208 DAIRY CHEMISTRY AND BIOCHEMISTRY
In the apical cytosol there are numerous protein-containing secretory
vesicles (Figure 4.33). EM studies suggest that they move to the apical
plasmalemma and fuse with it, releasing their contents by exocytosis.
Current ideas on intracellular transport of vesicles suggest participation of
cytoskeletal elements-microtubules and microfilaments. In mammary cells,
these structures are orientated from the basal to the apical membrane,
suggesting that they may act as ‘guides’ for vesicular movement. Alternative-
ly, vesicle transport may involve simple physical displacement as new
vesicles bud from the Golgi complex, or an ‘electrophoretic’ process,
dependent on a transcellular potential gradient.
Secretory vesicles seem to become attached to the cytoplasmic face of the
apical plasmalemma. The vesicles have a distinctive coat on their outer
surface which appears to react with appropriate receptors on the apical
membrane, forming a series of regularly spaced bridges. Presumably, these
bridges, and the contiguous vesicle and apical membrane material, are
subsequently eliminated and the vesicular contents released, but the process
seems to be very rapid and it has proved difficult to visualize the details of
the sequence by EM. However, secretory vesicle membrane becomes incor-
porated, however briefly, into the apical membrane as a consequence of
exocytosis.
Figure 4.34 Schematic representation of one apparent mechanism for exocytotic release of
secretory vesicle contents. (a) Vesicles assemble into a chain through ball-and-socket interac-
tion. The exit vesicle interacts with apical plasma membrane via a vesicle depression. (b) Linked
vesicles fuse together, apparently by disintegration of membrane in areas of fusion, resulting in
the formation of a continuum with the alveolar lumen. (c) Emptying of the vesicular chain
appears to result in collapse and subsequent fragmentation of the membrane. (From Keenan
and Dylewski, 1985.)