Ultrastructural changes during the yeastlike to mycelial-phase conversion of Blastomyces dermatitidis and Histoplasma capsulatum.

Fine details of the sequential anatomical events occurring during yeast to mold morphogenesis of the dimorphic fungal pathogens Blastomyces dermatitidis and Histoplasma capsulatum as seen in ultrathin sections are described and illustrated by electron micrographs. Discrete intracytoplasmic membrane systems intimately associated with the plasma membrane were observed to be formed within 6 to 8 hr after induction of the conversion process. Within 12 to 18 hr, an intermediate or transitional cell with Woronin bodies at the septum was formed from the converting yeastlike cell. Both cells were noted to contain increased numbers of mitochondria. At approximately 48 hr from the initial induction of the conversion stimuli, the newly forming hyphal cells were observed to produce postconversional intracytoplasmic membrane systems seen normally in the ultrastructural organization of the fully established mycelial-phase cell. These membrane systems appear to be associated with normal septal formation. Although minor variations of time were observed in the occurrence of the sequential events, it is suggested that yeastlike to mycelial-phase conversion of these two fungal pathogens proceeds via a similar mechanism of ultrastructural reorganization.