Substance P immunohistochemical study of the sensory innervation of normal subchondral bone in the equine metacarpophalangeal joint

Serial sections of bone and soft tissue from the metacarpophalangeal joints of 2 mature and 2 immature horses were evaluated for substance P immunoreactive sensory nerve fibers. Formalin-fixed specimens were sectioned, either nondemineralized or demineralized with formic acid or EDTA. Rabbit antiserum to substance P (SP) was used in the strep. tavidin-biotin-peraxidase complex method for immunolocalization of SP antigen, and staining with 3,3'- diaminobenzidine was used for permanent identification of SP fibers. Abundant sensory nerve fibers were identified in the joint capsule, synovial membrane subintimal layers, collateral ligaments, suspensory ligament and distal sesamoidean ligament attachments to the sesamoid bones, and the periarticular periosteal layers. Sparse SP-immunoreactive nerve fibers were found in subchondral bone plates of the metacarpus, proximal first phalanx, and dorsal articular surface of the sesamoid bones. Most SP fibers were associated with blood vessels in the small cancellous spaces and haversian canals of the subchondral bone. The deeper marrow spaces contained increased numbers of SP sensory fibers; a few appeared in small groups and as several SP-immunoreactive fibers in a larger nerve. Cortical bone contained only a few SP fibers in the haversian canals. Substance P fibers were not identified in the osteocytic lacunae, canaliculi, or the bony lamellae of the haversian systems of the subchondral bone plate, and its extension to the metaphyseal and diaphyseal cortical bone. Equine metacarpophalangeal joint soft tissues have an abundant sensory nerve supply, similar to that of other species. However, the subchondral bone plate also has sparse sensory nerve fibers, which is a unique finding, and may help explain signs of bone pain associated with disease states of the fetlock.