LOCALIZATION OF ALKALINE-PHOSPHATASE IN EQUINE GROWTH CARTILAGE

The aim of this study was to localise alkaline phosphatase (ALP) activ ity in equine growth cartilage both histochemically and ultrastructura lly. For histochemical studies, full thickness growth cartilage sample s were obtained from 6 anatomical sites from 16 horses and ponies rang ing in age from 90 d postconception to 12 years of age. For ultrastruc tural studies, samples were obtained from the lateral trochlear ridge of the distal femur of 3 animals ranging in age from 157 d postconcept ion to 12 months of age. Alkaline phosphatase in histological sections was localised using a substituted naphthol reaction which revealed en zyme activity around the cell surfaces of hypertrophic chondrocytes. A LP activity was quantified by determining the labelling index of ALP p ositive chondrocytes. The labelling index depended on (1) the age of t he animal (activity being highest in animals < 6 months old), and (2) the anatomical site (activity being lowest in the proximal phalanges a nd highest in the growth plate, reflecting the degree of long bone gro wth at each site). In order to demonstrate ALP activity ultrastuctural ly, two capturing agents were used: cerium and lead. This enzyme was v isualised by the precipitation of electron-dense salts at the site of activity. ALP activity was localised at low levels on the surface memb ranes of chondrocytes in the midproliferative zone. The enzyme activit y increased throughout the proliferative zone and was most intense on chondrocytes of the hypertrophic zone. ALP activity was seen to be con centrated on the matrix vesicles, the putative sites of mineralisation . This work demonstrates, for the first time in the horse, that ALP is a useful marker of incipient bone formation.