The urogenital ridge (which includes the gonad and attached mesonephros) has proved to be a unique system to analyze the role of vascularization in morphogenesis of an internal organ. During the period of vascularization, development of the testis and ovary diverge morphologically. Time-lapse imaging of the gonad provides novel insights into the mechanisms that incorporate vasculature into an organ at the stage when structural organization of the tissue is initiated. Moreover, the divergent development of the testis and ovary provides a basis of comparison to correlate vascular development with the sex specific morphological development of the tissue. We are currently incorporating different cellular markers in this analysis to simultaneously image the endothelium and other cells in the organ. As far as we know, this is the first system for imaging vascular development in an organ undergoing extensive epithelial morphogenesis (Coveney et al., 2008). We are optimistic that this work will lead to further insights into the interwoven processes of cell differentiation and paracrine signaling that transform a common bipotential primordium to the highly specialized testis or ovary.