On twenty carcinoids from various sites of origin, tissues which were formalin‐fixed and paraffin‐embedded for routine histology and three of these also from fresh tissues as control material were ultrastructurally evaluated. In all of these carcinoids primarily diagnosed on the basis of light‐microscopic morphology, specific secretory granules identifiable as those of an endocrine type were demonstrated, thus confirming one aspect of our recent hypothesis that carcinoids were a group of neoplasms originating from endocrine or related cells of several sorts distributed in the primitive gut system. The size, shape and range of density of carcinoid granules in paraffin‐embedded tissues were almost identical to those in conventionally processed tissue from fresh specimens, and valuable, if adequately processed, to confirm the final diagnosis, when fresh tissues were not available. Our present findings suggest as well that even a single carcinoid may be composed of endocrine or endocrine‐like cells of more than two types, although the possibility that they are in different stages of a functional cycle still exists.
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