Biology of signalling receptors in human
Ann Kristin Hansen
Hyaline cartilage is an avascular, aneural and alymphatic tissue covering the ends of long bones facilitating a frictionless movement and absorption of forces in the diarthrodial joint. The thickness of cartilage is related to the congruency of the joint, ranging from 1.2 mm in the congruent ankle joint to 2.17 mm in the incongruent knee joint of adults, while in adolescents knee joints the thickness range up to 4 mm. Cartilage consists of only 3‑4 % cells, while the bulk of the tissue is the surrounding matrix made up of collagen type II and glycosaminoglycan that provide structural architecture and captures water molecules. The tissue is spatially organised in a superficial, middle, deep and calcified zone. While most reports have named the chondrocyte as the sole cell type in cartilage, newer publications have reported progenitor cells residing in the superficial layer. The zonal organisation of the matrix facilitates the highly specialised mechanical properties of hyaline cartilage. The superficial zone is designed to handle the sheer forces of the moving joint with flattened chondrocytes and fibrils arranged parallel to the joint surface. The compressive forces are handled by the obliquely organised middle layer and particularly the deep layer where the cells are arranged in columns and the fibrils run perpendicular to the joint line. The tidemark is the basophilic line on histological sections separating the hyaline cartilage from calcified cartilage, while the cement line separates the calcified cartilage from the subchondral bone plate. The zonal organization is reflected in the chondrocytes exhibiting different phenotypes in the superficial, middle, deep and calcified zones.