Anatomical localization of SER and certain peptides in the developing gastrointestinal tract of the axolotl Ambystoma mexicanum

The developing gastrointestinal tract (GIT) of the axolotl, Ambystoma mexicanum, was investigated, by the use of standard immunohistochemical techniques, for the localization of immunoreactivity to serotonin (SER) and five regulatory peptides all known to occur in the mammalian GIT. The actual utilized developmental stages started at stage 42 and ended with late juvenile stage. Microwave pre-treatment of the sections was employed to improve the detection of antigens. Entero-endocrine cells reacted specifically with antisera to SER, substance P (SP), somatostatin (SS), gastrin/cholecystokinin (GAS/CCK) and neurotensin (NT). Enteric nerve fibers, on the other hand, reacted specifically with antisera to SER, SP, NT and calcitonin gene-related peptide (CGRP) starting at the first juvenile stage. Based on its nature, the present study represents the second investigation which emerged from the same laboratory and principally yields two new findings. Firstly, neither SER- nor any of the investigated regulatory peptides-immunoreactivity was detected at stages earlier than the pre-hatching stage (42). Secondly, GAS/CCK-immunoreactivity was detected chronologically earlier (at stage 42) and this is consistent with the previously known trophic effect for both hormones during the early development of the GIT. The study provides a description of the qualitative distribution and a semi-quantitative account of the relative abundance of the mentioned neuroendocrine substances within the developing GIT.