Osteal macrophages: a new twist on coupling during bone dynamics.

Osteoimmunological interactions are central to maintaining bone homeostasis and are key mechanisms in bone pathology. Macrophages are highly adaptable cells with pleiotropic actions. They have important roles in development, homeostasis and both innate and adaptive immunity. Macrophages can have broad ranging effects on bone, particularly in pathologic situations, but they are most commonly considered for their in vitro potential as an osteoclast precursor. We have recently shown that, like most tissues, the endosteum and periosteum contain a population of resident tissue macrophages (OsteoMacs) that impact on the bone formation process and are likely to play important roles in the bone niche. This review discusses the wider impact of macrophages in bone homeostasis and disease and proposes novel roles for OsteoMacs in bone modelling and remodelling.