Spermatogenesis - Human Reproduction Notes - Class 12

GAMETOGENESIS
The process of formation of haploid gametes in gonads for sexual reproduction by meiotic division.

SPERMATOGENESIS
It takes place in seminiferous tubules in 3 stages:
(i) Spermatocytogenesis
(ii) Meiosis I and Meiosis II
(iii) Spermiogenesis (spermateleosis).

Fig: Process of Spermatogenesis

• First two stages are continuous involving divisions in the primordial germ cells spermatogonia (12 µm size), while the last stage involves no division but only morphological changes.
• The primordial germ cells along the wall of seminiferous tubules (germinal epithelium) are called the spermatogonia. These diploid (2n) cell divide mitotically many times giving rise to type A spermatogonia and type B spermatogonia, the former serve as stem cells while later differentiate into primary spermatocyte (2n) becoming double in size;
• A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. What would be the number of chromosome in the spermatids?

SPERMIOGENESIS 
The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called spermiation.

SPERMIOGENESIS (SPERMTELEIOSIS)
The spermatids separate from each other; flagellum is formed and the organelles get rearranged forming the parts like head, with cap (acrosome) and tail.

HORMONAL CONTROL OF SPERMATOGENESIS
Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH). This, if you recall, is a hypothalamic hormone. The increased levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinising hormone (LH) and follicle stimulating hormone (FSH). LH acts at the Leydig cells and stimulates synthesis and secretion of androgens.
Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

Testosterone - Secreted by Leydig cells.
It is essential for:
(i) Sperm production
(ii) development of secondary sexual characters

Structure of Spermatozoa - Need diagramme of sperm
Structure of sperm has three parts - Head, neck, Middle piece, Tail

Fig: Spermatozoa

• Head(nucleus and Acrosome) - nucleus  is flat and oval in human sperm. It is composed of a large posterior nucleus (Nucleus of the sperm is very small)At the base of the nucleus in a pit like depression proximal centriole is present.
  Acrosome - Acrosome is formed from the Golgi complex. It contains digestive enzyme “spermlysin”. It is the caplike covering above the nucleus. It is surrounded by double membrane. Acrosome plays important role in penetration of ovum by sperm. 
• Neck - In between the head and the middle piece a small neck is present. Neck consists of both Proximal and distal contriole. Both the centrioles are at right angles to each other. Proximal centriole first induce cleavage in a fertilized egg. First spindle fibre forms from it. Distal centriole gives rise to the axial filament of the sperm. It has (9+2) microtubular arrangement.
• Middle piece - This is known as the energy-chamber of the sperm. Many mitochondria spirally surround the axonema, this is called “Nebenkern sheath”. This part provides energy to the sperm for locomotion.
  In middle-piece, cytoplasm is found in the form of a thin-sheet called Manchett.
• Tail - The longest and the fibrous part of the sperm is termed its tail.
  Sperm moves with the help of its tail.