Reproduction in Domestic Animals

VEGFA Isoforms as Regulators of Testis Vasculature 311Sex determinationTestis morphogenesis# Tail somites (ts)Da yspostcoitum(dpc)8 10–15 16–18 19–23 24–3010. 5 10.7–11.2 11.3–11.5 11.6–11.9 12–12.5Fig. 1. Sry-regulated events occurringduring sex determination andtestis morphogenesis using dayspost-coitum and tail somites asdevelopmental markers (modifiedfrom Cupp and Skinner 2005)MesonephrosGenital RidgeSRY mRN APrim ordialGe rm CellSertoli celldifferentiationSertoli cellproliferationMesonephric ce llmigrationCord formatio nWhat is known about formation of sex-specific vasculardevelopment?Very little is known about what regulates sex specificvascular development in the gonads. Pre-endothelialcells in the indifferent gonad express markers for bothveins (EphB4) and arteries (ephrin B2); however thischanges as each gonad develops sex-specific structures.In the testis, endothelial cells migrate from the mesonephrosand form vascular networks with arterialmarkers between cords and the coelomic vessel. Thecoelomic vessel is the major testicular artery that isformed when mesonephric endothelial cells migratefrom the mesonephros into the testis and is unique tothe testis. As endothelial cells differentiate a greaterpercentage develop into arteries and contribute to thecoleomic vessel than those which contribute to veinswithin the testis. The development of arteries may benecessary to aid movement of testosterone to thedeveloping male reproductive tract which allows for itsmaintenance and differentiation (Brennan et al. 2002).In contrast, the ovary develops similar amounts ofarterial and venous networks from endothelial cells.The origin of vasculature is different in the testis andovary with vascular networks developing throughangiogenesis (branching from existing vasculature inthe mesonephros) in the testis and through vasculogenesis(de novo or neovascularization) in the ovary(Brennan et al. 2002). It is likely that the testis andovary develop vascular patterns from different endothelialcell origins due to expression of sex-specifictranscription factors. Furthermore, sex-specific transcriptionfactors stimulate different growth factors tonot only initate blood vessel formation but organspecificsupport structures such as seminiferous cordsor oogonial cysts.Experiments using knockout mice demonstrate thatdisruption of vascular development also causes sexreversaland abnormalities in germ cell development.Mice that are null for Wnt4 (Jeays-Ward et al. 2003)and Follistatin (Fst) (Yao et al. 2004) have sex-reversedXX gonads with ectopic expression of a coelomic vesseland vasculature surrounding seminiferous cord-likestructures. The Wnt4 gene regulates expression of Fstso it is not surprising that mice null for both of thesegenes have the same phenotype. Follistatin is onlyexpressed in the XX gonad and in general acts to inhibitactivins. Therefore, there is potential for activins to beinvolved in formation of the coelomic vessel. In contrastInhibin beta b (Inhbb) XY null mice do not formcoelomic vessels 50% of the time. Thus, Inhbb null XYgonads are sometimes sex-reversed to XX vascularphenotypes. Activin A is composed of two inhibin betaa, Inhba, subunits while Activin B is composed of twoInhbb subunits. In the ovary, only Inhbb is expressedand not Inhba. Furthermore, expression of Inhbb isfourfold higher in the testis compared to the ovary at12.5 dpc. Mating of Wnt4 and Inhbb null mice recapitulatethe normal phenotype in XY gonads to form acoelomic vessel. Yet, matings between Inhbb and Fstnull mice did not recapitulate the XY vascular phenotype,thus Wnt4 suppression of Inhbb appears to beindependent of Fst inhibitory actions on activins (Jeays-Ward et al. 2003; Yao et al. 2006).During normal testis development, Sry antagonizesexpression of Wnt4 to suppress Fst and relieve inhibitionof Inhbb (Fig. 2). We can only speculate that Inhbbtranscriptional regulation is due to expression of Sox-9,or other growth factors downstream of Sry expression.Inhbb stimulates endothelial cell migration and formationof a coelomic vessel. In XX gonads with no Wnt4repression, Fst expression is increased and Inhbbexpression is down regulated to prevent endothelial cellmigration and formation of ovarian-specific vascularpatterns (Yao et al. 2004).AMHXYSrySox9FGF-9?InhbbActivin ABMPsTestis vascular patternsXXWnt4FstOvarian vascular patternsFig. 2. Genes demonstrated to be involved in sex-specific vascularpatterns through development of null mice or treatment of indifferentgonads in organ cultureÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag