Ovarian, uterine and embryo dynamics in horses versus ponies

Characteristics of the internal genitalia were compared between horses and ponies contemporaneously in a single study by a single operator, Diameter of the preovulatory follicle for 10 days before ovulation (Day 0) and cross-sectional area of the corpus luteum for 10 days after ovulation were measured in 12 horses and 12 ponies. Follicle growth rate and maximum diameter attained during growth and luteal tissue area were not different between mare types, However, ultrasonic echogenicity of luteal tissue was less on Day 0 (P<0.09) and Day 1 (P<0.01) in ponies than in horses, suggesting that under field conditions mare type should be considered when using the extent of luteal gland echogenicity for estimating the number of days postovulation and luteal function, Circulating concentrations of progesterone averaged over Days 3 to 10 were greater in ponies (10.3 ng/ml) than in horses (8.7 ng/ml) as suggested by a tendency (P<0.1) towards a type-by-day interaction. Luteal, uterine and embryonic characteristics during Days 0 to 40 of pregnancy were compared between 7 horses and 9 ponies. Luteal tissue area of the primary corpus luteum was not different between mare types during Days 0 to 30 and during luteal resurgence after Day 30. The interaction between mare type and day was significant for circulating progesterone concentrations; the interaction seemed to result primarily from higher progesterone levels in ponies during Days 6 to 12 and Days 34 to 40, Progesterone concentrations were about 25% higher in ponies on the approximate days that levels were at their highest in the two types of mares. These results indicate a need for further studies on the effect of mare size or type on circulating concentrations of progesterone and the consideration of size or type in clinically evaluating progesterone levels in pregnant mares, Apparent partial recovery of the corpus luteum of the previous estrous cycle occurred during early pregnancy in one pony. Increasing diameter of the embryonic vesicle and the extent of uterine tone were similar between horses and ponies, Although diameter of the uterine horns decreased in parallel for the two mare types during Days 0 to 40, the diameter was significantly less (2.2 mm difference averaged over all days) for ponies than horses, The temporal relationship between an expanding embryonic vesicle concurrently with decreasing uterine horn size and enhanced uterine turgidity is compatible with the postulated mechanism of embryo fixation, The smaller uterus in ponies with no difference in size of the conceptus between mare types could account for earlier fixation of the embryonic vesicle in ponies than in horses.