Immunofluorescence labelling of androgen and estrogen receptors in pig spermatozoa: A) AR red fluorescence in sperm proximal mid-piece. B) P450arom red brilliant light in the proximal tail of sperm with a diffuse labelling in the distal tail. C) ERα red fluorescence in the sperm mid-piece, together with a faint labelling in the tail. D) ERβ green intense light in the sperm acrosomal region. Inserts: immunonegative absorption controls. Scale bars: 5 μm.