Histomorphologic and morphometric evaluation of the uterine horns in nulliparous and multiparous Beagles

OBJECTIVE: To macroscopically, histomorphologically, and morphometrically compare uterine tissues obtained from nulliparous and multiparous dogs. ANIMALS: 12 dogs constituting 2 homogenous groups (7 nulliparous Beagles and 5 multiparous Beagles, each of which had at least 7 pregnancies). PROCEDURE: Serum concentrations of progesterone and estradiol-17beta were determined. Samples of uterine tissues were fixed, embedded in paraffin, and cut into serial cross sections. Morphometric analysis was performed on systematically selected trichrome-stained sections. RESULTS: Mean absolute volume of the uterine wall did not differ between groups. Volume of blood vessels was significantly higher and relative mean value for myometrial connective tissue was significantly lower in the multiparous dogs. Arteries in the myometrium of multiparous dogs had pronounced thickening of the intima (ie, pregnancy sclerosis) and a concomitant thinning of the smooth muscle layer (tunica media). Furthermore, the elastica interna of these arteries appeared to be disintegrating and was highly and irregularly folded. Adventitia of the myometrial veins in multiparous dogs contained numerous layers of elastic fibers; however, only a few layers were observed in the adventitia of corresponding veins in nulliparous dogs. CONCLUSIONS AND CLINICAL RELEVANCE: In this study, we documented that in contrast to other species, the uterus of nonpregnant dogs does not increase in size and volume even after at least 7 pregnancies. Furthermore, sclerotic alterations of uterine arteries are proof of at least 1 pregnancy. Results of this study may be useful in the evaluation of uterine diseases.