Natural Rescue and Resurgence of the Equine Corpus Luteum

• Three cases of rescue of an apparently regressed corpus luteum (CL; progesterone [P4] approximately 1 ng/mL) have been reported.
• Resurgence of the CL is indicated by an increase in dimensions and P4 output.
• Resurgence occurs after rescue of CL of estrous cycle and at 34 days of pregnancy.
• Rescue at the end of an estrous cycle is attributable to luteinizing hormone (LH).
• Resurgence at 34 days of pregnancy is attributable to the beginning of equine chorionic gonadotrophin (eCG) secretion

The recuperative powers of the equine corpus luteum (CL) are astonishing. The availability of transrectal ultrasonic imaging has led to three reports describing the rescue of the CL of the estrous cycle after the CL regressed to the dimensions and structure characteristic of the end of luteolysis. In two mares, a rescued CL increased in size and apparently in progesterone output during the postovulatory development of the CL of the next cycle or ensuing pregnancy. The other mare had a persistent CL (PCL). The CL from the ovulation at the beginning of the PCL decreased in area and in progesterone output to the size and function used to define the end of luteolysis. The transient regression occurred in the absence of a second ovulation or a follicle of preovulatory diameter but in the presence of an luteinizing hormone (LH) surge. Better known and documented is resurgence of the primary CL of pregnancy in cross-sectional area and progesterone output beginning on about 35 days after ovulation associated with the secretion of equine chorionic gonadotropin (eCG) from the developing endometrial cups. The demonstration of resurgence of the primary CL during pregnancy was preceded by about 40 years of misinformation and dogma about demise of the primary CL and replacement by secondary CL.