Zebrafish fins provide an excellent model to study cellular and molecular mechanisms of regeneration.

This chalkboard sketch conveys the concept that stem cells in adult epithelial tissues follow different trajectories for self‐renewal and differentiation depending on the context of their environment. Under conditions of homeostatic maintenance, stem cells transit through intermediate states to give rise to committed progenitors and mature cell types. During injury‐induced regeneration, stem cells become activated and traverse transient states unique to regeneration, before making the choice to self‐renew or differentiate. Finally, cellular plasticity, in which mature cells dedifferentiate to a multipotent state, can also contribute to cell replacement after injury.

Two scenarios by which persistent sex differences in gene expression can arise in the developing brain: long‐term changes in chromatin state lead to increased levels of transcription of a given gene, or developmental programming of cell types results in an increased number of cells expressing that gene. Without single‐cell resolution, these outcomes cannot be identified.