Morphology and stereology of the female canine urethra correlated with the urethral pressure profile.

Perfusion-fixed urethrae of five sexually intact nulliparous female beagles in anestrus, between 2 and 3 years of age, were morphologically and morphometrically investigated. Absolute and relative volumes of different tissue components were stereologically estimated per urethral quarter. A discrete smooth muscle sphincter could neither be identified in the bladder neck nor in the urethra. However, the circularly arranged smooth muscle fibers in the proximal half of the urethra were regarded as being capable of sphincteric function. The connective tissue, composed of collagenous and longitudinally orientated numerous elastic fibers, accounted for the most predominant constituent. It comprised 77.3 to 78.2% of the total volume in the proximal three urethral quarters. The vascular plexus was most prominently developed in the proximal two urethral quarters, and its relative and absolute volumes significantly exceeded those of the third quarter. The discovered direct connections between small arteries and the sinusoids of the vascular plexus emphasize its importance as a major contributory component to urinary continence. The striated musculus urethralis was essentially confined to the distal half of the urethra and most strongly developed in the fourth quarter, where it surrounded the urethra transversely at its ventral and lateral aspects. The caudal muscle fibers continued dorsally to the vagina. A transitional epithelium bounded the urethral lumen. Its superficial cell layer progressively flattened towards the external urethral orifice, changing into a stratified squamous lining as also seen in the vestibulum. Recorded pressures, or pressure changes, could neither be correlated with volumes nor with morphology of particular tissue elements in the different urethral quarters.