Functional adaptation of equine articular cartilage: the formation of regional biochemical characteristics up to age one year

Biochemical heterogeneity of cartilage within a joint is well known in matu re individuals. It has recently been reported that heterogeneity for proteo glycan content and chondrocyte metabolism in sheep develops postnatally und er the influence of loading. No data exist on the collagen network in gener al or on the specific situation in the horse. The objective of this study was to investigate the alterations in equine ar ticular cartilage biochemistry that occur from birth up to age one year, te sting the hypothesis that the molecular composition of equine cartilage mat rix is uniform at birth and biochemical heterogeneity is formed postnatally . Water content, DNA content, glycosaminoglycan content (GAG) and biochemical characteristics of the collagen network (collagen content, hydroxylysine c ontent and hydroxylysylpyridinoline [HP] crosslinks) were measured in immat ure articular cartilage of neonatal (n = 16), 5-month-old foals (n = 16) an d yearlings (n = 16) at 2 predefined differently loaded sites within the me tacarpophalangeal joint. Statistical differences between sites were analyse d by ANOVA (P < 0.01), and age correlation was tested by Pearson's product moment correlation analysis (P < 0.01). In neonatal cartilage no significan t site differences were found for any of the measured biochemical parameter s. This revealed that the horse has a biochemically uniform joint (i.e. the cartilage) at birth. In the 5-month-old foals and yearlings, significant s ite differences, comparable to those in the mature horse, were found for DN A, GAG, collagen content and hydroxylysine content. This indicates that fun ctional adaptation of articular cartilage to weight bearing for these bioch emical parameters takes place during the first months postpartum. Water con tent and HP crosslinks showed no difference between the 2 sites from neonat al horses, 5-month-old animals and yearlings. At both sites water, DNA and GAG decreased during maturation while collagen content, hydroxylysine conte nt and HP crosslinks increased. We propose that a foal is born with a uniform biochemical composition of ca rtilage in which the functional adaptation to weight bearing takes place ea rly in life. This adaptation results in biochemical and therefore biomechan ical heterogeneity and is thought to be essential to resist the different l oading conditions to which articular cartilage is subjected during later li fe. As collagen turnover is extremely low at mature age, an undisturbed fun ctional adaptation of the collagen network of articular cartilage at a youn g age may be of significant importance for future strength and resistance t o injury.