The synovial joint arises from an initial condensation of cells that subsequently develops into distinct skeletal structures, separated by the joint. Bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs) have a fundamental role during skeletogenesis, including joint formation. Development of the joint appears to be dependent on the differential expression/activity of the related BMP and GDF subfamilies. Gdf-5 is expressed in the developing joints and is necessary for the formation of some joints. In contrast, recent data has shown that antagonism of the BMP family is crucial for joint formation. Here, we review mechanisms of how BMP signalling may be antagonised/modified. We also describe the expression of Bmp-2 and Bmp-4 together with two BMP antagonists, chordin and noggin, during chick joint development. Finally, we discuss possible mechanisms of how a joint forms and the evidence that the joint is a 'signalling centre' that may coordinate the development of adjacent skeletal structures.

Recent experiments, showing that both cranial paraxial and splanchnic mesoderm contribute to branchiomeric muscle and cardiac outflow tract (OFT) myocardium, revealed unexpected complexity in development of these muscle groups. The Pitx2 homeobox gene functions in both cranial paraxial mesoderm, to regulate eye muscle, and in splanchnic mesoderm to regulate OFT development. Here, we investigated Pitx2 in branchiomeric muscle. Pitx2 was expressed in branchial arch core mesoderm and both Pitx2 null and Pitx2 hypomorphic embryos had defective branchiomeric muscle. Lineage tracing with a Pitx2 cre allele indicated that Pitx2 mutant descendents moved into the first branchial arch. However, markers of both undifferentiated core mesoderm and specified branchiomeric muscle were absent. Moreover, lineage tracing with a Myf5 cre allele indicated that branchiomeric muscle specification and differentiation were defective in Pitx2 mutants. Conditional inactivation in mice and manipulation of Pitx2 expression in chick mandible cultures revealed an autonomous function in expansion and survival of branchial arch mesoderm.

Wolf-Hirschhorn syndrome (WHS) is a multiple malformation syndrome characterized by mental and developmental defects resulting from the absence of a segment of one chromosome 4 short arm (4p16.3). Recently, Pitt-Rogers-Danks syndrome (PRDS), which is also due to a deletion of chromosome 4p16.3, has been shown to be allelic to WHS. Due to the complex and variable expression of these disorders, it is thought that WHS/PRDS results from a segmental aneusomy of 4p resulting in haploinsufficieny of an undefined number of genes that contribute to the phenotype. In an effort to identify genes that contribute to human development and whose absence may contribute to the phenotype associated with these syndromes, we have generated a transcript map of the 165-kb critical region and have identified a number of potential genes. One of these genes, WHSC2, which was identified with the IMAGE cDNA clone 53283, has been characterized. Sequence analysis defined an open reading frame of 1584 bp (528 amino acids), and transcript analysis detected a 2.4-kb transcript in all fetal and adult tissues tested. In parallel, the mouse homologue was isolated and characterized. Mouse sequence analysis and the pattern of expression are consistent with the clone being the murine equivalent of the human WHSC2 gene (designated Whsc2h). The data from sequence and transcript analysis of this new human gene in combination with the lack of significant similarity to proteins of known function imply that it represents a novel gene. Most importantly, its location within the WHSCR suggests that this gene may play a role in the phenotype of the Wolf-Hirschhorn/Pitt-Rogers-Danks syndrome.

Members of the fibroblast growth factor (FGF) family and growth and differentiation factor 5 (GDF-5) have been implicated in joint specification, but their roles in subsequent cavity formation are not defined. Cavity formation (cavitation) depends upon limb movement in embryonic chicks and factors involved in joint formation are often identified by their expression at the joint-line. We have sought support for the roles of FGF-2, FGF-4, and GDF-5 in cavitation by defining expression patterns, immunohistochemically, during joint formation and establishing whether these are modified by in ovo immobilisation. We found that FGF-2 exhibited low level nuclear expression in chondrocytes and fibrocartilage cells close to presumptive joints, but showed significantly higher expression levels in cells at, and directly bordering, the forming joint cavity. This high-level joint line FGF-2 expression was selectively diminished in immobilised limbs. In contrast, we show that FGF-4 does not exhibit differential joint-line expression and was unaffected by immobilisation. GDF-5 protein also failed to show joint-line selective labelling, and although immobilisation induced a cartilaginous fusion across presumptive joints, it did not affect cellular GDF-5 expression patterns. Examining changes in GDF-5 expression in response to a direct mechanical strain stimulus in primary embryonic chick articular surface (AS) cells in vitro discloses only small mechanically-induced reductions in GDF-5 expression, suggesting that GDF-5 does not exert a direct positive contribution to the mechano-dependent joint cavitation process. This notion was supported by retroviral overexpression of UDPGD, a characteristic factor involved in hyaluronan (HA) accumulation at presumptive joint lines, which was also found to produce small decreases in AS cell GDF-5 expression. These findings support a direct mechano-dependent role for FGF-2, but not FGF-4, in the cavitation process and indicate that GDF-5 is likely to influence chondrogenesis positively without contributing directly to joint cavity formation. Developmental Dynamics 235:826 -834, 2006.