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Embryonic neural crest cells have been traced to the primitive entoderm where they differentiate into a family of hormone-producing cells, APUD cells. The APUD cell concept explains many otherwise seemingly dissociated clinical circumstances involving endocrine glands and hormone production by tumors.
APUD-cells (A = Amine, P = Pprecursor, U = Uptake, D = Decarboxylation) are endocrine cells with numerous common characteristics: 1. They produce polypeptide hormones of low molecular weight. 2. They possess specific 1-amino acid decarboxylases to synthesize catecholamines and/or 5-hydroxytryptamine from precursors. 3. A part of the APUD-cell family primarily accumulates biogenic amines within their cytoplasm. 4. All APUD-cells are rich in specific secretory granules that are the morphological substrate for most of their common features.--Enzyme histochemically the APUD-cells represent a rather heterogenous population. The capacity to take up biogenic amines from the blood is confined to a smaller numer of APUD-cells. The concept of the APUD-cells representing a peripheral neurosecretory system (Pearse und Polak, 1971) appears to be premature. It is probable that in several APUD-cells the accumulation of biogenic amines only constitutes a phenomenon without any physiological relevance which, notwithstanding, could explain pathological reactions of APUD-cells.
Twenty-seven small cell carcinomas of the lung and three tumors of the large intestine with combined adenocarcinomatous and small cell and/or anaplastic carcinoid-type histologic features were studied by light and electron microscopy. It was shown that the small cells have morphologic characteristics of APUD cells. Also presented are the histologic features of a carcinoma of the lung with large cell undifferentiated carcinoma, adenocarcinoma, squamous cell carcinoma, and giant cell carcinoma areas in the primary site and in several metastatic foci. Two of the renal metastases showed small cell carcinoma. The combined tumors and the numerous other similar neoplasms described in the literature and reviewed here suggest an endodermal origin for digestive and respiratory tract APUD cells based on the hypothesis that cancer is a clonal proliferation, and mucous and squamous cell differentiation is an endodermal rather than neural crest characteristic. The ultrastructural features of tumors of cells of known neural crest origin, including a medullary carcinoma of the thyroid, three carotid body tumors, a pheochromocytoma, and two cutaneous melanomas were compared with those of other APUD cell tumors including small cell carcinomas of the lung, two bronchial carcinoids, a carcinoid of the appendix, and a carcinoid of the kidney. Cells of the latter group sometimes possessed cytoplasmic tonofibrils, round compact masses of cytoplasmic microfilaments, and ductal lumina. These features were lacking in the former group and may signify a different embryologic origin. The histologic, histopathologic, and embryologic evidence regarding the origin of digestive and respiratory tract APUD cells is reviewed, showing that the former are, and the latter probably are, of endodermal and not neuroectodermal origin.
Merkel cells of sinus hair follicles of nude mice were investigated by fluorescence and electron microscopy following pretreatment with amine precursors (L-Dopa, L-5-Hydroxytryptophan) and monoaminoxidase-inhibitors (Marsilid, Harmaline). Neither in control animals nor in pretreated animals any evidence for an involvement of Merkel cells in monoamine metabolism could be found. Therefore, the hypothesis that Merkel cells might share the most constant cytochemical characteristics of the cells of the APUD series, i.e., to take up amine precursors, should definitely be left. Following Harmaline treatment, however, Merkel cells were found fluorescent; cytophotometric analysis of the fluorescence emission spectra of formaldehyde-treated tissue ascertained that this fluorescence was due merely to a specific Harmaline fluorescence. The significance of Harmaline uptake in Merkel cells, most probably in the Merkel cell granules, is discussed.
The Merkel cell, a neural crest migrant to the skin, possesses a characteristic intranuclear rodlet, cytoplasmic membrane-bound granules, and horn projections and is usually associated with nerve terminations. It is also associated with all types of known organizations of sensory nerve endings in the skin and possesses characteristic enzyme reactions in relation to the ending. The Merkel cell is found in the skin and the oral mucous membranes, and similar cells are observed in the taste buds and pulmonary tissue. A comparison between the Merkel cell and the APUD cell system of polypeptide hormone and amine-producing cells reveals many structural and chemical similarities.
Argyrophilia and argentaffinity, as basic properties of APUD cells, were investigated in 50 normal and hyperplastic prostates, which included both autopsy and surgical specimens from patients of various ages. Normal prostates (including glands from 3 foetuses) had 62% of argyrophil-positive granules in the glandular epithelia, while only 44% of the hyperplastic glands were positive. Argentaffin-positive cells were found in 12% of the surgical hyperplastic cases. Both argyrophil and argentaffin cells were distributed in zones, often in lobule-like shapes, lying along the basal membrane. On the basis of these findings, there is a discussion on the possible roles played by the so-called APUD cells in hyperplastic and neoplastic growths of the prostate, such as carcinoid tumours (apudomas) or endocrine-associated syndromes in the course of prostatic cancer.
A variety of cells found in the pituitary and pineal glands, sympathetic nervous system and adrenal glands, the gut, pancreas, thyroid (C-cells), chemoreceptors (type I-Cells), lungs (P-cells), skin (melanocytes) and the urogenital tract have a common origin from the neural crest. These cells are programmed for neuro-endocrine function and, as a group, can be regarded as one of the physiological control systems. They secrete a variety of amine and peptide hormones and have common cytochemical characteristics from which the term APUD cell is derived. Tumours of these cells are referred to as 'apudomas' and may synthesise not only their own hormones but also those which are normally produced by other APUD cells. The relevant physiological properties of some of the peptides which have been described relatively recently are discussed and the principal clinical syndromes produced by the APUDomas are described.
A primary melanoma of the small intestine is reported. A possible explanation of its origin according to the APUD cell concept is suggested. This concept may lessen the doubts and confusion whenever an intestinal melanoma is discovered and a primary tumor at one of the more common sites can reasonably be excluded.
The development of Barbus conchonius is described with special attention to the differentiation of the gut. Amine precursor uptake and decarboxylation (APUD) are present in enteroendocrine cells during development, whereas these processes are lacking in adult specimens. The first APUD cells originate on the fourth day of development in the anterior part of the gut and on the fifth day in the caudal areas. The APUD facility of the cells disappears within 2 days, and after the 6th day APUD cells can no longer be distinguished in the intestinal epithelium. The first APUD cells were obsserved when four types of enteroendocrine cells were recognized with the electron microscope. These enteroendocrine cells contain granules of different electron densities, and microtubules and cilia can be observed. Some enteroendocrine-like cells are found below the basement membrane of the intestinal epithelium, indicating a possible extra-endodermal origin. APUD cells, except melanoblasts, have not been found migrating from the neural crest in ventral direction. The origin of the enteroendocrine cells of B. conchonius is discussed.
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Our studies show that the apparent number of Feyrter cells in the lung declines during the neonatal period in normoxic rabbits, and that in hypoxic animals a uniformly and significantly lower number of cells occurs as compared with the normoxic rabbits. There is some indication of degranulation of cells in the hypoxic groups. It is suggested that environmental and/or physiological factors associated with the start of extrauterine life, or lung development, may affect the apparent number and probable level of activity of these cells. These changes seem to be enhanced by hypoxia. Mast cells are scarce, and Feyrter cells are relatively more numerous along the airways. These cell types could possibly represent storage sites for 5-hydroxytryptamine, as suggested also by other investigators. Intraepithelial nerve fibres in bronchi and bronchioles were found but they were not limited to innervations of Feyrter cells or related cell bodies.
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