Insulin
and proinsulin are stored in secretory granules known as dense core vesicles
(DCVs) in pancreatic β-cells. Insulin granule trafficking is a key step in the insulin
secretion process. Abnormalities in the ultrastructure and distribution of DCVs
reflect different physiological conditions. However, because of its small size
(around 350 nm), it’s difficult to study the morphological parameters and
distribution of DCVs. We applied tomography based on scanning transmission
electron microscopy (STEM) to image 1 μm thick sections of β-cells of mouse pancreatic
islets which contain hundreds of intact insulin granules with resolution less
than 5 nm. After reconstruction, we calculated density and DCV size and
compared the distribution differences between wt and conditional knockout mice.
To calculate the distance between DCVs and plasma membrane, we used focused ion
beam scanning electron microscopy (FIB/SEM) to acquire large volumes (13.8×13.8×20 μm) of resin-embedded beta cells. DCVs were
recognized and distances were calculated automatically using self-designed
programs. In conclusion, we used STEM and FIB/SEM to study the 3D ultrastructure
and distribution of DCVs with high resolution, and conducted a statistic
analysis of different distributions of DCVs under different physiological
conditions.