Am. J. Respir. Cell Mol. Biol., Vol 15, No. 4, 10 1996, 529-539.
Microanatomy of secretory granule release from guinea pig tracheal goblet cells
TM Newman, A Robichaud and DF Rogers
Thoracic Medicine, National Heart and Lung Institute (Imperial College), London, United Kingdom.
The microanatomy of mucin granule release from epithelial goblet cells has
been investigated in guinea pig tracheae. Using a tannic acid arrest
procedure, granule release under basal conditions and after high K+ or
acetylcholine (ACh) application was arrested and a variety of granule
fusion sites were identified in ultrathin sections and freeze- fracture
replicas. Rather than there being subclasses of secretory cells containing
either electron-lucent granules (indicative of mucin) or smaller
electron-dense (serous) granules, the majority of secretory cells in both
control and treated groups contained granules with an electrondense core
surrounded by an electron-lucent region. Granule release sites were of
three principal types: (1) simple exocytosis, where the membranes of single
granules fused directly with the plasma membrane to give an "omega"
profile; (2) compound exocytosis, where granule membranes, fused together
intracellularly, were found in continuity with the plasma membrane; and (3)
apocrine-like secretion, which involved the loss of the central apical mass
of granules together with elements of the cell cytoplasm. In treated
preparations, there was an increase in the number of cells exhibiting
fusion sites; the percentage showing simple fusions fell from 82% to 59%
(with ACh) and 57% (with KCl), whereas the percentage of cells exhibiting
compound and apocrine-like secretion increased. Dense cores were frequently
retained at the sites of fusion and, despite the expansive decondensation
of mucin known to occur, there was also evidence of some retention of the
electron-lucent material.
