Que. Write an essay on Spermatogenesis?

Ans. Definition:  Formation of sperms is known as spermatogenesis.  It is  divided into two stages- formation of spermatids and spermiogenesis

 

1. FORMATION OF SPERMATIDS

This phase of spermatogenesis is further subdivided into three phases.

 

1. Multiplication phase:

The wall of the seminiferous tubule contains germinal epithelium which devides mitotically to form diploid spermatocytes

 

2. Growth phase:

During this phase, growth of spermatogonia takes place; their volume becomes double and they are now called primary spermatocytes which are still diploid in number.

 

3. Maturation phase:

Each primary spermatocyte enters into prophase of first meiotic division or maturation division which is followed by cytokinesis (cytoplasmic division). It results in the formation of two haploid secondary spermatocytes. Both secondary spermatocytes undergo Meiosis-II and produce spermatids. Thus, from each primary spermatocyte, four haploid spermatids are formed.

 

 

2. SPERMIOGENESIS:

The spermatids produced at the end of maturation phase are still not capable of functioning as male gametes. They have to undergo a process of differentiation. The process of gradual differentiation of stationary (non-motile) and round spermatid into active (motile) and elongated spermatozoon is called as Spermiogenesis.

 

It involves the following changes –

 

i.  Changes in the nucleus: the nucleus shrinks by losing water, RNA, nucleolus and the chromatin becomes closely packed. The spherical nucleus becomes elongated and narrow.

 

ii. Changes in the centriole: The spermatid contains two centrioles lying at right angles to each other. During spermiogenesis, they move and come to lie behind the nucleus.

One of them, called as proximal centriole enters the depression developed in the posterior part of the nucleus. The other one, called as distal centriole lies behind the proximal centriole and gives rise to the axial filament of the flagellum for which it serves as a basal granule.

 

iii. Changes in mitochondria: Mitochondria from different parts of the spermatid concentrate around the proximal part of the axial filament and form the middle piece of the sperm. Gradually, they fuse together forming two densely packed bodies one on each side of the axial filament. These get spirally twisted around the axial filament forming a sheath called as nebenkern in mammals.

 

iv. Acrosome Formation: The Golgi apparatus of an early spermatid consists of a series of cisternae concentrically around an aggregation of small vacuoles. One or more vacuoles enlarge, inside which a small dense body called proacrosomal granule develops and all of these fuse to form the Acrosomal Granule which forms the of the spermatozoon.

 

Structure of Spermatozoon

The structure of a typical mammalian sperm is described below. It is differentiated into 3 parts – Head, Middle Piece and Tail.

 

1. Head: It may be ovoid, flattened or spirally twisted in shape. It contains two parts viz.,

Acrosome and nucleus.

a. Acrosome: It is the anterior part of the head and continues on the sides of the nucleus. It contains a number of enzymes such as acid phosphatase, cathepsin and hyaluronidase which help the sperm in penetrating through the egg membrane.

b. Nucleus: It occupies most of the head. It contains the haploid component of DNA and is devoid of nucleolus, RNA and fluid contents.

2. Middle Piece: It is the middle part of the sperm and is connected to the head with a neck. It consists of 4 parts – centriole, mitochondria, manchette and ring centriole.

a. Centriole: Inside the neck, there are two centrioles. The anterior or proximal centriole is present in the posterior depression of the nucleus. During fertilization, it is donated to the egg and helps in the formation of the first mitotic spindle in the developing zygote during cleavage. Behind this, the posterior or distal centriole is present from which the axoneme of the tail sperm develops.

b. Mitochondria: They form the main part of the middle piece around which they are spirally twisted.

c. Manchette: It is the remainder of cytoplasm present around the mitochondrial sheath.

d. Ring centriole: The plasma membrane at the posterior end of muddle piece thickens

to form a boundary between the mid-piece and tail called Jensen’s Ring, Annulus or Ring centriole. It prevents mitochondria form slipping off into the tail.

3. Tail: It is the longest part of the sperm body. It has 9+2 microtubular axial filament. It helps in movement of the sperm during fertilization

 

Factors Affecting Spermatogenesis

This process is affected by minute changes in the hormone levesl. For example- testosterone is developed through the hypothalamus, Leydig cells, and pituitary gland. This process is very sensitive to changes in temperature, deficiency in the diet, alcoholism, exposure to drugs and the presence of disease can affect the rate of sperm formation adversely.