Transmission electron micrograph of ultrathin cryosections from the muscle layer of the foetal mouse ductus arteriosus. (a) Dual immunogold labelling for COX1 (particle, 10 nm) and microsomal PGE synthase (particle, 5 nm); note colocalisation of the two labels (arrows) in vesicle-like structures close to the nucleus (N). (b) Dual immunogold labelling for COX2 (particle, 10 nm) and microsomal PGE synthase (particle, 5 nm); colocalisation of the two labels (arrows) in proximity of the nucleus (N). (c) Dual immunogold labelling for COX1 (particle, 10 nm) and microsomal PGE synthase (particle, 5 nm) in the peripheral cytoplasm; colocalisation of the two labels in vesicle-like structures (arrows). (d) Dual immunogold labelling for COX2 (particle, 10 nm) and microsomal PGE synthase (particle, 5 nm) in the peripheral cytoplasm; colocalisation of the two labels (arrows). Note that in both perinuclear and peripheral cytoplasm, COX2 colocalised more frequently than COX1 with microsomal PGE synthase. Bar represents 0.1 μm.