Histology of the Bladder
References: (Benninghoff, 1993).
Urothelium: Mucosa of the Bladder
The mucosa of the urinary bladder is lined by a multilayered transitional epithelium (urothelium). The mucosa with a special GAG-layer enables the storage of urine with a high osmotic gradient to the blood.
Histology of the Transitional Epithelium:
multilayered epithelium with a maximum of seven cell layers [fig urothelium]. There are three different cell types: basal cell layer, intermediate cell layer and surface cell layer. The basal cell layer renews the urothelium by cell division. The new cells migrate to the lumen. The surface cells are of large epithelial cells, sometimes multinucleate, and up to 200 μm in size. The enormous elasticity of the urothelium, and in particular of the surface cells, is explained with subapical vesicles. The subapical vesicles, with increasing bladder stretch, are built into the cell membrane (exocytosis).
fig. Schematic drawing of the transitional epithelium (urothelium).
GAG layer of the bladder:
The surface cells are covered by a glycosaminoglycan layer (GAG). The function of the GAG layer is controversial, suggested tasks are a osmotic barrier function and also as an antibacterial coating of the urothelium (low adherence of the bacteria).
There is a high osmotic and chemical gradient between the urine and the blood plasma. The following mechanisms of the urothelium protect the passage of urine into the lamina muscularis: GAG-layer, surface cells (4 to 6 tight junctions), uroplakin proteins in the apical cell membrane, and a capillary plexus in the submucosa.
Despite the above mentioned factors there is a permeation of substances from the urine into the plasma. Passive diffusion as well as active transport cause the change of urinary concentrations in the urinary bladder over the time. This is most evident for water and urea.
Submucosa (Lamina Propria) of the Bladder Wall
The submucosa of the urothelium consists of connective tissue (collagen, elastin and other extracellular matrix proteins). Beneath the urothelium, there is a dense capillary plexus. In addition to the vascular supply of the urothelium, the capillary plexus serves as a barrier function. Substances, that penetrate through the urothelium, are removed and the muscular layer of the bladder wall is protected against urine. Due to the strong elasticity of the vascular plexus, the filling of the bladder does not increase the vascular resistance of the submucosal plexus. Only with an increasing tension of the bladder wall, due to exessive filling, the vascular resistance rises.
Tunica muscularis of the bladder
The tunica muscularis of the bladder is formed by the detrusor muscle (smooth muscle). The muscle fibers run irregularly in all directions, only at the bladder trigone the detrusor muscle consists of three layers (see trigone of the urinary bladder).
Histology of the smooth muscle fiber:
spindle-shaped muscle cells, connected to each other. The cells contain actin and myosin as contractile elements, but they are disorganized and do not form sarcomeres. The myofibrils move across the smooth muscle cell from cell wall to cell wall, producing a shortening and thickening of the muscle cell with contraction. In the relaxed state, smooth muscle cells are several hundred μm long and 5–6 μm thick.
Extracellular matrix of the tunica muscularis:
between the muscle cells, there is an elastic connective tissue stroma of collagen (e.g. I, III, IV) and elastin. The ratios of connective tissue proteins to each other and the relationship of smooth muscle cells to the extracellular matrix are essential for the elasticity (compliance) of the bladder.
Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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Benninghoff 1993 BENNINGHOFF, A.:
- Makroskopische Anatomie, Embryologie und Histologie des
München; Wien; Baltimore : Urban und Schwarzenberg, 1993