VAN 211: Veterinary Histology and Embryology (2+2)

TESTIS
(Seminiferous tubles) 

• Testis produces Spermatozoa (when the gland becomes mature and functional), and male sex hormone.
• The testis is a cytogenous gland i.e. it produces living germ cells. The testes are compound tubular glands.
• The gland is enveloped by a capsule of dense fibrous tissue, the tunica albuginea and serous layer, which is visceral layer of tunica vaginalis outside it. The capsule is rich in blood vessels.
• The capsule projects as thickening into the gland as mediastinum testis at its posterior edge and extends through its long axis from this mediastinum radiate septa - Septula testis towards the capsule dividing the gland parenchyma into a number of pyramidal or cone-shaped lobuli testis, whose apices converge towards the mediastinum testis.
• Each lobule is occupied by the terminal portions of seminiferous tubules. One to three of these occupy a lobule. They have an extremely tortuous course, rarely branch and are called convoluted seminiferous tubules.
• Sperms are formed in the convoluted tubule. The seminiferous tubule is 80-90cm. long and 100-200 microns thick with lumina of varying sizes. They start blindly at the periphery continue as tubuli contorti and run towards the mediastinum testis. At the mediastinum testis they become straight - the tubuli recti and then form a system of irregular, anastomosing structure with cavernous spaces, the rete testis.
• At the upper part of posterior edge 6 to 20 efferent ductules arise from the rete testis and emerge on the surface of testis. Through many convolutions they form vascular cones, whose apices point towards the mediastinum. The vascular cones form the head of the epididymis. They fuse to form a single ductus epididymis. This duct is highly coiled and forms the body and tail of epididymis.
• The duct gradually becomes straight to continue as ductus deferens. Tubuli recti, rete testis, epididymis and ductus deferens forms the excretory portion of the gland.

Seminiferous tubules

• Wall of the convoluted Seminiferous tubule is composed of lamellar collagenous connective tissue containing elastic fibres and condenses into a thin basement membrane.
• The seminiferous epithelium consists of two kinds of cells - spermatogenic cells and sertoli cells.
• Spermatogenic cells through proliferation and transformation produce sperms. They have indistinct boundaries and are not seen distinctly.
• Sertoli cells
  • These are tall, irregularly columnar cells and placed perpendicular to the basement membrane to which they are attached.
  • They have a distal ragged edge and project into the lumen of the tubule.
  • Basal portion contains a light staining vesicular oval nucleus. The cytoplasm shows a loose reticular structure.
  • The spermatogenic cell at certain stages of development embeds themselves into the cytoplasm of sertoli cells.
• Spermatogenic cells
  • These occur in 3 to 7 layers and include several generations of cells produced by division. The mother cells Spermatogonia make up the basal layer and by division give rise to many generations of spermatogonia.
  • These by growth, form Spermatocytes which occupy a few layers. Each primary spermatocyte soon divides to form two minute spermatids and make up the remaining layers. Spermatids become attached to the cytoplasm of sertoli cells.
  • Spermatogonia have dark staining spheroid nuclei and are round Primary spermatocytes also have round, dark staining spheroid nuclei. The secondary spermatocytes are smaller than the primary spermatocytes and almost immediately divide into two spermatids.
  • The nucleus of the spermatocytes will present a variable appearance, as the cells will be found in various stage of division. The division of primary spermatocyte is by meiosis or reduction division.
  • Each primary spermatocyte gives rise to two secondary spermatocytes by reduction or maturation division. Each secondary spermatocyte receives only half the number of chromosomes (haploid number) present in the primary spermatocyte.
  • Each secondary spermatocyte divides into two spermatids. The spermatids are small cells about half the size of secondary spermatocyte and have a round darkly staining nucleus.
  • Spermatids do not divide but by transformation form the specialized cells the Spermatozoa.
  • The tubuli recti and rete testis are lined by cuboidal or squamous epithelium. These receive the sperm produced by seminiferous epithelium.
  • The interstitial tissue i.e. the thin collagenous tissue between the tubules contains vessels, nerves and interstitial cells of Leydig. These cells are polyhedral. Nuclei are and spherical with a distinct nucleolus. Their cytoplasm stains light, due to dissolution of lipoid granules and droplets in ordinary preparations. They secrete testosterone or male hormone.
  • In undeveloped testis only spematagoina are present. Only with onset of sexual maturity several layers of sepermatogenic cells are seen.

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