Electron microscopy of the transitional conversion cell of Histoplasma capsulatum

Abstract

Aspects of the fine structure of the transitional conversion cell formed during the early stages of the yeast to mold morphogenesis ofHistoplasma capsulatum as seen in ultrathin sections are described and illustrated by electron micrographs. Formation of the transitional cell was observed to occur with the highest degree of frequency between the 18th and 24th hr following induction of the conversional stimulus, although many yeastlike cells were observed to undergo degeneration or to initiate conversion only to abort the process. Cytoplasmic streaming and organelle migration from the parent yeast to the transitional cell was observed to occur prior to septation. The cell wall of the transitional form is thinner than that of the yeast and appears to arise from the inner portion of the laminated cell wall adjacent to the plasma membrane of the converting yeastlike cell. Interseptal or Woronin bodies were observed in association with the septal pore of the completed septum and were observed in the cytoplasm of both the yeastlike and transitional cell. The presence of these structures support strongly the pre-hyphal character of the converting cell complex.