Histology of the Urinary System
Kidney: Each kidney is contained within a connective tissue capsule. Internally, the kidney possesses an outer cortex and inner medulla, surrounding a central renal pelvis.
The urinary system maintains homeostatic conditions in the blood. The basic physiological unit of the urinary system, the nephron, accomplishes this through three processes: filtration, reabsorption and secretion. Filtration is accomplished in a structure called a renal corpuscle, which contains a network of capillaries (glomerulus) and a receiving chamber (Bowman’s capsule). This filtrate then passes into a series of renal tubules, where it is processed into urine before it accumulates in the renal pelvis. Processing involves reabsorption and secretion between the tubules and capillaries. The renal tubule system terminates in ducts, which collect urine and drain it into the renal pelvis.
Ureter: The renal pelvis is continuous with the ureter, a tube that conducts urine from the kidney to the urinary bladder. The ureter is a typical tubular organ, with a tunica intima, a tunica media composed of two layers of smooth muscle and tunica externa of connective tissue.
Bladder: The urinary bladder must expand to accommodate and store urine. Its luminal surface contains superficial folds and its muscular wall muscle relaxes adaptively to allow additional expansion. This muscle contracts forcefully in the process of urination.
Slide 1: Capsule and Renal Cortex, scanning view
This is a low-magnification view of the superficial portion of the kidney.
In this image you can see the Renal Capsule (labeled A), which is composed of connective tissues, and the renal cortex (B). The renal cortex is filled with blood vessels, renal tubules, and renal corpuscles, where filtration takes place. In this image, the glomeruli are the slightly darker stained areas of the cortex.
Slide 2: Renal cortex, low power view
This slide shows a higher magnification view of structures in the renal cortex.
At a higher magnification, you can see the proximal and distal convoluted tubules (C) of the nephrons. This is where water and some nutrients are reabsorbed into the bloodstream from the filtrate. The filtrate is created in the renal corpuscle (B).
Slide 3: Renal corpuscle and tubules
The structures you identified in Slide 2 are shown at higher magnification in Slide 4. At this magnification, you can better see the orientation of the renal tubules.
The renal corpuscle is composed of the capsule that surrounds the structure (D) and the glomerulus (B) which is the filtration capillary. Glomeruli are one of the few capillary types in our body that connect arteries. This allows for the regulation of pressure to increase or decrease filtration. The tubules (C) tend to be coiled near the corpuscle, you should note that you see mainly circles to the left of the corpuscle. Compared to the longitudinal arrangement of tubules to the right of the corpuscle.
Slide 4: Cortex/Medulla Boundary
This slide shows the boundary between the renal cortex and medulla.
Although the separation is not demarcated by a line of connective tissues, the change from renal cortex (B) to medullary space (A) is indicated by the change in orientation of the renal tubules. As the tubules move into the medulla, notice that they all start to run in the same direction. These are mainly tubules from juxtamedullary nephrons. These long tubule limbs in the medulla are specially adapted to help with water retention.
Slide 5: Renal Medulla
This slide is a high magnification view of a portion of the renal medulla.
Higher magnification of the renal tubules in the renal medulla. Notice that they are lined with cuboidal epithelium and again, their orientation in linear and in the same direction.
Slide 6: Ureter
The ureter conveys urine from the renal pelvis to the urinary bladder. This slide is a low-power section through a ureter.
The ureters have a mucosal lining of transitional epithelium. Like the digestive tract, the tunica media has two layers of smooth muscle, a longitudinal and circular layer. These smooth muscle layers help to move urine into the bladder from the kidneys.
Slide 7: Urinary Bladder
This section shows a low-magnification view of the bladder.
The urinary bladder has the typical three-layer arrangement common of hollow internal organs. In this image, the lumen is towards the top of the picture. From the luminal surface, you can see the tunica intima separated into two sections, the thin mucosal layer that is exposed to the lumen and the submucosa which is filled with connective tissue. Towards the bottom of the image you can see the tunica media, which in this case is composed of smooth muscle.
Slide 8: Urinary Bladder
This section provides a high-magnification view of the luminal surface of the urinary bladder.
At a higher power, you can see the separation between the mucosal layer (A) and the submucosa. The luminal surface is transitional epithelium. Although it looks like it is stratified, all of the cells are attached to the basement membrane. This allows for maximal stretching of the lining of the bladder as it fills with urine. The submucosa consists of dense regular connective tissue.