is absent. The inner wall of the pollen grain is called the intine which is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell.

OR

Oogenesis starts during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary.

The three hormones linked with oogenesis are

• Estrogen (secreted by the ovaries)

• Progesterone (secreted by the corpus luteum in the ovary)

• Follicle-stimulating Hormone (secreted by the gonadotropic cells of the anterior pituitary gland)

• Luteinizing Hormone (produced by gonadotropic cells in the anterior pituitary gland).

Role of Gonadotropins (FSH and LSH):

(i) FSH stimulates the follicular development and also stimulates the follicle cells to secrete estrogen, a female sex hormone. The estradiol inhibits the secretion of FSH and stimulates the secretion of LH from the anterior pituitary.

(ii) Both LH and FSH attain a peak value in the mid of the cycle.

(iii) LH induces the Graafian follicle to burst and eject its egg into the fallopian funnel. This is called ovulation.

(iv) LH stimulates formation of corpus luteum from the ruptured follicle and secretion of progesterone from it.