THE AUSTRALIAN MERINO DISPLAYS DYSPLASTIC EPIPHYSEAL GROWTH PLATE MORPHOLOGY AND AN AGE-DEPENDANT CHONDROID TRANSFORMATION OF THE NUCLEUS PULPOSUS- EVIDENCE OF A CHONDRODYSTROPHIC PHENOTYPE

Abstract

Introduction: The Merino sheep breed has been used extensively for intervertebral disc research but it has not previously been documented that the breed displays a mild form of chondrodystrophy with disproportionate dwarfism. The ovine Merino intervertebral disc is similar to human and chondrodystrophic canine discs in structure, absence of notochordal cells in the adult structure, response to trauma, display of an age-dependant loss of proteoglycans and degenerative spinal pathology including Schmorl’s nodes. In contrast, non-chondrodystrophic breeds have a gelatinous nucleus pulposus, notochordal cells which may persist into adulthood and a low incidence of spinal disorders of discal origin thus are unsuitable as models of the human intervertebral disc.

Methods: Haematoxylin and Eosin, Toluidine blue stained and aggrecan and versican immunolocalised sections were examined by bright field and Nomarsky differential interference contrast microscopy.

Results: The ovine merino intervertebral disc undergoes an age-dependant chondroid transformation of the central nucleus pulposus with the appearance of cell nests of chondrocytic morphology within a hyaline cartilage-like matrix rich in aggrecan and type II collagen but deficient in versican and type I collagen. In contrast, the adjacent nucleus pulposus is a fibrocartilage rich in types I and II collagen, versican and aggrecan; the constituent cells are readily distinguished from the aforementioned cell clusters. The ovine femoral epiphyseal growth plate displays dysplastic changes with relatively short columns of flattened, columnar chondrocytes in the pre-hypertrophic region and a disorganised integration of the hypertrophic cells into metaphyseal endochondral bone in the distal growth plate.

Conclusions: These observations warrant the classification of the Australian merino as a chondrodystrophic breed. The Merino is a useful comparative animal model for the human intervertebral disc.