The expression of functional receptors for nerve growth factor in insulin-producing cell lines grown in vitro has recently been demonstrated. The possible importance of signals transduced via these receptors in the control of islet maturation has been proposed based on data obtained using an in vitro culture system. To further support this hypothesis, we have studied the expression of Trk-A, the high-affinity receptor for NGF, in vivo during the embryonic and fetal development of the rat pancreas. We have also examined the expression of NGF during the same period. Immunohistological analysis shows that at embryonic day 11 (E11), Trk-A is expressed by the epithelial cells of the presumptive pancreas. The few pancreatic endocrine cells present at that stage express Trk-A. At E12 and E16, Trk-A expression was detected in the developing ductal network. The endocrine cells located in the ducts express Trk-A while those that have migrated into the surrounding mesenchyme now stain negative for Trk-A. By E20, Trk-A expression by ductal cells has considerably decreased and can be detected only in small ducts closely associated with islet-like structures. These islet-like structures stain negative for Trk-A. After birth, insulin-positive cells arranged into islets re-express Trk-A. During the same period, NGF mRNA is found to be expressed in the developing pancreas. The expression of Trk-A and its ligand NGF in the pancreas during embryonic and fetal life suggests that NGF and its receptor could play an important role in the development of the pancreas.

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