HJstochemistry 61,213 221 (1979) Histochemistry 9 by Springer-Verlag 1979 Bombesin-Like Immunoreactivity in the Avian Gut and its Localisation to a Distinct Cell Type C.M. Timson 1 , J.M. Polak ~, J. Wharton ~, M.A. Ghatei 1, S.R. Bloom ~ L. Usellini 2, C. Capella 2, E. Solcia 3, M.R. Brown 4, and A.G.E. Pearse 1 Departments of Histochemistryand Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, W12 OHS, U.K., 2 Centro Diagnostica Istopatologica, Universit/~ di Pavia a Varese, Ospedale di Circolo, Varese, Italy, 3 Istituto di Anatomia e Istologia Patologica dell' Universit~di Pavia, via Forlanini, 14 18, Pavia, Italy, and 4 The Salk Institute, Peptide BiologyLaboratory, San Diego, CA 92112, USA Summary. The distribution of a bombesin-like immunoreactive peptide in the avian gastro-intestinal tract was analysed by combined radioimmu- noassay and immunocytochemistry. Radioimmunoassay of tissue extracts showed that the largest quantities of bombesin-like immunoreactivity were present in the proventriculus (64.5_+ 6.0 pmol/g) with smaller but still consid- erable amounts in the gizzard (40.0+ 6.0 pmol/g). Immunocytochemically the extractable bombesin-like immunoreactivity was localised in numerous endocrine cells. These, in the proventriculus, were found mainly in the deeper layers of the mucosa. Further study of these cells by the semi-thin/thin technique revealed the presence of characteristic secretory granules. The functional name BN is proposed for this cell type. Introduction Early classification of gut endocrine cells was based on their localisation and the ultrastructural appearance of their secretory granules, coupled with hormone distribution patterns obtained by bioassay and, later, radioimmunoassay (cf. Wiesbaden classification, Creutzfeldt et al., 1970). It was by this means, for example, that the G-cell was associated with gastrin in the antrum (Solcia et al., 1967). Technological advances have since made possible the immunocyto- chemical staining of endocrine cells. More recently, complete ultrastructural identification of many cell types has been achieved using the semi-thin/thin technique (Polak et al., 1975) on resin-embedded material. Bombesin, a recently discovered tetradecapeptide first isolated from amphib- ian skin (Anastasi et al., 1971), has been shown to have powerful pharmacolog- 0301-5564/79/0061/0213/$01.80