Plate 12.242 Urinary Bladder
Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Human, 10% formalin, H. & E., 22 x
The component parts of the wall of the urinary bladder are shown at low power. The innermost layer, the mucosa, consists of transitional epithelium, underlaid by a prominent lamina propria. The folds in the mucosa are seen in three dimension to be extensive longitudinal ridges and represent one means by which the bladder wall accommodates to distention. The thickness of the epithelium is also reduced upon stretching, as is the height of the luminal epithelial cells (dome, or umbrella cells, see Plate 24 for details of the epithelium). Thus, the overall topography of sections of the bladder will always depend upon the state of distention and turgidity of the organ at the time of fixation. The muscular layer of the bladder is generally recognized as consisting of three layers: (1) inner longitudinal, (2) middle circular or spiral; and (3) outer longitudinal. The definition of these as discrete layers is rendered difficult by the intertwining of muscle bundles and fascicles from adjacent layers, and by variation in the thickness of the respective layers in different parts of the organ (e.g., the formation of the internal sphincter by the internal longitudinal layer in the region of the trigone). This histological feature is in contradistinction to the well-defined muscle layers evident in preparations of the gastrointestinal tract. Such a muscular arrangement does, however, facilitate the occlusion of the bladder lumen upon voiding, and the avoidance of residual urine in the bladder, which is recognized as predisposing to bladder infections. The organ is covered by a connective tissue adventitia, except on its superior aspect, which bears a serosa of reflected peritoneum. Present within this layer are blood vessels, nerves, and underlying adipose tissue.
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