Evolution of the Concepts of Endometrosis, Post Breeding Endometritis, and Susceptibility of Mares

Simple Summary Our understanding about inflammation of the endometrium after mating and susceptibility of mares to endometritis has changed in the last 100 years since it was recognized for the first time. Initially, it was believed that bacteria introduced into the uterus during mating could infect the uterus until it was shown that sperm induce neutrophilia. It was realized that post breeding endometritis (PBE) is a physiological defense mechanism used to clean the uterus from excess semen and inflammatory by-products. In mares susceptible to endometritis, PBE can be prolonged beyond the normal duration of 24 h. Delayed uterine clearance due to conformational defects, deficient myometrial contractions, and failure of the cervix to relax is detected by intrauterine fluid accumulation and is an important reason for susceptibility to endometritis. Untreated prolonged PBE can lead to bacterial or fungal endometritis called persistent or chronic endometritis. Multiparous aged mares are more likely to be susceptible. When sperm arrive in the uterus, pro-inflammatory cytokines are released. They attract neutrophils and induce modulatory cytokines which control inflammation. However, persistence of neutrophils and pro-fibrotic cytokines can have deleterious effects in inducing endometrosis. In this paper, the pathogenesis of fibrosis is reviewed. Endometritis and endometrosis are interconnected influencing each other. In this paper, the evolution of our understanding about post breeding endometritis (PBE), the susceptibility of mares, and events leading to endometrosis are reviewed. When sperm arrive in the uterus, pro-inflammatory cytokines and chemokines are released. They attract neutrophils and induce modulatory cytokines which control inflammation. In susceptible mares, this physiological defense can be prolonged since the pattern of cytokine release differs from that of resistant mares being delayed and weaker for anti-inflammatory cytokines. Delayed uterine clearance due to conformational defects, deficient myometrial contractions, and failure of the cervix to relax is detected by intrauterine fluid accumulation and is an important reason for susceptibility to endometritis. Multiparous aged mares are more likely to be susceptible. Untreated prolonged PBE can lead to bacterial or fungal endometritis called persistent or chronic endometritis. Exuberant or prolonged neutrophilia and cytokine release can have deleterious and permanent effects in inducing endometrosis. Interactions of neutrophils, cytokines, and prostaglandins in the formation of collagen and extracellular matrix in the pathogenesis of fibrosis are discussed. Endometritis and endometrosis are interconnected, influencing each other. It is suggested that they represent epigenetic changes induced by age and hostile uterine environment.Peer reviewe