Downloaded from genesdev.cshlp.org on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW FGF signaling pathways in endochondral and intramembranous bone development and human genetic disease David M. Ornitz1,3 and Pierre J. Marie2 1Department of Molecular Biology and Pharmacology, Washington University Medical School, St. Louis, Missouri 63110, USA; 2Laboratory of Osteoblast Biology and Pathology, INSERM U349 affiliated CNRS, Hopital Lariboisiere, Paris cedex 10, France Over the last decade the identification of mutations in ized type II collagen-expressing chondrocytes and more the receptors for fibroblast growth factors (FGFs) has de- peripherally localized type I collagen-expressing peri- fined essential roles for FGF signaling in both endochon- chondrial cells (Kosher et al. 1986). At this stage, chon- dral and intramembranous bone development. FGF sig- drocytes begin to elaborate a specialized extracellular naling pathways are essential for the earliest stages of matrix containing type II collagen. Midway between the limb development and throughout skeletal develop- ends of this elongated cartilaginous template, chondro- ment. In this review, we examine the role of FGF signal- cytes exit the cell cycle, hypertrophy, and begin to syn- ing in bone development and in human genetic diseases thesize type X collagen in place of type II collagen that affect bone development. We also explore what is (Schmid and Linsenmayer 1985). presently known about how FGF signaling pathways in- To synthesize bone, a center of ossification forms in teract with other major signaling pathways that regulate the mid-hypertrophic zone by neovascularization of the chondrogenesis and osteogenesis. initially avascular cartilaginous template. The secretion and mineralization of a type I collagen-containing extra- cellular matrix is mediated by osteoblasts that are asso- Overview of skeletal development ciated with the newly developed vasculature. As bones grow, this center of ossification propagates toward the Skeletal elements are formed through two distinct de- two epiphyseal plates. velopmental processes. Endochondral ossification gives The epiphyseal growth plate consists of well-demar- rise to long bones that comprise the appendicular skel- cated zones of cells that follow an elegant developmental eton, facial bones, vertebrae, and the lateral medial program (Caplan and Pechak 1987; Hall and Miyake clavicles. Intramembranous ossification gives rise to the 1992; Olsen et al. 2000; Wagner and Karsenty 2001; flat bones that comprise the cranium and medial Karsenty and Wagner 2002). Proximally (toward the end clavicles. Both types of ossification involve an initial of a developing bone), a pool of chondrocytes (called the condensation of mesenchyme and the eventual forma- resting or reserve zone) supplies cells to a population of tion of calcified bone. However, intramembranous bone proliferating chondrocytes. Proliferating chondrocytes in formation accomplishes this directly, whereas endo- turn differentiate to form a transient pool of prehyper- chondral ossification incorporates an intermediate step trophic and then a more long-lived pool of hypertrophic in which a cartilaginous template regulates the growth chondrocytes. At the distal end of the epiphyseal growth and patterning of the developing skeletal element. plate, hypertrophic chondrocytes die by apoptosis and Development of endochondral bones initiates shortly are replaced by trabecular bone. In this manner, hyper- after the formation of the limb bud with the condensa- trophic chondrocytes provide a template for the forma- tion of loose mesenchyme, marked by expression of type tion of trabecular bone. II collagen (Fig. 1A; Kosher et al. 1986; Nah et al. 1988). In addition to the cartilaginous and trabecular core, Condensing mesenchyme forms an anlage for the endo- endochondral bone contains a hard outer shell of cortical chondral skeleton and can either branch or segment to bone (the bone collar), which surrounds the marrow cav- form individual skeletal elements (Hall and Miyake ity centrally and is contiguous with the perichondrium 1992, 2000). Differentiation of condensing mesenchyme proximally (Caplan and Pechak 1987). The perichon- gives rise to a proliferating population of centrally local- drium contains precursor pools of cells that give rise to osteoblasts that line the endosteal (inner) and periosteal 3Corresponding author. (outer) surface of cortical bone. Osteoblasts differentiate E-MAIL [email protected]; FAX (314) 362-7058. Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/ into osteocytes, which become embedded within corti- gad.990702. cal bone. The center of ossification in the perichondrium 1446 GENES & DEVELOPMENT 16:1446–1465 © 2002 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/02 $5.00; www.genesdev.org Downloaded from genesdev.cshlp.org on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press FGF pathways in skeletal development Figure 1. Endochondral and intramembranous bone development. (A) Schematic representation of a developing endochondral bone. Endochondral skeletal development begins with the formation of a mesenchymal condensation, expressing type II collagen (blue). Centrally, cells differentiate into chondrocytes, which hypertrophy and express type X collagen (purple). Progression to the mature growth plate accompanies development of the perichondrium (yellow), vascular invasion, and the formation of a center of ossification containing type I collagen-expressing osteoblasts (yellow). (B) Schematic representation of a developing intramembranous bone. Undifferentiated mesenchymal cells differentiate into osteoprogenitor cells expressing Cbfa1 (pink). Osteoprogenitor cells progress to mature osteoblasts that express Cbfa1 and type I collagen (yellow). These cells deposit and mineralize bone matrix. Osteoblasts either die by apoptosis or are embedded in the matrix, becoming osteocytes. is aligned with the center of ossification at the chondro– into osteoprogenitor cells and then into osteoblasts that osseous junction of the growth plate. A key feature of the express type I collagen, bone sialoprotein, and osteocal- growth of long bones is the coordination of chondrogen- cin, and synthesize bone matrix along the bone margins. esis and ossification in the epiphyseal growth plate and Growth and differentiation at the suture is regulated by osteogenesis in the perichondrium/periosteum (Fig. 1A). interactions between the mesenchyme, the osteogenic Cranial vault development is a complex process in- front, and the Dura mater, a tough, fibrous membrane volving cells of neural crest origin (Couly et al. 1993) and forming the outer envelope of the brain and the inner paraxial mesoderm that contribute to intramembranous lining of cranial bones and sutures. (see Fig. 3C below; bones of the cranial vault and sutures (Noden 1992; Op- Most et al. 1998; Opperman 2000; Morriss-Kay et al. perman 2000). Intramembranous bone growth begins 2001). Because sutures are the major sites of intramem- with the condensation of mesenchymal cells (Fig. 1B). branous bone growth during cranial vault development, Ossification centers are formed by direct bone matrix the events occurring at the suture are essential for the deposition-forming plates, which expand during develop- regulation of intramembranous ossification (Cohen ment but do not fuse at the junction with other cranial 2000a). bones (Hall and Miyake 2000). The junction between cal- varial bones is a functional structure called a suture, FGFs and FGF receptors—a brief introduction which is responsible for the maintenance of a separation between membranous bones and for regulating the ex- FGFs comprise a family of 22 genes encoding structur- pansive growth of the skull. In the vicinity of the suture, ally related proteins (Ornitz and Itoh 2001). Six subfami- a minority of osteogenic mesenchymal cells differentiate lies of FGFs, grouped by sequence similarities, tend to GENES & DEVELOPMENT 1447 Downloaded from genesdev.cshlp.org on September 26, 2021 - Published by Cold Spring Harbor Laboratory Press Ornitzand Marie share biochemical and functional properties and are ex- cations that are required for, and may actually regulate, pressed in specific spatial and developmental patterns. functional ligand–receptor interactions (Rapraeger 1995; Four distinct FGF receptor tyrosine kinase molecules Guimond and Turnbull 1999; Ornitz 2000; Allen et al. bind and are activated by most members of the FGF fam- 2001). ily. Alternative mRNA splicing produces FGF receptors with unique ligand binding properties (Johnson and Mutations in FGF receptors in chondrodysplasia Williams 1993; Ornitz et al. 1996). Alternative splicing is and craniosynostosis syndromes mostly tissue-specific, producing epithelial variants (b splice forms) and mesenchymal variants (c splice forms; The importance of FGF signaling in skeletal develop- Miki et al. 1992; Orr-Urtreger et al. 1993; Chellaiah et al. ment was first revealed with the discovery that a point 1994; Naski and Ornitz 1998). FGF activity and specific- mutation in the transmembrane domain of FGFR3is the ity are further regulated by heparan sulfate oligosaccha- etiology of Achondroplasia, the most common genetic rides, in the form of heparan sulfate proteoglycans. Hep- form of dwarfism in humans (Fig. 2; Rousseau et al. 1994; arin/Heparan sulfate, FGF, and an FGF receptor (FGFR) Shiang et al. 1994). Since this discovery, the etiology