ABSTRACT
Adult zebrafish fins develop and robustly regenerate an elaborately branched bony ray skeleton. During caudal fin regeneration, basal epidermal-expressed Sonic hedgehog (Shh) locally promotes ray branching by partitioning pools of adjacent progenitor osteoblasts (pObs). We investigated if and how Shh signaling similarly functions during developmental ray branching. As during regeneration, shha is uniquely expressed by basal epidermal cells (bEps) overlying pOb pools at the distal aspect of outgrowing juvenile fins. Lateral splitting of each shha-expressing epidermal domain followed by the pOb pools precedes overt ray branching. We use ptch2:Kaede fish and Kaede photoconversion to identify short stretches of shha+ bEps and neighboring pObs as the active zone of Hh/Smoothened (Smo) signaling. Basal epidermal distal collective cell migration continuously replenishes each shha+ domain with individual cells transiently expressing and responding to Shh. In contrast, pObs have constant Hh/Smo activity until differentiating. Hh/Smo inhibition using the small molecule BMS-833923 (BMS) prevents branching in all fins, paired and unpaired, with minimal effects on fin outgrowth or skeletal differentiation. Staggered addition of BMS indicates Hh/Smo signaling acts throughout the branching process. shha+ bEps and pObs are tightly juxtaposed at the site of Hh/Smo signaling, as with regenerating fins. We use live time-lapse imaging and cell tracking to find Hh/Smo signaling restrains the distal movement of bEps by apparent ‘tethering’ to pObs. We conclude short-range Shh/Smo signaling enables ray branching by re-positioning pObs during both fin development and regeneration. We propose instructive basal epidermal collective migration and Shh/Smo-promoted heterotypic cell adhesion between bEps and pObs directs fin skeleton branching morphogenesis.
Competing Interest Statement
The authors have declared no competing interest.
