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This paper is a personal recollection of the studies, conducted in Prof. Pearse's laboratory in London in the years 1965-1969, which led to the discovery of production of calcitonin by parafollicular C cells and medullary carcinomas of the thyroid. The author's intention is to underline the combination of technical excellence, brilliant intuition, dedication and serendipity which led to a series of major discoveries and, historically, established the pivotal role to be played by immunohistochemistry in endocrine research and diagnosis. The formulation of Pearse's APUD cell theory gave a formal credence to the existence of common endocrine mechanisms, molecular markers and structural features in dispersed cells, all belonging to a diffuse endocrine system. This represented a major breakthrough which primed, in the following years, the studies on polypeptide hormone-producing cells and tumours, thus paving the way to the endocrine histology and pathology as we know, and practice them today.

This review describes the problems of disperse endocrine system and APUD-system morphology, summarizes some debatable issues of single endocrine cell biology. The data presented refer to the history of both systems discovery, morphological methods of their study, developmental sources, their structural organization and physiological roles of their cells. The significance of single endocrine cells in the regulation of the organism functions is discussed.

As early as the 2nd century, Galen proposed that 'vital spirits' in the blood regulated human bodily functions. However, the concept of hormonal activity required a further 18 centuries to develop and relied upon the identification of 'ductless glands', Schwann's cell and the recognition by Bayliss and Starling of chemical messengers. Bernard's introduction of 'internal secretion' and its role in homeostasis laid a physiological basis for the development of endocrinology. Kocher and Addison recognized the consequences of ablation of glands by disease or surgery and identified their necessary role in life. Detailed descriptions of the endocrine cells of the gut and pancreas and their putative function were provided by Heidenhain, Langerhans, Laguesse and Sharpey-Schafer. Despite the dominant 19th century concept of nervism (Pavlov), in 1902, Starling and Bayliss using Hardy's term 'hormonos' described secretin and in so doing, established the gut as an endocrine organ. Thus, nervism was supplanted by hormonal regulation of function and thereafter numerous bioactive gut peptides and amines were identified. At virtually the same time (1892), Ramón y Cajal of Madrid reported the existence of a group of specialized intestinal cells that he referred to as 'interstitial cells'. Cajal postulated that they might function as an interface between the neural system and the smooth muscles of the gut. Some 22 years later, Keith suggested that their function might be analogous to the electroconductive system of the heart and proposed their role as components of an intestinal pacemaker system. This prescient hypothesis was subsequently confirmed in 1982 by Thuneberg and a decade later Maede identified c-Kit as a critical molecular regulator in the development and function of the interstitial cells of Cajal and further confirmed the commonality of neural and endocrine cells. The additional characterization of the endocrine regulatory system of the GI tract was implemented when Feyrter (1938) using Masson's staining techniques, identified 'helle Zellen' within the pancreatic ductal system and the intestinal epithelium and proposed the concept of a diffuse neuroendocrine system. Pearse subsequently grouped the various cells belonging to that system under the rubric of a unifying APUD series. Currently, the gut neuroendocrine system is viewed as a syncytium of neural and endocrine cells sharing a common cell lineage whose phenotypic regulation is as yet unclear. Their key role in the regulation of gastrointestinal function is, however, indubitable.

One hundred twenty patients with nonspecific ulcerative colitis (NUC) were examined. The patients were divided into three groups equal in number according to the severity of the disease. The controls were 24 practically healthy individuals, 64 patients with irritated bowel syndrome (IBS) without pathological changes in the colon mucosa, and 64 IBS patients with atrophic or inflammatory changes in the colon mucosa. The study showed that the development of NUC was associated with an abrupt decrease in the level of proliferation markers (cycline D, and proliferating cell nuclear antigen - PCNA) in the epithelial cells of the sigmoid colon. Their number progressively decreases in more severe cases, while cell apoptosis intensifies. Such changes in cell renewal are closely connected with the functional and structural rebuilding of the diffuse endocrine system, which manifests in an increase of the total number of apudocytes as well as serotonin- and melatonin-producing cells together with a decrease in the number of VIP-producing cells.

Many disturbances in electrolyte and hormonal balance in the body induced by functional impairment of renal parenchyma may affect the activity of amine precursor uptake and decarboxylation (APUD) cells, which constitute a very important link in the regulation of homeostasis. The aim of the present study was the morphological, immunohistochemical and ultrastructural estimation of enteroendocrine cells in the stomach of uremic rats. Fragments of gastric pylorus were collected 1, 2 and 4 weeks after nephrectomy. Paraffin embedded sections were stained with H + E and by silver impregnation. For identification of neuroendocrine cells, immunohistochemical reactions were performed using specific antibodies against somatostatin, synaptophysin, neuron-specific enolase and anti-calcitonin gene related peptide. The analysis showed an increased number of APUD cells in the stomach of uremic rats compared to control rats, which may be a morphological expression of their hyperfunction in the functional impairment of renal parenchyma. These results suggest that chronic renal failure can modulate the secretory processes of APUD cells.

Psilocybin poisoning produces biphasic reactions composed of a schizophrenic phase and a panic attack-like phase. There is a time lag of several hours between phases, which may be considered an accumulation time in certain sites between the gut and the brain. So far as 5-hydroxytryptamine (5-HT) congeners are concerned, no sites are to be found except the amine precursor uptake and decarboxylation (APUD) system. It is postulated that argyrophil cells (AC) in the foregut, neuroepithelial bodies (NEB) in the lung, and raphe nuclei (RN) in the brainstem axis are relevant to mental disorders. Schizophrenia might be due to the massive destruction of APUD cells, and the paroxysmal release of 5-HT with peptides and panneuroendocrine markers from NEB might be the cause of panic attack., (Copyright 2002 Harcourt Publishers Ltd.)

Removal of the pineal gland leads to structural and functional rearrangement of gastric endocrine cells and thyroid C-cells in albino rats, as was shown by immunohistological methods and morphometry. Injection of pineal peptides epithalone and epithalamine eliminated these changes. Biological activity of epithalone is believed to be higher than that of epithalamine.

An experimental morpho-functional assessment of rat APUD and RAA systems status in acute radiation injury (cerebral form) was performed. The named regulatory systems were found to display an actual momentary post-exposure reaction, followed by subsequent functional changes of a distinct phase character. The latter were shown to determine in many respects the clinical picture and in some cases the outcome of the disease.

V. cholerae infection with clearly pronounced diarrhea was reproduced in adult rabbits with the use of the RITARD system. The state of the APUD system of the intestine of the animals and morphological changes in internal organs in experimental cholera were described. As noted in this study, the manifestation of changes in the intestine and other organs, as well as the reaction of apudocytes to V. cholerae infection, were linked with the intensity of diarrhea and the time of the death of the animals.

In 24 patients with uncomplicated duodenal ulcer and 40 patients with acute bleeding duodenal ulcer morphofunctional status of the local paraendocrine adjusting system of the duodenal mucous membrane in the periulcer, ulcer and remote zones was studied with histochemical and electron-microscopic techniques. It is established, that ECL- and G-apudocytes hyperplasia and hyperfunction and vice versa D-cells reduction of the number and suppression of the secretory activity are reliable prognostic criteria of duodenal ulcer bleeding.

Duodenal endocrine apparatus was studied in rat during individual development using electron microscopy. Endocrinocyte differentiation was demonstrated to occur in embryonic period and it is characterised by fast cytotype number growth. During embryogenesis processes of endocrinocyte differentiation occur more intensely than in adult animals which is manifested by higher incidence of different types of incretory elements in embryos. By the moment of birth EC-, L-, S-, D-, D1-, P and I-cells can be identified in the duodenal epithelium at different stages of differentiation as well as "mixed" exo-endocrine cells. The presence of transitory cellular forms from agranular elements up to mature differentiated endocrinocytes and "mixed" exo-endocrine cells reflects cytogenetic processes in population of endocrine cells and allows to consider duodenal epithelium both structurally and functionally as a tissue with common histogenetic basis.

The current knowledge of the catecholaminergic innervation of the mammalian adrenal cortex is summarized, and macro- and microscopic neuromorphology, including the central nervous system connections of the adrenal cortex, is briefly discussed. Morphological and functional data on the catecholaminergic (i.e., noradrenergic) innervation of the adrenal cortex are reviewed. Experimental data suggest that in addition to the regulation of adrenal blood flow, the noradrenergic innervation has a primary influence on zona glomerulosa cells possibly via beta 1 adrenergic and dopaminergic receptors (DA2 subtype via inhibiting T-type Ca2+ channels) It is concluded that the local, modulatory effect of noradrenergic nerve fibres, terminating in the close vicinity of the zona glomerulosa cells, on the systemic renin-angiotensin-aldosterone and other peptide cascade may be influenced by neuropeptides, particularly neuropeptide Y and vasoactive intestinal peptide.

The paper presents a review of the results obtained by the authors on the study of external (gamma) and internal (I-131) radiation effects on the functional morphology and linkage of the diffuse neuroendocrine system (DNES) and amine precursor uptake and decarboxylation (APUD) cells of the stomach and duodenum. The investigations performed enabled us to determine that the morphological changes noted in APUD cells had a dose and time dependency. The present study supports the point of view that the radiation initiates serotonin release from APUD cells, which appears to initiate the mechanism of early postirradiation dysfunctions of the gastrointestinal tract and the subsequent adaptive response of DNES. Analysis of our results, together with a review of the literature, indicates that APUD cells actively participate both in pathogenesis of radiation injury and development of organ and tissue radiosensitivity.

Endocrine-paracrine (neuroendocrine, amine precursor uptake and decarboxylation [APUD]) cells of the prostato-urethral region are serotonin and peptide containing regulatory cells, which are part of a dispersed neuroendocrine regulatory system also known as the APUD system. These cells most likely regulate growth and differentiation, as well as the secretory functions of the prostate. Prostatic carcinoma exhibits neuroendocrine differentiation in 3 forms: 1) small cell neuroendocrine carcinoma, 2) carcinoid-like tumors and 3) conventional prostatic adenocarcinoma with focal neuroendocrine differentiation. Small cell carcinoma and carcinoid-like tumors are rather rare (1 to 2% of all prostatic malignancies) and generally pursue an aggressive course. Focal neuroendocrine differentiation in adenocarcinoma is extensive in 10% of the cases and may be present in virtually all adenocarcinomas to a minor degree. There are conflicting studies on the prognostic significance of focal neuroendocrine differentiation in prostatic carcinoma, although several suggest a poor prognosis. The finding that serum neuroendocrine markers predict initial insensitivity to or the development of resistance to hormonal suppression therapy, coupled with the recent observation that androgen receptor is not expressed in neoplastic neuroendocrine cells suggests that neuroendocrine differentiation directly results in resistance to hormonal manipulation therapy. Neuroendocrine differentiation in prostatic carcinoma raises the possibility of innovative modes of treatment. Future directions of research should concentrate on the quantitative analysis of serotonin and various peptides in prostatic malignancy, since high levels of constitutive secretion may not be appreciated by immunocytochemistry, as well as analysis of tumors for receptors to neuroendocrine products, which are necessary for these products to have a functional role. Finally, specific subtypes of neoplastic cells with neuroendocrine differentiation based on serotonin and peptide profiles should be analyzed.

This paper describes the morphological features and distribution of endocrine cells in the human fetal gastrointestinal mucosa between the 8-9th and the 36th the weeks of gestation. We used techniques of argentic impregnation, techniques with chromogranine and neuron specific enolase, electronmicroscopical techniques and immunohistochemical techniques as well (PAP and ABC) with antibodies for G cells, EG cells, D cells, S cells CCK (I) cells and EC cells. The first argyrophilic and/or argentaffin cells appear in the fetal gut of 8-9 weeks old. We describe the appearance, distribution, morphology of the identified endocrine cells with immunohistochemical techniques and we discuss the possible morphofunctional interrelations between them, but also with the neighbouring structures like lymphoid tissue in ileum, colon and appendix in embryofetal period.

The characteristic biochemical pathway of the APUDoma cell, namely amine precursor uptake and decarboxylation, are illustrated by the examples of serotonin and catecholamine metabolism. Increasing understanding of the origins of APUDomas as well as the biochemistry and physiology of the hormones they produce, has led to improved methods of detection, imaging and treatment of afflicted patients.

Medullary thyroid carcinoma, MTC, accounts for 8-10% of all thyroid malignancies. It occurs in the sporadic form (70-75%) and the familial variant (25-30%). In familial MTC the autosomal dominant type of heredity is involved. This variant is found frequently in association with other endocrine hyperplasias or tumours within the framework of the APUD system and is thus an indispensible condition of the MEN 2 syndrome, either MEN 2A (MTC and pheochromocytoma and/or hyperparathyroidism) or MEN 2B with the frequent concurrent presence of pheochromocytoma and a typical phenotype, in particular multiple neurofibromas on the lips and tongue. The type of MTC determines also to a certain extent the prognosis of the disease. MTC in the framework of MEN 2A are usually most benign, while the prognosis is poorest for MEN 2B. The sporadic variant has a poorer prognosis, as compared with differentiated thyroid carcinoma. The only effective and rational treatment of MTC is radical surgery. In all instances where we confirm the diagnosis before surgery we indicate bilateral total thyroidectomy. In the familial variant where the tumour is usually multifocal and in both lobes this indication is absolute, in sporadic forms with possible early intrathyroid dissemination of the primary tumour also to the other side, bilateral surgery is also indicated. Moreover we know that a certain ratio of assumed sporadic forms are genetically conditioned tumours and thus are familial. The histological finding of hyperplasia of the C-cells suggests the familial variant even when tumourous transformation did not occur yet.(ABSTRACT TRUNCATED AT 250 WORDS)

Basing on structural and functional signs, pit cells and large granule-containing lymphocytes (LGCL) can be considered as the same type of cells. LGCL take origin from bone marrow, circulate in the blood stream and get into tissues as pit cells or as interepitheliocytic lymphocytes. Therefore, on this ground for convenience of studying and understanding these elements of cell immunity a suggestion is made to distinguish the LGCL system, possessing natural killer activity and paracrinic secretion. An increased functional load to the organism results in an intensified getting of LGCL into places of structural reconstructions of tissues (regenerative active zones). The same effector elements--LGCL (pit cells)--in dependence of necessary functional ensuring under various conditions, produce in the surrounding parenchyma both a proliferative effect and its opposite action. The latter can be manifested as a monocellular colliquational necrosis, or as apoptosis. Apoptosis (active cellular death) is of greatest interest. Disturbances of this mechanism can facilitate to an excess proliferation of cells, or to their extreme death; malignant growth and transplant-versus-host reaction can be mentioned as examples, respectively. LGCL play an important role in the immunobiological supervision and are effectors of active ageing mechanisms. Studying of this important link in regulation of quantity of cell population adequate to functional load should help to purposively influence it for stimulating or repressing cell proliferation, depending on certain concrete purposes.

Methanol extracted skins from 84 species of amphibia were screened, measuring by RIAs: gastrin-CCK, VIP, calcitonin, GIP, PP and motilin. G-CCK-like immunoreactivity was found in 97.6%; VIP-like immunoreactivity in 41%; CT-like immunoreactivity in 34%; GIP-like immunoreactivity in 10%; PP-like immunoreactivity in 40% and MT-like immunoreactivity in 60% of the samples. The use of a sequence-specific radioimmunoassay and of gel-chromatography confirmed the caerulein-CCK-8-like nature of the immunoreactive material. Detected amounts of the other peptides (VIP, CT, GIP, PP, MT) were too low for bioassay or chromatographic studies, thus leaving the question open if they are due to some kind of unspecific interferences or, most likely, to species-specificity differences of the used antisera.

Besides their well-known actions, glucagon, ACTH, pentagastrin and insulin from the APUD series exert a direct action on the bone calcium content. Incubation with these substances of rat calvaria in vitro yields an evident stimulation of osteolysis with ACTH. Pentagastrin inhibits osteolysis. Glucagon and insulin inhibit parathormone-stimulated osteolysis, with no influence on the spontaneous one. Glucagon, resembling calcitonin, stimulates the 45Ca uptake from the incubation medium. The action of these substances completes the series of hormones influencing bone calcium metabolism, underlining possible interference actions of APUD-type hormones.

As sensitive radioimmunoassays for the detection of polypeptide hormones are developed, the exciting discovery of a diffusely distributed system of interrelated endocrine cells has begun a new era of endocrinology. This system, although anatomically disassociated, is bound together by a number of common features such as its biosynthetic mechanism, histochemical and ultrastructural features, and embryologic origin (Table I). The most prominent feature, however, is their biosynthetic pathways for hormone production, from which the acronym APUD has been derived. These are the capacity for Amine Precursor Uptake such as DOPA and then subsequent Decarboxylation, resulting in the synthesis of bioactive amines or polypeptide hormones. Hyperplasias or neoplasms of these cells are defined as apudomas. In the last ten years a great deal of research has rapidly altered the original concepts of this system, especially in terms of its embryologic origin, physiologic interrelationships, classification, as well as the addition of many new APUD cell members. These will be reviewed, and the origin, diagnosis, and treatment of each recognized apudoma will be synthesized in light of its membership within the APUD system.

The functional condition of lateral endocrine (melanocytes, mast cells) and neurogenic (adrenergic plexus) arterial apparatus of the cat brain during occlusion of the right common carotid artery for 5, 20, 35 min, was studied. The maximal decrease of the melanocytes occurred on the 20th min in the vessels of the right side of the Willis circle. An increased concentration of the mast cells and nervous fibers was found on the 20th min followed by its reduction by the 35th min on the side of ischemia. The reaction of the blood supply was evident as a gradual increase of the mast cells, longitudinal and transversal adrenergic fibers on the 35th min of the experiment.

Histochemical and ultrastructural examinations were performed on neuroepithelial bodies (N. E. B.) in the lung of the mouse and rat. The N. E. B. were identified as specialized groups of pale, columnar cells. They were located throughout the intrapulmonary airway. These cells displayed some special cytochemical properties also seen in the APUD (Amine Precursors Uptake and Decarboxylation) endocrine system, such as cytoplasmic argyrophilia and the capability of selective uptake of amine precursors. Ultrastructrally the N. E. B. were composed of a kind of granulated cell which had concentrations of specific cored vesicles in the basal cytoplasm. It seemed likely that the cells released the contents of the cored vesicles into the extracellular space by exocytosis. These cells made extensive and intimate contact with the mitochondria-rich nerve terminals which occurred mainly at the level of the supranuclear portions of the granulated cells. In this contact area, the membrane of the nerve terminals were apposed to that of the granulated cells with a consistent gap of about 20 nm. Among these membrane appositions, some membrane thickenings were observed in which cored vesicles were closely associated in the granulated cell cytoplasm. From these observations, it was conceived that the lung N. E. B. function both as receptor and endocrine organs and that their specific cored vesicles may have an intimate correlation with these dual functions.

The distribution, immunostaining profile and ultrastructural morphology of human female urethral and paraurethral endocrine-paracrine (APUD) cells was studied. Five urethras obtained from radical cystectomy specimens were stained with antisera to serotonin, neuron-specific enolase, chromogranin, bombesin, calcitonin, somatostatin, prostatic acid phosphatase, prostatic specific antigen and with the Churukian-Schenk argyrophil method. Numerous endocrine-paracrine cells were observed along the entire length of resected urethra, with these cells most numerous in the paraurethral ducts. The endocrine-paracrine cells were positive for serotonin, neuron-specific enolase, chromogranin and the argyrophil reaction. Scattered bombesin and very rare calcitonin immunoreactive cells were noted. The endocrine-paracrine cells were of the open (luminal extensions) and closed cell types and often had multiple long dendritic processes suggesting a primarily paracrine function. By electron microscopy the secretory granule morphology was highly variable. Autonomic innervation of endocrine-paracrine cells was noted. The relationship of female urethral and paraurethral endocrine-paracrine cells to those of the male prostate and urethra is discussed with speculation as to the functional role these cells may play.

We assessed the changes in argyrophil neuroendocrine (NE) cell numbers, intensity of 5-HT fluorescence, and arterial medial thickness in the lungs of neonatal rabbits under various oxygen treatments. NE cell numbers and 5-HT fluorescence in normoxic rabbits increased from 12 hr before to 1 day after birth, and NE cells declined thereafter to the 10th day. In acute hypoxic (520 mm Hg for 2-2.5 hr) 5-day-old rabbits, 5-HT fluorescence was decreased, whereas NE cell numbers and medial thickness were unchanged. Neonates hypoxic from birth had higher NE cell numbers and increased medial thickness at 3 and 5 days, whereas 5-HT fluorescence was decreased compared with that in normoxic controls. These chronically hypoxic neonates showed a dramatic drop in argyrophil NE cell numbers to below normal when they were exposed to normoxia for 1 hr, but cell numbers and medial thickness returned to normal at 4 and 24 hr, respectively. We also tested the effect of acute and chronic hyperoxia: 100% O2 for 2-2.5 hr caused a significant drop in detectable NE cell numbers, whereas 40% O2 in N2 caused no change; chronic 40% O2 in N2 caused a fivefold increase in argyrophil NE cells by day 5, and medial thickness was below normal; 5-HT fluorescence decreased in acute 100% and chronic 40% hyperoxia and was elevated in acute 40% O2.

The aspects of structure and function of the APUD-system under normal and pathological conditions are analysed. New terms and concepts (apudocyte, apudoblast, apudogenesis, apudopathies) applicable to the APUD-system are introduced. It is suggested that some apudocytes originate from polypotential stem cells. The dysfunctions of the APUD-system (apudopathies) are described, their classification is suggested, and the role of apudocytes in the endogenous mechanisms of oncogenesis is analysed.

The use of highly sensitive and selective methods (autoradiography, light and electron microscopy, radioimmunoassay) allowed identification of peptide hormones in both the specialized endocrine cells (apudocytes) of some internal organs and various structural elements of the central and peripheral nervous system. Current knowledge permits one to suggest the existence of a peptidergic innervation as one of the parts of the nervous system and to demonstrate the direct participation of these peptides in the pathogenesis of some diseases. Biological significance of these substances in supporting homeostasis determines the urgent need for a combined investigation of the structural-functional relationship between apudocytes and peripheral nerves under normal and pathologic conditions. This will promote new opportunities in the elucidation of the mechanisms of some pathologic processes.

Histochemical methods devised by Masson, Sevka and Dominichi and immunohistochemical methods were used to establish that immunization markedly changes both qualitative and quantitative characteristics of the hormonal production of APUD cells in immune organs. The differences in the function of the APUD-system of immunocompetent organs were recorded on the primary and secondary immune responses as well as during sensitization in the development of the immediate type hypersensitivity. The most intense changes occurring in different forms of immune response were noticed at the level of serotonin- and catecholamine-containing APUD cells of the immune system.

The neurally programmed APUD-cells from ectodermal origin are of peculiar significance in two respects: 1. They represent the largest endocrine gland of the whole organism and the peptides, which are secreted by the different APUD-cells, exercise a great influence on most of the motoric and secretory processes within the whole gastrointestinal tract just as on many metabolic processes as demonstrated on the so-called "enteropancreatic axis". Neoplasias of these APUD-cells, the "Apudomes", give rise to peculiar clinical syndromes. Early diagnosis of an apudome can be reached today by radioimmunologic detection of high concentrations of the responsible peptide in the blood enabling us to initiate an effective therapy at least for some of the known apudomes. 2. Beyond that the APUD-cells and their corresponding peptides are representing the so-called "peptidergic" division of the nervous system which acts as third-line effector to modulate not only the actions of the autonomic division but also of each other. The given example of the modulating effect of substance P or of the endorphines on the perception of pain is only one single aspect of the broad functional spectrum of these neuropeptides. The discovery, that the very same peptide can be detected in endocrine cells as well as in neurones, has opened one of the most fascinating and provocative developments in biology of the present time and some surprising new details can be expected in the next future.

Complex histological, histochemical, immunological and electron-microscopic studies made it possible to receive new data on functional morphology of the endocrine cells of the APUD system in norm, radiation exposure and tumor growth. The data obtained are being discussed and analysed and optimal methods of optimization of radiation and chemotherapy of malignant tumors are being developed.

Among the epithelial elements of the pulmonary airways are cells with features suggestive of endocrine function. Although these cells share a number of properties with the intestinal endocrine (argentaffine) cells, peptide hormones analogous to the multitude of those present in the gastrointestinal tract have yet to be identified in the lung. The oat cell carcinoma, which arises from this cell type, is of importance clinically, yet investigations into the basic properties and functions of this cell have been difficult and sparse, in part due to a lack of appreciation of the problem.