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

This research investigates the sensory nerve supply of normal subchondral bone in the equine metacarpophalangeal joint, noting that sensory nerve fibers are abundant in certain areas and sparse in others.

Overview of Study and Method

• The study was conducted on sections of bone and soft tissue from the metacarpophalangeal joints of four horses, two mature and two immature.
• These specimens were fixed with formalin and then sectioned. Some specimens were demineralized, either using formic acid or EDTA, and others were kept nondemineralized.
• Immunolocalization of Substance P (SP) antigens was achieved using rabbit antiserum to substance P. The method used was the streptavidin-biotin-peroxidase complex method.
• Permanent identification of SP fibers was made possible by staining with 3,3′-diaminobenzidine.

Findings

• Abundant sensory nerve fibers were identified in a number of parts of the joint, which include the joint capsule, synovial membrane subintimal layers, collateral ligaments, suspensory ligament and distal sesamoidean ligament attachments to the sesamoid bones, and periarticular periosteal layers.
• Less abundant SP-immunoreactive nerve fibers, or sparse ones, were found in the subchondral bone plates of the metacarpus, proximal first phalanx, and dorsal articular surface of the sesamoid bones.
• The majority of SP fibers were observed associated with blood vessels in the small cancellous spaces and haversian canals.
• Deeper marrow spaces contained increased numbers of SP sensory fibers. Some occurred in small groups and some were recognized as several SP-immunoreactive fibers in a larger nerve bundle.
• Only a few SP fibers were found in cortical bone, specifically in the haversian canals.
• SP fibers were not identified in the osteocytic lacunae, canaliculi, or the bony lamellae of the haversian systems of the subchondral bone plate, as well as its extension to the metaphyseal and diaphyseal cortical bone.

Conclusion

• The presence of an ample sensory nerve supply in the soft tissues of the equine metacarpophalangeal joint aligns with observations noted in other species, indicating similar physiological mechanisms across various species.