| This study examines the fundamental principles governing the early stages of skeletal morphogenesis. I specifically investigate the specification of mesenchymal progenitor cells to a skeletal fate. Using explant culture system, I determine that the pharyngeal endoderm was sufficient, but not necessary for specifying pre-chondrogenic identity. FGF signaling is both sufficient and required for specification of Sox9 expression and specification of prechondrogenic identity, as demonstrated by the addition of recombinant FGF protein or the FGF receptor inhibitor (SU5402) to explanted tissue, respectively. However, FGF signaling cannot maintain Sox9 expression or initiate the chondrogenic program as indicated by the absence of Col2a1 transcripts. BMP4 signaling induces and maintains Sox9 expression in the isolated mesenchyme, but only in combination with FGF signaling is capable of inducing Col2a1 expression, and thus, chondrogenesis. I propose that this represents a general mechanism of local signals specifying pre-chondrogenic identity and initiation of the chondrogenic program.;Additionally, I determine how specified mesenchymal cells integrate mechanical and molecular information from their environment forming a cartilage condensation. The classical model defines condensation based on increased cell density. Based on our results, we propose a revised model for skeletal condensation, which is based on differential cell shape changes and independent of increasing cell density. By disrupting cytoskeletal reorganization, I demonstrate that tension dependent dynamic cell shape changes drive condensation and modulate the response of the condensing cells to FGF, BMP and TGF-β pathways. Rho Kinase driven actomyosin contractions and myosin II generated differential cell cortex tension drive the cell shape changes and negatively TGF-&... |
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