What is spermatogenesis and how does it occur


The below article will provide you with the detail information about spermatogenesis. You will find the complete procedure of spermatogenesis. You will also find the different phases of spermatogenesis.

Introduction

The process by which gametes are produced by the gonads is called gametogenesis. The process of sperm formation is known as spermatogenesis and the process of formation of the ovum is called oogenesis . The gametes are the only link between generations and their nuclear DNA is the depository of all those genetic information which are required for the development of the new organism. During the process of gametogenesis, the germ cells undergo meiosis so that in gametes the number of chromosomes is reduced to half and gametes have the haploid number of chromosomes.

Spermatogenesis is the formation of sperms from the germinal epithelium of the seminiferous tubules of testes. The germinal epithelium contains cuboidal primordial germ cells and tall somatic cells called Sertoli cells or nurse cells. The Sertoli cells anchor the differentiating cells and provide nourishment to the developing sperms. Spermatogenesis complete in two phases:

  • Formation of spermatids
  • Formation of sperms or metamorphosis of spermatid



spermatogenesis
(Image courtesy: www.wikipedia.org)

Formation of spermatids

The formation of spermatids completes in the following three phases:
  • Phase of multiplication : The sperm mother cells or spermatogonia divide repeatedly by mitosis forming new sex cells. Some of these cells move towards the lumen of seminiferous tubules and enter to growth phase known as primary spermatocytes and some cells produced this way remain in their original condition and continue to divide giving rise to primary spermatocytes such cells are known as stem cells.
  • Phase of growth : In this phase, spermatocytes and their nuclei enlarge in size
  • Phase of maturation : Each primary spermatocyte undergoes first maturation division which is reductional division and two daughter cells are formed each with the haploid number of chromosomes. These daughter cells are called secondary spermatocytes. Secondary spermatocytes undergo second maturation division which is simple mitotic division and this way four daughter cells are formed which are known as spermatids. Thus from each primary spermatocyte, four spermatids are formed


Formation of sperms or metamorphosis of spermatid

The transformation of spermatids into spermatozoa is called spermiogenesis. The spermatozoa are then known as sperms. Spermatids formed as the result of maturation divisions are like typical haploid animal cell and in this form cannot function as the male gamete. During the process of metamorphosis, a non-motile spermatid is converted into motile sperm by following changes:
  • Spermatids increase lengthwise in size.
  • The nucleus shrinks by losing water from nuclear sap and assumes different shapes in different animals. The shape of the nucleus determines the shape of the sperm head. The RNA contents of the nucleus are greatly reduced and DNA becomes more concentrated and chromatid becomes closely packed.
  • The two centrioles of the spermatid get arranged one after the other behind the nucleus. The anterior one is called the proximal centriole and the posterior as distal centriole. The distal one changes into the basal body and gives rise to the axial filament of the sperm.
  • The mitochondrial spiral is formed around the axial filament. This mitochondrial sheath is called nebenkern which remains confined to the middle piece of the sperm.
  • The Golgi complex undergoes some changes to form acrosome which covers the nucleus as a cap. During acrosome formation, the Golgi complex gets concentrated near the anterior end of the nucleus and soon forms a dense pro-acrosomal granule which gets attached to the anterior end of the nucleus and enlarges to form acrosome.
  • The cell membrane of the spermatid remains as an envelope over the entire mature sperm including its tail.


All these changes together result in a typical sperm containing the head, middle and tail. During the process of differentiation and development, sperms have their head embedded in the Sertoli cells from which they get nourishment.

Conclusion

So in the process of spermatogenesis, a spermatocyte converted into four sperms. The process of spermatogenesis takes place in the testes and starts at the age of 12 years. The testes of human being form sperms lifetime. The process of spermatogenesis takes place in the presence of GTH (Gonadotropic hormone) secreted by Pituitary gland .


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