We use the skin vasculature model as a favorable model system to study mechanisms that coordinate arterial differentiation and patterning of vascular branching. We previously observed that arterial vessels and peripheral sensory nerves (PNs) develop alongside each other in the embryonic limb skin. This co-patterning is developed by PN-mediated signal(s) that instructively guide the arterial branching network (Mukouyama et al. 2002). Therefore, the limb skin vasculature affords an attractive system in which to study the nature of neuronal signals that control vascular network formation. Using the tissue-specific knockout technology, we have begun to dissect out the PN-derived signals that participate in integrating both branching networks. We showed that PN-derived vascular endothelial growth factor (VEGF)-A functions to control arteriogenesis-arterial differentiation and smooth muscle cell association (Mukouyama et al. 2005). We have discovered that PN-derived C-X-C motif chemokine ligand (CXCL) 12 controls the nerve-blood vessel alignment. Our data establish that two distinct mechanisms underlie the congruence of nerve and arterial vessel branching: VEGF-A controlling arterial differentiation, and Cxcl12 controlling vessel branching and alignment with nerves (Li et al. 2013). These studies pose additional fundamental questions in branching morphogenesis and patterning during angiogenesis and neurogenesis: 1) What specifies the correct timing of these guidance and differentiation signals to form the conserved and stereotyped branching patterns of the neuro-vascular network? 2) How do endothelial cells (ECs) migrate and assemble into vessels of the appropriate size and shape along nerves? 3) What determines the choice of either axon extension along blood vessels or innervation to blood vessels? 4) How does the neuro-vascular network influence organ physiology, regeneration, and diseases? We are engaged in a new project for studying the role of the neuro-vascular association during tissue repair or in disease conditions. Whole-mount immunofluorescence microscopy has revealed that adult ear skin maintains the neuro-vascular bundle, suggesting that the association reflects the mutual requirement of nerve and vessel in the function and maintenance of both networks. Using this adult ear skin vasculature model, we are currently studying peripheral nerve regeneration and re-vascularization in the ear skin regeneration/wound healing. We have recently developed a whole-mount high-resolution imaging technique to visualize processes of innervation and vascularization in the gut. We are particularly interested in villus morphogenesis, the functional absorptive units of the small intestine. We will attempt to examine whether defective innervation or vascularization influences villus morphogenesis (Hatch &Mukouyama. 2014).
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