EC-cell, serotonin-producing carcinoids are formed by characteristic rounded nests of closely packed tumour cells, often with peripheral palisading (Type A) {1775}. Often, within the solid nests, rosette type, glandular-like structures are detected. This variant of the fundamental structure designated as mixed insular + glandular (A + C) structure seems prog-nostically more favourable than the pure

Fig. 4.16 Multiple carcinoids of the ileum with mesenteric lymph node metastases.

type A structure {1780}. In areas of deep invasion with abundant desmoplastic reaction, the cell nests may be oriented into cords and files. Mesenteric arteries and veins located near the tumour, or away from it, may be thickened and their lumen narrowed or even occluded by a peculiar elastic sclerosis, which may lead to ischaemic lesions in the intestine {72}. Most tumour cells are intensely argyrophilic and reactive with chromo-granin A and B antibodies. In about 30% of cases, a variable number of cells is also reactive for prostatic acid phos-phatase {211}.

The identification of tumour cells as EC-cells can be accomplished using histo-chemical methods for serotonin, including argentaffin, diazonium, and immuno-histochemical tests. Because serotonin occurs in some non EC-cell and related tumours {655}, electron microscopic examination of serotonin-immunoreactive tumours (particularly those failing to react with histochemical tests) can confirm their EC-cell nature by detecting characteristic pleomorphic, intensely osmiophilic granules {1778}. Substance P and other tachykinins, such as neurokinin A, are reliable markers of a fraction of jejuno-ileal EC-cell tumours {144, 1173}; foregut (gastric, pancreatic and duodenal) EC-cell tumours remain mostly unreactive {1780}. Minor populations of enkephalin, somatostatin, gastrin, ACTH, motilin, neurotensin, glucagon/gli-centin, and PP/PYY immunoreactive cells, unassociated with pertinent hyperfunc-tional signs, have been reported in some ileal and jejunal tumours mostly composed of EC-cells {1173, 2168}. Dopamine and norepinephrine have also

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