Ultrastructural changes during formation and germination of microcysts in Polysphondylium pallidum, a cellular slime mould

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Abstract
Ultrastructural changes during encystation of Polysphondylium pallidum, a cellular slime mould, include an increase in fibrillar material in the cytoplasmic matrix, the formation of cytoplasmic microprojections at the cell periphery and the occurrence of tiny vesicles and some larger vacuoles near the cell periphery. The cyst wall appears first as a fluffy, loose network of fibrils. In the mature cyst it consists of a dense inner and a somewhat looser outer layer. It contains inclusions of apparently cytoplasmic origin. Electron-dense material lines the cell periphery beneath the plasma membrane. Excystment of the myxamoeba is accompanied by a swelling of aggregated vacuoles and polyvesicular bodies, the disappearance of the peripheral cytoplasmic lining, and a general loosening of the wall texture. For a limited period the cisternae of the rough endoplasmic reticulum appear highly dilated. The loosened wall eventually breaks and the myxamoeba emerges by extending pseudopodia through the rupture, leaving the entire cyst wall behind.