Luteal function induced by transvaginal ultrasonic-guided follicular aspiration in mares

Ultrasonic-guided transvaginal follicle aspiration was performed in 58 crossbreed mares in order to determine whether aspiration of various dominant follicle diameters resulted in luteal tissue capable of producing progesterone (P4). The mares were randomly assigned to three groups according to follicular diameter (25–29 mm; 30–35 mm and >35 mm). Mares that had ovulations naturally served as controls. The serum progesterone (P4) concentrations in the aspirated mares were greater (P < 0.0001; r2 = 0.6687; CV = 21.52) in mares with natural ovulation compared to mares with aspirated follicles regardless of groups. Serum P4 concentration in aspired mares with follicular diameter of 25–29 mm declined 0.365 ng/ml/day (P = 0.0065) from the day of aspiration (D0) up to D8. In mares with follicle diameter of 30–35 mm, serum P4 concentration increased (0.258 ng/ml/day; P = 0.001), as well as in the mares with follicles >35 mm diameter (0.481 ng/ml/day; P < 0.0001), and in mares with natural ovulation (1.236 ng/ml/day; P < 0.0001). Out of the 25 mares with follicular aspirations that formed Corpora hemorragica (P4 >1 ng/ml), 23 (92%) had greater (>2 ng/ml) serum P4 concentrations on Day 8 after aspiration. Of these 23 mares, 75% were in the 25–29 mm group, 9/10 (90%) in the 30–35 mm group, and 11/11 (100%) of the mares in the >35 mm follicular diameter group had luteinization (P4 >2 ng/ml). These results suggest that a functional Corpus luteum can be induced in mares using follicular aspiration and that a minimum 35 mm follicular diameter is needed to reach a progesterone serum concentration compatible with that of a Corpus luteum produced by natural ovulation.