Biology Reference
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Each depression measures between 100 and 180 nm in diameter, and
typically three to four depressions are found within a pit. The basolateral
membranes in acinar cells are devoid of such structures. High-resolution
AFM images of depressions in live acinar cells further reveal a cone-shaped
morphology, and the depth of each cone measures 15-35 nm. Subsequent
studies over the years demonstrate the presence of depressions in all secretory
cells examined. Analogous to pancreatic acinar cells, examination of resting
growth hormone (GH)-secreting cells of the pituitary
and chromafin cells
of the adrenal medulla 14 also reveals the presence of pits and depressions
at the cell plasma membrane. The presence of depressions or porosomes in
neurons, astrocytes, β-cells of the endocrine pancreas and mast cells has also
been elucidated, demonstrating their universal presence.
Exposure of pancreatic acinar cells to a secretagogue (mastoparan)
results in a time-dependent increase (25-45%) in both the diameter and
relative depth of depressions. Studies demonstrate that depressions return
to resting size on completion of cell secretion.
8
No demonstrable change
in pit size is detected following stimulation of secretion. 6 Enlargement of
depression diameter and an increase in its relative depth after exposure to
secretagogue correlated with secretion. Aditionally, exposure of pancreatic
acinar cells to cytochalasin B, a fungal toxin that inhibits actin polymerization
and secretion, results in a 15-20% decrease in depression size and a
consequent 50-60% loss in secretion.
6,7
Results from these studies suggest
depressions to be the fusion pores in pancreatic acinar cells. Furthermore,
these studies demonstrate the involvement of actin in regulation of both the
structure and function of depressions. Similarly, depressions in resting GH
cells measure 154 ± 4.5 nm (mean ± SE) in diameter, and following exposure
to a secretagogue, there is a 40% increase in depression diameter (215 ±
4.6 nm;
6
The enlargement
of depression diameter during cell secretion and its subsequent decrease,
accompanied by loss in secretion following exposure to actin depolymerizing
agents, 6 also suggested them to be the secretory portal. A direct determination
that depressions are indeed the portals via which secretory products are
expelled from cells was unequivocally demonstrated using immuno-AFM
studies ( Fig. 5.3 ) .
p
< .01), with no appreciable change in pit size.
8
Localization at depressions of gold-conjugated antibody to
secretory proteins inally provided the direct evidence that secretion occurs
through depressions. ZGs contain the starch-digesting enzyme amylase.
AFM micrographs of the speciic localization of gold-tagged amylase-speciic
antibodies ( Fig. 5.3 ) at depressions, following stimulation of cell secretion,
7
7,10
conclusively demonstrated depressions as the cellular secretory portal.
Similarly, in somatotrophs of the pituitary gland, gold-tagged GH-speciic
antibody found to selectively localize at the depression openings following
stimulation of secretion
8
established these sites too to be the secretory portal
 
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