Microfluidic platform for the formation of neuromuscular junctions (NMJs). (A-B) Design of the microfluidic platform and the muscle and neuronal chambers. Scale bar: 1 μm. (C) Fluorescent image showing neurite extension from the optogenetic motoneuron neurosphere to the skeletal microtissue. Scale bar: 500 μm. (D) Evolution of axonal growth from the neurosphere to the muscle tissue during the first week in co-culture (days 1, 2, 3, 5 and 7). (E-F) Confocal images showing innervation of the skeletal micro tissues and muscle striation after 10 (E) and 20 days in co-culture (F) (green = ChR2-YFP, red = α-actinin, blue = DAPI). Scale bars: 100 μm.