SPERMATOGENESIS AND SPERMIOGENESIS DEFINITION

SPERMATOGENESIS

In multicellular organisms the reproductive process commences with the production of gametes. The gametes are the sex cells that develop inside the gonads, the testes in males and ovary in females.

The term gametogenesis includes both spermatogenesis and oogenesis.

In vertebrates as well as in higher invertebrates the testis, the site of production of sperm are composite organs, consisting of a number of seminiferous tubules. Sections of the testis would reveal the seminiferous tubules with sperm development in different stages. This means that the spermatogenesis is a continuous process and one can observe the various developmental stages of sperm in a single testis.

Spermatogenesis can be broadly divided into three phases

(1) multiplication phase

(2) growth phase

(3) maturation phase

1. Multiplication phase

The initial cells in the germ line are known as primordial germ cells (PGC). The PGC which arise at some distance from the prospective gonads migrate into them and are known as stem cells. The stem cells divide by mitosis and give rise to spermatogonial cells. The multiplication phase marks the production of a number of spermatogonia inside the testis.

Stages in spermatogenesis

                   

2. Growth Phase

Growth phase is characterised by the acquisition of the structural and functional characteristics of distinct sex cells. Also there is a pronounced increase in the size of spermatogonial cells which are now known as primary spermatocytes.

3. Maturation Phase

The maturation phase characterises the transformation of diploid primary spermatocytes into haploid spermatids. The primary spermatocyte divides meiotically to produce two secondary spermatocytes cach with a haploid nucleus. Each of the secondary spermatocytes undergoes second meiotic division to produce two haploid spermatids. Each spermatid undergoes an intracellular differentiation to produce a spermatozoa. The process of intracellular differentiation of a spermatid into a spermatozoa is known as spermiogenesis.

seminiferous tubules in a human testis

SPERMIOGENESIS

At the end of the meiosis the spermatids appear as simple spherical cells with a centrally located nucleus. Their differentiation into sperm requires an extensive morphological transformation. The various cellular organelles like mitochondria, golgi body and centrosomes contribute to such a transfor The first step involves the formation of an acrosomal structure from the golgi body. The acrosome lies proximal to the nucleus and forms a cap over the sperm nucleus. As the cap is formed the nucleus rotates and the acrosomal cap faces the basal membrane of the seminiferous tubule. This rotation is essential because the flagellum is to arise from the centriole on the other side of the nucleus, Subsequently the nucleus flattens and condenses and there is a loss of cytoplasm. The mitochondria tend to form a ring around the base of the flagellum and become the neck region of the sperm. The fully formed sperm enters the lumen of the seminiferous tubule.

In humans it takes 74 days to complete the process of spermatogenesis.