Megasome biogenesis in Leishmania amazonensis: a morphometric and cytochemical study

Megasomes are large lysosomes found in the amastigote stage of Leishmania s pecies belonging to the mexicana complex. The biogenesis of megasomes was i nvestigated by transmission electron microscopy during the transformation o f promastigotes into the amastigote form of L. amazonensis maintained in ax enic cultures. Mainly small vacuoles with low electron density were found i n the promastigote and early intermediate forms. Morphometrical analysis sh owed an increase in the volume density of these structures during the trans formation process. Cysteine proteinase was localized in this structure by i mmunocytochemical assay. Membrane-bounded structures filled with electron-d ense material were also found in significant amounts from the 2nd day on. T hese structures were relatively abundant, both in axenic and lesion-derived amastigotes, but not in stable long-term axenic amastigote culture. A thre e-dimensional reconstruction of lesion-derived amastigotes and axenic amast igotes of L. amazonensis demonstrated that megasomes comprise almost 5% of the total cell volume. In addition, the development of other organelles was examined during the transformation process.