Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 310–316 (2008); doi: 10.1111/j.1439-0531.2008.01179.xISSN 0936-6768A Proposed Role for VEGF Isoforms in Sex-Specific Vasculature Development in theGonadRC Bott, DT Clopton and AS CuppAnimal Science Department, University of Nebraska-Lincoln, Lincoln, NE, USAContentsMany scientists have expended efforts to determine whatregulates development of an indifferent gonad into either atestis or ovary. Expression of Sry and upregulation of Sox9 arefactors that initiate formation of the testis-specific pathway toallow for both sex-specific vasculature and seminiferous cordformation. Migration of mesonephric precursors of peritubularmyoid cells and endothelial cells into the differentiatingtestis is a critical step in formation of both of these structures.Furthermore, these events appear to be initiated downstreamfrom Sry expression. Sertoli cell secretion of growth factorsacts to attract these mesonephric cells. One hypothesis is that agrowth factor specific for these cell linages act in concert tocoordinate migration of both peritubular and endothelial cells.A second hypothesis is that several growth factors stimulatemigration and differentiation of mesonephric ‘stem-like’ cellsto result in migration and differentiation into several differentcell lineages. While the specific mechanism is unclear, severalgrowth factors have been implicated in the initiation ofmesonephric cell migration. This review will focus on theproposed mechanisms of a growth factor, Vascular EndothelialGrowth Factor, and how different angiogenic and inhibitoryisoforms from this single gene may aid in development oftestis-specific vascular development.IntroductionWhy study gonadal differentiation and sex-specificvascular development?Infertility affects 40–70 million couples; of thoseapproximately 50% of the infertility problems areattributed to male-related factors which include: lowsperm count, abnormal spermatogenesis, and reducedandrogen production (Skakkebaek 2004). In the past12 years, the incidence of male infertility cases hasincreased at an alarming rate resulting in the developmentof a new syndrome – testicular dysgenesis syndrome.The reasons for the increase in testicularabnormalities are not understood but may be a resultof genetic or environmental disruption of cell differentiationduring testis morphogenesis (Skakkebaek 2004).Formation of testicular cords, and sex-specific vasculature,are the two morphological hallmarks that distinguisha testis from an ovary. Testis development isinitiated by Sertoli cell differentiation and expression ofSry, causing mesonephric cell migration into the differentiatingtestis to form seminiferous cords. Removal ofthe mesonephros or blockage of mesonephric cellmigration prevents cord formation. Thus, the migrationof mesonephric derived cells, which include pre-endothelialand pre-peritubular cells, are critical to testisdevelopment.What is known about embryonic testis morphogenesis?In the mouse, the Sry gene is expressed in embryos at10.5 days post-coitus (dpc) and ceases after 12.5 dpc.Sry induces expression of genes which cause differentiationof the Sertoli cell lineage from precursorsomatic cells in the coelomic epithelium (Fig. 1) (Cuppand Skinner 2005). So even though Sry function isimportant, expression in the rodent is brief. The Sertolicell is the first cell of the testis to differentiate (Magreand Jost 1991). After differentiation, Sertoli cells beginto proliferate and simultaneously move into the gonadproper, forming aggregates with primordial germ cells.Proliferation of Sertoli cells increases the size of thetestis, and this proliferation appears to be solelydependent on the expression of Sry (Schmahl et al.2000). Without differentiation and proliferation of theSertoli cells, the indifferent gonad would not developinto a testis.After Sertoli cell differentiation, testicular cords andsex-specific vasculature develop to establish adult testismorphology. In the mouse, these events occur during11.5–12 dpc and are complete by 12.5 dpc while in therat these events occur between embryonic day 13.5–14(Martineau et al. 1997). Induction of cord formation isinitiated by cell migration from the adjacent mesonephrosinto the developing testis to surround theprimordial germ and Sertoli cell aggregates (Buehret al. 1993; Merchant-Larios et al. 1993). Mesonephriccell migration is the result of Sry regulated expressionof paracrine growth factors secreted by Sertoli cells(Martineau et al. 1997). Paracrine growth factors act aschemo-attractants inducing mesonephric cell migrationand cord formation (Ricci et al. 1999; Cupp et al. 2000,2002, 2003; Colvin et al. 2001). If mesonephric cellmigration is blocked, or if the mesonephros is removed,no cords will form. The population of cells that migratefrom the mesonephros is proposed to be pre-endothelialand ⁄ or pre-peritubular cells (Buehr et al. 1993; Merchant-Larioset al. 1993). Therefore, in addition tosurrounding the Sertoli-primordial germ cell aggregatesto form testis cords, the migrating mesonephric cellsmay also initiate the formation of vasculature withinthe developing testis. Platelet-derived growth factor(Uzumcu et al. 2002a) and VEGF (Bott et al. 2006)have been demonstrated to be necessary for cordformation; however, only VEGF has been demonstratedto be critical for both vascular development andcord formation.Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag