Double-membraned vesicles and their possible role in the ontogeny of microbodies of Basidiobolus haptosporus.

A concomitant but transient occurrence of large numbers of double-membraned vesicles with immature, culturally induced microbodies was demonstrated in thin sections of Basidiobolus haptosporus soon after transfer of the fungus to a defined medium containing xanthine or its catabolites as the sole source of nitrogen. Double-membraned vesicles were rapidly formed as derivatives of endoplasmic cisternae later to occur free in the cytoplasm, often as the most predominant cytoplasmic inclusion. Densitometric measurements revealed that heavy metal-binding cytoplasmic constituents were concentrated within the vesicular lumen. Most of the double-membraned vesicles appeared to undergo degeneration; they were rare to absent in thin sections of older cells. At times, double-membraned vesicles were seen connected to newly formed microbodies in a manner suggesting ontogenic relationships. The outer vesicular membrane was continuous with the single limiting membrane of the microbody via a short tubule. We interpret these observations as empirical ultrastructural evidence to suggest that some double-membraned vesicles become specialized and function as precursor inclusions in the biogenesis of new microbody populations committed to purine salvage.