RESUMEN
OBJECTIVE: To determine the effects of melatonin on rat endometrium morphology and embryo implantation. DESIGN: Experimental study. SETTING: Federal University of São Paulo, Brazil. ANIMAL(S): Forty female rats. INTERVENTION(S): GI: control, GII: sham-operated, GIII: pinealectomized, and GIV: pinealectomized rats that received melatonin during 3 months. The GI, GII, and GIII groups received the vehicle of melatonin (NaCl + ethanol). At the end of the treatment, the animals were killed during the estrous phase; the uterus was removed for morphometric analysis. Urine was collected for 6-sulfatoxymelatonin. Blood was collected for estrogen (E) and progesterone (P) level determinations. In a second experiment, female rats were used to evaluate the endometrial embryo implantation. MAIN OUTCOME MEASURE(S): Endometrial morphology and embryo implantation. RESULT(S): GIII presented the highest values for endometrial area and thickness index, number of endometrial glands, and eosinophils. The number of vessels of groups I, II, and IV was fewer than that of GIII. The highest number of eosinophils was detected in GIII in comparison to other groups. The implantation rate in GIII was the lowest of all groups. This implantation rate was significantly increased and restored toward normal in GIV. CONCLUSION(S): Our data suggested that, in nonphotoperiodic animals such as rats, melatonin may positively affect the endometrial morphology and improve embryo implantation.