Murine monocytes: Origins, subsets, fates, and functions

Monocytes are a conserved population of leukocytes that are present in all vertebrates, with some evidence of a parallel cell population in fly hemolymph (1). Monocytes are defined by their location in the bloodstream, their phenotype and nuclear morphology, as well as by their characteristic gene and microRNA expression signatures (2-5). In mice, monocytes represent 4% of the nucleated cells in the blood, with considerable marginal pools in the spleen and lungs that can be mobilized on demand (6,7). Within the blood, monocytes, and in particular the classical Ly6C+mouse subset, exhibit a characteristically short half-life of 20 h (8), akin to that of similar ephemer neutrophils (9).