Various cells in skeletal muscle have known myogenic potential [1–4], but their physiological roles in its formation are unclear. The general consensus holds that satellite cells are required for muscle regeneration [5–7]. However, in a recent article in Development, Kostallari and colleagues demonstrated that pericytes are also indispensable for postnatal growth of skeletal muscle [8]. Using a transgenic mouse model for selective diphtheria toxin-induced depletion of NG2+ pericytes, they found that pericyte ablation led to myofiber hypotrophy. Their report is the first to show that skeletal muscle formation in vivo depends on myogenic cells other than satellite cells. Future studies should test whether depleting the skeletal muscle of other interstitial cells with myogenic potential will affect muscle formation; for example, CD133+ [9]; PW1+ interstitial cells [10]; and muscle side population (SP) cells [11].

The skeletal muscle microenvironment is very heterogeneous, with such distinct cell types as fibroblasts, adipocytes, Schwann cells, and blood cells, all of which may influence and, in turn, be influenced by local structural and biochemical cues. Studies should determine how depleting each specific cell population affects skeletal muscle function and regenerative capacity. For instance, Kardon’s group showed that connective tissue fibroblasts without myogenic potential are necessary for muscle regeneration [5].

Kostallari and colleagues also found that pericytes form a niche for satellite cells, similar to the niche in the bone marrow of mesenchymal stem cells for hematopoietic stem cells [12, 13]. During postnatal skeletal muscle development, the distance between pericytes and satellite cells became progressively shorter. After pericyte depletion, the previously quiescent satellite cells were activated. When indirect co-cultures were used to assess the functional interactions between pericytes and satellite cells, the authors found that pericytes both promoted myogenic differentiation and induced satellite cell quiescence. Furthermore, using tissue-specific mouse mutants, they demonstrated that pericytes stimulate muscle growth through IGF1 and control satellite-cell quiescence through ANGPT1 [8], but whether the reverse occurs has not been shown. Do satellite cells form a niche for pericytes? How are pericyte numbers and functions affected after satellite-cell depletion? What signaling molecules/cytokines influence the communication between these cell types? Do satellite cells produce signals that inform such pericyte stem-cell functions as angiogenesis/myogenesis?

Skeletal muscle pericytes are heterogeneous, and two major subpopulations have been distinguished: type-1 (Nestin-GFP-/NG2+/PDGFRβ+) and type-2 (Nestin-GFP+/NG2+/PDGFRβ+) [1, 3, 14–17]. Kostallari et al. propose that the pericytes in the satellite-cell niche are type-2. However, most of their experiments were done in mice and affected both pericyte subtypes, which both express NG2 proteoglycan. Pericyte subtypes have distinct functions, and only type-2 pericytes contribute to muscle formation [2]. Recombination-based lineage tracing and ablation of a specific pericyte subpopulation, rather than all NG2+ pericytes, may better explain this complex microenvironment. Unfortunately, the only marker so far found to be differentially expressed in pericyte subpopulations is nestin-GFP, which is also expressed by other cell types. Tracking pericyte fate or ablating a subtype in vivo will require the discovery of markers that only one pericyte subpopulation expresses, perhaps using single-cell microarrays or RNAseq analysis. Pericytes that do not express NG2 were described in other tissues [18]; we do not know whether they exist in skeletal muscle vessels and what their function might be.

Kostallari and colleagues provide two important new insights into skeletal muscle biology: (1) satellite cells are not the only cells essential for skeletal muscle growth; and (2) the skeletal muscle cellular niche plays an important role in satellite-cell maintenance.