Degenerating and regenerating skeletal muscles contain several subpopulations of macrophages with distinct spatial and temporal distributions.

Macrophages of different phenotypes can be detected using a panel of antibodies. We have used such antibodies to demonstrate that the macrophages in freeze-lesioned skeletal muscles are heterogeneous, with each subtype having a distinct location within the lesion as well as distinct times of arrival and departure from the lesion. ED1+ monocytes and macrophages began invading the lesion within 3 h and were abundant until necrotic tissue had been removed. In some macrophages, the ED1 antigen aggregated into a single or a few clumps and such cells persisted in the regenerated area for at least 21 d. ED2+/Ox6-/ED1-/RM1- cells are one of the major subpopulations of resident macrophages within skeletal muscle. Cells of this phenotype accumulated in the epimysia and perimysia surrounding the lesions but did not penetrate into the lesion until extensive phagocytosis had occurred (usually 1 or 2 d). ED2+ cells were subsequently concentrated in the regenerating connective tissues and empty remnants of phagocytosed fibres. They only rarely invaded necrotic tissue, even when immediately adjacent to it, suggesting that this type of macrophage has a specialised function which is unrelated to removal of damaged tissue. The ED2+ macrophages were CD4+ and it is probably that macrophages of this type have been previously misclassified as CD4+ T cells. Skeletal muscles also contain numerous Ox6(Ia)+/ED2- resident macrophages. Unlike ED2+ macrophages, Ox6+ macrophages invaded the damaged muscles half a day after lesioning and were abundant in necrotic tissue. As regeneration occurred, the Ox6+ macrophages became restricted to the connective tissues of the muscle, which is their normal location.