Chromaffin Cells in the Amphibian Urodele Triturus carnifex Show Ultrastructural Features Indicative of a Vesicle‐Mediated Mode of Cell Degranulation

Abstract

Recently, suggestive evidence for piecemeal degranulation (PMD), a particulate pattern of cell secretion accomplished by vesicle-mediated extracellular transport of granule-stored material, has been provided by electron microscopy investigations in chromaffin cells of different vertebrate species. In this study, chromaffin tissue from the interrenal gland of the amphibian urodede Triturus carnifex has been investigated by quantitative transmission electron microscopy in search for ultrastructural clues indicative of a vesicle-mediated mode of cell degranulation. Interestingly, a single type of chromaffin cell is recognizable in T. carnifex, which undergoes seasonal variations in its adrenaline and noradrenaline granule ratio according to an annual cycle that matches the trend of changes in secondary sexual characters. In this study, we looked for a series of ultrastructural changes regarded as highly specific for PMD. We calculated the percentage of (i) resting, unaltered granules (type 1 granules); (ii) granules with changes indicative of progressive release of secretory materials, that is, granules with lucent areas in their cores, reduced electron density, disassembled matrices, and residual cores (type 2 granules); and (iii) membrane empty containers (type 3 granules) in chromaffin cells of T. carnifex during the annual cycle. We found a significant increase in type 2 and 3 granules, accompanied by a significant decrease in type 1 granules, in the April and November samples. During the same seasonal periods, the number per net cytoplasmic area of 30-100-nm diameter electron-dense vesicles was found to be significantly augmented, and there was also an augmented percentage of chromaffin granules showing blebs or protrusions in their profiles. These ultrastructural data are indicative of an increased vesicle-mediated transport of chromaffin Grant sponsor: MIUR

Suggestive evidence of a vesicle‐mediated mode of cell degranulation in chromaffin cells. A high‐resolution scanning electron microscopy investigation

“…This study provides high‐resolution scanning images of rat adrenal chromaffin cells. Some of these 3D ultrastructures were previously investigated with conventional TEM and regarded as highly suggestive of PMD in chromaffin cells (Crivellato et al. 2003, 2004, 2005, 2006, 2009a,b).…”

“…Ultrastructural criteria for detecting PMD by means of electron microscopy include specific granular and cytoplasmic changes. Both kinds are recognizable in chromaffin cells (Crivellato et al. 2003, 2004, 2005, 2006, 2009a,b).…”

“…This study provides high-resolution scanning images of rat adrenal chromaffin cells. Some of these 3D ultrastructures were previously investigated with conventional TEM and regarded as highly suggestive of PMD in chromaffin cells (Crivellato et al 2003(Crivellato et al , 2004(Crivellato et al , 2005(Crivellato et al , 2006(Crivellato et al , 2009a. First described in the early 1970s, PMD is a slow, particulate and possibly selective process of cell secretion that is especially suitable for the outward transport of small quanta of highly active granule components to avoid local or systemic toxic effects (Dvorak, 2005a,b).…”

“…As a result, PMD would accomplish discharge of secretory constituents from storage granules without granule-to-granule and granule-to-plasma membrane fusion events and without direct granule opening to the cell exterior. It is of note that ultrastructural clues diagnostic of PMD have been identified with transmission electron microscopy (TEM) in chromaffin cells from different species including man, mouse, rat, chicken embryo, newt and fish (Crivellato et al 2003(Crivellato et al , 2004(Crivellato et al , 2005(Crivellato et al , 2006(Crivellato et al , 2009a. In chromaffin cells, this suggestive degranulation pathway may represent a versatile model of cell secretion.…”

“…On the other hand, it would exhibit all of the properties of a subtle mechanism for releasing small, and possibly selective, amounts of granule constituents for autocrine and ⁄ or paracrine functions. This may be the case in Anuran amphibians, where the adrenal homologue is composed of closely interconnected aminergic chromaffin and inter-renal steroidogenic cells (Crivellato et al 2009a).…”