- Osteochondrosis (OC) is a major source of lameness in horses, involving abnormal maturation of cartilage during the first year of life. Previous studies have shown that cartilage canals may be associated with early OC lesions with accumulation of large numbers of small rounded chondrocytes surrounding the canals. Altered expression of paracrine factors, parathyroid hormone-related peptide (PTH-rP) and Indian hedgehog (Ihh), as well as the additional factors, vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), and matrix metalloproteinase-13 (MMP-13), may play a role in the development of OC. The objective of this study was to elucidate the expression of Ihh, PTH-rP, VEGF, PDGF, and MMP-13 in cartilage of prepubescent foals to determine how this process becomes disrupted in early osteochondrosis prior to the onset of clinical signs. The hypothesis was that
osteochondrosis develops as a result of increased expression of these regulatory molecules via cartilage canals during development, leading to failure of cells surrounding cartilage canals to mature and calcify. Cartilage was harvested from femoropatellar joints of foals ≤6 months old. Laser-capture microdissection was used to capture cells surrounding the cartilage canals and osteochondral junction. Equine-specific Ihh, PTH-rP, VEGF, PDGF-A, MMP-3, MMP-13 and 18S mRNA expression levels were evaluated by real-time qPCR. Whole cartilage RNA expression levels were also evaluated. Spatial tissue protein expression was determined by immunohistochemistry.
There was significantly increased Ihh, MMP-3, and MMP-13 whole cartilage gene expression in early OC cartilage compared to normal controls. Protein expression of VEGF, PDGF-A, MMP-13, Ihh, and PTH-rP was mainly observed in the superficial and deep cartilage layers, as well as along the osteochondral junction and cartilage canals. In laser-captured samples, there was significantly increased MMP-13 and PDGF-A gene expression in chondrocytes adjacent to cartilage canals and increased PDGF-A gene expression in osteochondral junction chondrocytes of OCD-affected foals compared to controls. Increased Ihh, MMP-3, and MMP-13 gene expression in early OC whole cartilage provided stronger evidence for their association with OC, while significantly increased PDGF-A and MMP-13 gene expression surrounding OCD cartilage canals and increased PDGF-A gene expression at the osteochondral junction in this study suggests that pathways involving endochondral maturation and invasion of the ossification front are altered in early osteochondrosis.