NEONATAL EXPOSURE TO PROGESTERONE AND ESTRADIOL ALTERS UTERINE MORPHOLOGY AND LUMINAL PROTEIN-CONTENT IN ADULT BEEF HEIFERS

Exposure of the developing urogenital tract to steroids can affect str ucture and function of adult tissues and compromise reproductive perfo rmance. This study was conducted to determine 1) if exposure of neonat al heifer calves to progesterone (P) and estradiol benzoate (E), deliv ered from a commercial growth-promoting implant, would affect adult ut erine morphology or uterine luminal protein content; and 2) whether su ch effects would be related to neonatal age at the first exposure. At birth (Day 0), 20 crossbred beef heifers were assigned to 1 of 4 treat ment groups (n = 5 per group), defined by age at implant placement. He ifers either received an implant on Days 0, 21 or 45, or served as unt reated controls. The heifers were maintained together and slaughtered at 15 mo of age, during the luteal phase of an induced estrous cycle, when reproductive tracts and blood samples were obtained. Peripheral p lasma P concentrations were determined by RIA. Uterocervical wet weigh ts were recorded, and uterine luminal flushings (ULF) were assayed for total protein. Cross-sections of uterine tissues were evaluated histo morphometrically to determine myometrial and endometrial areas and rel ative endometrial gland density. Treatment did not affect plasma P con centrations (3.2+/-0.5 ng/ml). Regardless of age at treatment, neonata l PE exposure reduced uterocervical wet weight by 35% (112.8 < 173.9+/ -13.9 g; P<0.01), myometrial area by 23% (125.3 < 162.8+/-8.5 mm2; P<0 .02), and endometrial area by 27% (33.3 < 45.4+/-2.7 mm2; P < 0.09) co mpared with the untreated controls. Endometrial gland density was redu ced (P<0.01) by 40% in treated heifers. This effect was related to age at implant placement. Uterine gland density was reduced (P<0.01) by 6 5% in heifers treated at birth, while reductions of 22 and 33% were ob served for heifers treated on neonatal Day 21 or 45, respectively. Con sistently, ULF protein content was lower (P<0.01) in the treated heife rs (2.67 < 4.98+/-.72 mg/ULF). Thus, exposure of newborn calves to PE can have profound effects on adult uterine morphology and environment, the extent of which may depend upon the developmental period when exp osure occurs. The potential of such alterations to affect reproductive performance in adult beef heifers remains to be investigated.