Macrophages in the external muscle layers of mammalian intestines

The literature on macrophages in the muscularis externa of mouse, rat, guinea pig, cat, dog and human gut is reviewed. In smaller mammals macrophages are regularly situated in two locations: in the serosa and at the level of Auerbach's plexus between the longitudinal and circular muscle layers. In addition a few solitary cells are present at the level of the deep muscular plexus. At the level of Auerbach's plexus the macrophages occur as a constant and regularly distributed cell population with intimate associations between macrophages and interstitial cells of Cajal. Morphologically they differ from most resident macrophages in being irregular in shape with 4-6 primary cytoplasmic processes, which branch and give a stellate appearance. They have been demonstrated with endocytotic markers (trypan red, FITC-dextran, cholera toxin), immunocytochemically with macrophage antibodies (F4/80, M1/70) and antibodies against MHC class-II antigen, GABA and cGMP. In muscularis externa of the human gut a regularly distributed cell population of macrophages is not obvious. However, a phenotypically distinct subgroup is identified by light microscopy with the pan macrophage antibodies (EBM11, C3b1 and partly by p150.95), and shows MHC class-II antigen. By electron microscopy muscularis externa macrophages, in all species investigated, appear to be endocytically downregulated, and since they are lysozyme, prostaglandine H synthase (both constitutive and activated) and acid phosphatase negative, they appear to be inactivated cells. Both origin and function of these cells are unknown. They may be immuno-competent, participate in a neuroimmune axis, tissue growth and modulation or other regulations of specific cell functions.