Foetal and postnatal equine articular cartilage development: magnetic resonance imaging and polarised light microscopy.

Adult articular cartilage (AC) has a well described multizonal collagen structure. Knowledge of foetal AC organisation and development may provide a prototype for cartilage repair strategies, and improve understanding of structural changes in developmental diseases such as osteochondrosis (OC). The objective of this study was to describe normal development of the spatial architecture of the collagen network of equine AC using 1.5 T magnetic resonance imaging (MRI) and polarised light microscopy (PLM), at sites employed for cartilage repair studies or susceptible to OC. T2-weighted fast-spin echo (FSE) sequences and PLM assessment were performed on distal femoral epiphyses of equine foetuses, foals and adults. Both MRI and PLM revealed an early progressive collagen network zonal organisation of the femoral epiphyses, beginning at 4 months of gestation. PLM revealed that the collagen network of equine foetal AC prior to birth was already organised into an evident anisotropic layered structure that included the appearance of a dense tangential zone in the superficial AC in the youngest specimens, with the progressive development of an underlying transitional zone. A third, increasingly birefringent, radial layer developed in the AC from 6 months of gestation. Four laminae were observed on the MR images in the last third of gestation. These included not only the AC but also the superficial growth plate of the epiphysis. These findings provide novel data on normal equine foetal cartilage collagen development, and may serve as a template for cartilage repair studies in this species or a model for developmental studies of OC.