Lineage relationships in muscle stem cells: linking functional fate to molecular state.

Muscle stem cells (MuSCs) represent a subset of satellite cells (SCs) and constitute the stem cell compartment of the skeletal muscle tissue. MuSCs and SCs are quiescent under homeostatic conditions but activate upon injury to drive regeneration and repopulation of the stem cell pool. Heterogeneity has become a central question in MuSC biology, prompting efforts to define MuSC lineage progression through both classical and modern experimental approaches. However, the relationship between the resulting lineage progression models remains unclear. Classical lineage-tracing and transplantation studies uncovered functional heterogeneity and suggested a hierarchical organization. Next-generation sequencing and single-cell technologies later resolved molecular heterogeneity and suggested a dynamic model of continuous state transitions. These two models are often interpreted independently, and their interrelationship is often overlooked. In this review, we critically evaluate the current evidence on the functional and molecular heterogeneity of the MuSC compartment during the quiescence-to-activation transition, emphasizing lineage progression models and the contributions of the regenerative niche and return-to-quiescence mechanisms. We specifically address the link between functional fate and molecular state, proposing a lineage progression model in which these concepts are not mutually exclusive, but rather are intertwined depending on the level and specific transition. Recognizing how functional stemness and dynamic molecular state transitions are related should encourage future experimental designs that combine lineage tracing and transplantation with single-cell multi-omics to obtain a holistic view of MuSC function. This is necessary to develop a better understanding of MuSC deficits in disease and aging, and pave the way for the development of therapeutic strategies targeting functional fate and molecular state links comprehensively.