Influence of ovariectomy on the canine striated external urethral sphincter (M. urethralis): a stereological analysis of slow and fast twitch fibres.

Slow and fast twitch fibres were stereologically analysed in the morphologically defined and most strongly developed ventral and ventrolateral region of the external urethral sphincter (M. urethralis) using five sexually intact anestrous and five ovariectomized nulliparous beagles. The estimated mean total cross-sectional area of the investigated part of the muscle consisted of 4.2 mm2 (15.1%) type I fibre, 10.0 mm2 (32.1%) type II fibre, and 16.3 mm2 (52.8%) connective tissue in the control group. The corresponding absolute mean value of type I fibres (3.7 mm2/13.5%) was statistically lower in ovariectomized animals. No significant difference between groups was observed in the relative number of transverse profiles of type I and II fibres; type I fibres comprised 23.8% and type II 76.2% of all muscle fibres in the sexually intact group, but 21.8% and 78.2% in the ovariectomy group, respectively. The ovariectomized dogs exhibited a statistically significant lower type I and II fibre number and a concomitant slightly larger mean single profile area (diameter) of fibre type II compared with the control animals. The significantly reduced number and decreased total cross-sectional area of the fatigue-resistant type I fibres in ovariectomized dogs suggest a predominant weakening of the fibre type I portion of the M. urethralis as consequence of ovariectomy. The effect could be mediated by sex hormonal factors and may contribute to the development of postspaying urinary incontinence in female dogs.