  We have developed a modular system for high-resolution microscopy at high
hydrostatic pressure. The system consists of a pressurised cell of volume ~100
microlitres, a temperature controlled holder, a ram and a piston. We have made
each of these components in several versions which can be interchanged to allow
a wide range of applications. Here, we report two pressure cells with pressure
ranges 0.1-700MPa and 0.1-100MPa, which can be combined with hollow or solid
rams and pistons. Our system is designed to work with fluorescent samples
(using a confocal or epifluorescence microscope), but also allows for
transmitted light microscopy via the hollow ram and piston. The system allows
precise control of pressure and temperature [-20-70C], as well as rapid
pressure quenching. We demonstrate its performance and versatility with two
applications: time-resolved imaging of colloidal phase transitions caused by
pressure changes between 0.1MPa and 101MPa, and imaging the growth of
Escherichia coli bacteria at 50MPa. We also show that the isotropic-nematic
phase transition of pentyl-cyanobiphenyl (5CB) liquid crystal provides a
simple, convenient and accurate method for calibrating pressure in the range
0.1-200MPa.
