The development of autonomic innervation in bone and joints of the rat

The development of autonomic nerves in the hindlimb skeleton, was studied in rats from gestational day (G) 15 to postnatal day (P) 24 by immunoreactivity to neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). Control labelling with antisera to neurofilaments, protein gene-product 9.5 (PGP 9.5), and nerve terminals, synaptophysin (SYN), showed nerve fibres at G15 and nerve terminals at G19 in the perichondrial tissue. From P4, nerve fibres and terminals were observed within the bone organ. Noradrenergic sympathetic nerves, containing NPY, were first discerned at birth, G21, in the perichondrial tissue and within the bone organ at P4. Autonomic cholinergic nerve fibres, indicated by immunoreactivity to VIP, exhibited a similar temporal and regional occurrence. The diaphyseal parts were first supplied with autonomic nerves at P4. The nerve fibres extended into the metaphyses at P6-8 and finally into the epiphyses at P10, concomitant with the first signs of mineralization. Vascular as well as non-vascular nerve fibres were seen. The study shows that developing bone organ is supplied with autonomic nerves from birth, and the the growth of nerves parallels the mineralisation process. Previous studies have demonstrated that NPY potently inhibits parathyroid hormone (PTH) induced effects on osteoblastic bone cells and that VIP is a strong inductor of bone resorption. NPY and VIP also exert vasoregulatory effects. The combined findings suggest an autonomic influence on bone development.