Microscopic and Ultrastructural Features of the Gill‐Associated Neuromast of the Common Carp ( Cyprinus carpio ), with the Connection to Telocyte

Light microscopy (LM) and transmission electron microscopy (TEM) were used to examine the structure of the gill-associated neuromast of the common carp (Cyprinus carpio), as well as its histochemistry and immunohistochemistry. LM showed H&E-stained neuromasts as pale, spherical structures in the gill epithelium. PAS (Periodic Acid-Schiff) staining showed modest neuromast reactivity but substantial goblet cell and microvilli glycocalyx positivity, while bromophenol blue showed high sensory cell protein content. The neuromast had sensory hair cells, a mantle, supporting cells, and basal cells. The neuromast center contained sensory hair cells and supporting cells. Mantle cells formed the outer sheath. Neuromast basal cells were at the bottom. The basal neuromast nerve terminal extends upward to innervate the sensory hair cells. The afferent nerve terminal had several secretory vesicles. Afferent nerve terminals were myelinated in the basal neuromast and lost their myelin sheath in the apex. The lamina propria efferent nerve terminal was partially myelinated. Prolongations identified telocytes in light microscopy by Toluidine blue, methylene blue and iron haematoxylin. They were linked to the neuromast nerve terminal. Telocyte cell bodies and podomeres were detected by TEM. They touched afferent and efferent nerve terminals. The surprise detection of CD3 and mAb 3A5 positivity in gill neuromast sensory hair cells indicates a novel, non-immunological function for this established T-cell marker and suggests an important connection between the immune and sensory systems. In conclusion, gill-associated neuromasts may detect the water stream entering the gills to adjust respiratory rates. Telocytes may regulate neuromast function.