The timing of the maternal recognition of pregnancy signal is specific to individual mares

Abstract

Over several breeding seasons, eighteen inseminated mares were examined at 8(±1)h intervals from late oestrus until after ovulation. The time of ovulation was determined as 4h before the first detection of ovulation. Examinations for pregnancy began at 8h intervals between 236 to 260h (late Day 9 to 10) post ovulation. When present an embryo was reduced by compression with a total of 132 reductions on Days 10 (n=15), 11 (n=47), 12 (n=36), 13 (n=27), 14 (n=5) and 15 (n=2). Ten mares were used on multiple occasions, undergoing between 3 and 16 embryo reductions each. Mares were subsequently examined, first to determine non-pregnancy, and then to determine whether luteolysis or luteostasis occurred. Luteostatic mares maintained an ultrasonically identifiable corpus luteum, and between Days 22 and 25 were returned to oestrus by administration of PGF2α. Increasing numbers of mares became luteostatic following embryo reduction as the number of days post ovulation increased. The earliest initiation of luteostasis occurred when an embryo was reduced at 252h (10.5 days). Between the 8h examinations on Days 10 to 12 there was an increase in the proportion of mares undergoing luteostasis. When mares were used on multiple occasions, individual animals had aset time point within a ±8h window post ovulation when they responded to the MRP signal and after which, but not before, embryo reduction resulted in luteostasis. For the eight mares which had embryos reduced ≥6 occasions, the time when luteostasis occurred was consistent in an individual but varied between mares between Day 10 (244–252h) and Day 14 (348h). For example, one mare which did not recognise the presence of an embryo until Day 14 (348h), had 10 reductions undertaken between Days 11 to 13 (268–332h) and all resulted in luteolysis, but all 3 reductions at or after Day 14 (348h) resulted in luteostasis. Despite the late timing of when the MRP signal in this animal was recognised, no spontaneous embryo loss had been seen in three previous pregnancies. These results suggest that the timing of the luteolytic cascade is variable between mares but relatively constant in its timing in an individual animal. This would imply a consistent set of physiological processes that program the endometrium for the luteolytic cascade in any given animal. Further data is needed to investigate whether oestrous cycle length and variation in the oestrous and dioestrous phases of the mare play a role in initiating the mechanisms that enable the uterus to respond to the MRP signal.