Lamina propria: The connective tissue of rat urinary bladder mucosa.

To describe and illustrate the structure of the propria, the bladder of adult rats was fixed in controlled conditions of distension and examined by light and electron microscopy. The lamina propria, ~50 µm thick in the distended bladder, consists of a superficial part (the cellular component), adjacent to the urothelium, rich in nerves, capillaries, fibroblasts and thin bundles of collagen, and a deep, thicker part (the fibrous component), adjacent to the detrusor, rich in large collagen fibres and with few fibroblasts. In the cellular part there is an extensive plexus of afferent nerve fibers and a dense capillary network (with numerous pericytes), lying close to the urothelium, that is unique to the bladder. The main resident cells are fibroblasts, adhering to each other at the end of laminar extensions without forming specialized junctions. The deep part of the lamina propria is made of thick collagen fibers, interwoven and crisscrossing each other, with a few fibroblasts in the interstitial spaces between them. In summary, the superficial part of the lamina propria has most of the bladder afferent nerves, contains many fibroblasts and has a network of suburothelial capillaries. The deep part as a whole forms an ovoid balloon of woven fibrous material that is acted upon by the detrusor musculature attached to its outer surface. The lamina propria is a strong fibrous barrier between urothelium and musculature. The abundance of collagen points to the main role for its fibroblasts, that is, the production of collagen fibrils, assisting the mechanical role of the lamina propria.