Several different types of spermatogonia have been identified A-0 through A-4, intermediate [IN], and B. Only the discriminating anatomist can actually distinguish among types of spermatogonia. Mitosis ends when a B spermatogonium yields two primary spermatocytes. The diploid number of primary spermatocytes is halved during meiosis. A primary spermatocyte is transformed into two secondary spermatocytes during meiosis I - these cells then in turn are converted into 1N spermatids during meiosis II.
The second meiotic division is rapid and therefore very few secondary spermatocytes can be identified in histological sections. Spermatocytes and spermatids tend to be larger than their ancestral spermatogonia. Males have an almost unlimited capacity to produce germ cells; this is accomplished by replenishment of A spermatogonia early in mitosis. Although the mechanics of renewal are not totally understood, it appears that a stem cell A-0 divides into an A-1 spermatogonia and an operative copy of itself.
The A-1 cell becomes dedicated to spermatocytogenesis and the A-0 cell is kept in reserve for future divisions.
Endogenous damage to the the genetic material eg. Throughout spermatocytogenesis cells retain a rounded configuration Figure Spermatids undergo a dramatic change in form during spermiogenesis - into the streamline spermatozoa adapted for fertilization. Spermiogenesis involves nuclear condensation, formation of the acrosomal cap, and development of a tail. The acrosome is derived from the Golgi apparatus. Centrioles points of organization of spindle fibers migrate to a postnuclear region after the completion of meiosis.
The distal centriole provides a template for accretion of cytoskeletal elements comprising the contractile lattice of the tail. Mitochondria become concentrated into the sheath of the middle piece. Cells do not divide during spermiogenesis Figure Spermatogenic cycle and wave.
If one closely examines serial cross-sections of a seminiferous tubule you will discover that sperm cells differentiate in distinctive associations. Each spermatogenic association has been classified as a stage of the seminiferous epithelial cycle.
A spermatogenic cycle is defined as the time it takes for the reappearance of the same stage within a given segment of the tubule. Each stage of the cycle follows in an orderly sequence along the length of the tubule.
The distance between the same stage is called the spermatogenic wave. One tubule can contain numerous complete waves. Each primary spermatogonia undergoes a series of mitotic divisions to produce a cluster of secondary spermatogonia encapsulated within a cyst.
The cyst arises from a Sertoli cell associated with the original germ cell. Clusters of cells resulting from divisions of the original germ cell maintain a consistent stage of development within the cyst Secondary spermatogonia are smaller than primary spermatogonia with large lightly basophilic nuclei and little cytoplasm. Primary spermatocytes, the result of another round of mitotic divisions, are smaller still with increasingly basophilic nuclei.
Primary spermatocytes undergo the first meiotic division to produce secondary spermatocytes. Another muscle, the cremaster muscle , consists of skeletal muscle fibers and controls the position of the scrotum and testes. When it is cold or a man is sexually aroused, this muscle contracts to pull the testes closer to the body for warmth. Each testis is an oval structure about 5 cm long and 3 cm in diameter.
A tough, white fibrous connective tissue capsule , the tunica albuginea , surrounds each testis and extends inward to form septa that partition the organ into lobules. There are about lobules in each testis. Each lobule contains 1 to 4 highly coiled seminiferous tubules that converge to form a single straight tubule, which leads into the rete testis. Short efferent ducts exit the testes. Interstitial cells cells of Leydig , which produce male sex hormones, are located between the seminiferous tubules within a lobule.
Sperm are produced by spermatogenesis within the seminiferous tubules. A transverse section of a seminiferous tubule shows that it is packed with cells in various stages of development. Interspersed with these cells, there are large cells that extend from the periphery of the tubule to the lumen. These large cells are the supporting, or sustentacular cells Sertoli's cells , which support and nourish the other cells. During this division, random inclusion of either parental chromosome and chromosomal crossover both increase the genetic variability of the gamete.
Each cell division from a spermatogonium to a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development. Not all spermatogonia divide to produce spermatocytes; otherwise, the supply would run out.
Instead, certain types of spermatogonia divide to produce copies of themselves, thereby ensuring a constant supply of gametogonia to fuel spermatogenesis. The creation of spermatids from secondary spermatocytes. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen in histological preparations. At this stage, each spermatid begins to grow a tail and develop a thickened midpiece where the mitochondria gather and form an axoneme.
Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is packaged with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive. The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosome. One of the centrioles of the cell elongates to become the tail of the sperm.
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