Live and apoptotic intraepithelial neutrophils in mammary epithelial cells. Lactating C57BL/6 mice where infused with 106 CFUs of viable E. coli DH5α bacteria. H&E staining of paraffin sections (A) and transmission electron microscopy imaging (B–D) of mammary tissues 24 h after challenge. Mastitis is characterized by massive recruitment of neutrophils into the alveolar spaces (white arrow in A; see also Additional file 4B) and alveolar epithelial cells containing intraepithelial neutrophils (yellow arrows in A). Morphologically normal (white arrow in B, C) and apoptotic neutrophils (yellow arrows in B, D) are visible in mammary alveolar epithelial cells. Microvilli are visible on the apical membrane of the mammary epithelial cells (white arrow in B) and milk space is marked by asterisk in B. Scale bars 50 µm (A), 10 µm (B), 2000 nm (C) and 2 µm (D). All images are representative of the entire sample. The histological morphology and pathology results were very similar for each gland in a given mouse and between mice.