Transcellular trafficking in which various molecules are
transported across the interior of a cell, is commonly classified as
transcytosis. However, historically this term has been used
synonymously with transudation. In both cases transcellular
trafficking starts with the internalization of proteins or other
compounds on the basal or basolateral side of a cell and
continues by their transport across the interior to the apical pole
(or vice versa) where they are subsequently released. This allows
a cell to release products which are synthesized elsewhere. Here,
we discuss the common features of both transcytosis and
transudation, and that which differentiates them. It appears that
transcytosis and transudation are identical in terms of vesicular
import and endosomal sorting of cargo, but completely differ in
the re-secretion process. Specialized epithelial cells re-release
substantial quantities of the endocytosed material, and often also
a great variety. Some recent studies indicate that this is achieved
by non-canonical apocrine secretion rather than by the regular
vesicular mechanism of exocytosis, and takes place only on the
apical pole. This massive re-release of endocytosed proteins, and
potentially other compounds via the apocrine mechanism should
be considered as transudation, distinct from transcytosis.