Maturation Times of Pancreatic Beta Cell Secretory Granules Estimated from Serial Block-Face Electron Microscopy

Abstract

We have used serial block-face electron microscopy (SBEM) [1] to study the maturation of secretory granules in mouse pancreatic islets of Langerhans [2], which are micro-organs 100–200 micrometers in diameter containing ~1,000 cells, mainly consisting of insulin-secreting \uf062-cells and glucagon-secreting \uf061cells, whose purpose is to control blood glucose. Here, we explore the possibility of deriving information about the maturation times for \uf062-cell secretory granules by analyzing the 3D ultrastructure of whole cells at snapshots in time. It is assumed that the \uf062-cells are in homeostasis, and that the morphology of immature granules containing proinsulin can be readily distinguished from the morphology of mature insulin granules. For islets in homeostasis, the rate of loss of proinsulin from immature \uf062-cell granules is equal to the rate of formation of insulin packaged in mature granules, and the rate of insulin loss from mature granules is equal to the rate of insulin secretion from the cell, which can be determined from pulsed Sradiolabeling of sulfur-containing amino acids [3].