Background & Aims:  Letrozole, as an aromatase inhibitor drug, is administrated to induce ovulation in individuals with anovulation disorder and, at the same time, it is used to enhance pregnancy success. The current study was designed to investigate the effect of Letrozole on follicular growth/atresia, as well as uterine histology. For this purpose, 15 mature female NMRI mice were assigned into 3 groups (NO=5 in each group): Control, Control sham, which received double distilled water, orally and experimental group received 0.5 mg/kg letrozole, orally.
Materials & Methods:  Following 14 days, the animals were euthanized and, the follicular number and percentage of atresia at different stages of growth and the morphometric changes of uterine tissue were analyzed and compared between groups.
Results:  The animals in experimental group represented no significant changes in primary, secondary, tertiary follicles, but exhibited a remarkable (p<0.05) reduction in graafian follicles number per two ovaries versus control and control-sham animals. Moreover, the letrozole -received mice exhibited diminished percentages of graafian follicles compared to control and control-sham animals. Finally, the endometrial thickness, as well as endometrial glands distribution per one mm² of tissue were decreased in experimental group when compared to control and control-sham animals.
Conclusion: Letrozole significantly reduces the graafian  follicles atresia and via this mechanism up-regulates the ovulation ratio. However, it is able to prolong uterine development during a menstrual cycle.

Keywords: Letrozole, NMRI, Ovary, Ovarian follicles, Uterine

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