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Histology of Testis

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Histology of Testis HISTOLOGY OF TESTIS DR ADITY SARBAJNA, DEPARTMENT OF ZOOLOGY, SURENDRANATH COLLEGE INTRODUCTION • The main functions of the male reproductive system, are to produce spermatozoa, androgens (sex hormones - principally testosterone) and to facilitate fertilisation, by introducing spermatozoa into the femal genital tract (copulation). The male reproductive system includes the testis, genital ducts, accessory sex glands and penis. After following this topic, you should know the structure and function of these structures. The pair of testes produces spermatozoa and androgens. Several accessory glands produce the fluid constituents of semen. Long ducts store the sperm and transport them to the penis. The male reproductive system consists of paired testes and genital ducts, accessory sex glands and the penis. The testes and ducts are shown in the diagram. TESTIS ORGANIZATION The testis is surrounded by a thick capsule, the tunica albuginea, from which a conical mass of connective tissue, the mediastinum testis, projects into the testis. The tunica albuginea is covered externally by a serosa. From the mediastinum, delicate fibrous septa radiate towards the tunica albuginea and divide the parenchyma of the testis into about 300 lobuli testis, which communicate peripherally. Each lobule contains 1- 4 convoluted seminiferous tubules Interstitial tissue between the convoluted tubules is continuous with a layer of loose vascular connective tissue, the tunica vasculosa testis, which is found beneath the tunica albuginea. Each seminiferous tubule continues near the mediastinum into a straight tubule, a tubulus rectus. The straight tubules continue into the rete testis, a labyrinthine system of cavities in the mediastinum. These tubules are enclosed by a thick basal lamina and surrounded by 3-4 layers of smooth muscle cells (or myoid cells). The insides of the tubules are lined with seminiferous epithelium, which consists of two general types of cells: spermatogenic cells and Sertoli cells. MALE REPRODUCTIVE SYSTEM: SPERMATOGENESIS The production of sperm and eggs/ova (gametes) is a procedure called gametogenesis (spermatogenesis and oogenesis). Gametogenesis involves two rounds of meiotic cell division, in which one diploid cell gives rise to 4 haploid cells. This diagram shows the processes involved in spermatogenesis. The germinal (seminiferous epithelium) of the seminiferous tubules contains spermatogenic cells and Sertoli cells. The spermatogenic cells divide by mitosis, then meiosis to form gametes, which mature into sperm by the process of spermiogenesis. Unusually, the developing spermatogenic cells remain connected by cytoplasmic bridges, until they have formed a mature spermatozoan. There are three types of spermatogonia - • Type A (dark) which are reserve stem cells • Type A (pale) which are renewing stem cells • type B spermatogonia which are differentiating progenitors, and form spermatocytes. It takes about 2 months for pale A spermatogonia to form new spermatozoa, the total production time for mature human spermatozoa is 74 days. SPERMATOCYTES Primary spermatocytes lie in the cell layer luminal to the spermatogonia. They appear larger than spermatogonia. They immediately enter the prophase of the first meiotic division, which is extremely prolonged (about 22 days!). A large number of primary spermatocytes is always visible in cross-sections through seminiferous tubules. Cell divisions, from the formation of primary spermatocytes and onwards, to the production of the spermatocytes, are incomplete. The cells remain connected by bridges of cytoplasm. The completion of the first meiotic division results in the formation of Secondary spermatocytes Secondary spermatocytes are smaller than primary spermatocytes. They rapidly enter and complete the second meiotic division and are therefore seldom seen in histological preparations. Their division results in the formation of Spermatids. Spermatids lie in the luminal part of the seminiferous epithelium. They are small (about 10 µm in diameter) with an initially very light (often eccentric) nucleus. The chromatin condenses during the maturation of the spermatids into spermatozoa, and the nucleus becomes smaller and stains darker. The terminal phase of spermatogenesis is called spermiogenesis and consists of the differentiation of the newly formed spermatids into Spermatozoa Spermatozoa:The mature human spermatozoon is about 60 µm long and actively motile. It is divided into head, neck and tail. SPERMATOZOA The head (flattened, about 5 µm long and 3 µm wide) chiefly consists of the nucleus (greatly condensed chromatin!). The anterior 2/3 of the nucleus is covered by the acrosome, which contains enzymes important in the process of fertilisation. The posterior parts of the nuclear membrane forms the so-called basal plate. The neck is short (about 1 µm) and attached to the basal plate. A transversely oriented centriole is located immediately behind the basal plate. The neck also contains nine segmented columns of fibrous material, which continue as the outer dense fibres into the tail. The tail is further divided into a middle piece, a principal piece and an end piece. The axonema (the generic name for the arrangement of microtubules in all cilia) begins in the middle piece. It is surrounded by nine outer dense fibres, which are not found in other cilia. In the middle piece (about 5 µm long), the axonema and dense fibres are surrounded by a sheath of mitochondria. The middle piece is terminated by a dense ring, the annulus. The principal piece is about 45 µm long. It contains a fibrous sheath, which consists of dorsal and ventral longitudinal columns interconnected by regularly spaced circumferential hoops. The fibrous sheath and the dense fibres do not extend to the tip of the tail. Along the last part (5 µm) of the tail, called the end piece, the axonema is only surrounded by a small amount of cytoplasm and the plasma membrane. This diagram shows a single seminiferous tubule, and cells in the interstitial tissue outside the tubule. In thisconnective tissue between the seminiferous tubules there are large cells with eosinophilllic cytoplasm called Leydig cells. There are also cells called myoid cells surrounding the basement membrane, which are squamous contractile cells, and generate peristaltic waves in the tubules. SERTOLI CELLS Like all epithelial cells, the Sertoli cells are avascular. Sertoli cells suport the germ cell progenitors and help to transfer nutrients from the nearby capillaries. The developing spermatogonia rely on the Sertoli cells for all of their nourishment. The blood-testis barrier formed by the Sertoli cells effectively isolates the developing spermatogonia, spermatocytes, spermatids and mature spermatozoa from blood. Differentiating spermatozoa nestle in pockets in the peripheral cytoplasm of these cells. Sertoli cells also produce testicular fluid, including a protein that binds to and concentrates testosterone, which is essential for the development of the spermatozoa. They also help to translocate the differentiating cells to the lumen, and phagocytose degenerating germ cells and surplus cytoplasm remaining from spermiogenesis. Blood-testis barrier: Importantly, the basal regions of the lateral borders of the Sertoli cells are connected to each other by continuous tight junctions. These divide the tubules into two separate compartments. The mitotic spermatogonia remain in the basal compartment. Differentiating progeny enter the adluminal compartment, and are sealed off from the basal compartment. If novel antigens are expressed on the haploid cells, then it is less likely that they will be detected by the immune system in this sealed off compartment. The developing cells are also in a very protective environment. LEYDIG CELLS Leydig cells are 'interstitial' cells (as they lie between the tubules). They have pale cytoplasm because they contain many cholesterol-lipid droplets. The Leydig cells make and secrete testosterone, in response to lutenising hormone from the pituitary. The cholesterol is used in the first step of testosterone production. Testosterone promotes production of spermatozoa, secretion from the accessory sex glands, and acquisition of male secondary characteristics. This process does not start until puberty when LH stimulates the Leydig cells to produce testosterone. FSH stimulates the Sertoli cells to secrete androgen- binding protein into the lumen of the seminiferous tubules. Binding of testosterone in the lumen provides a local testosterone supply for the developing spermatogonia.
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