Cartilage canals in the chicken embryo: ultrastructure and function

In this study the detailed morphology and the function of cartilage canals in the chicken femur are investigated. Several embryonic stages (e 13.5, 16, 19, and 20) are examined by means of light microscopy, electron microscopy (TEM), and immunohistochemistry (VEGF, type I and II collagen). Our results show that cartilage canals originate from the perichondrium and form a complex pattern. Two types of canals are distinguishable: shell canals and communicating canals. Shell canals are in the reserve zone and are arranged in successive layers. Communicating canals spring from the shell canals and pass down into the proliferative zone and into the hypertrophic zone. These canals are conical shaped and are orientated nearly in parallel to the long axis of the femur. Cartilage canals comprise venules, arterioles, capillaries (mature and immature), and undifferentiated mesenchymal cells. No canal wall in the sense of an epithelium is elaborated. VEGF is detected in both types of canals and macrophages are found at the end of the cartilage canals. We conclude that the growth factor stimulates angiogenesis and that the latter cells erode the matrix ahead of the canals and thus enable the advancement of the vessels. The results clearly show that the canal matrix differs from the remaining cartilage matrix. The canal matrix contains type I collagen, few type II collagen fibrils and proteoglycans are lacking. In contrast, in the cartilage matrix type II collagen and proteoglycans are abundant but no type I collagen is found. Communicating canals are surrounded by a distinct layer of type I collagen indicating that osteoid is formed around these canals. Hypertrophic chondrocytes label for type I collagen and it seemed possible that chondrocytes adjacent to the communicating canals differentiate into bone-forming cells. Our results provide evidence that cartilage canals are involved in nourishment of the cartilage as well as in the ossification process.