Exocrine pancreas under experimental conditions

Summary At least four types of endocrine-like cells have been detected histochemically in the mucosa of the human colon and rectum, i.e. argentaffin cells storing 5-hydroxytryptamine (5 HT) and non-argentaffin cells reacting with glucagon, somatostatin and bovine pancreatic peptide (BPP) antibodies. Ultrastructurally, four main types and three rare types of endocrine-like cells have been identified. Among the former cells were: (1) argentaffin EC₁ cells, known to store 5 HT and substance P, (2) poorly argyrophil L cells, corresponding to the glucagon-immunoreactive cells storing enteroglucagon or glucagon-like immunoreactivity (GL1), (3) inconstantly argyrophil F-like cells, possibly corresponding to BPP-immunoreactive cells, and (4) fairly argyrophil H cells of unknown function. Rare D cells, corresponding to somatostatin cells, N cells, corresponding to neurotensin cells, and P cells, of unknown function, have been also found.Supported in part by the Italian National Research Council (Grant N. 76.01558.04)     

Neuron-specific enolase in mucosal endocrine cells and carcinoid tumours of the small intestine: A comparative study with neuron-specific enolase immunocytochemistry and silver stains

Summary Endocrine cells of human small intestinal mucosa, small intestinal carcinoids and carcinoid liver metastases were stained with an immunocytochemical technique using an antiserum against neuron-specific enolase (NSE), with the argyrophil technique of Grimelius and with the argentaffin technique of Masson. In the normal mucosa, scattered NSE-immunoreactive cells were seen mainly in the deeper parts of the crypts. These cells, as shown in the same sections, corresponded to the argentaffin and/or argyrophil cells indicating that they were of endocrine type.All intestinal carcinoids (16 cases) displayed NSE immunoreactivity. However, this reaction did not correlate on the cellular level with the silver techniques employed. Thus, many tumour cells were NSE immunoreactive but lacked an argentaffin or argyrophil reaction and vice versa. On the light microscopical level the silver techniques reveal the presence of neurohormonal granules in the tumour cells, while the NSE immunoreactivity appears to disclose neuroendocrine differentiation of the tumour cells irrespective of their hormone and granular content.Out of 13 carcinoid liver metastases, eight displayed strong NSE immunoreactivity, three were weakly stained and two were unreactive. Consecutive or the same tumour sections showed an argentaffin and argyrophil reaction in all carcinoid metastases. Since silver staining provides one type of information and NSE immunocytochemistry another, they provide in combination a good discriminator for neuroendocrine tumours.     

Application of silver stains to cytologic specimens of neuroendocrine tumors metastatic to the liver

Cytologic specimens of neuroendocrine tumors metastatic to the liver were examined with regard to their silver staining properties after the application of argentaffin and argyrophil staining techniques (Masson, Grimelius and Sevier-Munger). In tumors with a content of serotonin (small intestine carcinoids), the presence of this substance was demonstrated cytologically as an argentaffin reaction in individual tumor cells; however, formalin fixation was a prerequisite for positive staining. Melanin in malignant melanoma cells displayed a positive argentaffin reaction, irrespective of the fixation used (air drying, formalin, Bouin's fluid or acetone-alcohol). Thus, serotonin and melanin can be distinguished in cytologic samples of neuroendocrine tumors by the use of the Masson argentaffin reaction with different fixatives. The nonargentaffin-positive neuroendocrine tumor cells were weakly stained or unreactive with the Grimelius argyrophil technique. The Sevier-Munger argyrophil technique was negative or gave a disturbing nonspecific background staining reaction that was difficult to interpret in the cytologic samples. Thus, the Grimelius method appears to be the most useful silver stain for identifying neuroendocrine tumor cells in cytologic material, irrespective of their hormone content, since both argentaffin-positive and argentaffin-negative cell samples were stained at least to some degree.     

Electron microscopic classification of amine-producing endocrine cells by selective staining of ultra-thin sections

Summary The argyrophil, argentaffin and chromaffin reactions were performed directly on ultra-thin sections for examination in the electron microscope. Glutaraldehyde fixation was appropriate for the argentaffin and chromaffin reactions; additional fixation with osmium tetroxide, however, caused impairment of these reactions. Fixation with formaldehyde, but not with glutaraldehyde, was adequate for the argyrophil reaction; post-fixation with osmium tetroxide did not affect this staining. At the light microscopic level the staining reactions were correlated with fluorescence histochemistry according to the method of Falck and Hillarp. The techniques described were used to study certain amine-producing endocrine cell systems: adrenal medullary cells and thyroid parafollicular cells of the mouse, gastric endocrine cells from the oxyntic gland area of the mouse, rat and rabbit. All these cells stained argyrophil. The adrenal medullary cells and one cell type in the oxyntic gland area of the rabbit were strongly argentaffin and chromaffin. The remainder of the cells were non-argentaffin and non-chromaffin but could be induced to give an argentaffin (and chromaffin) reaction after injection of the animals with l-3,4-dihydroxyphenylalanine or l-5-hydroxytryptophan, a treatment which is known to result in the accumulation of the highly reducing dopamine and 5-hydroxytryptamine, respectively, in these endocrine cells. Without exception the precipitates formed in all the staining reactions accumulated selectively over the secretory granules of the cells.The techniques described permit differential staining of consecutive ultra-thin sections for electron microscopic characterization of one and the same cell. They will provide information necessary for correlative studies of the stainable cells at the light and electron microscopic levels.     

SGC (small granule chromaffin) cells in the mouse adrenal medulla: light and electron microscopic identification using semi-thin and ultra-thin sections.

Adrenal glands of the mouse, fixed either in glutaraldehyde followed by osmium tetroxide or in a mixture of potassium dichromate and glutaraldehyde, and embedded in Epon 812, were investigated by light and electron microscopy. An argentaffin reaction was applied to semi-thin sections for light microscopy and to ultra-thin sections for electron microscopy. Since the mature secretory granules in the Small Granule Chromaffin (SGC) cell were argentaffin and were mainly located along the cell membrane, this cell was clearly distinguishable under the light microscope both from the A (adrenaline) cell whose secretory granules were non-argentaffin and from the NA (noradrenaline) cell whose cytoplasm was rich and was filled with large, strongly argentaffin granules. Chromaffinity of the SGC cell was demonstrated under the light microscope. The SGC cell was intensively stained with toluidine blue without revealing metachromasia. It was demonstrated at the EM level that not only the secretory granules but also the synaptic-like vesicles in the SGC cell contained argentaffin substances. Possible functional relationship between the secretory granules and the synaptic-like vesicles was discussed.     

Amine-producing endocrine-like cells in the epithelium of urethra and prostate of the guinea-pig a chemical,fluorescence histochemical,and electron microscopic study

Summary The urethra and prostate of the guinea-pig contain at least two types of endocrine-like cells in the epithelium. The predominant type is argentaffin and stores 5-hydroxytryptamine. Treatment with reserpine or a dopa decarboxylase inhibitor markedly reduces the 5-hydroxytryptamine content of this cell type. The other less numerous cell type, which is argyrophil but not argentaffin, is devoid of 5-hydroxytryptamine but can be induced to store dopamine if supplied with dopa. Both cell types occur disseminated in the urethral epithelium, whilst only the argyrophyl, non-argentaffin cell type devoid of 5-hydroxytryptamine is found in the prostate. At the ultrastructural level the argentaffin cell type contains numerous electron-dense cytoplasmic 800–1000 Å granules. These granules are argentaffin, suggesting that they are the storage site for 5-hydroxytryptamine. The cells sometimes reach the urethral lumen via a narrow neck, the apex being endowed with microvilli. This arrangement suggests that the cells are capable of responding to stimuli in the urethral lumen. Preliminary attempts to test the effect of depriving or loading guinea-pigs with water failed to induced changes in the 5-hydroxytryptamine content of the urethral endocrine-like cells.     

Argyrophilia in ovarian serous tumors. A comparative study in 127 epithelial ovarian tumors.

The distribution of argyrophil cells in epithelial ovarian tumors was studied in 127 cases. The results showed that not only mucinous tumors and endometrioid tumors contained argyrophil cells, but also some serous tumors expressed argyrophilia. 31% of serous tumors including 40% of serous adenocarcinomas contained variable numbers of argyrophil cells. Argyrophilia has been demonstrated in mucinous tumors, endometrioid tumors and Brenner tumors before. However, this is the first time the presence of argyrophilia in serous tumors has been noticed. Moreover, the argyrophil cells in 5 serous carcinomas showed reactivity with Neuroendocrine (chromogranin A) antibody but not with serotonin. The expression pattern of argyrophilia in the serous tumors was different from that of the mucinous tumors; in the former, argyrophil granules appeared in apical portions or throughout the cytoplasm of single or clustered cells. In addition, the argyrophilia in some serous tumors and endometrioid tumors decreased after diastase digestion. Ultrastructurally, no typical neurosecretory granule was found in the argyrophilic serous tumors. The findings in this study suggest that argyrophilia could be quite frequently found in ovarian epithelial tumors and in itself is not a very specific differential characteristic of carcinoid tumors. The argyrophilia found in a variety of epithelial ovarian tumors might lend additional support to the histogenesis and close relationship between the common epithelial tumors of the ovary.     

Distinct patterns of chronic gastritis associated with carcinoid and cancer and their role in tumorigenesis.

A series of 60 gastric endocrine tumors comprised 44 body-fundus argyrophil carcinoids, of which 23 arose in a background of hypergastrinemia and type A chronic atrophic gastritis (A-CAG), mainly with histologic patterns suggestive of an autoimmune process. Only 22 percent (compared with 19 percent of 58 tumor-free A-CAG cases) of 36 carcinoids and 21 percent of 19 A-CAG carcinoids investigated had Helicobacter pylori (HP) colonization, against 50 percent of 14 CAG-associated neuroendocrine carcinomas or mixed endocrine-exocrine tumors, 84 percent of 150 cases with early gastric cancer (p < 0.001 versus carcinoids), mostly with B- or AB-type CAG, 76 percent of 97 tumor-free AB-CAG, and 95 percent of 151 tumor-free B-CAG cases. Secondary hypergastrinemia and local mechanisms activated by chronic autoimmune gastritis are among factors involved in the pathogenesis of relatively indolent CAG-associated carcinoids, whereas active HP gastritis in cooperation with environmental carcinogens may likely cause more severe epithelial transformation, leading to ordinary cancer and, possibly, to neuroendocrine carcinomas or mixed endocrine-exocrine tumors.     

Ultrastructural similarity of endocrine-like cells of the human lung and some related cells of the gut

Summary The ultrastructure of endocrine-like cells of the human lung was compared to the ultrastructure of endocrine-like cells of the stomach and pancreas in both adult and foetal material.Three types of endocrine-like cells were found in the human foetal lung. Type 1 or P₁ cells contained very small granules (about 110 nm) of two varieties, cored and vesicular; type 2 or P₂ cells with cored granules measuring about 130 nm; and type 3 cells with cored granules of about 180–190 nm. In the adult lung only one type P_a cells with cored granules could be found.Cells resembling foetal P₁ cells were not found in foetal or adult gastric mucosa, or in the pancreas. In the gastric mucosa cells resembling pulmonary P_a or P₂ cells were moderately represented and often difficult to distinguish from each other. Thus, they were grouped together as gastric P cells. Cells with granules resembling those of pulmonary type 3 cells were found most numerous in the adult oxyntic mucosa. Cells resembling gastric P cells (and pulmonary P₂ cells) were rather numerous in foetal pancreas, but very rare in adult pancreas. Few cells containing granules somewhat resembling those of pulmonary type 3 cells were present in both foetal and adult pancreas.The results were discussed in respect to 1) the similarities between some gastric or pancreatic carcinoids and lung carcinoids, 2) the gastro-pancreatic P cells as a separate cell population, 3) the possible secretion by the lung endocrine-like cells of active substances, either amines or peptides, 4) the similarity between the secretory granules of P_a and P₁ cells and neurosecretory granules of the hypothalamus and between P₂ cells and some endocrine cells of the pituitary.Supported in part by the Italian National Research Council (Grants N. 75.00630.04 and N. 76.01558.04)     

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.     

A novel endocrine cell type in the guinea-pig gastric mucosa: cellular source of pro-enkephalin-derived peptides. A histochemical, immunohistochemical, and electron microscopical characterization

A novel endocrine cell type has been identified in the guinea-pig gastric mucosa which preferentially occurs in the oxyntic area. Cells of this type exhibit immunoreactivities for bovine adrenal medulla dodecapeptide (BAM-12P) and in many cases for Met-enkephalin and are thus presumed to contain a pro-enkephalin-like precursor protein. Systematic immunohistochemical investigations show that these cells do not contain immunoreactivities for various enteric hormones, neuropeptides and biogenic amines (serotonin, histamine). However, they do contain immunoreactivity for chromogranin A, an acidic glycoprotein which is common to the majority of entero-endocrine cells. Using silver impregnation techniques BAM-12P immunoreactive cells prove to be argyrophil, but fail to react argentaffin. On the electron microscopical level, these cells contain a well-developed endoplasmic reticulum and Golgi apparatus and numerous polymorphous secretion granules which measure about 290 nm in diameter. The secretion granules are ovoid or pear-shaped but largely plump compared to those of enterochromaffin cells. Light and electron microscopical findings indicate that BAM-12P immunoreactive cells constitute an endocrine cell population of the gastric epithelium in addition to the "established" endocrine cells hitherto known in this location.     

多疣壁虎肠道内分泌细胞的分布及形态学观察

用光镜和电镜对多疣壁虎肠道嗜银细胞的分布及形态做了初步的观察,结果显示,十二指肠嗜银细胞密度最高,大肠次之,空、回肠较低;作者认为嗜银细胞的分布与动物生活环境是相关的,嗜银细胞形态多样,位于肠上皮细胞之间和固有层结缔组织中,电镜下,嗜银细胞内充满足电子致密颗粒,十二指肠和大肠嗜银细胞颗粒大小和密度不同。根据内分泌特点的不同,可分为开放型和闭合型两类。本文还对多疣壁虎肠道嗜银细胞的分布及形态特点做了探讨。     

Gut hormones in Salamandra salamandra

Summary Histological, cytochemical and immunocytochemical methods were used in light and electron microscopical studies to demonstrate the presence of a neuroendocrine system in the gut of the urodele, Salamandra salamandra.Cytochemical stains capable of detecting peptide-producing endocrine cells demonstrate cells reacting with Masson's silver (argentaffin) method, Grimelius' argyrophil silver method, masked metachromasia method and the lead haematoxylin stain.Using antisera raised to a variety of mammalian gut peptides, cells containing bombesin-, gastrin-, somatostatin-, substance P- and glucagon-like immunoreactivity were identified; vasoactive intestinal polypeptide- and substance P-like immunoreactivities were found in nerve fibres in the submucous and myenteric plexus. No immunoreactivity was detected for motilin, gastric inhibitory polypeptide, cholecystokinin or secretin.The ultrastructure of the immunoreactive cells and nerves was revealed by the semithin/thin method. All the cells identified contained numerous electrondense secretory granules, which varied in their chracteristic morphological structure from one cell type to another.The evidence collected in this study indicates that a complex neuroendocrine system regulating gut function is present in this amphibian and may have developed prior to the emergence of the phylum.     

Endocrine-like cells of the pulmonary epithelium of the human adult lung

Summary The morphological and histochemical characteristics of endocrinelike cells of the pulmonary epithelium of the right lower lobe of 12 human adult lungs were studied.Few cells were reactive to the argyrophil silver method of Grimelius and of Sevier and Munger and cells with a similar morphology and distribution emitted a green or yellow fluorescence after treatment of the lung epithelium with the amine precursors L-DOPA or L-HTP, respectively. A greater number of cells seems to be demonstrated by electron microscopy. The cells were characterized by small, round secretory granules showing a central dense core and a very thin clear halo between the core and the surrounding membrane.The cells are thought to be related to the endocrine-like cells of the pulmonary epithelium of the human foetal lung and to cells of carcinoids of larger bronchi.     

Immunoreactive atrial natriuretic polypeptides in the adrenal medulla and sympathetic ganglia

Atrial natriuretic polypeptide (ANP)-like immunoreactivity was found in the rat adrenal gland by using indirect immunofluorescence and peroxidase-antiperoxidase techniques. ANP-like immunostaining was present in most of chromaffin cells with varying degrees of immunoreactivity. The majority of medullary cells displayed very intense immunostaining, and several clusters revealed weaker immunostaining. No staining was found in the adrenal cortex or in the nerve fibers in this organ. In the consecutive sections treated for dopamine-beta-hydroxylase (DBH), apparently all medullary cells had intense immunofluorescence for DBH and its distribution pattern was very similar to that for ANP-like immunoreactivity. While phenylethanolamine N-methyltransferase (PNMT) immunoreactive cells largely corresponded to the intensely stained ANP-like immunoreactive cells, suggesting that adrenaline cells contained a large amount of ANP-like substance, noradrenaline cells contained a smaller amount of this substance than adrenaline cells. Ultrastructural study showed that end-products due to the immunoreaction with the ANP antiserum were primarily associating with chromaffin granules. In addition, the presence of ANP-like immunoreactivity was investigated in several sympathetic ganglia of the rat. No principal ganglion cells were ANP-positive, whereas a few small intensely fluorescent (SIF) cells were ANP-immunoreactive. The present findings suggest that catecholamines coexist with ANP which has a natriuretic and vasodilating effect, in adrenal medullary cells and SIF cells in several rat sympathetic ganglia, but not in principal ganglion cells.     

Gastrointestinal carcinoid tumours. Histogenetic, histochemical, immunohistochemical, clinical and therapeutic aspects

The increased knowledge of the pathobiology of gastrointestinal carcinoid (neuroendocrine) tumours and the improved therapeutic possibilities have brought a demand for more precise diagnosis. Although the carcinoid tumours can often be tentatively recognized in routinely processed microscopic slides, their more accurate identification requires additional diagnostic procedures. General neuroendocrine markers such as the argyrophil reaction of Grimelius and immunohistochemistry with application of antibodies against chromogranin A and of neuron-specific enolase are discriminatory staining methods which are used to reveal the neuroendocrine origin of almost all highly differentiated carcinoid tumours of the gastrointestinal tract. Mid-gut carcinoids, which predominate among these tumours almost unexceptionally contain serotonin. This biogenic amine can be demonstrated by the argentaffin reaction of Masson, serotonin immunoreactively or by formalin-induced fluorescence. The characteristic staining pattern of mid-gut carcinoids is almost invariably preserved in the metastatic deposits and consequently the staining methods for identifying serotonin can also be used on metastases to reveal a primary mid-gut carcinoid. The enterochromaffin-like (ECL) cell carcinoids of the body and fundic area of the stomach often seen in association with pernicious anaemia are argyrophil with the Sevier-Munger silver stain. Other neuroendocrine tumours, viz. antral, duodenal and rectal carcinoids should be studied by a battery of relevant peptide hormone antisera for adequate diagnosis. During the last decade new peptide hormones have been found in circulation in patients with carcinoid tumours, but serotonin and urinary 5-HIAA are still the most important markers for carcinoids of the mid-gut origin. Other clinically useful tumour markers are chromogranin A + B, pancreatic polypeptide, human chorionic gonadotropin alpha and beta subunits. For localizing procedures, angiography is the most reliable investigative method for primary tumours in the gut, whereas CT-scan and ultrasound investigations are good for detection of liver metastases. During the last five years, the therapy for malignant carcinoid tumours has been considerably improved. Chemotherapy has only revealed objective response rates in about 10-30% of the patients giving median survivals from start of therapy of about 10 months. Recently treatment with alpha interferons and the new somatostatin analogue octreotide have given objective responses in 50-75% of patients with malignant mid-gut carcinoid tumours. These patients have now a median survival from start of therapy of 70 months when treated with alpha interferons. In the future new therapies will come into use such as monoclonal antibodies and perhaps also agents blocking different growth factors.     

Identification of enterochromaffin cells in adjacent Epon-embedded sections at light and electron microscopic levels

Summary Methods for light and electron microscopic comparison of individual argentaffin and argyrophil enterochromaffin cells (EC) in the sheep duodenal mucosa are described. These silver procedures were applied for light microscopy to Epon-embedded sections. The adjacent sections were examined with the electron microscope. The most specific characteristics of the argentaffin and argyrophil EC in electron microscopy are highly osmiophilic cytoplasmic granules. In one cell type these granules are smaller and more roundish than in the another type. These two cell types are stainable both by the argentaffin and argyrophil reactions. No essential difference can be observed in the localization of these elements. It is suggested that both cell types belong to the enterochromaffin system. Both silver methods are also suitable for the light microscopic identification of other intestinal structures in sections adjacent to that sectioned for electron microscopy.This work was supported by a grant from the Yrjö Jahnsson Foundation, Helsinki, Finland.The electron microscopic observations were carried out in the Electron Microscope Laboratory, University of Helsinki.     

On argyrophil reactions of endocrine cells in the human fetal pancreas

Summary The endocrine cells in the pancreas of five human fetuses with gestational ages of 18–20 weeks were examined by light and electron microscopy with special regard to argyrophil reactions. B-cells and typical A and D-cells were easily identified electron microscopically on the basis of their typical secretory granules. In the Grimelius argyrophil silver stain, a concentration of silver grains over the less electron dense peripheral mantle of the A-cell secretory granules was observed by electron microscopy. In the Hellerström and Hellman modification of the argyrophil Davenport alcoholic silver stain, silver grains were concentrated over the internal structures of the D-cell secretory granules. With this stain an accumulation of silver grains was also seen at the surface of the A-cell secretory granules. The argyrophil reaction of the A-granules was less pronounced than in the D-cells. In addition to B-cells and A- and D-cells, two other types of endocrine cell were observed by electron microscopy. These cells were argyrophil with the silver impregnation method of Grimelius. The electron microscopic findings at least partly explain the frequent overlapping between the two staining methods observed at the light microscope level.This study was supported by the Swedish Medical Research Council (Project No. 102)     

In situ hybridization study of chromogranin A and B mRNA in carcinoid tumors.

The distribution of the mRNAs for chromogranin A and B was analyzed by in situ hybridization with 35S-labeled oligonucleotide probes in formalin-fixed paraffin-embedded carcinoid tumor tissues. All the 15 mid-gut carcinoid tumors examined contained both mRNAs for chromogranin A and B at high level in tumor cells. Sixteen of 18 bronchial carcinoid tumors but only 2 of 5 rectal carcinoid tumors expressed one or both species of chromogranin mRNAs. The same tendency was seen with the argyrophil reaction according to Grimelius where most of the mid-gut tumor cells were uniformly stained, while considerable variation in reactivity was seen in some of the bronchial and rectal carcinoid tumor cells. The sequential sections were stained with a monoclonal antibody against chromogranin A and a polyclonal antiserum which reacts with both chromogranins. The expression of the mRNA for chromogranin A on the carcinoid tumors was almost concordant with that of chromogranin B as well as with the chromogranin A protein, whereas almost all tumors stained positively with the polyclonal antibodies. Analyses of mRNA expression of chromogranin A before and after interferon therapy on 4 patients with mid-gut carcinoids indicated an inhibition at pre-translational level. In conclusion, the mRNAs for chromogranin A and B are good markers for the carcinoid tumors, especially of mid-gut origin. Fore-gut, mid-gut and rectal carcinoid tumors are different in their endocrine properties regarding the expression of the chromogranins.     

Quantification of endocrine cells and ultrastructural study of insulin granules in the large intestine of opossum Didelphis aurita (Wied-Neuwied, 1826)

This study aimed to investigate the distribution of argyrophil, argentaffin, and insulin-immunoreactive endocrine cells in the large intestine of opossums (Didelphis aurita) and to describe the ultrastructure of the secretory granules of insulin-immunoreactive endocrine cells. Fragments of the large intestine of 10 male specimens of D. aurita were collected, processed, and subjected to staining, immunohistochemistry, and transmission electron microscopy. The argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells were sparsely distributed in the intestinal glands of the mucous layer, among other cell types of the epithelium in all regions studied. Proportionally, the argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells represented 62.75%, 36.26%, and 0.99% of the total determined endocrine cells of the large intestine, respectively. Quantitatively, there was no difference between the argyrophil and the argentaffin endocrine cells, whereas insulin-immunoreactive endocrine cells were less numerous. The insulin-immunoreactive endocrine cells were elongated or pyramidal, with rounded nuclei of irregularly contoured, and large amounts of secretory granules distributed throughout the cytoplasm. The granules have different sizes and electron densities and are classified as immature and mature, with the mature granules in predominant form in the overall granular population. In general, the granule is shown with an external electron-lucent halo and electron-dense core. The ultrastructure pattern in the granules of the insulin-immunoreactive endocrine cells was similar to that of the B cells of pancreatic islets in rats.