what is called to the duration between development of primary spermatocyte from spermatogonoum?
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Seminiferous Tubules
Spermatogenesis takes place within the seminiferous tubules, which, in humans, are ~200 μm in diameter and have a total length of ~600 meters occupying ~60% of the testis volume (Fig. 136-1).
The terminal ends of the seminiferous tubules in the mediastinum empty via straight tubular extensions termed tubuli recti. Depending on the species, individual seminiferous tubules may be highly convoluted (e.g., human), or they may form numerous relatively linear segments linked by cranial and caudal hairpin turns (e.g., rodent testes).3
Within the epithelium of the seminiferous tubules, germ cells undergo spermatogenesis, which commences with the spermatogonia that lie adjacent to the basement membrane of the tubules and divide by mitosis. Spermatogonia, as well as renewing themselves, give rise to cells that lose contact with the basement membrane and commence the process of meiosis, now called primary spermatocytes. Having completed the first meiotic division, these cells give rise to daughter cells called secondary spermatocytes, which divide to complete meiosis to form round spermatids.
The round spermatids do not divide but undergo a complex metamorphosis, called spermiogenesis, to become spermatozoa that are released into the lumen of the seminiferous tubule by a process called spermiation.
Interspersed between the germ cells within the seminiferous epithelium are the supporting cells, called Sertoli cells,which extend from the basement membrane of the tubule to the lumen like a tree with its trunk abutting on the basement membrane and its branches being interspersed between the germ cells. The physical relationship between the nondividing Sertoli cells in the adult testis and the various types of dividing and differentiating germ cells is difficult to appreciate by light microscopy since the Sertoli cell cytoplasmic extensions between the germ cells are thin. The complexities of this association have, however, been thoroughly described in ultrastructural studies (see reviews elsewhere4,5) to reveal a dynamic convoluted architecture. As germ cells progress through spermatogenesis, they are progressively moved apically through the seminiferous epithelium separated by processes of Sertoli cell cytoplasm that create pockets, or recesses between the Sertoli cells. The most mature germ cells, the spermatozoa, are ultimately released into the lumen of the seminiferous tubule (Figs. 136-2, 136-3, and 136-4).
Where adjacent Sertoli cells interface with each other above the basal spermatogonia, a specialized tight cell junction is formed preventing intercellular transport of substances, thus creating basal and adluminal compartments of the seminiferous tubules. These tight junctions effectively form the blood-testis barrier, which can open to enable spermatogonia to lose their connection with the basement membrane and enter the adluminal compartment.
The sperm and luminal fluid are moved by irregular contractions of the peritubular myoid cells that lie on the external surface of the tubules through the mediastinum into the rete testis. The rete testis is a maze of anastomosing spaces within the mediastinum and drains into the epididymis. The morphology of the rete is species-specific,6 but can generally be divided into three principal zones. The septal rete is composed of straight tubules that empty into the mediastinal rete, a network of anastomosing channels. These, in turn, drain into the extratesticular rete, which is characterized by wider spaces in continuity with the 6 to 12 fine efferent ductules leading to the head of the epididymis.
Spermatogenesis takes place within the seminiferous tubules, which, in humans, are ~200 μm in diameter and have a total length of ~600 meters occupying ~60% of the testis volume (Fig. 136-1).
The terminal ends of the seminiferous tubules in the mediastinum empty via straight tubular extensions termed tubuli recti. Depending on the species, individual seminiferous tubules may be highly convoluted (e.g., human), or they may form numerous relatively linear segments linked by cranial and caudal hairpin turns (e.g., rodent testes).3
Within the epithelium of the seminiferous tubules, germ cells undergo spermatogenesis, which commences with the spermatogonia that lie adjacent to the basement membrane of the tubules and divide by mitosis. Spermatogonia, as well as renewing themselves, give rise to cells that lose contact with the basement membrane and commence the process of meiosis, now called primary spermatocytes. Having completed the first meiotic division, these cells give rise to daughter cells called secondary spermatocytes, which divide to complete meiosis to form round spermatids.
The round spermatids do not divide but undergo a complex metamorphosis, called spermiogenesis, to become spermatozoa that are released into the lumen of the seminiferous tubule by a process called spermiation.
Interspersed between the germ cells within the seminiferous epithelium are the supporting cells, called Sertoli cells,which extend from the basement membrane of the tubule to the lumen like a tree with its trunk abutting on the basement membrane and its branches being interspersed between the germ cells. The physical relationship between the nondividing Sertoli cells in the adult testis and the various types of dividing and differentiating germ cells is difficult to appreciate by light microscopy since the Sertoli cell cytoplasmic extensions between the germ cells are thin. The complexities of this association have, however, been thoroughly described in ultrastructural studies (see reviews elsewhere4,5) to reveal a dynamic convoluted architecture. As germ cells progress through spermatogenesis, they are progressively moved apically through the seminiferous epithelium separated by processes of Sertoli cell cytoplasm that create pockets, or recesses between the Sertoli cells. The most mature germ cells, the spermatozoa, are ultimately released into the lumen of the seminiferous tubule (Figs. 136-2, 136-3, and 136-4).
Where adjacent Sertoli cells interface with each other above the basal spermatogonia, a specialized tight cell junction is formed preventing intercellular transport of substances, thus creating basal and adluminal compartments of the seminiferous tubules. These tight junctions effectively form the blood-testis barrier, which can open to enable spermatogonia to lose their connection with the basement membrane and enter the adluminal compartment.
The sperm and luminal fluid are moved by irregular contractions of the peritubular myoid cells that lie on the external surface of the tubules through the mediastinum into the rete testis. The rete testis is a maze of anastomosing spaces within the mediastinum and drains into the epididymis. The morphology of the rete is species-specific,6 but can generally be divided into three principal zones. The septal rete is composed of straight tubules that empty into the mediastinal rete, a network of anastomosing channels. These, in turn, drain into the extratesticular rete, which is characterized by wider spaces in continuity with the 6 to 12 fine efferent ductules leading to the head of the epididymis.
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