Development of rat tibia innervation: colocalization of autonomic nerve fiber markers with growth-associated protein 43.

Development of autonomic innervation of the tibia was investigated in rat fetuses on gestational days (GD) 17-21 and in juvenile animals on postnatal days (PD) 1-28. Double immunofluorescence combined with confocal microscopy was applied to study colocalization of neuronal growth- associated protein 43 (GAP-43) and panneuronal marker protein gene product 9.5 (PGP) with markers of the autonomic nervous system: neuropeptide Y (NPY) and dopamine beta-hydroxylase (DbetaH) for adrenergic, as well as vasoactive intestinal polypeptide (VIP) and vesicular acetylcholine transporter (VAChT) for cholinergic fibers. The first GAP-43-immunoreactive (GAP-IR) nerve fibers were seen on GD17 in the perichondrium of the proximal epiphysis. Further GAP- and PGP-IR innervation appeared in the perichondrium/periosteum of the diaphysis and in the distal epiphysis (GD19), then in the bone marrow and in the intercondylar eminence (GD21). On PD1, NPY-IR and DbetaH-IR fibers appeared within the diaphyseal periosteum and on PD4 within the bone marrow. From PD14, GAP-43 immunoreactivity of NPY-positive fibers decreased. From PD7 on, NPY-IR fibers were observed in cartilage canals of both epiphyses and in the intercondylar eminence. In secondary ossification centers, NPY-IR fibers were seen from PD10, and in the bone marrow of the epiphyses from PD14. First VIP-IR and VAChT-IR fibers were observed on PD4 within the periosteum, bone marrow and patellar ligament. From PD10 on, VIP-positive fibers were seen in the intercondylar eminence, and from PD14 in secondary ossification centers. GAP-43 proved to be superior to PGP 9.5 as marker of growing nerve fibers, mostly due to its earlier appearance. The presence of specific nerve fibers may suggest possible involvement of autonomic innervation in regulation of bone development.