Facial morphogenesis involves directed outgrowth of the facial primordia until they appose and fuse with one another. Currently, outgrowth of primordia is considered to be driven primarily by bulk tissue displacement caused by proliferation gradients in the mesenchymal tissue. However, our lab and others have found that other mechanisms, such as directed cellular migration and epithelium guided growth, likely contribute as well. In the limb bud, tensile forces generated by the ectoderm have been shown to direct cellular orientation and growth, and proliferation gradients across the limb bud cannot accurately model morphogenesis without incorporating directed cellular behaviors. Our lab has previously shown that mesenchymal cells in the face are polarized and that activation of fibroblastic growth factor (FGF) signaling disrupts this polarity while causing aberrant outgrowth of the primordium. The extracellular matrix (ECM) around cells provides the structural scaffolding required to generate and guide force through tissue, and the ECM in the epithelium may guide or constrain growth regionally. I hypothesize that the ECM directs morphogenesis in the face via regional differences in composition and compliance. I will test this hypothesis with two main aims. In the first aim I will examine how FGF signaling affects ECM physical properties and cellular organization in the facial primordium. I will quantitatively map the composition of ECM proteins and ECM compliance across the face with confocal microscopy and atomic force microscopy, then relate regional differences in these measures to facial shape change.
This aim will elucidate mechanisms by which molecular signaling of fibroblastic growth factor controls morphogenesis through the ECM and cellular organization. In the second aim, I will increase or decrease tissue compliance directly by altering collagen crosslinking, then repeat the same analyses as the first aim.
The second aim will determine if ECM stiffness directs outgrowth directly while the mesenchyme passively conforms around it, or if there is feedback between ECM and cells that actively remodel the environment to direct growth. Failure of the facial primordium to appose and fuse leads to facial clefting. Understanding the role of ECM in directional growth is important for understanding the etiology of birth defects in the face.
Cellular growth rate alone cannot explain the directed tissue growth that is required for the face to form correctly. The physical properties of tissue determine how force is distributed ? likely guiding and restricting growth of different regions of the face at different stages of development. The proposed research will determine the role of tissue physical properties in face formation by a) measuring face tissue physical properties after increasing or decreasing growth factor signaling required for correct face formation, and b) measuring changes in face shape after directly altering tissue physical properties.
