Animal Reproduction Science 222 (2020) 106605 Available online 16 September 2020 0378-4320/© 2020 Elsevier B.V. All rights reserved. Review article A review of in vivo and in vitro studies of the mare endometrium Riley E. Thompson a, b , Christopher Premanandan c , Budhan S. Pukazhenthi b, **, Brian K. Whitlock a, * a Department of Large Animal Clinical Sciences, University of Tennessee, Knoxville, TN, USA b Center for Species Survival, Smithsonian Conservation Biology Institute, Front Royal, VA, USA c Department of Veterinary Biosciences, The Ohio State University, Columbus, OH, USA A R T I C L E INFO Keywords: Equine Endometrium Uterus Cell culture Mare In vitro ABSTRACT The inner layer of the uterus, the endometrium, is responsible and necessary for many repro- ductive functions. Normal reproductive cyclicity, maternal recognition of pregnancy, maternal interaction with the embryo, and interaction of the reproductive tract with pathogens are dependent on the endometrium. Although most studies have been conducted in vivo using live animals, recent advances in in vitro approaches could facilitate future research in a laboratory setting with minimal effect on animals. Many reproductive studies have been performed in vivo and in vitro in equids, but new in vitro methods to study the endometrium of mares remain un- explored. In this review, there is a description of the normal anatomy and physiology of the mare endometrium in vivo, in vitro endometrial cell culture techniques that have been previously described for the mare, and opportunities for future reproductive research using in vitro methods. 1. Introduction The endometrium is an important component of reproductive signaling about which there is little information. In many species, the ovaries are seemingly the component of the female reproductive tract on which there is the greatest research focus. In mares, more research is needed to further understand normal physiology, pathological conditions, and therapeutics of the pathology of the endometrium. The endometrium interacts directly with the embryo in early pregnancy and later with the placenta and contains glands that secrete histotroph, or “uterine milk,” which is a nutrient source for the conceptus (Amoroso, 1952). There can be pathologic effects at the endometrium that can interfere with establishment of pregnancy and/or maintenance of a pregnancy throughout the gestational period. Developing approaches to study the endometrium in vitro could allow the assessment of multiple treatments to be performed simultaneously rather than conducting evaluations on individual mares during multiple estrous cycles. In addition, mares can either be evaluated individually by keeping a mare’s cells separate or reducing individual variation by pooling cells from multiple mares. Furthermore, if cryopreservation is utilized, a repository of endometrial tissue could be utilized for these in vitro reproductive eval- uations to occur throughout a yearly period (Thompson et al., 2019). Because mares are seasonally anestrus, reproductive cycles only occur for about half of the year (Nagy et al., 2000). If a cryo-repository were available, endometrial tissue collected during different stages of the estrous cycle and of various pathologies could be stored, and samples could be thawed as needed for conducting * Corresponding author at: 2407 River Drive, Knoxville, TN, 37996, USA. ** Corresponding author at: 1500 Remount Road, Front Royal, VA, 22630, USA. E-mail addresses: pukazhenthib@si.edu (B.S. Pukazhenthi), bwhitloc@utk.edu (B.K. Whitlock). Contents lists available at ScienceDirect Animal Reproduction Science journal homepage: www.elsevier.com/locate/anireprosci https://doi.org/10.1016/j.anireprosci.2020.106605 Received 24 February 2020; Received in revised form 9 September 2020; Accepted 10 September 2020