Spermatogenesis

Introduction to spermatogenesis

It is the formation of haploid, microscopic and functional male gametes, spermatozoa from the diploid reproductive cells, spermatogonia present in the testes of the male organism.

Period: In the seasonally breeding animals, the testes undergo testicular cycle in which testes and their spermatogenic tissue become functional only in the specific breeding season. So in some seasonally breeding mammals like bat, otter and llama, testes enlarge, become function functional and descend into the scrotum in the building season while become reduced, non functional and ascend in t the abdomen in other season. Spermatogenesis is a continuos process and is completed in about 74 days.

                                                          Picture of spermatogenesis
                       

Formation of Spermatid

Mechanism of spermatogenesis is divides into two parts. These are formation of spermatid and spemioteliosis.  

Formation of spermatid:

It is divided in to three phases.

• Multiplicative phase: It involves the rapid mitotic division of diploid primary or primordial germ cells present in germinal epithelium of the seminiferous tubules of the testes. These cells are undifferentiated and have large and chromatin rich nucleus. This forms large number of diploid and rounded sperm mother cells called spermatogonia, each having a prominent nucleus.
• Growth phase: It is characterised by spermatocytogenesis in which a diploid spermatogonia increases in size by the accumulation of nutritive materials in the cytoplasm and replication of DNA and form diploid primary spermatocyte.
• Maturation Phase: it is characterised by meiosis. The diploid primary spermatocyte undergoes meiosis first and forms two haploid cell secondary spermatocytes, each containing 23 chromosomes.

Spermioteliosis

The Trans formation of a non motile, rounded and haploid spermatid into a function and motile spermatozoan is called spermioteliosis. The main aim is to increase the sperm motility. It involves the following changes.

• Nucleus becomes condensed, narrow and anteriorly pointed due to loss of material like RNAs, nucleolus and most of proteins.
• A part of Golgi body spermatid forms the acrosome, while the lost part of Golgi body is called Golgi rest.
• Centrioles of spermatid form the neck of sperm.
• Distal centriole gives rise to axoneme.
• Mitochondria form a spiral ring behind the neck around the distal centriole and proximal part of axoneme. This is called nebenkern.
• Most of cytoplasm is lost but some cytoplasm forms sheath of tail of sperm.