Spermiogenesis

Spermatid turns to spermatozoon. Spermatid has organelles of typical mammalian cell ie spherical nucleus, Golgi complex, mitochondria, paired centrioles and endoplasmic reticulum. Granules synthesised in the Golgi complex fuse forming a large acromosal vesicle. When this covers the anterior aspect of the condensed nucleus its referred to as the acrosome. Centrioles which migrate to the pole of the nucleus opposite the acrosome form the axial filament from which the tail of the spermatozoon develops. Mitochondria aggregate in the proximal region of the filament forming the middle piece of the spermatozoon. Excess portions of cytoplasm shed from spermatids are collectively known as residual bodies. The cytoplasmic divisions of spermatogonia are incomplete as spermatocytes remain attached by cytoplasmic bridges. Spermatogenic cells develop in close association with Sertoli cells in the seminiferous tubules. The germ cells are almost completely surrounded by the cytoplasm of sertoli cells which nourish and support them during differentiation. Tight junctions between sertoli cells divide seminiferous tubules into basal and adluminal compartments thereby preventing entry of cells involved in immunological response. At the completion of spermiogenesis immature spermatozoa are extruded from their intimate relationship with sertoli cells into the lumen of the seminiferous tubules = spermiation.  Prior to their release most of the cytoplasm of the spermatozoa is shed and phagocytosed by sertoli cells. A small amount of cytoplasm (the protoplasmic droplet) remains attached to the middle of the immature spermatozoon. In the seminiferous tubules the spermatozoa are immotile but carried passively by the tubular fluid to the rete testis. From here they are conveyed by efferent ductules to the epididimys through ciliary action of duct epithelium and contractions of smooth muscle in the duct wall. The epididymis which is a long tightly convoluted tube is anatomically divided into three regions, head, body and tail. During their passage spermatozoa undergo a maturational process which confers on them the ability to fertilise an ovum. As they mature they undergo a number of morphological and physiological changes. Including alterations in nuclear chromatin, changes in the composition of plasma membrane and loss of the protoplasmic droplet. They acquire the ability to propel themselves forward. Some seminiferous and duct fluid is absorbed resulting in an increased concentration of spermatozoa in the remaining fluid. Mature spermatozoa capable of fertilisation are stored in the tail of the epididymis prior to ejaculation. Most of the unejaculated spermatozoa are gradually discharged into the urinary system, a small percentage which remain in the epididymis undergo degenerative change and are phagocytosed. The transport of spermatozoa through the epididymis due to contractions of the smooth muscle of the epididymal duct wall takes up to 12 days in the bull and ram and up to 14 in the boar and stallion. With increased frequency of ejaculation, transport time may be reduced.

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