RESUMEN
The ultrastructural changes which occur during the mycelium to yeast transition in Histoplasma capsulatum induced by a temperature shift from 25 degrees C to 34 degrees C are described and compared to those observed after a temperature shift from 25 degrees C to 37 degrees C. 24 hours after the temperature shift to 34 degrees C only 8% of the cells are lysed. However, many mitochondria have lost their characteristic elongated form and have become rounded. Vesicular cristae which are no longer oriented parallel to the long axis of the mitochondria are also observed. In contrast a temperature shift from 25 degrees C to 37 degrees C induces lysis of 70% of the cells; mitochondria are rarely observed in the remaining cells. These ultrastructural changes can be correlated with the uncoupling of oxidative phosphorylation and the production of heat shock proteins.