Abstract
The gastric chief cells of the hibernating bats (Rhinolophus ferrum-equinum nippon) were observed with the electron microscope. The chief cells contain an extensive granular endoplasmic reticulum and many mitochondria. In the supranuclear region a well developed Golgi apparatus and various number of secretory granules (zymogen granules) are seen. Many microvilli protrude into the lumen from the apical surface of cell. The intercellular interdigitations on the lateral cell border is poorly formed. Near the luminal end of the lateral cell border there occurs“junctional complex” which is composed of zonula occludens, zonula adhaerens and macula adhaerens (desmosome). Besides, several desmosomes are found also in deeper part of the cell border. Basal infoldings are scarcely seen at the cell base. In the matrix of the microvilli a number of electron opaque filaments run along the long axis of them, to be continued with a faint filamentous structure in the apical cytoplasm subjacent the luminal cell surface.
The unit membrane structure (triple-layered structure) of the plasma membrane is clearly identified both in the apical and the lateral cell boundaries, but it is rather indistinct at the cell base. The thickness of each layer of the plasma membrane varies to some extent in different regions; the inner dense layer gives fairly constant value of about 30Å, while the outer dense layer shows a relatively variable thickness in various regions. In the apical and microvillous region, each of the three layers of the plasma membrane measures about 30Å in thickness. The triple-layered membrane can be identified also in Golgi apparatus and mitochondria, whereas it is rather difficult to discern the three layers in the limiting membrane of the granular endoplasmic reticulum.
The secretory granules are divided into two types in regard to their limiting membrane: One is covered by the triple-layered membrane (85Å), and the other by a thinner monolayered membrane (40Å). The majority of the mature secretory granules and the small immature ones found in the vicinity of the Golgi apparatus are covered by the triple-layered membrane, while the monolayered one covers as a rule only some of larger secretory granules found in the apical cytoplasm. The triplelayered limiting membrane may be transformed into the monolayered during the process of maturing of the secretory granules.
The matrix of the secretory granules has a moderate electron density and a homogeneous compact texture. The small immature granules mentioned above are relatively opaque showing a somewhat coarse texture. The large, electron lucent granules which are found in the apical cytoplasm and show sometimes a coarse texture are regarded as highly matured secretory granules or as secretory vacuoles.
Two modes of secretory granule extrusion have been confirmed in the gastric chief cells of the bat. In the first case, the secretory granules are extruded into glandular lumen by means of reversed pinocytosis; in the second, the electron lucent content of matured secretory granules in the apical cytoplasm probably either permeates through the altered or monolayered limiting membrane, or flow out through the interrupted portion of the limiting membrane into the surrounding cytoplasm, and thereafter the secretion may be emptied by diffusion through the plasma membrane into the glandular lumen. In the last stage of this extrusion process it may be possible that the secretion remaining in the apical cytoplasm is taken up in smooth surfaced vesicles scattered around there to be later released by reversed pinocytosis into the lumen.
In the bats secretory function of the gastric chief cells seems to continue to some extent even in the hibernating period. The intermediary or prosecretory granules are found among the Golgi vacuoles and the immature secretory granules and show intermediate morphological characteristics between the both.
