The Journal of Cell Biology, Vol 84, 172-183, Copyright © 1980 by The Rockefeller University Press
Internalization of macromolecules from the medium in Suctoria
MA Rudzinska
Takophrya infusionum like all other Suctoria lacks an oral cavity. Its
feeding apparatus consists of tentacles, long narrow tubes through which
the contents of the living prey are ingested. For normal growth,
reproduction, and longevity of clones, Tokophrya needs supplements deriving
from the medium in addition to living prey. Since Tokophrya lacks a mouth,
these supplements can reach the cytoplasm only through the complex
structure of the cortex, which is composed of a three- membraned pellicle
and a dense epiplasm. In addition, external to the cortex, an extraneous
coat covers the whole organism. Only the outer pellicular plasma membrane
is continuous; the other two and the epiplasm are interrupted by the outer
plasma membrane which invaginates at intervals forming the so-called pits.
The invaginated plasma membrane dips down into the cytoplasm where it
extends to form a saccule. Experiments with cationized ferritin and
Thorotrast provide evidence that internalization of these macromolecules
takes place through the pits by pinocytosis. The membrane of the saccules
of the pits forms invaginations which pinch off giving rise to small,
flattened vesicles containing the tracers. The tracers were never found
free in the cytoplasm but exclusively in the flat vesicles. These vesicles
are thus the vehicles transporting macromolecules from the medium to the
cytoplasm. The saccules of the pits are the natural loci of pinocytosis and
together with the flattened vesicles perform an important function in
Suctoria, supplying the organisms with macromolecules from the medium.
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