Immunoelectron-microscopic demonstration of histamine depletion in the gastric enterochromaffin-like cells of rats treated with α-fluoromethylhistidine

We conducted an immunoelectron-microscopic study for histamine (HA) in the enterochromaffin-like (ECL) cells of normal rats and rats given alpha-fluoromethylhistidine (alpha-FMH, 3 mg/kg per hour) via osmotic minipumps over a period of 24 h. The indirect immunoperoxidase procedure utilized a mouse monoclonal antibody (mAb), AHA-2, which is produced against glutaraldehyde-conjugated HA. alpha-FMH is a potent and irreversible inhibitor of the HA-forming enzyme histidine decarboxylase and is known to reduce tissue HA concentrations in several tissues. The present study clearly demonstrated that HA immunoreactivity, which was found to a high degree in the cores of the granules and secretory vesicles and in the cytoplasm of ECL cells of control rats, was completely abolished from the corresponding compartments in the cells of alpha-FMH-treated rats. Furthermore, treatment with alpha-FMH drastically lowered the number of secretory vesicles and was associated with larger cores in the granules of the ECL cells. These results seem to support the idea of a HA-pathway mechanism, emphasizing that the granules in normal ECL cells take up HA from the cytosol during its transport from the Golgi zone to the more peripheral portion of the cell and condense it in their cores, thus forming mature secretory vesicles. However, the present study showed that not only the secretory vesicles but also almost all the granules seen in ECL cells were already loaded with HA in their cores, suggesting that the newborn granules very rapidly take up HA from the cytosol. Also suggested was the fact that HA depletion impairs the maturation of the granules into secretory vesicles.     

ECL cell morphology.

Using immunohistochemistry at the conventional light, confocal and electron microscopic levels, we have demonstrated that rat stomach ECL cells store histamine and pancreastatin in granules and secretory vesicles, while histidine decarboxylase occurs in the cytosol. Furthermore the ECL cells display immunoreactivity for vesicular monoamine transporter type 2 (VMAT-2), synaptophysin, synaptotagmin III, vesicle-associated membrane protein-2, cysteine string protein, synaptosomal-associated protein of 25 kDa, syntaxin and Munc-18. Using electron microscopy in combination with stereological methods, we have evidence to suggest the existence of both an exocytotic and a crinophagic pathway in the ECL cells. The process of exocytosis in the ECL cells seems to involve a class of proteins that promote or participate in the fusion between the granule/vesicle membrane and the plasma membrane. The granules take up histamine by VMAT-2 from the cytosol during transport from the Golgi zone to the more peripheral parts of the cells. As a result, they turn into secretory vesicles. As a consequence of stimulation (e.g., by gastrin), the secretory vesicles fuse with the cell membrane to release their contents by exocytosis. The crinophagic pathway was studied in hypergastrinemic rats. In the ECL cells of such animals, the secretory vesicles were found to fuse not only with the cell membrane but also with each other to form vacuoles. Subsequent lysosomal degradation of the vacuoles and their contents resulted in the development of lipofuscin bodies.     

Enterochromaffin-like cells in the rat stomach: effect of α-fluoromethylhistidine-evoked histamine depletion

Summary In the rat, gastric histamine is stored predominantly in the enterochromaffin-like (ECL) cells, which are located basally in the oxyntic mucosa. The functional significance of histamine in the ECL cells is a matter of speculation. In this study the effect of depletion of histamine on the properties and ultrastructure of the ECL cells was examined. Histamine synthesis was inhibited with -fluoromethylhistidine (3 mg·kg^-1·h^-1) given via osmotic minipumps over a period of 24 h. The treatment reduced the histidine decarboxylase activity (approximately 20% remaining) and histamine concentration (less than 20% remaining) in the oxyntic mucosa, as well as the intensity of histamine- and chromogranin A-immunostaining in the ECL cells, compared to control rats. The cytoplasmic (secretory) granules/vesicles were greatly reduced in number and size following -fluoromethylhistidine administration. The histamine immunostaining of the mast cells, which occurs at the mucosal surface and in the submucosa, appeared unaffected. We conclude that ECL cell histamine accounts for at least 80% of the total oxyntic mucosal histamine in the rat and that it represents a more mobile pool than mast cell histamine. The reduction in the number and size of the ECL cell granules/vesicles following histamine depletion is in accord with the idea that they represent the storage site for histamine.     

Immunocytochemical localization of histamine in enterochromaffin-like (ECL) cells in rat oxyntic mucosa: a transmission electron microscopy study using monoclonal antibodies and conventional glutaraldehyde-based fixation.

Histamine (HA), contained in the enterochromaffin-like (ECL) cells of the gastric mucosa in animals, plays an important role in gastric acid secretion, although methods for its exact morphological localization are still lacking. We used a pre-embedding indirect immunoperoxidase approach to define the fine structural localization of HA in rat oxyntic mucosa that was fixed with a glutaraldehyde-based fixative and HA monoclonal antibodies (MAbs AHA-1 and 2). Transmission electron microscopy showed that the peroxidase endproduct not only was concentrated in the cores of cytoplasmic granules but also was distributed to a high degree in the cytoplasm peripheral to the granules of the ECL cells. These results suggest that in ECL cells HA is enzymatically synthesized in the cytoplasm, then is transported and stored in the cores of the granules before its release from the basal lamina. The present HA immunoelectron microscopic method with MAbs would be applicable more generally to the ultrastructural identification of HA-containing cells.     

Effect of α-fluoromethylhistidine-evoked histamine depletion on ultrastructure of endocrine cells in acid-producing mucosa of stomach in mouse,rat and hamster

The oxyntic mucosa of the mammalian stomach is rich in endocrine cells, such as ECL cells, A-like cells, somatostatin cells, D₁/P cells and, in some species, enterochromaffin cells. The various endocrine cell types can be distinguished on the basis of their characteristic cytoplasmic granules and vesicles. The ECL cells contain numerous large secretory vesicles and relatively few, small electron-dense granules and small clear microvesicles. We have suggested that in the rat the ECL cells contain most of the gastric histamine with the secretory vesicles as the major histamine storage site in these cells. α-Fluoromethylhistidine is an irreversible inhibitor of histidine decarboxylase, the histamine-forming enzyme. We have previously shown that this enzyme inhibitor depletes histamine from the ECL cells in the rat and reduces the number of secretory vesicles in the cytoplasm. In the present study, we have examined whether α-fluoromethylhistidine affects the ECL cells in other species and whether it affects other types of endocrine cells in the oxyntic mucosa of the rat. Mice, rats and hamsters were treated with the inhibitor (3 mg/kg per h) via minipumps subcutaneously for 24 h. This treatment lowered the oxyntic mucosal histamine concentration by 65–90% and the number and volume density of the secretory vesicles by 85–95% in the ECL cells of the three species examined. In contrast, the number and volume density of granules and microvesicles were not greatly affected. No evidence was found for an effect of α-fluoromethylhistidine on A-like cells, somatostatin cells or D₁/P cells of the rat stomach, suggesting that, unlike the ECL cells, they do not contain histamine. Received: 18 January 1996 / Accepted: 23 May 1996     

Secretory organelles in ECL cells of the rat stomach: an immunohistochemical and electron-microscopic study

ECL cells are numerous in the rat stomach. They produce and store histamine and chromogranin-A (CGA)-derived peptides such as pancreastatin and respond to gastrin with secretion of these products. Numerous electron-lucent vesicles of varying size and a few small, dense-cored granules are found in the cytoplasm. Using confocal and electron microscopy, we examined these organelles and their metamorphosis as they underwent intracellular transport from the Golgi area to the cell periphery. ECL-cell histamine was found to occur in both cytosol and secretory vesicles. Histidine decarboxylase, the histamine-forming enzyme, was in the cytosol, while pancreastatin (and possibly other peptide products) was confined to the dense cores of granules and secretory vesicles. Dense-cored granules and small, clear microvesicles were more numerous in the Golgi area than in the docking zone, i.e. close to the plasma membrane. Secretory vesicles were numerous in both Golgi area and docking zone, where they were sometimes seen to be attached to the plasma membrane. Upon acute gastrin stimulation, histamine was mobilized and the compartment size (volume density) of secretory vesicles in the docking zone was decreased, while the compartment size of microvesicles was increased. Based on these findings, we propose the following life cycle of secretory organelles in ECL cells: small, electron-lucent microvesicles (pro-granules) bud off the trans Golgi network, carrying proteins and secretory peptide precursors (such as CGA and an anticipated prohormone). They are transformed into dense-cored granules (approximate profile diameter 100 nm) while still in the trans Golgi area. Pro-granules and granules accumulate histamine, which leads to their metamorphosis into dense-cored secretory vesicles. In the Golgi area the secretory vesicles have an approximate profile diameter of 150 nm. By the time they reach their destination in the docking zone, their profile diameter is between 200 and 500 nm. Exocytosis is coupled with endocytosis (membrane retrieval), and microvesicles in the docking zone are likely to represent membrane retrieval vesicles (endocytotic vesicles).     

Thyroid medullary carcinoma of the Djungarian hamster Phodopus sungorus. Ultrastructural evidence for the production of normal and atypical intracellular granules

We have carried out an electron microscopic and immunocytochemical study of thyroid medullary carcinoma arising spontaneously in the Djungarian hamster, Phodopus sungorus. At the ultrastructural level the cytoplasm of tumor cells contained numerous round to slightly elongated, dense-cored secretory granules. The number of secretory granules differed from cell to cell in the tumor, being scanty in some cells but more or less abundant in most. Electron microscopic-immunocytochemistry demonstrated that all dense-cored secretory granules in all tumor cells exhibited calcitonin immunoreactivity. In approximately 10% of the tumor cells, unusual star-shaped secretory vesicles were also found in the cytoplasm. These vesicles contained a small, but well-defined, lucent core surrounded by a region of finely granular material of greater electron density. The outer contour of these unusual vesicles was stellate rather than smooth. They appeared to originate not from the Golgi complex, but from the rough endoplasmic reticulum. These atypical stellate vesicles did not show any calcitonin immunoreactivity. Furthermore, in a small number of tumor cells (approximately 1%) a third type of membrane enclosed structure was found. These were conspicuous rods 1-5 micron in length with tapering ends and a crystalline substructure. The presence of both normal and atypical secretory granules in some tumor cells suggests that carcinogenic transformation may interfere with the normal synthesis and assembly of secretory products by the cell.     

Functionally impaired, hypertrophic ECL cells accumulate vacuoles and lipofuscin bodies

ECL cells in the oxyntic mucosa of stomach control gastric acid secretion by mobilizing histamine in response to gastrin. They respond to gastrin also with hypertrophy and hyperplasia. ECL cells exhibit functional impairment upon long-term gastrin stimulation. The impairment is manifested in a gradual decline of the activity of the histamine-forming enzyme per individual ECL cell and in a failure of gastrin to mobilize histamine. The mechanism behind this impairment is unknown. In the present study, rats were treated with the proton pump inhibitor pantoprazole for 45 days to induce sustained hypergastrinemia. The ECL cells were isolated from normogastrinemic and hypergastrinemic rats and size-separated from other mucosal cells by the elutriation technique. The total ECL cell number was twofold higher in hypergastrinemic rats than in normogastrinemic rats, and most of the cells appeared in elutriation fractions where large cells predominate. The ECL cells of the different fractions were analyzed by quantitative electron microscopy. Normal-sized ECL cells from hypergastrinemic rats displayed a reduced number of secretory vesicles (probably because of degranulation) compared with normal-sized ECL cells from normogastrinemic rats. Hypertrophic ECL cells from hypergastrinemic rats had an unchanged number of secretory vesicles, supporting the view that such cells fail to respond to gastrin with degranulation. Although both normal-sized and hypertrophic ECL cells from hypergastrinemic rats contained vacuoles, those in the hypertrophic ECL cells were larger and more numerous. In addition, hypertrophic ECL cells were found to contain numerous, prominent lipofuscin bodies which are the presumed end product of crinophagia. Conceivably therefore, large vacuoles and lipofuscin bodies cause functional impairment of the hypertrophic ECL cells.     

Immunohistochemical observations of immunoglobulin A in the Paneth cells of germ-free and formerly-germ-free rats

Summary The localization of secretory immunoglobulin A (IgA) in Paneth cells was immunohistochemically studied in germ-free (Gf) and ex-Gf rats that had been injected with feces obtained from specific-pathogen-free (SPF) rats. In Gf as well as SPF rats, the secretory granules of Paneth cells and the brush borders of crypt cells exhibited IgA immunoreactivity. At 12 and 24 h after inoculation, it was found that, concomitant with the occurrence of considerable degranulation, the IgA immunoreactivity in Paneth cells disappeared, except of the margin of supranuclear vacuoles. In contrast, the IgA immunoreactivity of the crypt-cell brush borders was unchanged. Four days after inoculation, secretory granules exhibiting IgA immunoreactivity reaccumulated in Paneth cells. The present study suggests that Paneth cells regulate the bacterial milieu in the intestine by releasing secretory granules containing IgA into the crypt lumen.     

Immunohistochemical observations of immunoglobulin A in the Paneth cells of germ-free and formerly-germ-free rats

The localization of secretory immunoglobulin A (IgA) in Paneth cells was immunohistochemically studied in germ-free (Gf) and ex-Gf rats that had been injected with feces obtained from specific-pathogen-free (SPF) rats. In Gf as well as SPF rats, the secretory granules of Paneth cells and the brush borders of crypt cells exhibited IgA immunoreactivity. At 12 and 24 h after inoculation, it was found that, concomitant with the occurrence of considerable degranulation, the IgA immunoreactivity in Paneth cells disappeared, except of the margin of supranuclear vacuoles. In contrast, the IgA immunoreactivity of the crypt-cell brush borders was unchanged. Four days after inoculation, secretory granules exhibiting IgA immunoreactivity reaccumulated in Paneth cells. The present study suggests that Paneth cells regulate the bacterial milieu in the intestine by releasing secretory granules containing IgA into the crypt lumen.     

Ultrastructural changes in the mammary glands of white rats during the estrus cycle

Secretory granules of milk protein were found in the apical cytoplasm of some ductal cells in the mammary gland of non-lactating albino rats. During the estrous period, the diestrous phase, the number of "coated" vesicles increased essentially. Some of the "coated" vesicles contained secretory granules and granular debris. The functional relationship of the "coated" vesicles to lysosomes and to some other cytoplasmic structures is discussed. The results suggest that during the estrous phase formation and release of secretory products prevail, whereas at the diestrous the process of their removal and destruction is predominant.     

Unique features of secretory granules observed in the pituitary growth hormone-secreting (GH) cells of the musk shrew (Suncus murinus L.)

Summary Unique rod-shaped secretory granules were observed among oval or spherical secretory granules in GH cells of the anterior pituitary gland of musk shrew using the protein A-gold procedure combined with electron microscopy. The rod-shaped and spherical secretory granules were both immunoreactive by the immuno-gold method using antiserum to sheep GH. The rod-shaped secretory granules, which seem to be formed directly from the Golgi vesicles, extend from several hundred to several thousand nm in length. They often touch each other and fuse. The spherical secretory granules are also unique in that they may also fuse with loss of dense contents to leave empty circular membrane profiles in the cytoplasm. Both the rodshaped and spherical secretory granules are secreted from the cell by exocytosis.     

Localization of 5′-nucleotidase activity in the parotid acinar cells of a rat treated with isoproterenol

Summary Cytochemical localization of 5- nucleotidase (AMPase) has been investigated in the parotid acinar cells of rats at various stages of exocytic secretion induced by an administration of isoproterenol (IPR).In the resting stage, the acinar cells show AMPase activity located on the baso-lateral and luminal plasmalemma, and in the earliest secretory stage the luminal plasma membranes are devoid of the enzymatic activity. However, these particular regions exhibit AMPase activity during the advanced stages of secretion, and the AMPase positive membranes become absorbed into the cytoplasm by endocytic activity. The absorbed membrane components then seem to be degraded by the action of lysosomes.The intracellular fate of the endocytic vacuoles has been examined by the aid of ferritin particles introduced retrogradely through ductal lumina. Ferritin containing vacuoles are distributed in the cytoplasm, and these droplets change into secondary lysosomes. No tracer particles are recognized in the internal space of the Golgi lamella and its associated vesicles.The results suggested that in the exocytic secretion of parotid acinar cells, AMPase originating from plasma membrane intermingles with the membranes derived from secretion granules, and is translocated into cytoplasm by an endocytic mechanism. The internalized membrane components are, at least partly, degraded by lysosome action.     

Effects of colchicine on the ultrastructure of mouse taste buds

Summary Effect of colchicine on the ultrastructure of taste bud cells was studied in the mouse. In untreated mice microtubules were abundant throughout the entire cytoplasm of type-III cells, but only in the apical cytoplasm of type-I cells. After 2 h of colchicine treatment, no microtubules were observed in any taste bud cells; dense secretory granules in the apical cytoplasm of type-I cells mostly disappeared, and instead, numerous phagosomes appeared. It is suggested that colchicine causes an interruption of the transport of the secretory granules in type-I cells from the Golgi apparatus to the membrane of the apical surface, from which release occurs. In type-III cells, after 4 or 5 h of treatment, dense-cored vesicles scattered throughout the cytoplasm tended to increase in number; they were often observed to accumulate in the vicinity of the Golgi apparatus. Five hours after treatment with 5-hydroxy-l-tryptophan (5-HTP) following colchicine pretreatment, monoamine specific fluorescent cells and vesicles with highly electron-dense cores of type-III cells were still present. On the other hand, 5 h after 5-HTP treatment alone both fluorescent cells and vesicles with highly electron-dense cores had already disappeared. These observations suggest that the treatment with colchicine interrupts the transport of densecored vesicles of type-III cells to synaptic areas, in which those vesicles are presumed to discharge the neurotransmitter substance.     

Effect of a low calcium diet on the ultrastructure of the parathyroid gland of chicks

Summary The effect of a low calcium diet on the ultrastructure of the parathyroid gland in the chick was examined. Two-week-old White Leghorn chicks were fed a low calcium diet (calcium content 0.63%) for two weeks. In these chicks, the parathyroid glands are grossly enlarged. Hypertrophy and hyperplasia of the chief cells are evident. The plasma membranes between adjacent cells are relatively straight but interdigitate in some places. Chief cells contain occasional membrane-limited secretory granules (150–350 m in diameter) and with contents of variable electron density. Secretory granules are distributed randomly but some are closely applied to the plasma membrane. There is an increase in the development of the rough-surfaced endoplasmic reticulum. The Golgi complex is enlarged and consists of cisternae arranged in concentric layers, smooth-surfaced and coated vesicles and condensing vacuoles. Dilatations of the cisternae at several points are observed. Mitochondria and filaments are also encountered. These morphological features suggest that low calcium intake stimulates the synthetic activity of the chief cells of the chick parathyroid.     

The cytomorphology of goblet cells of the fetal intestine. Studies of the large intestine of cattle (Bos primigenius taurus)

In the region of the base of the intestinal crypts undifferentiated goblet cells display a configuration and constellation of organelles and membrane structures that are indicative of their importance for function. These images at this stage of development deliver a scenario of the mechanism of secretory granule production: aggregates of protein vesicles from the "transitional elements" (PALADE) of the granular endoplasmic reticulum are, so to speak, rolled up on the trans side of the Golgi apparatus by inversion of peripheral membrane segments of the innermost Golgi lamellae, thereby forming corpuscles. The origin of the capsulated vacuoles, which contain vesicles as single elements or as conglomerates, is well established. Their capsule consists of a trilaminar external and external and internal membrane; between them lies condensed material of the Golgi apparatus. In the opinion of the present author, the development of the ensheathed vacuoles represents a basic, more general mechanism. In contrast, the further steps of synthesis, for the formation of secretory granules, are more heterogeneous. Condensation of the vesicles and the inner capsular membrane results in the formation of a prosecretory granule, which in the basic element in the process of secretory granule production. The prosecretory granules develop singly or by fusion with other granules to give primary secretory granules. The complexity of this mechanism of secretory granule formation, however, becomes evident when considering the apposition of capsulated vacuoles and prosecretory--primary--secondary secretory granules, of prosecretory and primary secretory granules as well as prosecretory granules and secondary secretory granules. Generally, primary granules show a tendency to become secondary secretory granules or to fuse with them. During maturation of the goblet cells the secretory granules fuse to form larger mucous bodies in the theca by fusion of the laminae of the membranes; a final product, there is a homogeneous mucous mass devoid of membranes.     

On degeneration of peptidergic neurosecretory fibres in the albino rat

Three types of degenerating peptidergic neurosecretory fibres have been found in the posterior pituitary of chronically dehydrated albino rats. "Dark" neurosecretory fibres and their swellings contain neurosecretory granules, neurotubules, shrunken mitochondria and diffusely distributed fine dense material. Some swellings are filled with synaptic vesicles and/or conglomerations of dense membranes. The transitional forms exist between these fibres and extracellular accumulations of electron dense material. Synaptic vesicles, single neurosecretory granules, lipid-like droplets and lamellar bodies occur in the latter. Some neurosecretory fibres and swellings have numerous polymorphous inclusions arising due to degradation of secretory inclusions and organelles, mitochondria and neurotubules in particular. "Dark" neurosecretory elements and those with numerous polymorphous inclusions are enveloped by pituicyte cytoplasm. Sometimes the plasma membranes both of the pituicytes and neurosecretory fibres are destroyed or transformed into a multi-membrane complex. It is assumed that pituicytes may phagocytize degenerating neurosecretory elements. N urosecretory fibres with a locally dissolved neuroplasm and/or large lucent vacuoles seem to be due to axonal degeneration by the "light" type. These neurosecretory elements, the largest of them in particular, may transform into large cavities bordered by a membrane and containing flake-like material and single-membrane vacuoles. Degeneration of neurosecretory elements seems to occur mainly due to hyperfunction of the hypothalamo-hypophysial neurosecretory system.     

Light and electron microscopic identification of the histamine-storing argyrophil (ECL) cell in murine stomach and of its equivalent in other mammals

Summary A peculiar type of cell, the ECL cell, accounts for a large portion of nonenterochromaffin endocrine cells in the gastric oxyntic glands of several mammals, including rat, mouse, guinea pig, rabbit, cat, dog, pig, and man. The ECL cell is characterized mainly by its secretory granules with irregular cores heavily reacting to Grimelius' silver, Sevier-Munger silver and phosphotungstic acid, while failing to react, in all species but the cat and rabbit, to Masson's argentaffin method. In the rat and mouse, the ECL cell seems to correspond exactly to the argyrophil non-argentaffin histamine-storing enterochromaffin-like cell of Håkanson and Owman (1967); in the remaining species, ECL cells seem to account for only a part of the gastric argyrophil enterochromaffin-like cells described by Håkanson and coworkers. Besides sporadic amines, ECL cell granules store non-amine components, whose possible nature is discussed.This investigation was supported in part by Farmitalia S.p.A., Milano.     

Functional morphology of the pars intermedia of the rat hypophysis as revealed with the electron microscope

Summary The pars intermedia of the hypophysis of normal and experimental rats was studied by electron microscopy. Observations of the hypophysis at various intervals following formalin induced stress or adrenalectomy indicate the existence of a functional relationship between the posterior lobe, the pars intermedia, and the adrenal cortex.Glandular cells of the normal pars intermedia are divided into two types, i. e., the light and dark cells. The former type dominates in number and is characterized by a large amount of cytoplasm filled with clear vesicles 250–350 m in diameter. Dark secretory granules smaller than 300 m are few in number and restricted to the Golgi region.After a single injection of formalin, the clear vesicles of the light cell dimmish and dark secretory granules varying in opacity increase in number. Transition from dark granules to clear vesicles is suggested. Three to five days after adrenalectomy, the light cells contain an abundance of moderately dense vesicles which are smaller than the larger more electron lucent vesicles of the normal light cells. The moderately dense vesicles are about 200 m in diameter and are extremely abundant filling the entire cytoplasm of the light cells 7 days after adrenalectomy.Bundles of unmyelinated nerve fibers are often observed in the pars intermedia, and a typical neuroglandular synapse was found in the pars intermedia of a sham-operated animal suggesting neural control of the secretion process of pars intermedia cells.The author wishes to express his hearty thanks to Dr. K. Kurosumi for his guidance throughout this work.     

Reversibility of omeprazole-evoked changes in the ultrastructure of ECL cells in the rat stomach

The ECL cells are histamine- and peptide hormone-producing endocrine cells in the rat oxyntic mucosa. They are rich in secretory vesicles and also contain microvesicles and electron-dense granules. They operate under the control of circulating gastrin. In the present study, we examined the ECL-cell ultrastructure after long term treatment with omeprazole, which is known to induce hypergastrinemia, and after withdrawal of the drug. Rats received omeprazole (400 µmol/kg per day, orally) for 16 days and were killed 1, 5, 20, or 40 days after the last dose of the drug. Oxyntic mucosal specimens were processed for electron microscopy. Electron micrographs of ECL-cell profiles were analyzed planimetrically. The ECL-cell profile area increased promptly in response to omeprazole, the secretory vesicles and granules were reduced in number and volume density, the microvesicles were unchanged in number but reduced in volume density, and vacuoles appeared. Within a week after stopping the omeprazole treatment, the numbers and volume densities of secretory vesicles and microvesicles returned to pre-stimulation values. Also, the vacuoles disappeared promptly. The ECL-cell profile area decreased below the pre-stimulation level within five days after stopping treatment, while, in contrast, the granules increased in number and volume density. Somewhat surprisingly, the cell size and the granule compartment did not return to normal until 40 days after stopping treatment.