Kidney

ORIENTATION  IN THE SLIDE

Kidney is a large organ and for that reason a slide contains only a small part of its parenchyma. Outer surface is covered by a  capsule from a dense connective tissue (in interactive slide depicted in brown). Both lateral edges follow to a deep medulla. A superficial part of the parenchyma under the capsule is called the cortex. the cortex contains renal corpuscles (depicted in green), which are well visible even at the lowest magnification. A boundary between cortex and medulla (dashed line) contains relatively  large transverse sections of arcuate arteries (red). Medulla is a little bit lighter; there are no renal corpuscles, but it contains many renal tubules (see Fig. 5). A tissue covered by a rectangle at the interface between the cortex and the medulla is shown at higher magnification in Fig. 3.

Fig. 2. Interactive slide of the kidney: Low magnification. Moving a cursor to an image shows structures in the slide in different colours. Illustration legend can be found in the text in left. Author of the  microphotograph: J. Mokrý.

Parenchyma of the kidney is divided in cortex and medulla. The most conspicuous structure is renal corpuscle (green) that occurs only in the renal cortex. Renal corpuscles are of spherical shape; because of their large size (200 μm) they are well visible at low magnification. Corpuscles in the vicinity to cortico-medullary boundary belong to the so called juxtamedullary nephrons - their glomerulus is of large size. In the renal corpuscle here are two types of tubules: proximal and distal tubules – see Fig. 4). The amount of interstitial tissue comprises less than 5% parenchyma. At border between cortex and medulla, large blood vessels can be found: arcuate arteries and veins. Arcuate artery (red) belongs to muscular arteries; in Fig. 3 a vein is depicted in blue. The medulla contains many tubules, mainly loops of Henle and collecting ducts (see Fig. 5).

Fig. 3 Interactive slide of the renal parenchyma. Moving a cursor to an image shows structures in the slide in different colours. Illustration legend can be found in the text in left; haematoxylin-eosin staining. Author of the  microphotograph: J. Mokrý.

RENAL CORPUSCLE

Renal corpuscle is the most conspicuous structure of the renal cortex. It is composed of Bowman's capsule and glomerulus. In the Bowman's capsule, the parietal layer is best visible: it is lined by simple squamous epithelium (in Fig. 4 indicated in grey line). The visceral layer covers the glomerulus. Glomerulus (red) is formed by a tuft of fenestrated capillaries; it contains erythrocytes, endothelial cells, podocytes and intraglomerular mesangial cells. The urinary space (of Bowman; in Fig 4 in white) occurs between parietal and visceral layers. An area with a thickest urinary space is called the urinary pole; here a squamous epithelium of the parietal layer is continuous with the epithelial lining of the proximal tubule (light blue). An opposite area is called the vascular pole - here the urinary space become narrow and disappears and parietal layer is continuous with a visceral layer. In the vascular pole, the afferent arteriole enters the Bowman capsule and efferent arteriole leaves it. In the vicinity to renal corpuscle, renal parenchyma contains proximal and distal tubules (visible in transverse and longitudinal sections). Proximal convoluted  tubules (purple) have irregular lumen (in some it is indicated by a black line). Epithelium is formed by voluminous columnar nephrocytes – as a result of a cross section reveals only few nuclei. Distal tubules have a regular lumen (asterisk); nephrocytes are smaller (these tubules contain more cell nuclei) = simple cuboidal epithelium. The cytoplasm is paler than in proximal tubules. In both tubules, cell boundaries are not well visible. The site where a loop of distal tubule comes in a contact with a vascular pole, constitutes the macula densa (green): cell nuclei are densely packed, and the cells assume a columnar shape.

Fig. 4. Structures in renal cortex: Interactive slide. Moving a cursor to an image shows structures in the slide in different colours. Illustration legend can be found in the text in left.
Author of the  microphotograph: J. Mokrý.

Renal medulla contains tubules of different sizes seen in transverse, oblique or longitudinal sections. Identification of particular tubules is facilitated after classification of a lining epithelium and by comparison of tubules diameters. The smallest lumen has the loop of Henle; its thin segment is lined with simple squamous epithelium (some thin segments are indicated by a black line). Thick segment of loop of Henle is lined by simple cuboidal epithelium (green); size of lumen and of tubule can be compared to thin segments. Thin segments look similar to thin-walled blood vessels vasa recta (red) – they can be distinguished according to red blood cells in their lumen. Collecting ducts have a larger lumen lined by a simple cuboidal to columnar epithelium (blue). Their epithelial cells are very pale with well visible boundaries; apical cytoplasm is convex (bulges in the lumen).

Fig. 5. Structures in renal medulla: Interactive slide. Moving a cursor to an image shows structures in the slide in different colours. Illustration legend can be found in the text in left. Author of the  microphotograph: J. Mokrý.