Occurrence of neurotensin-immunoreactive cells in the digestive tract of lower vertebrates and deuterostomian invertebrates. A correlated immunohistochemical and radioimmunochemical study.

In the mucosal epithelium of the digestive tract of two marine teleost bony fish, one cartilaginous fish, one cyclostome, and in that of two of three representatives of deuterostomian invertebrates studied, endocrine cells of open type were found, exhibiting immunoreactivity with antisera against C-terminal sequences of mammalian neurotensin and of the structurally closely related amphibian neurohormonal peptide xenopsin. From these observations, and from those of previous studies, it is suggested that neurotensin cells do not occur in the digestive tract mucosa until at the evolutionary level of the more highly developed deutoerostomian invertebrates. Three evolutionary stages seem to exist in the distribution pattern. The first stage, characterized by few, widely scattered cells, is found in the uro- and cephalochordates, the cyclostomes, the cartilaginous fish, and the stomachless bony fish. In the second stage, comprising the remaining submammalian classes, including more highly developed bony fish, the typical distribution pattern is that of numerous neurotensin immunoreactive cells in the antrum, pylorus, and duodenum. The final stage of neurotensin evolution is found in higher mammals and is characterized by a great density of neurotensin immunoreactive cells in the ileum.