Morphological, histochemical and stereological analysis of the female canine M. urethralis.

The M. urethralis was morphologically investigated in ten medium-sized female dogs of different breeds and age, as well as histochemically and stereologically analysed in a homogeneous group of five female beagles. Macroscopically, the muscle was essentially confined to the distal third and most strongly developed in the fourth quarter of the urethra. Here, it surrounded the urethra transversely at the ventral and lateral aspects, passing with its caudal fibres dorsally onto the vagina. The muscle fibres were assembled in groups of different sizes and usually separated by thick connective tissue septae. Based on the myofibrillar actomyosin ATPase (mATPase) reaction, type I and two main subtype II fibres could be differentiated. Type II fibres were, however, indistinguishable by their metabolic enzyme activities since both subclasses displayed oxidative-glycolytic properties. The subtype II fibres containing the more acid-labile mATPase activity were classified as IIA, whereas the other main subtype was designated IIS (subtype) and considered as peculiar to the dog. In addition, the investigation revealed a rare fibre type exhibiting the histochemical profile of IIC fibres. There was no evidence of classical glycolytic IIB fibres. The M. urethralis was composed of 24% type I and 76% type II fibres with an average diameter of 34.9 and 28.5 microns, respectively. Connective tissue constituted 52.8% of the total muscle volume. Due to the predominance of fast twitch II fibres, the urethral muscle is not designed to maintain a high tone over long periods of time. Its primary role is to function rapidly and intermittently guaranteeing urinary continence during stress situations when other continence factors are overburdened.