Renal corpuscle
- The glomerulus consists of a number of separate capillaries connecting an afferent arteriole with an efferent arteriole. These vessels are usually closer together where they enter and leave the glomerulus and this end is known as the vascular pole of the renal corpuscle.
- The endothelial cells of the glomerular capillaries are extremely thin and the cytoplasm is fenestrated. The endothelium rests on a basement membrane, which fuses with the basement membrane of the visceral layer of Bowman’s capsule, which closely invests the glomerulus.
- The Bowman’s capsule consists of two layers of epithelium, a visceral layer closely investing the glomerulus and a parietal layer made up of simple squamous epithelium. Between the two layers of capsule is a space where provisional urine, first formed as glomerular filtrate collects.
- The visceral layer of the capsule is made up of cells whose nuclei project into the capsular space and their cytoplasm shows numerous foot-like processes, which are in contact with the basement membrane. Hence these cells are termed as podocytes. The arrangement of podocytes with the fenestrated endothelium of glomerular capillaries, leave only the basement membranes as the barrier between the blood in the capillaries and capsular space.
- The afferent arterioles of the glomerulus show special features. It lacks a distinct tunica adventitia and there is no internal elastic membrane. In the tunica media instead of plain muscle are myoepithelioid cells having pale staining afibrillar cytoplasm. These cells exhibit granules demonstrable by special techniques. These are referred to as the Juxta-Glomerular cells or apparatus (JG cells).
- It is now established that these JG cells secrete a substance known as Renin, which acts on hypertensinogen in the blood to form hypertensin (angiotensin), which produces vaso-constriction of blood vessels and increases blood pressure.
Neck of the tubule
- Here the epithelium is of the cuboidal variety.
- It is a very short segment with a diameter of 34 microns.
Proximal convoluted tubules (PCT)
- It is the longer and broadest portion of the renal tubule arising as a continuation of the neck.
- In the pars convoluta goes towards the surface of the cortex runs a tortuous course and then enters medullary ray to pass down into the medulla runs straight course (termed as the straight or medullary) segment of PCT to be continued by the descending limb of Henle.
- The diameter of PCT is about 45-60 microns and is lined by high pyramidal cells with granular cytoplasm and spherical nucleus located in the basal part. The lateral margins of the cells interdigitate so that cell boundaries are indistinct. The apices of the cells show a brush border (stereo-cilla), which is indistinct in ordinary preparations.
Descending limb of Henle
- (Thin segment of loop of Henle). It is the continuation of the medullary segment of PCT and extends down into the medulla for varying distances. The extent of the thin segment and length of the loop varies greatly in different tubules.
- The diameter varies form 10-17 microns. It is lined by flattened squamous cells with bulging nuclei (resembling a capillary) with a faint staining cytoplasm. The nuclei of the opposite sides alternate.
- Loop of Henle: It is present in the medulla and may show the structure of descending or ascending segments.
Ascending limb of Henle (ALH)
- (thick segments of Henle’s loop) is 25-40 microns in diameter. From the medulla, it passes up to the cortex (in pars radiata), reaches the renal corpuscle of its nephron, attaches itself to the vascular pole and is then continued as the distal convoluted tubule.
- The ALH is lined by cuboidal cells with granular cytoplasm and spherical nuclei. Cell outlines are distinct.
Distal convoluted tubule (DCT)
- It is present in pars convoluta of the cortex and begins at the vascular pole of the Renal corpuscle and after passing a winding course becomes continuous with an arched collecting tubule (in the pars convoluta itself). It is much less convoluted than the PCT and the diameter varies form 35-53 microns.
- It is lined by cuboidal cells with granular cytoplasm stains intensely than the cells of PCT. The cell borders are fairly distinct and they do not show a brush border. In cross sections cells are more numerous, nuclei are placed close and lumen is wider than that of the PCT.
- At the Vascular pole of the Renal corpuscle, the wall of the DCT is in close contact with afferent arteriole with JG cells. This wall of the tubule shows numerous nuclei close together and the cells are also taller and this zone of the tubule is referred as Macula Densa.
- Between it and the glomerulus proper in the concavity between the efferent and afferent arterioles is a collection of small with pale nuclei. This group of cells is referred to as polkissen. The functional significance of Macula Densa and polkissen is not clear.
- This DCT terminates the nephron or secretory segment of the renal tubule.
Arched collecting tubule
- 22.60 microns in diameter. It starts form the periphery of the medullary ray and reached the apex of pyramid. The diameter progressively increases.
- The epithelium is cuboidal gradually increasing in height with clear lightly stained cytoplasm and deeply stained nuclei with definite cell out lines.
Papillary ducts of Bellini
- Diameter 200-300 microns. Columnar cells with clear basophilic cytoplasm line these.
- Near termination at the renal papilla the epithelium become two layered and it may become transitional as the duct opens into a minor calyx.
