Abstract: An in vitro chamber for sustaining organ tissue samples during optical study with, for example a microscope, has a tray assembly with a plurality of wells, each of which may be provided with a clampable sleeve or a vertically adjustable thimble such that an associated organ tissue sample contacts a nutrient medium contained in a reservoir. The tray assembly rests upon a heater assembly having a water-jet-driven magnetic stirrer magnetically coupled with a driven magnetic stirrer in the nutrient reservoir to maintain a substantially uniform temperature in both the heater assembly and the nutrient reservoir. A thermally insulating collar surrounding the tray assembly and the heater assembly maintains relative positions thereof. A transparent cover encloses the top of the tray assembly so that air borne exchanges with the wells and nutrient splashes are substantially contained.