Human Reproduction — NCERT Solutions

Oogenesis is the process of formation of a mature female gamete (ovum).

Briefly:

• It begins during embryonic development when oogonia are formed in the fetal ovary.
• Oogonia enter meiosis and become primary oocytes, arrested in prophase I.
• Each primary oocyte is surrounded by granulosa cells and forms a primary follicle.
• Follicles develop into secondary and then tertiary follicles.
• In the tertiary follicle, the primary oocyte completes meiosis I to form a large secondary oocyte and a small first polar body.
• The tertiary follicle becomes the Graafian follicle.
• Ovulation releases the secondary oocyte (ovum).

The corpus luteum forms from the remaining parts of the Graafian follicle after ovulation. Its main function is to secrete large amounts of progesterone. Progesterone maintains the endometrium, which is necessary for implantation and pregnancy.

The endometrium is the inner glandular layer of the uterus. It lines the uterine cavity and undergoes cyclical changes during the menstrual cycle. A properly maintained endometrium is necessary for implantation of the fertilised ovum and for pregnancy.

The acrosome is the cap-like structure on the sperm head. It is filled with enzymes that help the sperm enter the ovum through the zona pellucida during fertilisation.

The sperm tail helps in movement. The middle piece of the sperm contains mitochondria that provide energy, and the tail enables motility, which is essential for fertilisation.

The fimbriae are finger-like projections on the infundibulum of the fallopian tube. Their function is to collect the ovum after ovulation.

This statement is false. Androgens are synthesised and secreted by Leydig cells, not Sertoli cells. Sertoli cells provide nutrition to the germ cells.

This statement is true. Sertoli cells provide nutrition to the developing germ cells in the seminiferous tubules, including spermatozoa during their development.

This statement is false. Leydig cells are found in the testes, in the interstitial spaces outside the seminiferous tubules, not in the ovary.

This statement is true. Leydig cells synthesise and secrete androgens, which are testicular hormones.

This statement is false. Oogenesis takes place in the ovary, not in the corpus luteum. The corpus luteum forms after ovulation from the ruptured follicle.

During pregnancy, all events of the menstrual cycle stop and there is no menstruation. Therefore the statement is true.

The chapter states that the presence or absence of hymen is not a reliable indicator of virginity or sexual experience because the hymen may be torn for many reasons other than intercourse. Therefore the statement is true.

The menstrual cycle is the reproductive cycle in female primates (including humans). It is the cycle of events from one menstruation to the next, lasting about 28/29 days on average. One ovum is released around the middle of the cycle.

It is regulated by pituitary and ovarian hormones. The main hormones are LH and FSH from the pituitary, and oestrogens and progesterone from the ovary. During the follicular phase, LH and FSH stimulate follicular development and estrogen secretion; the LH surge causes ovulation, and the corpus luteum secretes progesterone to maintain the endometrium.

Parturition is the process of delivery of the foetus at the end of pregnancy. It occurs due to vigorous contractions of the uterus, leading to expulsion of the baby through the birth canal.

It is induced by a complex neuroendocrine mechanism. The hormones involved include oxytocin, which causes stronger uterine contractions, and signals from the foetus and placenta that trigger the foetal ejection reflex. The chapter summary also mentions cortisol and oestrogens as involved in induction of parturition.

This is not correct because the sex of the baby is determined by the father, not the mother. The female has XX sex chromosomes, so all ova carry only X. The male has XY chromosomes, so sperms are of two types: X-bearing and Y-bearing. If an X-sperm fertilises the ovum, the child is XX and a girl; if a Y-sperm fertilises it, the child is XY and a boy. Therefore, the mother does not determine whether the baby will be a daughter or a son.

In a human female, generally one egg is released by the ovary in a month.

• If the mother gave birth to identical twins, they develop from one ovum fertilised once and then the embryo splits, so the answer would still be 1 egg.
• If the twins were fraternal twins, they develop from two different ova fertilised separately, so 2 eggs would have been released.

So, the answer changes for fraternal twins but not for identical twins.

A female dog that gave birth to 6 puppies would have released about 6 eggs, because each puppy generally develops from a separate ovum fertilised by a sperm. In a litter, the number of eggs released usually corresponds to the number of offspring produced.