235 MORPHOMETRIC DETERMINATION OF ENDOMETRIAL LEUKOCYTE MIGRATION DURING DIFFERENT STAGES OF THE EQUINE ESTROUS CYCLE

Abstract

Uterine defenses against bacterial challenge are more efficient during estrus than during diestrus. The aim of the present study was to test the hypothesis that the magnitude of endometrial leukocyte migration after instillation of semen into the uterine lumen is greater during estrus than during diestrus. Eight Nooitgedacht mares of proven fertility were used in the study. Each mare received a different treatment (Rx) during each of 4 estrous cycles, with a rest cycle after each Rx. Two Rxs were performed during diestrus and 2 during estrus. One Rx for each stage of the cycle was a control Rx (no challenge to the endometrium). At time zero (T0) of challenged cycles, a single aliquot of 13 mL raw semen, frozen-thawed without addition of any cryoprotectant or extender, was instilled into the uterus. An endometrial biopsy was taken 6 and 48 h after T0 and endometrial cytology and culture were performed at 48 and 120 h after T0. An image analyzer was used to record the total number of cells, round cells, neutrophils, and eosinophils per unit surface area of epithelium, stratum compactum (SC), and stratum spongiosum (SS). The relative number of round cells, neutrophils, and eosinophils were expressed as proportions of the number of each cell type per total number of cells. The use of an image analyzer made the collection of quantitative data from histologic sections possible. Without respect to challenge or time, the total numbers of epithelial cells per mm epithelium and the total number of cells (fibroblasts, round cells, neutrophils, and eosinophils) and of round cells per mm² of SS were significantly greater during diestrus than during estrus; however, no such difference could be demonstrated for the SC. The stage of the estrous cycle had no meaningful influence on any other (measured or calculated) variable. During challenged cycles, absolute and relative numbers of neutrophils were significantly greater in the epithelium, SC, and SS than during control cycles. There was an interaction (not always reaching significance) between Rx and time with regard to the absolute and relative numbers of neutrophils in epithelium and SS and round cells in the epithelium. Numbers of neutrophils and round cells were significantly higher 6 h after Rx than 48 h after Rx in challenged cycles, but did not differ during control cycles. During challenged cycles, the stage of the estrous cycle when Rx occurred had no effect on the duration of the induced endometritis, the occurrence of positive cytology or culture results, or the type of bacteria that were cultured. Regardless of the stage of their cycles when they were challenged, all mares rid themselves of the opportunistic pathogens placed into the uterine lumen within one estrous cycle. It is concluded that the stage of the estrous cycle did not influence the magnitude of the endometrial leukocyte response to a standardized challenge with semen in these reproductively sound mares. A similar study will be required to test whether this conclusion also holds true for mares that are susceptible to endometritis.