Four-dimensional analysis of vascularization during primary development of an organ, the gonad

Abstract

Time-lapse microscopy has advanced our understanding of yolk sac and early embryonic vascularization. However, it has been difficult to assess endothelial interactions during epithelial morphogenesis of internal organs. To address this issue we have developed the first time-lapse system to study vascularization of a mammalian organ in four dimensions. We show that vascularization of XX and XY gonads is a highly dynamic, sexually dimorphic process. The XX gonad recruits vasculature by a typical angiogenic process. In contrast, the XY gonad recruits and patterns vasculature by a novel remodeling mechanism beginning with breakdown of an existing mesonephric vessel. Subsequently, in XY organs individual endothelial cells migrate and reaggregate in the coelomic domain to form the major testicular artery. Migrating endothelial cells respect domain boundaries well before they are morphologically evident, subdividing the gonad into 10 avascular regions where testis cords form. This model of vascular development in an internal organ has a direct impact on the current dogma of vascular integration during organ development and presents important parallels with mechanisms of tumor vascularization.organogenesis ͉ ovary ͉ testis I nsights into vascular development and patterning in internal organs have historically relied on xenograft models and static analysis (1-7). However, this approach does not elucidate dynamic interactions between endothelial cells and other cells of developing organs. Advances in time-lapse imaging have improved understanding of vasculogenesis, flow-induced vascular remodeling, the genetic programming of angiogenesis, and many other facets of vascular development in the mouse and chick yolk sac and during establishment of the body plan in zebrafish (8-17). However, culturing internal organs throughout critical and prolonged periods of development has been difficult. The technical challenges of organ culture, compounded by simultaneous live imaging, is a major hurdle in understanding how endothelial cells remodel and integrate in an internal organ undergoing morphogenesis.For more than a decade the urogenital ridge (UGR) explant model has provided a unique system to analyze morphogenesis of an internal organ (18,19). The fact that the UGR can be successfully explanted to culture at the critical sex-determination stage [11.5 days postcoitum (dpc)], using conditions that maintain the normal morphological structure of the gonad, has been the driving force behind its broad adoption in the sexdetermination field. However, with respect to internal organ vascularization, the UGR has received little attention as a model system. In the UGR explant model the gonad retains contact with the mesonephros, the source of the endothelium, and vascularization ex vivo occurs similarly to vascularization in vivo (20). During the period of vascularization, development of the testis and ovary diverge morphologically. Whereas the XX gonad shows few morphological changes during this period, the XY gonad undergoes dramatic epi...

Uncovering the behaviors of individual cells within a multicellular microvascular community

“…Our method for tracking the behaviors of individual cells during tissue morphogenesis was useful both at the population level, where the dynamics of the collective population could be precisely quantified, as well as at the single-cell level, which highlighted the differences in behavior among individual cells. In our initial application of this method to studying the mechanism of microvasculogenesis, this method has provided, at the population level, additional granularity about how a system of endothelial cells form microvessels (33). For example, our observed sequence of events agrees with previous studies in which endothelial cells exhibited significant displacement before starting to elongate (14,15); in an in vivo study, endothelial cells also migrated into distinct spatial regions before undergoing morphological changes and vascular development (33).…”

“…In our initial application of this method to studying the mechanism of microvasculogenesis, this method has provided, at the population level, additional granularity about how a system of endothelial cells form microvessels (33). For example, our observed sequence of events agrees with previous studies in which endothelial cells exhibited significant displacement before starting to elongate (14,15); in an in vivo study, endothelial cells also migrated into distinct spatial regions before undergoing morphological changes and vascular development (33). Our analysis is also consistent with studies that identified a period in which the vascular plexus underwent a slight deformation after formation (14).…”

Yolk-sac–derived macrophages regulate fetal testis vascularization and morphogenesis

“…S6B), an Angiopoietin receptor expressed in macrophages associated with tumor neovascularization (36-38); and Tie2-Cre (Fig. S6C), as we previously reported using a Cre-responsive GFP reporter strain (39). Tie2-Cre is not expressed in hematopoietic cells of the yolk sac (40), suggesting that the expression of Tie2-Cre is not merely the result of the labeling of early hematopoietic progenitors but reflects de novo Tie2 expression within differentiated macrophages.…”

“…The vasculature plays an instructive role in testis morphogenesis (5)(6)(7)(8)39). The first step of this process, the breakdown of the mesonephric vascular plexus, is dependent on gonadal-mesonephric macrophages.…”

“…Kdr-mCherry + material contained within macrophages colocalized with CD68 (also called "macrosialin"), a marker of macrophage-specific lysosomes and endosomes of the phagocytic pathway (43,44) (Fig. 5H), suggesting that macrophages may physically break down the vascular plexus to free individual endothelial cells, which then can migrate from the mesonephros into the gonad (39).…”