Abstract:
The endocrine pancreas of the rock bass (Ambloplites
rupestris) was examined by light and electron microscopy. Two
cell types with staining properties similar to mammalian A and
B cells, and a third, non-staining cell type were found in the
spherical pancreatic islets that were surrounded by a connective
tissue capsule and embedded in two small masses of exocrine
tissue.
From an analysis of the ultrastructure of the A and B
cells, a secretory cycle for each of these cell types was proposed.
The secretory cycle of the A cell consisted of three well defined
stages:
(1) A cell production stage: during which A granule
formation occurred in the sacs of the Golgi apparatus and the
cell was characterized by the presence of numerous secretory
granules, some elements of lamellar endoplasmic reticulum, and a
homogeneously granular nucleus. The cytoplasm contained few
distended cisternae, variable numbers of free ribosomes, microtubules
and small vesicles.
(2) A cell release stage: during which the release of
A granules occurred and the cell usually contained several large
distended cisternae and variable numbers of secretory granules.
Granule release mechanisms included exocytosis, by which individual
granules were released into the extracellular space after their
membranes fused with the plasmalemma, and emiocytosis, by which one or more granules were released into a large cisterna whose
membrane fused with the plasmalemma and formed a pore through
which the cisternal contents passed out of the cell.
(3) A cell reorganization stage: during which the
changeover from the release stage to the production stage occurred
and the reorganization of organelles and membrane structures took
place. The cell contained few secretory granules and numerous
small endoplasmic reticular cisternae. The cytoplasm exhibited
less electron density than either of the other two stages.
The A granule after formation underwent a series of
morphological changes which were described in four numerically
identified phases.
The secretory cycle of the B cell consisred of two stages:
(1) B cell production stage: during which the B granule
formation occurred in the sacs of the Go1gi apparatus. The cell
was characterized by an irregular outline, the presence of numerous
secretory granules, and an irregularly shaped nucleus which contained
variable amounts of clumped chromatin. The cytoplasm contained
moderate amounts of lamellar endoplasmic reticulum studded with
ribosomes, several small vesicles, and an active Go1gi apparatus.
(2) B cell release stage: during which the release of B
granules occurred. The cell contained a rounded nucleus with
dispersed chromatin, several distended endoplasmic reticular
cisternae and a variable number of secretory granules. Granule
release occu~ by emiocytosis and exocytosis similar to that found
for the A cell.
