Gametogenesis
The primary sex organs, the testes in males and ovaries in females, produce gametes, i.e., sperm and ovum respectively, through the process called gametogenesis. In the testes, the immature male germ cells called spermatogonia produce sperm by the process of spermatogenesis, which begins at puberty. The spermatogonia present in the inner wall of the seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium is diploid and contains 46 chromosomes. Some of the spermatogonia periodically differentiate and develop into primary spermatocytes. A primary spermatocyte undergoes the first meiotic division, or meiosis-I, which is a reduction division and results in the formation of two equal haploid secondary spermatocytes, each containing 23 chromosomes. The secondary spermatocytes undergo the second meiotic division, or meiosis-II, to produce four equal haploid spermatids. Thus, one primary spermatocyte gives rise to four spermatids. The spermatids subsequently undergo differentiation and transform into mature spermatozoa or sperms by a process called spermiogenesis. The spermatozoa are finally released from the seminiferous tubules by a process called spermiation.
Diagram: Seminiferous Tubules
Spermatogenesis begins at puberty because of a significant increase in the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins, luteinising hormone (LH) and follicle-stimulating hormone (FSH). LH acts on the Leydig cells and stimulates the synthesis and secretion of androgens. The androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates the secretion of certain factors which support and help in the process of spermatogenesis.
Structure of a Sperm
A sperm is a microscopic structure consisting of a head, neck, middle piece and tail. A plasma membrane surrounds the entire sperm. The head contains an elongated haploid nucleus containing the chromosomal material. The anterior part of the head is covered by a cap-like structure called the acrosome. The acrosome contains enzymes that help the sperm in the process of fertilisation of the ovum. The middle piece contains numerous mitochondria, which produce the energy required for the movement of the tail. The tail provides motility to the sperm, which is essential for fertilisation. A human male ejaculates approximately 200–300 million spermatozoa during a single ejaculation. For normal fertility, at least 60 per cent of the sperm should have normal shape and size, while at least 40 per cent should show vigorous motility.
Diagram: Structure of a Sperm
The spermatozoa released from the seminiferous tubules are transported through the accessory ducts. The secretions of the epididymis, vas deferens, seminal vesicles and prostate gland are important for the maturation and motility of sperm. The sperm together with the seminal plasma constitute the semen. The functions of the male reproductive ducts and accessory glands are maintained by the testicular hormones called androgens.
Oogenesis
The process of formation of a mature female gamete is called oogenesis and it is markedly different from spermatogenesis. Oogenesis begins during the embryonic development of the female. During fetal development, a large number of female germ cells called oogonia are formed in each fetal ovary. No new oogonia are formed or added after birth. These cells multiply by mitotic division and enter prophase-I of meiosis, where they remain temporarily arrested. These cells are called primary oocytes. Each primary oocyte becomes surrounded by a layer of granulosa cells and forms a primary follicle. A large number of primary follicles degenerate from birth to puberty. Consequently, at puberty, only about 60,000–80,000 primary follicles remain in each ovary.
Diagram: Ovary
The primary follicles become surrounded by additional layers of granulosa cells and develop into secondary follicles. The secondary follicle subsequently develops into a tertiary follicle, which is characterised by the presence of a fluid-filled cavity called the antrum. The thecal layer becomes organised into an inner theca interna and an outer theca externa. During this stage, the primary oocyte present within the tertiary follicle increases in size and completes its first meiotic division. This is an unequal division and results in the formation of one large haploid secondary oocyte and a tiny haploid first polar body. The secondary oocyte retains most of the cytoplasm and nutrients of the primary oocyte.
The first polar body may undergo further division, although its exact fate is not completely certain. The tertiary follicle subsequently develops into the mature follicle called the Graafian follicle. The secondary oocyte becomes surrounded by a new membrane called the zona pellucida. The mature Graafian follicle finally ruptures and releases the secondary oocyte from the ovary. This process is called ovulation.
Gametogenesis: Spermatogenesis and Oogenesis
In spermatogenesis, the diploid spermatogonium contains 46 chromosomes and gives rise to a primary spermatocyte. The primary spermatocyte undergoes meiosis-I to form two haploid secondary spermatocytes, each containing 23 chromosomes. Meiosis-II produces four haploid spermatids, which differentiate into spermatozoa. In oogenesis, the diploid oogonium develops into a primary oocyte containing 46 chromosomes. The primary oocyte undergoes meiosis-I to form a large haploid secondary oocyte containing 23 chromosomes and a small first polar body. The secondary oocyte is the cell released during ovulation and completes its meiotic process in association with fertilisation.
Thus, one primary spermatocyte produces four haploid spermatozoa, whereas one primary oocyte gives rise to one large functional female gamete along with polar bodies. The unequal division during oogenesis allows the secondary oocyte to retain most of the cytoplasm and nutrients of the original primary oocyte.
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