Segmental territories along the cardinal veins generate lymph sacs via a ballooning mechanism during embryonic lymphangiogenesis in mice

Abstract

During lymphangiogenesis in the mammalian embryo, a subset of vascular endothelial cells in the cardinal veins is reprogrammed to adopt a lymphatic endothelial fate. The prevailing model of lymphangiogenesis contends that these lymphatic precursor cells migrate away from the cardinal veins and reassemble peripherally as lymph sacs from which a lymphatic vasculature is generated. However, this model fails to account for a number of observations that, as a result, have remained anecdotal. Here, we use optical projection tomography, confocal microscopy and in vivo live imaging to uncover three key stages of lymphatic vascular morphogenesis in the mouse embryo at high resolution. First, we define territories or "pre-lymphatic clusters" of Prox1-positive lymphatic endothelial progenitor cells along the antero-posterior axis of the cardinal veins. Second, these pre-lymphatic clusters undergo progressive extrusion ("ballooning") to generate primitive lymph sacs. Third, lymphatic vessels emerge by a combination of mechanisms including sprouting from the lymph sacs and direct delamination of streams of cells from the cardinal veins. Our data support a new model for lymphatic vascular patterning and morphogenesis, as a basis for identifying the molecular cues governing these processes.

Lymphatic endothelial progenitors bud from the cardinal vein and intersomitic vessels in mammalian embryos

“…Podoplanin expression cannot be detected in the Prox1-expressing LECs outside of the CV before E10.5 (data not shown). In fact, in agreement with previous reports, 18 For personal use only. on May 8, 2018.…”

“…7,8 Our results showing that the ISVs provide an additional source of LEC progenitors help explain the large and fast increase in the number of Prox1-expressing LECs migrating along the mesenchymal tissue leading to the formation of the primitive lymph sacs. Recently, a ballooning process was proposed to explain the process of lymph sac formation 18 ; however, our detailed analysis failed to visualize these type of structures. Our current data add important information to this process by showing that the ISVs are another important source of LEC progenitors and that Prox1 activity is required for LECs to bud from the CV.…”

Getting out and about: the emergence and morphogenesis of the vertebrate lymphatic vasculature

“…PROX1-positive LEC progenitors exit the walls of the veins at multiple sites throughout the embryo and migrate away (commencing ~E10.0-E11.5) in a dorsolateral direction to form lymph sacs and superficial lymphatic vessels ( Fig. 1A) (Wigle and Oliver, 1999;Oliver, 2004;François et al, 2012;Yang et al, 2012;Hägerling et al, 2013). This event was originally proposed in the early nineteenth century by Florence Sabin (Sabin, 1902) and recently validated by lineage-tracing experiments in mice (Srinivasan et al, 2007).…”

“…aISV, arterial intersomitic vessel; IL, intestinal lymphatic vessels; LL, lateral lymphatic vessels; PCV, posterior cardinal vein; PL, parachordal lymphangioblast; VS, venous sprout. REVIEW Development 140 (9) progenitors migrating away from the anterior region (van der Putte, 1975;Wigle and Oliver, 1999;Karkkainen et al, 2004;François et al, 2012). Ultimately, the mouse embryo develops its lymphatic vasculature progressively from anterior to posterior (van der Putte, 1975;Wigle and Oliver, 1999;Yang et al, 2012;Hägerling et al, 2013).…”

“…Three recent studies employing high resolution whole-mount confocal microscopy techniques have described the migratory paths taken by LECs during lymphatic vascular development in the mouse embryo and have also uncovered previously unappreciated morphogenetic events during the exit of LEC progenitors from the veins (François et al, 2012;Yang et al, 2012;Hägerling et al, 2013). The first study by François and colleagues suggested that within the wall of the CV, PROX1-positive LEC progenitor cells assemble in defined territories along the anteroposterior axis of the veins, termed pre-lymphatic clusters (PLCs).…”