Immunohistochemical and ultrastructural localization of CGRP-positive nerve fibers at the epiphyseal trabecules facing the growth plate of rat femurs.

We performed immunohistochemical and ultrastructural studies to disclose a possible relationship between nerve fibers and bone metabolism. Immunohistochemical distribution of calcitonin gene-related peptide (CGRP)-positive nerve fibers during bone development was assessed in the femurs of rats. CGRP-positive nerve fibers were denser in the epiphysis than in the metaphysis. These nerve fibers particularly ran along the epiphyseal trabecules facing the growth plate and came in contact with osteoclasts. Many osteoclasts at the epiphyseal trabecules facing the growth plate contained abundant toluidine blue and periodic acid-Schiff-positive granules. Electron microscopy revealed that these osteoclasts have many membrane-bound, electron-dense granular structures and dilated cisterns of rough endoplasmic reticulum containing electron-dense material. They were often surrounded by clear cells displaying features of nerve fiber and had no ruffled border. Furthermore, ultrastructural observations revealed electron-dense structures coating the cytoplasmic side of plasma membranes of the nerve fibers. We also observed coated pits in the cytoplasm of the osteoclasts facing the nerve fibers. To further clarify the role of innervation, we compared trabecules of rats undergoing denervation of the sciatic nerve with those from unoperated rats. Denervation resulted in a significant increase in the number of cement lines on the epiphyseal trabecules facing the growth plate. These results suggest that the osteoclasts at the epiphyseal trabecules facing the growth plate are in part regulated by CGRP-positive nerve fibers. Thus, CGRP-positive nerve fibers could be a crucial element in bone metabolism during bone growth and development.