Gametogenesis

The process of formation of gametes (sperm and ovum) by meiosis in the primary sex organs i.e. testis in males and ovaries in females is called gametogenesis. The formation of sperm by the testis is called spermatogenesis and the formation of the ovum by ovaries is called oogenesis. 

Spermatogenesis

The meiotic process by which the four haploid spermatids having 23 chromosomes are produced from the diploid male germ cells having 46 chromosomes inside the seminiferous tubules of the testis is called spermatogenesis. It begins at the time of puberty. It has the following phases;

1. Multiplication Phase: The germ cells present on the inside wall of seminiferous tubules multiply by mitotic division to form a large number of spermatogonia (diploid-46 chromosomes).
2. Growth Phase: Spermatogonia grow and increase in size to form a large number of primary spermatocytes (diploid-46 chromosomes).
3. Maturation Phase: Primary spermatocytes undergo 1st meiotic division (reduction division) leading to the formation of 2 secondary spermatocytes (haploid). These secondary spermatocytes undergo the 2nd meiotic division (equational division) to produce 4 haploid spermatids.

The spermatids will then undergo spermiogenesis in which the transformation of spermatids into spermatozoa (sperms) takes place.  After spermiogenesis, sperm heads get embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation. The human male ejaculates about 200 to 300 million sperms during coitus in which, for normal fertility, at least 60 % of sperm cells must have normal shape and size and at least 40 % of them must show vigorous motility.

Structure of Sperm

A mature sperm is a 0.06mm long, microscopic structure whose whole body is enveloped by a layer of the plasma membrane. Its body is composed of a head, neck, middle piece, and tail. 

1. Head: It contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome that is filled with enzymes (Hyaluronidase) that help in the fertilization of the ovum. 
2. Middle piece: It possesses numerous mitochondria, which produce energy for the movement of the tail that facilitates sperm motility. 
3. Tail: It helps in sperm movement.

Hormonal Control of Spermatogenesis or the Male Reproductive System

When a male reaches puberty the process of spermatogenesis starts due to the significant increase in the secretion of the Gonadotropin-releasing hormone (GnRH), which then acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins; luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

1. LH (luteinizing hormone): It acts at the Leydig cells and stimulates the synthesis and secretion of androgens; the testosterone. Androgens, in turn, stimulate the process of spermatogenesis.
2. FSH (follicle stimulating hormone): It acts on the Sertoli cells and stimulates the secretion of androgen binding factors and inhibin. Androgen binding factor binds with androgen which helps in the process of spermiogenesis. Whereas inhibin inhibits the secretion of LH and FSH when not required. 

Oogenesis

The process by which a mature female gamete or ovum is formed is called Oogenesis. Unlike making gametogenesis this process gets initiated during the embryonic development stage and includes the following phases;

1. Multiplication Phase: At the fetal stage, about 2 million diploid gamete mother cells (oogonia) are formed within each ovary and then no more oogonia are formed after birth.
2. Growth Phase: Oogonia undergo meiotic division and enter into prophase-I where it gets temporarily arrested and is called the 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 leaving only 60000 to 80000 primary follicles in each ovary. The primary follicles develop into secondary follicles by getting surrounded by more layers of granulosa cells and a new theca. The secondary follicle then develops a fluid-filled cavity called an antrum thus becoming the tertiary follicle. Then, the primary oocyte within the tertiary follicle grows in size.
3. Maturation Phase: The primary oocyte then completes its 1st meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle changes into the mature Graafian follicle and the secondary oocyte form 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. 

Structure of Ovum/human egg

The ovum is a round, non-motile, and haploid cell that has a centrally located nucleus and dense cytoplasm. The cytoplasm stores food material required for the entire process of development. The ovum has four layers of covering; the plasma membrane, the Vitelline membrane, the zona pellucida, and the corona radiata.

Hormonal Control of Oogenesis

The hypothalamus secretes the GnRH to stimulate the pituitary gland for the secretion of luteinizing hormone and follicle-stimulating hormone. LH stimulates the corpus luteum to secrete progesterone which when rises to undesirable levels inhibits the release of GnRH thus stopping the whole process. FSH stimulates the formation of estrogen and the development of the primary oocyte to form a secondary oocyte. 

NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic information from each parent, resulting in the formation of a zygote.