Question

Describe the process of spermatogenesis including cells involved and time length.

Answer 1

Ans :

Stages of spermatogenisis :  

Spermatogenesis is the process of the gradual transformation of germ cells into spermatozoa. It occurs mainly within the seminiferous tubules of the testes and can be divided into three phases, each of which is associated with different germ cell types:

1.Proliferative phase: spermatogonia - spermatocytes

2.Meiotic phase: spermatocytes -   spermatids

3.Differentiation phase (also known as spermiogenesis): spermatids -   spermatozoa

1.Proliferation phase

Stem or A spermatogonia located in the basal region of the tubular epithelium undergo mitosis. The progeny of these divisions maintain their own numbers as well as giving rise to several interconnected B spermatogonia (the number of these arising from a single A spermatogonia is species dependent). B spermatogonia divide to give rise to primary (1^o) spermatocytes. All descendants of a B spermatogonium remain connected by cytoplasmic bridges, forming a syncytium - like cell clone which undergoes synchronous development.

2.Meiotic phase

Each 1^o spermatocyte divides to give rise to two short-lived secondary (2^o) spermatocytes, which in turn give rise to two spermatids each. The spermatids contain a haploid number of chromosomes (half the number of a somatic cell). 1^o spermatocytes are the largest cells in the spermatogenic series and are located approximately midway within the seminiferous epithelium.

The process of meiosis occurs over a long period, with prophase of the first meiotic division taking up to three weeks

3.Differentiation phase

This phase is also known as spermiogenesis.

Spermatids undergo transformation into spermatozoa. Many changes occur within the cells, the three major ones being:

i) formation of the acrosome, which covers the cranial part of the head. The acrosome will contain hydrolytic enzymes to allow fusion of sperm and egg for fertilisation.

ii) condensation of nuclear chromatin in the head to form a dark-staining structure

iii) growth of the tail opposite the acrosome, and loss of excess cytoplasmic material which is shed as a residual body. The body is phagoctosed by the Sertoli cells

Spermatogenesis is controlled by a complex feedback mechanism involving the hypothalamus, anterior pituitary and testes. Gonadotrophic releasing hormone (GnRH) is released by the hypothalamus in a pulsatile manner and travels through portal vessels to the anterior pituitary, where it acts of the gonadotrophic cells. These cells respond to the stimulation by producing either follicle stimulating hormone (FSH) or luteinising hormone (LH) depending on the pattern of GnRH secretion.