Cyst wall formation in the ciliated protozoan Colpoda cucullus: cyst wall is not originated from pellicle membranes

Abstract

Ultrastructural changes during encystment (resting cyst formation) of Colpoda cucullus were
observed with special reference to cyst wall formation. Within 1.5 h after encystment induction,
fragmentation of some of the mitochondria occurred, followed by the appearance of a number of
net-like globules in the cytoplasm, which were expelled to outside and then involved in cell-to-cell
or cell-to-substratum adhesion. The cells were transformed into a spherical shape, and a number
of ellipsoidal vacuoles in which ectocyst precursor (amorphous substance) was contained
appeared near the cell surface (some of these opened to the outside). In this stage, the ectocyst
(outermost layer) was completed. In 3 ~ 10 h toluidine blue-stained substance (TBS), which was
probably the precursor for the first synthesized layer of endocyst (en-1), was released from a
point near the cell surface and diffused over the cell surface (diffused into between the ectocyst
and plasma membrane). Thereby, the ectocyst was lined by the en-1. Thereafter, several layers
of endocyst were periodically formed for 1 ~ 2 weeks. Finally a number of reserve grains were
accumulated, and cilia were resorbed.