HISTOLOGICAL ANALYSIS OF EQUINE CARTILAGE DEVELOPMENT

Abstract

Osteochondrosis (OC) is a major problem in the horse industry. This developmental orthopedic disease is defined as a focal disturbance in endochondral ossification and considered as a dynamic process; most often lesions appear and disappear again during cartilage development in the first months postpartum. Remarkably is seems that after the age of 8 months the situation remains stable. There are several factors supposed to play a more or less significant role in the etiology of this multifactorial disease. Internationally a lot of research has been done by many scientists, focused on the etiology and pathology of equine OC. However, it is still unclear which process in the developing cartilage is the one that makes a lesion temporary or permanent. In literature only a few papers can be found which specify on the details of equine cartilage development. Therefore, in this study our research aim was to perform a thorough histological analysis of developing cartilage tissue. The studied samples were derived from 11 foals without clinical signs of orthopedic disease at the time of dead. In these tissue samples we investigated the following cartilage characteristics: cartilage thickness, cellularity, vascularity and collagen orientation. Cartilage thickness of both the lateral and medial femoral condyle was measured and compared in relation to each other and under influence of maturation. Age dependent cartilage cellularity and blood vessel distribution of the specimen was measured and possible differences between the lateral and medial femoral condyle were measured. The collagen structure was studied with a picrosirius red staining using PLM. Besides collecting detailed information about the development of cartilage, early histological changes that could be precursors for OC lesions in later life were not found. Cartilage thickness was found to be related to age, in both femoral condyles. Cartilage thickness was significantly higher in the medial condyle compared to the lateral condyle. Cell density was found to decreases during maturation which is in line with the expectations, the location (lateral or medial) does show an unexpected difference. Cell density appears to be higher in the lateral femoral condyle. No significant difference in cell density could be found for the different sub regions in the cartilage of the foals. Developing cartilage is found to become avascular at the age of approximately 5 months. Data show a sigmoidal relationship between vascularity and age, indicating a limited start value for amount of blood vessels in the tissue during development. There are no significant differences between the lateral and medial condyle and no significant differences between the different sub regions in the cartilage. Against expectations, we found that collagen orientation is already present as early as 10 months gestation and is identical to the orientation known for articular cartilage in adult animals. Collagen structure in the superficial zone of the cartilage was static and similarly orientated in tissue of all ages (age range: 10 months gestation - 6 months and 15 days post-partum), whereas only the underlying non-oriented part of the cartilage is gradually taken over by bone formation during joint maturation.