Crypt cell proliferative micronests in rectal carcinoids. An immunohistochemical study.

Gastrointestinal endocrine cells are situated both in the epithelium as well as in the subepithelium, especially in relation to enteric nerves. This has complicated efforts at delineating the histogenesis of gastrointestinal carcinoids. However, gastrointestinal carcinoids themselves are a heterogeneous group made up of various subsets, and as such may have different modes of origin. The present study investigated the histogenesis of rectal carcinoids because this has not been adequately addressed. Nine rectal carcinoids together with sex- and age-matched controls were stained with silver stains and various immunoreagents. The number of intraepithelial endocrine cells per unit length of mucosa in the carcinoid group was compared with the controls using the Student t test. Our results showed that there was no evidence of diffuse intraepithelial endocrine cell hyperplasia associated with these carcinoids. In six of the cases, however, there were focal areas where the carcinoids abutted onto the mucosal epithelium, and in another two cases there were focal areas depicting crypt cell proliferative micronests. These findings suggest that most conventional rectal carcinoids arise from localized areas of crypt cell proliferation rather than from diffuse areas of intraepithelial endocrine cell hyperplasia. Furthermore, rectal carcinoids appear to be constituted of a heterogenous population of endocrine cells rather than a monoclonal population of cells with each cell expressing a multiplicity of hormones.