Organogenesis of the gonad is unique in that both the testis and ovary are derived from an initially bipotential tissue, the genital ridge. Development diverges when Sry is expressed during a narrow window of time between 10.5 and 12.5 days postcoitum (dpc) in the XY gonad, resulting in rapid morphological changes that produce a characteristic testis morphology by 12.5 dpc. At this stage, the organization of testis cords defines two compartments of the testis. Inside testis cords are the Sertoli cells, which support the development of primordial germ cells, are the cell type that expresses Sry, and are believed to orchestrate testis organogenesis. Peritubular myoid cells surround the cords and participate with Sertoli cells in the production of basal lamina. The interstitial compartment lies between testis cords and contains steroid-secreting Leydig cells, fibroblasts, and the characteristic vasculature of the testis. Although no direct targets of Sry have been identified, several developmental processes in the gonad depend on Sry and occur soon after Sry is expressed. One early event is an increase in proliferation of cells at the coelomic surface of the XY gonad (Schmahl et al. 2000). Proliferation occurs in two stages. The first stage of proliferation occurs between 11.2 and 11.5 dpc and gives rise to Sertoli cell precursors. The second stage of proliferation occurs between 11.5 and 12.0 dpc and gives rise to other, uncharacterized somatic cells of the testis (Karl and Capel 1998; Schmahl et al. 2000). The adjacent mesonephros is an additional source of cells for the XY gonad. Migration of cells from the mesonephros is dependent on Sry (Capel et al. 1999) and is required for organization of testis cords (Buehr et al. 1993; Merchant-Larios et al. 1993; Martineau et al. 1997; Tilmann and Capel 1999). The migrating cell population includes peritubular myoid cells and many endothelial and perivascular cells that contribute to divergent vascular development in the XY gonad (Brennan et al. 2002). The origin of fetal Leydig cells has not been clarified, but several sources have been suggested, which include the coelomic epithelium (Karl and Capel 1998), a migrating mesonephric population (Merchant-Larios and Moreno-Mendoza 1998; Nishino et al. 2001) that may include neural crest cells (Middendorff et al. 1993; Mayerhofer et al. 1996), or a common early precursor of the adrenal steroid cells (Hatano et al. 1996). Very little is known about the genetic signaling pathways operating downstream of Sry that control the cellular mechanisms of testis organogenesis. Using a candidate approach, we investigated the platelet-derived growth factor (PDGF) family of ligands and receptors, which have well-characterized roles in the proliferation and migration of mesenchymal cell types, particularly smooth muscle and endothelial cells (Betsholtz et al. 2001). There are two identified PDGF receptors, α and β, and four ligands, A, B, C, and D. Different combinations of ligands and receptors can homo- and heterodimerize and activate several distinct and overlapping intracellular pathways (for review, see Heldin and Westermark 1999; Klinghoffer et al. 2001). PDGFR-α has a broader specificity for ligand binding and can bind PDGF-A, PDGF-B, and PDGF-C homodimers as well as PDGF-AB heterodimers, whereas PDGFR-β can bind only PDGF-B and PDGF-D homodimers. Generally, Pdgf ligands are expressed by epithelial or endothelial cells, whereas mesenchymal cells express Pdgf receptors. Proliferation and migration of mesenchymal cells in response to PDGF signaling contribute to the morphogenesis and integrity of epithelial and endothelial structures as shown by genetic disruptions that affect tissues such as the lung, the kidney, and the intestine (Soriano 1994; Bostrom et al. 1996; Lindahl et al. 1998; Karlsson et al. 2000). Previous expression data (Gnessi et al. 1995) and recent genetic data have provided some insight into the involvement of PDGFs in the postnatal development of the testis. In the Pdgf-A−/− testis, very few adult Leydig cells develop, leading to a reduction in testis size and spermatogenic arrest by 32 days postnatal (dpn). Adult Leydig cells are not derived from fetal Leydig cells, but instead arise as a separate, distinct cell population after birth (Ariyaratne et al. 2000). Although fetal stages were not examined, two pieces of evidence implied that fetal Leydig cell development was normal. First, normal numbers of fetal Leydig cells were reported in Pdgf-A−/− gonads at 10–25 dpn, just before they are replaced by adult Leydig cells. Second, testicular descent and masculinization occurred normally, indicating that fetal Leydig cells were present during fetal development and produced testosterone. (Gnessi et al. 2000). The role of PDGF signaling in the embryonic testis has not been investigated in detail. A recent study using the PDGFR tyrosine kinase inhibitor, AG1296, implicated PDGF signaling in rat fetal testis cord development (Uzumcu et al. 2002). Large, dilated cords were noted in the treated samples, although the cellular and molecular basis for the defect was not characterized, and the role of specific PDGF receptors and ligands was not addressed. Our examination of gonads from Pdgfr-β−/− mice (Soriano 1994) revealed no overt defects in early XY or XX development at 11.5–13.5 dpc, ruling out a critical role for this receptor (K. Tilmann, unpubl.). These data led us to focus our study on PDGFR-α and its ligands PDGF-A, PDGF-B, and PDGF-C. We found that Pdgfr-α and Pdgf-A display a sexually dimorphic expression pattern in the gonad. To examine the potential roles of these factors in gonad development, we analyzed gonad development in Pdgfr-α−/− mice. These studies uncovered a sex-specific role for PDGFR-α signaling in promoting cord formation, proliferation, endothelial cell migration, and Leydig cell differentiation. We also provide data placing PDGF signaling in a parallel pathway with Dhh, which is expressed in Sertoli cells and is the only other fetal Leydig cell factor identified thus far (Yao et al. 2002).