Toward a molecular understanding of skeletal development

Much of our knowledge about cartilage and bone has come from descriptive anatomy, endocrinology, and cellular studies of bone turnover. Recent approaches have led to the identification of local factors that regulate skeletal morphogenesis. Molecular and biochemical studies of bone and cartilage cells in vitro, gene inactivation in mice, and the identification of genes responsible for mouse and human skeletal abnormalities have documented the importance of specific growth and differentiation factors, extracellular matrix proteins, signaling mediators, and transcription factors in bone and cartilage development. The successful convergence of mouse and human genetics in skeletal biology is illustrated in this issue of Cell with two papers that show that mutations in collagen type Xl cause chondrodysplasia both in cho/cho mice as well as in patients with Stickler syndrome (Li et al., 1995; Vikkula et al., 1995). In general, recent results emphasize the need to view skeletal development at various integrated levels of organization and illustrate how single gene products affect development at these different levels. Pattern information determines not only the body plan of the early skeleton but also the shape of each individual skeletal element. In addition, the sequence of events during bone growth and development must be temporally and spatially controlled to ensure correct proportions of bony elements. Positional information must also regulate the establishment of bone internal structure throughout growth, while local homeostatic mechanisms must maintain bone integrity throughout adult life. Lastly, a complex extracellular matrix must generate skeletal tissues with specific biomechanical properties. Ultimately, the morphogenesis of the skeleton derives from the regulated differentiation, function, and interactions of its component cell types. Three major cell types contribute to the skeleton: chondrocytes, which form cartilage; osteoblasts, which deposit bone matrix; and osteoclasts, which resorb bone. Chondrocytes and osteoblasts are of mesenchymal origin, whereas osteoclasts derive from the hematopoietic system. Once embedded in bone matrix, osteoblasts mature into terminally differentiated osteocytes. The activity and differentiation of osteoblasts and osteoclasts are closely coordinated during development as bone is formed and during growth and adult life as bone undergoes continuous remodeling. More specifically, the formation of internal bone structures and bone remodeling result from coupling bone resorption by activated osteoclasts with subsequent deposition of new matrix by osteoblasts (Figure 1). Bone remodeling also links bone turnover to the endocrine homeostasis of calcium and phosphorus, since the mineralized bone matrix serves as the major repository for these ions in the body. Descriptive embryology and anatomy distinguish two types of bone development: intramembranous and endochondral. Intramembranous ossification occurs when mesenchymal precursor cells differentiate directly into bone-forming osteoblasts, a process employed in generating the flat bones of the skull as well as in adding new bone to the outer surfaces of long bones. In contrast, endochondral bone formation entails the conversion of an initial cartilage template into bone and is responsible for generating most bones of the skeleton. Cartilage templates originally form during embryogenesis when mesenchymal cells condense and then differentiate into chondrocytes. These cells subsequently undergo a program of hypertrophy, calcification, and cell death. Concomitant neovascularization occurs, and osteoclasts and osteoblasts are recruited to replace the cartilage scaffold gradually with bone matrix and to excavate the bone marrow cavity. Longitudinal bone growth takes place through a similar pattern of endochondral ossification in the growth plates located at the epiphyses (ends) of long bones. In these epiphyseal plates, the calcified, hypertrophic cartilage provides a scaffold for the formation of new trabecular bone. Ultimately, all remaining cartilage is replaced by bone except at the articular surfaces of the joints (Figure 2). Skeletal Patterning Classical embryology has shown that three distinct embryonic lineages contribute to the early skeleton. The neural crest gives rise to the branchial arch derivatives of the craniofacial skeleton, the sclerotome generates most of the axial skeleton, and the lateral plate mesoderm forms the appendicular skeleton. Transplantation studies have indicated that information regarding the number and ana-