Pressure-sensitivity of endoplasmic reticulum membrane and nucleolus as revealed by electron microscopy

Murine erythroleukemia cells submitted to high hydrostatic pressure (up to 110 MPa, 17 min. at room temperature) remain viable (Mentré et al., 1997) but present, at the ultrastructure level, perturbations which are documented in this work. In cells submitted to 50 MPa, endoplasmic reticulum membranes displayed a characteristic rigid aspect, whereas plasma membrane remained unaffected up to 110 MPa. This result is in agreement with: i) the liquid-crystalline --> gel state transition undergone by phospholipid bilayers under pressure, which involves the structuration of water at the polar-apolar interface; ii) the low concentration in cholesterol of endoplasmic reticulum membranes compared to plasma membrane, and iii) the ability of cholesterol to protect membranes against the effects of high hydrostatic pressure. Nucleoli displayed a remarkable compact aspect above 80 MPa, involving the disappearance of vacuoles and the diminution of fibrillar component, but the retention of granular component. Pressure inhibition of translation is advanced as a possible cause of this perturbation.