The male reproductive system is composed of many components. Rete testis can be best defined as the tubules that carry sperms from the seminiferous tubules to efferent ducts. They are located at the cranial pole of the testis. It helps to mix the sperm cells in the fluid secreted by Sertoli cells.

Define menstrual cycle. Describe the various phases of menstrual cycle.

The reproductive cycle in the female primates (e.g. monkeys, apes and human beings) is called menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. One ovum is released (ovulation) during the middle of each menstrual cycle. The cycle starts with the menstrual phase, when menstrual flow occurs, and it lasts for 3-5 days. The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other underlying causes like stress, poor health etc. The menstrual phase is followed by the follicular phase. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy all events of the menstrual cycle stop and there is no menstruation. In the absence of fertilisation, the corpus luteum degenerates.

What is oogenesis? With the help of suitable diagram, describe the steps involved in this process.

The process of formation of a mature female gamete is called oogenesis which is markedly different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles. The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa. It is important to draw your attention that it is at this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte forms a new membrane called zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.

What is fertilisation? State its significance.

Fertilisation: Fertilisation is a process in which a haploid male gamete (sperm) fuses with a haploid female gamete (egg cell) to form a diploid zygote. In humans, fertilisation occurs in the fallopian tube of the female reproductive tract. Significance of fertilisation: Fertilisation restores the diploid number of chromosomes in a zygote. This process helps in the determination of the sex or gender of the foetus. It helps in the combination of the characters of two parents results in genetic variation among the offspring. This phenomenon has a significant role in evolution.

How is polyspermy prevented during fertilisation?

Polyspermy is a phenomenon in which the ovum is fertilized by more than one sperm which should be prevented to maintain the diploid status of the fertilized egg (zygote). Once the sperm enters the ovum, the entry of other sperms is prevented either by increasing concentrations of cytoplasmic Ca 2+ or by the formation of a fertilization membrane by cortical granules. The egg membrane becomes thick during the cortical reaction due to cortical granule exocytosis. Thus, monospermy is maintained and polyspermy is prevented.