During vertebrate development, T cells migrate from the thymus to colonise various tissues where they play a cruicial role in tissue homeostasis and wound healing. Here, Tanner Robertson, Anna Huttenlocher and colleagues characterise leukocyte migration within heterogeneous tissue environments in zebrafish. First, the authors use a previously described pan-T-cell reporter to identify an epidermal T-cell population on the outer scale layer of adult zebrafish that is distinct in morphology and function from the T cells within the previously characterised tessellated lymphoid network. These scale T cells resemble mice dendritic epidermal T cells: they maintain a dendritic morphology, round up in response to nearby tissue damage and are among the first T-cell precursors to leave the thymus and traffic to the developing epidermis. Further, while T cells in the thymus and intestines migrate with F-actin rich pseudopods at their leading edge, skin T cells that are confined between the outer epidermal layers and the basement membrane accumulate F-actin at the rear edge and form a cytoplasmic bleb at the leading edge to propel themselves using retrograde actin flow and myosin-dependent contraction. Together, this work reveals how leukocytes adapt diverse migratory modes in response to their microenvironment in vivo.
