Development 100, 661-671 (1987) Printed in Great Britain © The Company of Biologists Limited 1987 661 The effect of pancreatic mesenchyme on the differentiation of endocrine cells from gastric endoderm BEVERLEY KRAMER 1 , ANN ANDREW 2 , B. B. RAWDON 1 and P. BECKER 3 'Department of General Anatomy, School of Dentistry, University of the Winvatersrand, 1 Jan Smuts Avenue, Milner Park, Johannesburg 2001, South Africa 2 Department of Anatomy, Medical School, University of the Winvatersrand, 7 York Road, Parktown, Johannesburg 2193, South Africa 3 Institute for Biostatistics, South African Medical Research Council, PO Box 17555, Hillbrow 2038, South Africa Summary To determine whether mesenchyme plays a part in the differentiation of gut endocrine cells, proventricular endoderm from 4- to 5-day chick or quail embryos was associated with mesenchyme from the dorsal pancreatic bud of chick embryos of the same age. The combinations were grown on the chorioallantoic membranes of host chick embryos until they reached a total incubation age of 21 days. Proventricular or pancreatic endoderm of the appropriate age and species reassociated with its own mesenchyme pro- vided the controls. Morphogenesis in the experimental grafts corre- sponded closely to that in proventricular controls, i.e. the pancreatic mesenchyme supported the develop- ment of proventricular glands from proventricular endoderm. Insulin, glucagon and somatostatin cells and cells with pancreatic polypeptide-like immuno- reactivity differentiated in the pancreatic controls. The latter three endocrine cell types, together with neurotensin and bombesin/gastrin-releasing polypep- tide (GRP) cells, developed in proventricular controls and experimental grafts. The proportions of the major types common to proventriculus and pancreas (somatostatin and glucagon cells) were in general similar when experimental grafts were compared with proventricular controls but different when experi- mental and pancreatic control grafts were compared. Hence pancreatic mesenchyme did not materially affect the proportions of these three cell types in experimental grafts, induced no specific pancreatic (insulin) cell type and allowed the differentiation of the characteristic proventricular endocrine cell types, neurotensin and bombesin/GRP cells. However, an important finding was a significant reduction in the proportion of bombesin/GRP cells, attributable in part to a decrease in their number and in part to an increase in the numbers of endocrine cells of the other types. This indicates that mesenchyme may well play a part in determining the regional specificity of popu- lations of gut endocrine cells. Key words: epithelio-mesenchymal interaction, gut endocrine cells, regulatory peptides, cytodifferentiation. Introduction Hardly anything is known about factors concerned in the differentiation of gut endocrine cells - yet such information is important because these cells give rise to endocrine tumours. In approaching the problem, we showed that the progenitors of the cells are scattered through the digestive tract of the chick at early stages of development (Rawdon, Kramer & Andrew, 1984a). This mitigates against a single local induction of stem cells either for gut endocrine cells in general or for each endocrine cell type individually. The possibility that mesenchyme underlying the en- doderm of the gut may be responsible is indicated by three experiments in which endocrine cells appeared in epithelia normally lacking such cells at the stages studied. Yasugi (1976) found glucagon cells in avian allantoic epithelium associated with intestinal or pan- creatic mesenchyme and Haffen, Lacroix, Kedinger & Simon-Assmann (1983) noticed endocrine cells in gizzard epithelium combined with duodenal fibro- blasts from neonatal rats. Furthermore, it has been shown that intestinal mesenchyme from fetal rats, when combined with cultured endodermal cells, induced the differentiation of the main epithelial cell types including endocrine cells (Kedinger et al. 1986).