GAMETOGENSIS - SPERMATOGENESIS

STD-12      UNIT-6     CHA-3

HUMAN REPRODUCTION

GAMETOGENSIS - SPERMATOGENESIS

GAMETOGENSIS

• Gametogenesis is the process by which male and female sex cells or gametes , i.e .. sperms and ova are formed respectively in the male and female gonads ( testes and ovaries ) . 
• The gametes differ from all other cells ( = somatic cells ) of the body as their nuclei contain only half the number of chromosomes found in the nuclei of somatic cells . 
• Meiosis forms the most significant part of process of gametogenesis . Gametogenesis for the formation of sperms is termed as spermatogenesis , while that of ova is called oogenesis . 
• Both spermatogenesis and oogenesis comprise similar phases of sequential changes viz , 
• multiplication phase 
• growth phase and 
• maturation phase . 

SPERMATOGENESIS

• The process of formation of sperms is called spermatogenesis . 
• It occurs in the seminiferous tubules of the testes . 
• The seminiferous tubules are lined by germinal epithelium The germinal epithelium consists largely of cuboidal primary or primordial germ cells (PGC ) and contains certain tall somatic cells called Sertoli cells ( = nurse cells ) . 
• Spermatogenesis includes formation of spermatids and formation of spermatozoa .

Formation Of Spermatids

• Multiplication Phase . 
• At sexual maturity , the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia ( Gr. sperma = seeds , gonna generation ) . 
• Spermatogonia ( 2N ) are of two types : type A spermatogonia and Type B spermatogonia 
• Type A spermatogonia serve as the stem cells which divide form additional spermatogonia . 
• Type B spermatogonia are the precursors of sperms
• Growth Phase .
• Each type B spermatogonium actively grows to a larger primary spermatocyte by obtaining a nourishment from the nursing cells .
• Maturation Phase . 
• Each primary spermatocyte undergoes two successive divisions , called maturation divisions . 
• The first maturation division is reductional or meiotic Hence , the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes . 
• Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form , four haploid spermatids , by each primary spermatocyte . 

• Formation of Spermatozoa from Spermatids ( Spermiogenesis ) . 
• The transformation of spermatids into spermatozoa is called spermiogenesis or spermateliosis . 
• The spermatozoa are later on known as sperms . 
• Thus four sperms are formed from one spermatogonium . 
• After spermiogenesis sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation . 

• Hormonal Control of Spermatogenesis . 
• Spermatogenesis is initiated due to increase in gonadotropin - releasing hormone ( GnRH ) by the hypothalamus . 
• GnRH acts on the anterior lobe of pituitary gland to secrete luteinizing hormone ( LH ) and follicle stimulating hormone ( FSH ) . 
• LH acts on the Leydig's cells of the testes to secrete testosterone 
• FSH acts on Sertoli cells of the seminiferous tubules of the testes to secrete an androgen binding protein ( ABP and inhibin . ABP concentrates testosterone in the seminiferous tubules . 
• Inhibin suppresses FSH synthesis . 
• FSH acts on spermatogonia to stimulate sperm production . 

• Significance of Spermatogenesis . 
• During spermatogenesis , one spermatogonium produces four sperms . 
• Sperms have half the number of chromosomes . After fertilization , the diploid chromosome number is restored in the zygote .
• maintains the chromosome number of the species 
• During meiosis I crossing over takes place which brings about of the basic relationship of the organisms . Variation 
• Spermatogenesis occurs in various organisms . 
• Thus it supports the evidence of the basic relationship of the organisms

Spermatozoon ( Sperm ) 

• The sperms are microscopic and motile cells .
• Sperms remain alive and retain their ability to fertilize an ovum ( egg ) from 24 to 48 hours after having been released in the female genital tract.
• A typical mammalian sperm con sists of a head , neck , middle piece and tail . 

• Head . 
• It contains anterior small acrosome and posterior large nucleus Acrosoma is formed from Golgi body of the spermatid . 
• Acrosome contains hyaluronidase proteolytic enzymes which are popularly known as spermlysins that are used to con tact and penetrate the egg ( ovum ) at the time of fertilization 
• Neck 
• It is very short and is present between the head and middle piece . 
• It contains the proximal centriole towards the nucleus which plays a role in the first cleavage of the zygote and the distal centriole which gives rise to the axial filament of the sperm .
• Middle piece . 
• The middle piece of human sperm contains the mitochondria coiled round the axial filament called mitochondrial spiral . 
• They provide energy for the movement of the sperm . 
• So it is the power house of the sperm 
• At the end of the middle piece there is a ring centriole ( annulus ) with unknown function . 
• Posterior half of nucleus , neck and middle piece of sperm are covered by a sheath called manchette . 
• Tail . 
• The tail is several times longer than the head . In its most part called main piece , the axial filament is surrounded by a thin layer of cytoplasm . 
• The part behind the main piece is called end piece which consists of naked filament alone .
• The sperm swims about by its tail in a fluid medium .

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