Reproduction, an important process in human beings involves the fusion of the male and female gametes called sperm and ovum. These gametes are formed by a process called gametogenesis. In males, the process of gametogenesis occurs in the testes and is called spermatogenesis whereas in females, gametogenesis is called oogenesis and occurs inside the ovary.
Interestingly, spermatogenesis and oogenesis begin at different ages. Spermatogenesis begins at puberty, usually between 12 and 13 years of age and is divided into two stages – spermatocytogenesis and spermiogenesis. Spermatocytogenesis, the first stage, occurs in the seminiferous tubules whose inner walls are lined with spermatogonia, the diploid male germ cells containing 46 chromosomes. Some of these spermatogonia divide mitotically to form two daughter cells. Of these two daughter cells, one remains the spermatogonium while the other one becomes the primary spermatocyte. This primary spermatocyte divides meiotically to form two equal cells called secondary spermatocytes. During ejaculation, sperm from the epididymis are transported via the vas deferens to the urethra in the penis. As the sperms pass through the different tubes, it mixes with fluids secreted by the seminal vesicles and prostate gland to form a mixture called semen, which is ejaculated by the penis.
The formation of sperm is triggered when the hypothalamus secretes high levels of gonadotropin-releasing hormone or GnRH. FSH, on the other hand, acts on the Sertoli cells to stimulate the secretion of substances that nurture sperm cells during spermatogenesis. While spermatogenesis occurs at puberty, oogenesis is triggered before a woman’s birth when the germinal epithelium cells in the ovary of the foetus divide mitotically to form gamete mother cells called oogonia. A few oogonia divide again to form two immature cells called primary oocytes. The primary oocyte then gets enveloped by a layer of squamous granulosa cells and together, they make up the primordial follicle. The oocyte further develops as the foetus develops but the granulosa cells do not change till puberty. In fact, primary oocytes enter the prophase of meiotic division and remain suspended till puberty. As the tertiary follicle develops into a mature follicle or Graafian follicle, the antrum grows and the secondary oocyte and surrounding granulosa cells are pushed towards one end of the follicle. The granulosa cells secrete a glycoprotein substance that forms a membrane, the zona pellucida, around the secondary oocyte.
The secondary oocyte or ovum is released from the Graafian follicle when it ruptures due to a surge in LH in the body. This process, called ovulation, causes the oocyte or ovum to be placed in the oviduct, where it awaits fertilisation by sperm. In this way, the ovum and sperm are formed through a long and intricate process called gametogenesis.