Hyaline cartilage is present as a precursor to bones in the fetus. This cartilage is replaced by bone tissue except where one bone contacts another, as in a movable joint. In these locations, cartilage persists and covers the end of each bone as articular cartilage, providing a smooth, well-lubricated surface against which the end of one bone moves on the other in the joint. In addition, cartilage, being capable of interstitial growth, persists in weight-supporting bones and other long bones as a growth plate as long as growth in length occurs. The role of hyaline cartilage in bone growth is considered briefly below and in more detail in Plates 13 and 14.
Developing skeleton, fetal foot, rat, H&E ×85.
This section shows the cartilages that will ultimately become the bones of the foot. In several places, developing ligaments (L) can be seen where they join the cartilages. The nuclei of the fibroblasts within the ligaments are just barely perceptible. They are aligned in rows and are separated from other rows of fibroblasts by collagenous material. The hue and intensity of color of the cartilage matrix, except at the periphery, are due to the combined uptake of 212 the H&E. The collagen of the matrix stains with eosin; however, the presence of sulfated glycosaminoglycans results in staining by hematoxylin. The matrix of cartilage that is about to be replaced by bone, such as that shown here, becomes impregnated with calcium salts, and the calcium is also receptive to staining with hematoxylin. The many enlarged lacunae (seen as light spaces within the matrix where the chondrocytes have fallen out of the lacunae) are due to hypertrophy of the chondrocytes, an event associated with calcification of the matrix. Thus, where these large lacunae are present, i.e., in the center region of the cartilage, the matrix is heavily stained.
This figure also shows that the cartilage is surrounded by perichondrium, except where it faces a joint cavity (JC). Here, the bare cartilage forms a surface. Note that the joint cavity is a space between the cartilages whose boundaries are completed by connective tissue (CT). The connective tissue at the surface of the cavity is special. It will constitute the synovial membrane in the adult and contribute to the formation of a lubricating fluid (synovial fluid) that is present in the joint cavity. Therefore, all the surfaces that will enclose the adult joint cavity are derived originally from the mesenchyme. Synovial fluid is a viscous substance containing, among other things, glycosaminoglycans; it can be considered an exudate of interstitial fluid. The synovial fluid could be considered an extension of the extracellular matrix, as the joint cavity is not lined by an epithelium.
CARTI LAG E AN D TH E DEVE LOPI NG S KE LETON KEY B, bone C, cartilage CT, connective tissue JC, joint cavity L, ligament MC, marrow cavity arrowhead, calcified cartilage
Developing skeleton, fetal fnger, human,  thionine-picric acid ×30.