29.

(a) Explain the process of oogenesis in humans.

(b) State three differences between red muscle fibre and white muscle fibre.

(c) Mention a cause and symptom of each of the following:

(i) Amphysema (ii) Renal calculi (iii) Diarrhoea

The process of formation of a mature female gamete (ovum) is called oogenesis. It occurs in the ovaries (female gonads). It consists of three phases: multiplication, growth and maturation.

(a) Multiplication phase:

In the foetal development, certain cells in the germinal epithelium of the ovary of the foetus are larger than others. These cells divide by mitosis, producing a couple of million egg mother cells or oogonia in each ovary of the foetus. No more oogonia are formed or added after birth. The oogonia multiply by mitotic divisions forming the primary oocytes.

(b) Growth phase:

This phase of the primary oocyte is very long. It may extend over many years. The oogonium grows into a large primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells to form primary follicle. A large number of these follicles degenerate during the period from birth to puberty. So at puberty only 60,000- 80,000 primary follicles are left in each ovary. The fluid filled cavity of the follicle is called antrum.

(c) Maturation phase:

Like a primary spermatocyte, each primary oocyte undergoes two maturation divisions, first meiotic and the second meiotic. The results of maturation divisions in oogenesis are, however, very different from those in spermatogenesis. In the first, meiotic division, the primary oocyte divides into two very unequal haploid daughter cells— a large secondary oocyte and a very small first polar body or polocyte.

In the second maturation division, the first polar body may divide to form two second polar bodies. The secondary oocyte again divides into unequal daughter cells, a large ootid and a very small second polar body. The ootid grows into a functional haploid ovum. Thus from one oogo­nium, one ovum and three polar bodies are formed. The ovum, is the actual female gamete. The polar bodies take no part in reproduction and, hence, soon degenerate.

In human beings, ovum is released from the ovary in the secondary oocyte stage. The maturation of secondary oocyte is completed in the mother’s oviduct (Fallopian tube) usually after the sperm has entered the secondary oocyte for fertilization.

In humans (and most vertebrates), the first polar body does not undergo meiosis II, whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. How­ever, it then stops advancing any further; it awaits the arrival of sperm for completion of meiosis II.

b. 

(i) Amphysema : Amphysema is caused by Cigarette smoking and acute inflammation of bronchi and lungs. Symptom : Pulmonary hypertension.

(ii) Renal calculi : is caused by excessive calcium intake, decrease in water intake. Symptom : Acute pain when these kidney stones get stuck in the water.

(iii) Diarrhoea : Diarrhoea is caused by a flagellate protozoan Giardia intestinalip. Infection occurs by taking cysts with food and drinks. The parasite grows on the mucous membrane of the intestine and reduce the absorption of food. Symptom : Frequent and profuse defection of liquid faeces.