A who's who of cell types in skeletal muscle.

Cells are the basic unit of life. In multicellular organisms, cells are organized into tissues. This enables a division of labor where tissues perform complex tasks via coordinated actions of specialized cell types. Accordingly, the cellular collective determines tissue function. A comprehensive overview of which cell types exist in solid tissues is lacking. Using skeletal muscle as a model, we discuss basic principles for cell type classification, summarize 62 unified definitions of cell types in terms of lineage, molecular signatures, and function from the literature, and discuss how these cell types contribute to muscle function. For cell types for which quantitative data was available, we compare abundances in immunohistology and single cell and single-nucleus RNA sequencing data, revealing cell types that are commonly over- or underrepresented in each method. The result is a cell type resource that will serve as a benchmark for single-cell studies of skeletal muscle.