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The cells of the innate immune system are the sentinels of tissue homeostasis, acting as ‘first responders’ to cellular damage and infection. Although the complex interplay of different immune cells during the initial inflammatory phases of infection and repair has been documented over many decades, recent studies have begun to define a more direct role for specific immune cells in the modulation of tissue repair. One particular cell of the innate immune system, the macrophage, has emerged as a central integrator of the complex molecular processes that drive tissue repair and, in some cases, the development of specific cell types. Although macrophages display directed orchestration of stem cell activities, bidirectional cellular crosstalk mechanisms allow stem cells to regulate macrophage behaviour within their niche, thus increasing the complexity of niche regulation and control. In this Review, we characterize the roles of macrophage subtypes in individual regenerative and developmental processes and illustrate the surprisingly direct role for immune cells in coordinating stem cell formation and activation.

Muscle precursors need to be correctly positioned during embryonic development for proper body movement. In zebrafish, a subset of hypaxial muscle precursors from the anterior somites undergo long-range migration, moving away from the trunk in three streams to form muscles in distal locations such as the fin. We mapped long-distance muscle precursor migrations with unprecedented resolution using live imaging. We identified conserved genes necessary for normal precursor motility ( six1a , six1b , six4a , six4b and met ). These genes are required for movement away from somites and later to partition two muscles within the fin bud. During normal development, the middle muscle precursor stream initially populates the fin bud, then the remainder of this stream contributes to the posterior hypaxial muscle. When we block fin bud development by impairing retinoic acid synthesis or Fgfr function, the entire stream contributes to the posterior hypaxial muscle indicating that muscle precursors are not committed to the fin during migration. Our findings demonstrate a conserved muscle precursor motility pathway, identify dynamic cell movements that generate posterior hypaxial and fin muscles, and demonstrate flexibility in muscle precursor fates.

Stem cells are undifferentiated cells that play crucial roles during development, growth and regeneration. Traditionally, these cells have been primarily characterised by histology, cell sorting, cell culture and ex vivo methods. However, as stem cells interact in a complex environment within specific tissue niches, there has been increasing interest in examining their in vivo behaviours, particularly in response to injury. Advances in imaging technologies and genetic tools have converged to enable unprecedented access to the endogenous stem cell niche. In this Spotlight article, we highlight how in vivo imaging can probe a range of biological processes that relate to stem cell activity, behaviour and control.