Spermatogenesis

The process of spermatogenesis occurs in the seminiferous tubules of the testes.

The whole developmental process of proliferation of spermatogonia or the sperm mother cells to mature spermatozoa or sperm is called as spermatogenesis. The functional spermatozoa or sperms have haploid number of chromosomes.

A Part of Testis Enlarged to show Different Stages of Spermatogenesis and Spermiogenesis

Photomicrographs of Section through the Human Testis showing Seminiferous Tubules and Leydig (interstitial) Cells

The process of spermatogenesis is divided into the following phases

• Multiplication phase

• Growth phase

• Maturation phase

• Metamorphosis of spermatid

Multiplication Phase

Multiplication phase also known as Spermatocytogenesis. The sperm mother cells present in the germinal epithelium of the seminiferous tubules divide repeatedly by mitosis to form large number of diploid rounded sperm mother cells which are called as spermatogonia. Some of these sex cells move towards the lumen of seminiferous tubules and enter the growth phase. These cells are called primary spermatocytes. The primary spermatocytes are diploid and contain (44 + XY) chromosomes.

Some of the sex cells produced by the division of spermatogonia remain in their original condition and continue to divide giving rise to primary spermatocytes. Such cells are known as stem cells.

Growth Phase

During this phase the spermatocyte as well as its nucleus enlarges in size. It gets ready to undergo maturation division.

Maturation Phase

Each diploid primary spermatocyte undergoes meiosis I, which is a reduction division. Two daughter cells are formed each with 'n' number of chromosomes. The daughter cells are called secondary spermaotcytes. The secondary spermatocytes are haploid and much smaller comparitively, containing (22+X) or (22+Y) chromosomes.

The secondary spermaotcytes undergo the second meiotic division (equational). This results in the formation of four daughter cells known as spermatids.

The Process of Spermatogenesis and Spermiogenesis in Man

Metamorphosis or Spermiogenesis

The spermatids formed as a result of maturation division is a typical animal cell with all the cell organelles present in it. In this form it cannot function as a male gamete. So many changes take place to change the non-motile spermatid into motile spermatozoan. The main aim of the changes is to increase the motility of the sperm. The changes are

• The nucleus shrinks by losing water and DNA becomes closely packed.

• An acrosome is formed from the golgi complex.

• An axial filament of the tail of the spermatozoan is formed from the distal centriole of the spermatid.

• A mitochondrial ring is formed from the mitochondria around the distal centriole and is called as nebenkern.

• Much of the cytoplasm of the spermatid is lost and the remaining cytoplasm forms a sheath around the mitochondrial spiral. This sheath is known as manchette.

During the process of differentiation the developing sperms have their head embedded in the sertoli cells (rurse cells) from which they obtain their nourishment.

Spermatogenesis is controlled by FSH (follicle stimulating hormone) and the gonadotropins.

In man, there are two types of sperms produced. 50% of the sperms have the X chromosome and are called as gynosperms. 50% of the sperms have the Y chromosome and are called as androsperms.