Biology Reference
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(b)
(a)
(c)
(d)
(f)
(e)
Figure 5.2.
Porosomes or previously referred to as “depression” at the plasma
membrane (PM) in pancreatic acinar cell and at the nerve terminal. (a) AFM micrograph
depicting “pits” (yellow arrow) and “porosomes” within (blue arrow), at the apical
PM in a live pancreatic acinar cell.
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(b) To the right is a schematic drawing depicting
porosomes at the cell PM, where membrane-bound secretory vesicles called zymogen
granules (ZGs) dock and fuse to release intravesicular contents.
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(c) A high-resolution
AFM micrograph shows a single pit with four 100-180 nm porosomes within.
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(d)
An electron micrograph depicting a porosome (red arrowhead) close to a microvilli
(MV) at the apical PM of a pancreatic acinar cell. Note the association of the porosome
membrane (POM, yellow arrowhead) and the zymogen granule membrane (ZGM) (red
arrow head) of a docked ZG (inset). Cross section of a circular complex at the mouth of
the porosome is seen (blue arrow head).
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(e) The bottom left panel shows an electron
micrograph of a porosome (red arrowhead) at the nerve terminal, in association with
a synaptic vesicle (SV) at the presynaptic membrane (Pre-SM). Notice a central plug
at the neuronal porosome opening.
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(f ) The bottom right panel is an AFM micrograph
of a neuronal porosome in physiological buffer, also showing the central plug (red
arrowhead) at its opening.
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It is believed that the central plug in neuronal porosomes
may regulate its rapid close-open conformation during neurotransmitter release. The
neuronal porosome is an order of magnitude smaller (10-15 nm) in comparison with
porosome in the exocrine pancreas.
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