Histochemical study of the heart of the axolotl (Ambystoma mexicanum)

We have investigated the presence of cells containing monoamines, substance P, and neuron-specific enolase (NSE) in the heart and in the pericardial wall of a urodele amphibian, the axolotl. Fibers containing substance P-like immunoreactivity were present in the heart but not in the pericardial wall. Also present in the heart were small branched cells, which stained metachromatically with toluidine blue. Similar cells were found in the peritoneum and were tentatively identified as mast cells. NSE-immunoreactive fibers were found both in the heart and in the pericardial wall. Small intensely fluorescent (SIF) cells of the pericardial wall contained a high concentration of norepinephrine but no other monoamines, substance P, or NSE. Comparison with data available for the mudpuppy, Necturus maculosus, a closely related amphibian species, suggests that the innervation of the heart in the axolotl is substantially different.