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Receptor mediated endocytosis is a process in which extracellular
molecules are taken into the cell via coated vesicles that pinch from
the plasma membrane. In Paramecium the resulting vesicles fuse
with flattened cisterna called early endosomes (ee). Early
endosomes accumulate just under the cortex of the cell. Sorting of
proteins destined to be returned to the plasma membrane after
separating from cargo occurs in the early endosomes and is facilitated
by the formation of secondary coated pits (cop) and vesicles
(cov) on the early endosomes that are smaller than the coated
and uncoated preendosomal vesicles that arise from the plasma
membrane. These small coated pits pinch off and subsequently lose
their coats to form carrier vesicles (cav) that are roughly
100nm in diameter. These vesicles move deeper into the cell where they
bind to microtubular ribbons. sp, septal pore. EM taken on
11/4/68 by R. Allen with Philips 300 TEM. Neg. 20,500X. Bar = 0.5µm.
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