Veterinary Physiology-III

SPERMIOGENESIS

• Transformation of round, non motile spermatids into potentially spermatozoa by a series of progressive morphologic changes is called as spermiogenesis. These changes include
  • Development of an organelle, the acrosome that allows penetration of the oocyte
  • Development of mitochondria to furnish energy during movement in the female tract
  • Formation of a tail to aid movement within the female reproductive tract
• A considerable amount of the cytoplasm is extruded from the cell during spermatogenesis. Remnants of the cytoplasm are sometimes observed attached to the spermatozoa and are referred to as cytoplasm droplets.
• The presence of cytoplasmic bridges that have held developing clones of spermatozoa together during development disappear at the end of spermiogenesis.
• Spermiogenesis starts in the seminiferous tubules and completes in the epididymis. Spermiogenesis occurs within the cytoplasm of Sertoli cells and then released into the tubular lumen
• The various stages of development of spermatid transformation of are divided into 4 phases

Golgi phase

• Formation of proacrosomal granules within the golgi apparatus
• Coalescence of the granules into a single acrosomal granule
• The adherence of the resultant acrosomal granule to the nuclear envelope
• The early stages of tail development at the pole opposite that of the adherence of the acrosomal granules
• The proximal centriole migrates closer to the nucleus, where it is thought to form a basis for attachment of the tail to the head.

Cap phase

• Spreading of the adherent acrosomal granule over the surface of the spermatid nucleus. This process continues nearly two third of the anterior portion of the each spermatid is covered by a thin double layered membranous sac that closely ache adheres to the nuclear envelop.
• The developing axonemal (central core of tail 9+2) components of the tail which are formed from the elements of distal centriole elongated well beyond the periphery of the cytoplasm

Acrosomal phase

• The acrosomal phase of spermiogenesis is characterized by the major changes in the nuclei, the acrosome and the tail of developing spermatids
• The developmental changes are facilitated by rotation of each spermatid so that the acrosome is directed towards the basement or the outer wall of the seminiferous tubule and the tail towards the lumen.
• The nuclear changes include condensation of the chromatin into dense granules and reshaping of the spheroid nucleus into an elongated flattened structure.
• The acrosomes which is closely adherent to the nucleus, also condenses and elongates to correspond to the shape of the nucleus. These modification in nuclear and acrosomal shape appear to be molded by the surrounding sertoli cells
• The cytoplasm is displaced to the caudal aspect of the nucleus where it surrounds the proximal portion of the developing tail within this cytoplasm, microtubules associate to form a temporary cylindrical sheath.
• The annulus (ring like structure) first forms near the proximal centriole and then during subsequent development migrates posterior along the tail.
• The mitochondria which were previously distributed throughout the cytoplasm of the spermatid begin to concentration close to the axoneme where they form the sheath that characterizes the middle piece of the tail.

Maturation phase

• It involves the final transformation of the elongated spermatids into the cells to be released in the lumen of the seminiferous tubules
• Sperm head (nucleus and acrosome) becomes its elongated shape characteristic of each species
• Chromatin condensation reaches the final stage
• A fibrous sheath and underlying nine coarse fibres are formed around the axoneme the fibrous sheath covers the axoneme from the neck to the beginning of end piece.
• The annulus migrates distally from its position adjacent to the nucleus along the tail to a point where it will subsequently separate the middle piece from the principle piece of the tail
• The mitochondrion become tightly packed into a continuous sheath extending from the neck to the annulus.
• The remaining cytoplasm forms a residual body and the sperm is ready for release.

Click here for structure of sperm