SYNOPSIS Motile cells and cysts of Polytomella agilis, obtained over the entire growth cycle, were examined by electron microscopy. In typical late log phase cells there is a concentric arrangement of the internal organelles around the centrally located nucleus. Lying just beneath the plasma membrane is a peripheral band of elongate mitochondria. Numerous well defined Golgi bodies are also distributed around the nucleus. Vesicles associated with the Golgi body increase in size with distance from the secretory edges of the organelle. Cytoplasmic membranes with associated ribosomes are found between the mitochondrial and Golgi regions. A layer of slender membrane-limited structures is located near the mitochondrial layer. These organelles, which resemble proplastids, become highly branched during late log and early stationary phase, reaching maximum development in late stationary and early pre-cyst stages.
Large storage granules of varying density are found within the cell. The PAS-positive granules have been isolated and shown to contain starch. There is an increase in the amount of this storage material as the cells enter the stationary phase.
The remainder of the cytoplasmic matrix is finely granular and contains numerous free ribosomes except in the region of the anterior papilla. Four flagella arise from basal bodies at the anterior end of the cell.
The cyst is characterized by a thick multi-layered cell wall whose electron density obscures the limiting plasma membrane. Large storage granules are located close to and often in contact with the periphery of the cell, suggesting their involvement in the process of cell wall deposition.
Altho mitochondria can still be seen in the mature cyst, other cytoplasmic organelles often appear atypical. The mature cyst has an irregular profile possibly due to shrinkage from dehydration.