Histopathology of the endometrium in healthy, metritic and septic metritic cows at 5–10 days post-partum. Scale bars in all images are 50 µm. Colors mentioned in brackets refer to the arrowheads. Healthy cows: A–C H&E stained sections, showing intact epithelium layer (black) with vacuoles (red) and PMN infiltration between cells and in the endometrium (green). A continuous hemorrhage line (yellow) can be seen adjacent to luminal epithelium. D Blue MPO immunohistochemistry with red Sytox orange counterstain shows minimal PMN presence. E Gram stain shows small amounts of bacteria (blue) on epithelium and minimal tissue invasion. Metritic cows: F–H H&E stain shows that the luminal epithelium is missing. Hemorrhages are deeper and multifocal (yellow). Foci of PMN infiltration are seen (green). Clumps of bacteria can also be seen (blue). I MPO immunohistochemistry shows PMN infiltration. J Gram stain shows clumps of bacteria on the luminal side (blue, inset), as well as phagocytosis of bacteria by PMN (green). Septic metritic cow: K–M H&E stained sections show absence of luminal epithelium. Blood vessel congestion (purple) and thrombosis (orange) are widespread, as is the massive PMN infiltration (green) and luminal bacteria presence (blue). N MPO immunohistochemistry confirms massive PMN infiltration. O Gram stain shows heavy bacterial load (blue) on luminal side, serosa (left inset) and invasion of endometrium (right inset).