Neuroendocrine differentiation in gastric adenocarcinomas associated with severe hypergastrinemia and/or pernicious anemia

Patients with hypergastrinemia secondary to achlorhydria have an increased risk of developing ECL cell carcinoids and gastric adenocarcinomas. Hypergastrinemia is central in the pathogenesis of ECL cell carcinoids, but the link between gastrin and gastric carcinomas is controversial. During neoplastic transformation ECL cells may, however, lose many of their neuroendocrine characteristics, making them difficult to recognise as neuroendocrine with conventional immunohistochemical techniques. Neuroendocrine differentiation was therefore examined in eight gastric adenocarcinomas found in seven patients with severe hypergastrinemia and/or pernicious anemia using a monoclonal antibody towards chromogranin A and immunohistochemistry without and with a sensitive signal amplification technique. The Sevier-Munger method was used as a more specific marker of ECL cells. Seven of the carcinomas contained scattered neuroendocrine tumour cells. When using signal amplification, an increase in the number of immunoreactive neoplastic cells was seen. In many tumours, clusters or confluent sheets of such cells were disclosed, suggesting a neuroendocrine and ECL cell origin. These tumours may therefore be ECL cell carcinomas and hypergastrinemia may thus be involved in the tumourigenesis.     

Smooth Muscle Cell Abnormalities Associated with Gastric ECL Cell Carcinoids

The histologic and immunohistochemical study of 45 ECL cell gastric carcinoids and of the extratumoral gastric mucosa revealed four variants of smooth muscle cell abnormalities: (1) hypertrophy of muscularis mucosae trapped within the tumors, a finding occurring in 76.5% of cases; (2) proliferation of stromal smooth muscle cells originating from the muscularis mucosae and mostly associated with tumor invasion of the submucosa (seen in 93.9% of cases with abundant stromal component of the tumors); (3) occurrence of frequent, prominent aggregates of smooth muscle cells in the lamina propria of the antral (but not of the fundic) mucosa of the stomach (found in 41.7% of cases); and (4) increased thickness of the extratumoral muscularis mucosae in the fundic (but not in the antral) mucosa of patients with gastric carcinoids. In addition, localized muscle cell proliferation was also associated with foci of micronodular hyperplasia of endocrine cells in the extratumoral mucosa. These findings were neither observed in control cases of gastric adenocarcinoma, gastric peptic ulcer, and duodenal peptic ulcer (10 unselected cases from each group) nor were they observed in 10 subjects with normal gastric mucosa collected at autopsy. With the possible exception of the increased thickness of the extratumoral fundic muscularis mucosae, which may be influenced by the mucosal inflammatory process, it is suggested that the present findings represent a proliferative response of smooth muscle cells to basic fibroblastic growth factor whose production by gastric carcinoids and their precursor lesions has recently been demonstrated.     

Histamine in gastric carcinoid tumors: Immunocytochemical evidence

Gastric carcinoid tumors and nontumorous corpus mucosa from 4 patients suffering from chronic atrophic gastritis associated with pernicious anemia were characterized histochemically, with special reference to the cellular localization of histamine. Tissue sections were also examined for argentaffinity using the Masson technique, for argyrophilia using the Grimelius and Sevier-Munger techniques, and for chromogranin A and serotonin immunoreactivities. The majority of the tumor cells showed the staining characteristics of enterochromaffinlike cells: That is, they exhibited the argyrophil reaction with the Grimelius and Sevier-Munger techniques but lack of argentaffinity, positive histamine and chromogranin A immunostaining, but no serotonin immunoreactivity. Numerous histamine-immunoreactive mast cells were present in the stroma of the tumors and also in the surrounding mucosa. Our findings support the view that gastric carcinoids in patients with hypergastrinemia due to chronic atrophic gastritis are histamine-producing tumors derived from hyperplastic enterochromaffinlike cells.     

Gastric Carcinoids of Argyrophil ECL Cells

Histochemical and ultrastructural studies were carried out in four gastric carcinoids, two of which were associated with atrophic gastritis and pernicious anemia. All tumors showed intense argyrophilia and vesicular granules resembling those of endocrine enterochromaffinlike (ECL) cells in normal human gastric mucosa. Tumor cells were found to be unreactive to all the 18 available antiserums to gut hormones, including gastrin, somatostatin, glucagon, and pancreatic polypeptide. The tumors were interpreted as ECL cell argyrophil carcinoids with various degrees of differentiation and atypia.     

Gastric Carcinoids of Argyrophil ECL Cells

Histochemical and ultrastructural studies were carried out in four gastric carcinoids, two of which were associated with atrophic gastritis and pernicious anemia. All tumors showed intense argyrophilia and vesicular granules resembling those of endocrine enterochromaffinlike (ECL) cells in normal human gastric mucosa. Tumor cells were found to be unreactive to all the 18 available antiserums to gut hormones, including gastrin, somatostatin, glucagon, and pancreatic polypeptide. The tumors were interpreted as ECL cell argyrophil carcinoids with various degrees of differentiation and atypia.     

Endocrine Precursor Lesions of Gastroenteropancreatic Neuroendocrine Tumors

This review focuses on precursor lesions of gastrointestinal and pancreatic neuroendocrine tumors (GEP-NETs). There are three conditions that are associated with hyperplastic changes in endocrine cells preceding GEP-NETs: autoimmune chronic atrophic gastritis or multiple endocrine neoplasia type 1 (MEN1) with gastric enterochromaffin-like (ECL) cell hyperplasia; MEN1 with gastrin and somatostatin cell hyperplasia in the duodenum and glucagon cell hyperplasia in the islets of the pancreas; and inflammatory bowel disease with endocrine cell hyperplasia in the colon. In gastric ECL cell hyperplasia, it is assumed that hypergastrinemia promotes the growth of the ECL cells of the corpus mucosa and leads to hyperplasia and neoplasia. In the duodenum and the pancreas, the MEN1-associated germline mutation of the menin gene obviously causes hyperplasia of the gastrin and somatostatin cells (duodenum) and the glucagon cells (pancreas), resulting in multifocal development of tumors. These tumors show allelic deletion of the MEN1 gene, whereas the precursor lesions retain their heterozygosity. The endocrine cell hyperplasia in the colon described in inflammatory bowel disease has neither a genetic nor a definite hormonal background.     

An Unusual Case of Multiple Gastric Carcinoids Associated with Diffuse Endocrine Cell Hyperplasia and Parietal Cell Hypertrophy

We describe a case of multiple gastric carcinoid tumors in a 47-year-old Japanese man. The patient had markedly elevated serum gastrin levels (>800 pg/mL), which were suppressed by secretin-pancreozymin administration. In the partial gastrectomy specimen, a total of 19 carcinoids arose from diffuse linear and micronodular hyperplasia of the oxyntic mucosal endocrine cells. The carcinoid cells were chromogranin A-positive. Except for a very small number of serotonin-positive cells in several carcinoids, none of the 19 reacted with a battery of antibodies to other bioactive neuroendocrine substances. The most prominent findings in this case were peculiar fundic glands that were distended with a proteinaceous substance and lined with large hypertrophic parietal cells. At the ultrastructural level, these cells showed poorly developed intracytoplasmic canaliculi and vesicotubular profiles, yet their large cytoplasm had numerous mitochondria. On the basis of our histological and ultrastructural findings we suggest that an intrinsic HCl secretion abnormality of the parietal cells may be responsible for the patient's hypergastrinemia. Since there have been no reports on similar parietal cells changes, it is possible that our case may represent a pathological entity not previously described.     

Dedifferentiation of enterochromaffin-like cells in gastric cancer of hypergastrinemic cotton rats

The role of enterochromaffin-like (ECL) cells in gastric carcinogenesis is not fully understood. Spontaneous tumours developing in hypergastrinemic female cotton rats have an adenocarcinoma phenotype, but numerous cells in the dysplastic mucosa as well as in the carcinomas are positive for neuroendocrine markers. In the present study of female cotton rats with 2 and 8 months' hypergastrinemia, the oxyntic mucosa of the stomach was examined histologically and immunolabelled for histidine decarboxylase (HDC) and pancreastatin, and hyperplastic and neoplastic ECL cells were evaluated by electron microscopy. These animals developed hyperplasia of the oxyntic mucosa in general and of the ECL cells in particular after 2 months and dysplasia and carcinomas after 8 months. The immunoreactivity of the ECL cells in the oxyntic mucosa was increased at 2 months and declined at 8 months. These histological changes were associated with progressive loss of secretory vesicles and granules in ECL cells. We suggest that ECL cells in hypergastrinemic cotton rats dedifferentiate with time and that the gastric carcinomas may develop from ECL cells.     

Autoimmune gastritis. A clinicopathologic study of 25 cases

The histopathological diagnosis of autoimmune gastritis (AG) in its early stages can be a diagnostic challenge. Even some advanced cases with complete atrophy of the corpus mucosa may be difficult to recognize. To establish the diagnosis of autoimmune gastritis, several histological features should be assessed and combined with immunostains for enterochromaffin cell-like (ECL) cells and G-cells. The main histological criteria include a mononuclear infiltrate within the lamina propria, foci of destruction of oxyntic glands, intestinal metaplasia (IM), pyloric metaplasia, and parietal cell pseudohypertrophy. These criteria were evaluated in our series of 25 patients with achlorhydria and/or megaloblastic anemia. Some of our patients presented with nonspecific gastrointestinal symptoms. The age ranged between 46 and 79 years; one male patient was only 31 years old. Histologically, the corpus mucosa displayed in all cases chronic inflammation with focal complete IM and advanced pyloric metaplasia. In 4 patients, oxyntic glands were destructed in some sites. There was a pancreatic metaplasia of acinar type in 2 patients and a minimal focal pseudohypertrophy of parietal cells in the 31-year-old man. A tubular adenoma with a low-grade dysplasia was found in one female patient. Immunohistochemically, chromogranin-A highlighted linear or nodular hyperplasia of ECL cells in 19 patients, and adenomatoid ECL hyperplasia in one case (80%). In the remaining cases hyperplasia of ECL cells could not be recognized from their normal count. In 13 cases (52%) a few ECL cells were seen also in IM. Regarding associated pathology, in one woman with nodular ECL cell hyperplasia, a gastric carcinoid was removed endoscopically. The reaction with gastrin antibody revealed in 11 cases (44%) a small number of G cells in IM in the corpus mucosa. In 18 patients, antral mucosa was examined as well. In 8 patients, the mucosa was normal; in 10 cases, a mild chronic inactive gastritis was diagnosed, and in 15 patients G-cell hyperplasia was found. In accordance with other studies, we show that the diagnosis of AG may be established microscopically in endoscopic specimens of the gastric body mucosa when histologic features and immunohistochemical detection of ECL and G cell hyperplasia are combined.     

G‐cell hyperplasia of the stomach induces ECL‐cell proliferation in the pyloric glands in a paracrinal manner

An inhibitory mechanism toward gastrin hypersecretion is significantly different between G‐cell hyperplasia and gastrinoma despite the common clinical manifestations; hypergastrinemia and its related persistent gastric ulcers. We recenlty studied the G‐cell, d ‐cell and ECL‐cell density in a case of G‐cell hyperplasia. The 70‐year‐old patient has been treated for persistent gastric ulcers with a markedly increased plasma gastrin (5600 pg/mL). The stomach was surgically resected because of the obstruction associated with ulcer scars. The number of G‐cells in the pyloric glands was quantified on the surgical specimens and G‐cell hyperplasia was histolopathologically identified. Immunostainig of histidine decarboxylate revealed the presence of ECL‐cell hyperplasia in the pyloric glands and its density was significantly and positively correlated with G‐cell density. Somatostatin immunoreactive cells (d ‐cells) increased in their number in the oxyntic glands. These results all indicated that hypersecretion of gastrin in G‐cell hyperplasia could induce ECL‐cell proliferation in a paracrinal manner. In addition, relatively non‐prominent endocrinological features in the G‐cell hyperplasia compared to gastrinoma could be also related to the paracrinal somatostatin inhibitory effects upon ECL‐cells in the pyloric glands.     

Endocrine neoplasm of the stomach. Study of thirteen cases

A Thirteen patients with primary endocrine neoplasm of the stomach were studied for 20 years. Six patients were male and 7 female with an age range of 33 to 77, mean age 57 years. Nine cases corresponded to well differentiated carcinoids and four to neuroendocrine carcinomas. Of the former, three were sporadic and six were associated with atrophic gastritis. These two forms of neoplasm showed important differences: those associated with atrophic gastritis had hypergastrinemia, all of the multiple small tumors confined to the corpus and fundus were well differentiated carcinoids associated with intestinal metaplasia and G cell hyperplasia in antrum and ECL cell hyperplasia in corpus and fundus. Tumors were clinically benign, with an excellent prognosis. All patients are currently alive with no evidence of neoplasm. In only one of these cases, antiparietal cell antibodies were documented; in three of them, extensive intestinal metaplasia probably due to Helicobacter pylori infection was found. In contrast, sporadic carcinoids were large isolated tumors originating in the antrum or corpus. Two patients died as a consequence of the neoplasm; all of them were moderately differentiated and in none of the cases we found evidence of endocrine hyperplasia. All were positive for generic endocrine markers and were focally positive to some of the specific hormone markers. Al four neuroendocrine carcinomas had a clinical course similar to that of gastric adenocarcinomas and were poorly differentiated large tumors. We conclude that gastric carcinoids associated with atrophic gastritis have an excellent prognosis. On the other hand, neuroendocrine carcinomas have a very poor prognosis with fatal outcome of patients. Sporadic carcinoids have an intermediate prognosis.     

The regulation of gastric acid secretion – clinical perspectives

The purpose of this review, based upon 40 years of research, is to clear old controversies. The gastric juice is a strong acid with active enzymes (pepsin and lipase); ideal for killing swallowed microorganisms. Totally isolated rat stomach and histamine determination. Human gastric carcinomas were examined for ECL cell differentiation because tumours found in rodents after dosing with inhibitors of acid secretion were reclassified to be of ECL cell origin. The gastrin receptor is localized to the ECL cell only, where gastrin stimulates the function and growth. Drug‐induced hypo‐acidity induces hypergastrinaemia and ECL cell hyperplasia responsible for rebound acid hypersecretion. Every condition with long‐term hypergastrinaemia disposes to ECL cell neoplasia. In man, both atrophic gastritis and gastrinoma lead to ECL cell carcinoids. Proton pump inhibitors induce hypergastrinaemia with ECL cell hyperplasia and ECL cell carcinoids that disappear when stopping treatment. The gastrin antagonist netazepide induces regression of ECL cell carcinoids due to atrophic gastritis. Human gastric carcinomas of diffuse type, particularly the signet‐ring subtype, show ECL cell differentiation, suggesting involvement of gastrin in the carcinogenesis. Helicobacter pylori (Hp) causes gastritis and peptic ulcer, and when infecting the antrum only gives a slight hypergastrinaemia with acid hypersecretion predisposing to duodenal ulcer, but protecting from gastric cancer. When Hp infection spreads to oxyntic mucosa, it induces atrophy, reduced acid secretion and marked hypergastrinaemia and cancer.It is remarkable that the interaction between Hp and gastrin may explain the pathogenesis of most diseases in the upper gastrointestinal tract.     

Malignant Enterochromaffinlike Cell Carcinoid of the Gastric Stump: An Ultrastructural Study

A metastasizing carcinoid of the gastric stump found 25 years after Billroth II gastric resection for duodenal ulcer is described. Electron microscopy and optical endocrine cell staining proved the tumor to be composed of enterochromaffinlike (ECL) cells. This unusual combination further shows that, at variance with most of these tumors, ECL cell carcinoids may develop also in a condition excluding a trophic effect of gastrin. This case emphasizes the malignant behavior of gastrin-independent ECL cell tumors.     

ECL‐cell carcinoids and carcinoma in patients homozygous for an inactivating mutation in the gastric H+K+ATPase alpha subunit

A family with a missense variant of the ATP4A gene encoding the alpha subunit of the gastric proton pump (H⁺K⁺ATPase) has recently been described. Homozygous siblings were hypergastrinemic (median gastrin 486 pM) and had gastric tumours diagnosed at a median age of 33 years. In the current histopathological study, we further characterized the tumours found in the gastric corpus. The tumours had the histological appearance of carcinoids (NET G1 or G2) and were immunoreactive for the general neuroendocrine markers chromogranin A (CgA) and synaptophysin as well as the ECL‐cell markers vesicular monoamine transporter 2 (VMAT2) and histidine decarbozylase (HDC). One of the tumours consisted of a NET G2 component, but also had a component with glandular growth, which morphologically was classified as an intestinal type adenocarcinoma. Many glands of the adenocarcinoma contained a large proportion of cells positive for neuroendocrine markers, especially the small vesicle marker synaptophysin and the cytoplasmic enzyme HDC. In conclusion, patients homozygous for an inactivating ATP4A mutation develop gastric ECL‐cell carcinoids in their 3rd or 4th decade. The adenocarcinoma may be classified as neuroendocrine with ECL‐cell differentiation.     

Omeprazole: its influence on gastric acid secretion, gastrin and ECL cells

The H+,K+-ATPase inhibitor omeprazole is a highly effective gastric antisecretory agent, both in animals and man, with a long duration of action. These properties are shared by a number of recently described histamine H2-receptor antagonists. In life-long oncogenicity studies of these H2-receptor antagonists, as well as with the H+,K+-ATPase inhibitor omeprazole, gastric enterochromaffin-like cell (ECL cell) hyperplasia and carcinoids have been found. The purpose of this paper is to summarize available evidence for the "Gastrin Hypothesis" to explain the development of ECL-cell hyperplasia. The hypothesis may be outlined as follows: 1) Inhibition of gastric acid secretion leads to elevated antral pH and, secondarily, to release of gastrin from the antral gastrin cells into the blood stream. 2) Gastrin causes both general hypertrophy of the oxyntic mucosa and hyperplasia of the ECL cells in the oxyntic mucosa. That this sequence of events occurs not only with omeprazole but also with other effective gastric antisecretory agents has been verified in the rat by giving the H2-receptor antagonist ranitidine as a continuous infusion. Ranitidine caused a hypergastrinemia of a similar magnitude as that seen after omeprazole, provided that the acid secretion was inhibited to a similar degree. At similar gastrin levels, ECL-cell hyperplasia of the same magnitude developed during both ranitidine and omeprazole treatment. Antrectomy prevented the development of ECL-cell hyperplasia during omeprazole treatment, indicating that the hyperplasia was not due to the drug treatment per se, but rather to the hypergastrinemia. Both the hypergastrinemia and the ECL-cell hyperplasia were found to be reversible.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pattern of gastric endocrine cells in microcarcinoidosis — an immunohistochemical study of 14 gastric biopsies

A total of 14 gastric biopsy specimens from patients with microcarcinoidosis were analysed by immunohistochemical methods to evaluate the pattern of endocrine cell hyperplasia and dysplasia. All the patients had type A gastritis (autoimmune gastritis). Nonantral proliferations of gastric endocrine cells were classifed according to Solcia et al. All 14 cases had hyperplasia and 13 (92.9%) of them, dysplasia of gastric endocrine cells; 9 (64.3%) of the 14 were found to have showed a coexisting invasive gastric carcinoid at the time of diagnosis of microcarcinoidosis. The patients with invasive carcinoids had higher degrees and more complex forms of endocrine dysplasia (precarcinoid lesions). The average size of the foci of the microcarcinoidosis in gastric biopsies was 0.14±0.09 cm in the patients without invasive carcinoid, as against to 0.5±0.24 cm in the group of patients with associated invasive carcinoid. Microcarcinoid gastric biopsies about 0.5 cm in size, are suggestive of adjacent invasive carcinoid. However, even frankly invasive ECL carcinoids seem to be clinically less dangerous than was thought until recently.     

Achylia and the development of gastric carcinoids

Summary Twenty-four patients with gastric carcinoids were examined retrospectively with regard to the presence of achylia and pernicious anaemia (PA). Six patients (25%) had achylia preoperatively and half of these had an associated PA. The gastric carcinoids occurring in an achylic stomach were almost exclusively located to the corpus area and tended to be multiple. Further, the majority of cells in most of them were argyrophil (Sevier-Munger) positive, indicating that they were of the enterochromaffin-like cell type. At present, achylia with or without concomitant PA appears to be the most distinct condition associated with gastric carcinoids. The pathogenesis of these tumours which develop in an achylic stomach is discussed.The term carcinoid is used here to mean a primary endocrine tumour of the digestive tract irrespective of the presence or absence of 5-hydroxytryptamine (5-HT) (Dawson 1976)     

Goblet cell carcinoid of the appendix: a specific type of carcinoma

AIMS: Goblet cell carcinoid is a poorly understood tumour of the appendix. The aim of this study was to determine whether it should be regarded as a separate entity or as a variant of classical carcinoid. METHODS AND RESULTS: The immunohistochemical expression pattern of 21 markers and the mutation status of KRas codon 12 were determined in 16 goblet cell carcinoids and compared with 14 classical carcinoids, 19 colonic adenocarcinomas and 10 appendiceal mucinous cystadeno (carcino)mas. The results were subjected to a stepwise linear discriminant analysis. Goblet cell carcinoids were significantly different from the control groups. The most important markers for discriminating between the groups were CEA (classical carcinoid versus all others), KRas mutation (present in all mucinous cystadeno (carcino)mas), beta-catenin (goblet cell carcinoid versus left sided colonic adenocarcinoma) and chromogranin (goblet cell carcinoid versus right sided colonic adenocarcinoma). Expression of Math1 and HD5 was similar in goblet cell carcinoid and colonic adenocarcinoma but absent in classical carcinoid. CONCLUSION: The results suggest that goblet cell carcinoids should be regarded as a separate entity. The formerly used term 'crypt cell carcinoma' may be more appropriate because it reflects the more aggressive clinical behaviour of these tumours as well as their greater similarity to adenocarcinomas rather than to carcinoids.     

Chromogranin A in gastric neuroendocrine tumours: an immunohistochemical and biochemical study with region-specific antibodies

The aim of the present study was to investigate ECLomas and enterochromaffin-like (ECL) cell hyperplasia in gastric human mucosa regarding the immunohistochemical expression of chromogranin A (CgA) epitopes and to measure the same CgA epitopes in plasma samples. Eight gastric biopsies from ECLomas, seven of type I and one of type III, and biopsies from one patient showing only ECL cell hyperplasia were included in the study. Our results revealed a varying expression of region-specific CgA epitopes in the ECLomas regarding both the frequency of immunoreactive cells and intensity of immunoreactivity. CgA284–301 (pancreastatin) was not revealed in any neoplasm, whereas CgA361–372 (catestatin) was expressed in all ECLomas. However, the number of immunoreactive cells to vesicular monoamino transporter 2 (VMAT 2) or the commercial monoclonal CgA (CgA250–284) antibodies were generally higher. The plasma concentrations of the region-specific CgA radioimmunoassays differed considerably, with highest concentrations of CgA1–17 and CgA116–130 epitopes and the lowest with the CgA17–37, CgA63–76, CgA238–247 and CgA441–424 epitopes. No relationship was found between tissue expression and plasma concentration of CgA epitopes. In conclusion, this study shows that VMAT 2 and the commercial CgA antibodies seem more useful for histopathological diagnosis of ECLomas than the antibodies to the other CgA regions.     

Ultrastructural characterization of fundic endocrine cell hyperplasia associated with atrophic gastritis and hypergastrinaemia

Summary Clinical and experimental evidence indicates that carcinoid tumours of the stomach fundic mucosa represent another example of hormone-dependent neoplasm, gastrin being the hormone involved in tumour induction. In this context hyperplasia of fundic endocrine cells associated with chronic atrophic gastritis (CAG) and hypergastrinaemia is regarded as the most frequent preneoplastic lesion. However, the cell type involved in this hyperplasia has not been clarified. To elucidate this problem fundic endocrine cells were characterized ultrastructurally in 9 patients from which endoscopic gastric biopsies were obtained. ECL cells were the most frequent cell type in 8 cases, in 4 of which they were more numerous than all other cell types taken together. D₁ cells were the most frequent type in one case while they were inconspicuous in the other cases. P cells were found with a frequency in each case intermediate between that of ECL cells and that of D₁ cells. These results indicate that fundic endocrine cell hyperplasia occurring in hypergastrinaemic CAG is in most cases cytologically similar to that found in other hypergastrinemic conditions, in which the gastrin-dependent ECL cells were already found to prevail. They also explain why fundic carcinoids arising in CAG are mostly composed of ECL cells. The relation between ECL, D₁ and P cells, if any, remains obscure.Abbreviations EC enterochromaffin cells, producing 5-hydroxytryptamine - ECL enterochromaffin-like cells - D somatostatin producing cells - D₁ cells with small granules showing some characteristics of granules of D cells - P cells with small granules similar to those of pulmonary (P) endocrine cells - X gastric endocrine cells with large, dense granules. Unless specified, the secretory product of these cells is unknown Supported by grants from the Italian Ministry of Public Education and from the A.I.R.C. (Associazione Italiana per la Ricerca sul Cancro)