A microscopic system, consisting of a transmitted light microscope and constant-pressure flow vessel, is specifically designed to enable highly resolved high-pressure primary breakup investigations at engine-like densities. At first measurements diesel and two alternative fuels are injected with a standard diesel injector at different ambient pressures. Reynolds and Ohnesorge number are varied over a wide range and several individual mechanisms are identified: an asymmetric outlet flow, the formation of sheets, ligaments, and droplets, and finally the growth of azimuthal and transversal surface disturbances. Wavelength magnitudes of transversal disturbances are in good agreement with existing analytical theories of boundary layer instabilities. Azimuthal disturbances are associated with nozzle internal cavitation. For the given high Weber number azimuthal disturbances are mainly responsible for the overall shape and subsequent breakup of the jet core.