SUMMARY: The fine structure of the yeast-like cells of Histoplasma capsulatum Darling and H. duboisii Vanbreuseghem has been studied and compared by electron microscopy. The micro-morphology of single and budding cells, as well as some details of the budding process, are described. No evidence of a capsule or slime layer on the outside of the cell wall of either species was demonstrated. In both species the cell walls were found to be laminated and in general slightly thicker in H. duboisii than in H. capsulatum. The possibility of the laminations being associated with the presence of chitin in the framework of the cell walls is discussed. The cells of the two species, strikingly similar in structure, were found to be highly differentiated, resembling animal cells in some of their micromorphological features, namely in the presence of endomembrane systems, in mitochondrial numbers and forms, and in nuclear fine structure.
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