The histogenesis of carcinoid tumours of the rectum

`Carcinoid'' tumours of the rectum have been described which are classified on histological appearances into three main varieties: `true'' carcinoid or argentaffinoma, `atypical'' or non-argentaffin carcinoid, and `composite carcinoid. The histogenesis of these tumours is discussed and it is suggested that non-argentaffin carcinoids of the rectum have an essentially similar histogenesis to certain tumours of the lung, pancreas, and gastrointestinal tract. The difference in growth, pattern, and staining reactions are a reflection of differing directions and levels of differentiation of the parent epithelium. The functional implications of this hypothesis are discussed.