Pial collaterals develop through mosaic colonization of capillaries by arterial and microvascular endothelial cells

Abstract

Collaterals are unique blood vessels present in many healthy tissues that cross-connect distal-end arterioles of adjacent arterial trees, thus providing alternate routes of perfusion. Stroke patients with superior pial collateral flow respond better to treatments and present with an overall improved prognostic outcome. However, how pial collaterals develop in the embryo and how they reactivate upon stroke remains unclear. Here, using lineage tracing in combination with three-dimensional imaging, we demonstrate that mouse embryos employ a novel mechanism to build pial collaterals, distinct from their outward remodeling following stroke. Endothelial cells (ECs) of arterial and microvascular origin invade already existing pre-collateral vascular structures in a process which we termed mosaic colonization. Arterialization of these pre-collateral vascular segments happens concurrently with mosaic colonization. Despite having a smaller proliferative capacity, embryonic arterial cells represent the majority of cells that migrate to form nascent collaterals; embryonic microvascular cells, despite their higher proliferative potential, form only about a quarter of collateral endothelial cells. Moreover, postnatal collateral growth relies much more on self-replenishment of arterial cells than on microvascular contribution. Following ischemic injury, pial collateral outward remodeling relies on local cell proliferation rather than recruitment of non-arterial cells. Together, these findings establish distinct cellular mechanisms underlying pial collateral development and ischemic remodeling, raising the prospect for future research to identify novel, collateral-specific therapeutic strategies for ischemic stroke.

Development of Pial Collaterals by Extension of Pre-existing Artery Tips

“…We demonstrate the formation of collateral arteries from pre-existing arterial lineage as observed 9,10,13,22 or proposed 6 earlier. While this manuscript was in progress, Perovic et al reported that two separate vascular lineages populate pial collaterals.…”

“…While this manuscript was in progress, Perovic et al reported that two separate vascular lineages populate pial collaterals. 10 We also observed that well-formed pial collaterals are majorly populated by artery cells, and minimally by capillaries. Intra-vital imaging of embryonic brains reveal that pre-existing artery cells extend along defined microvascular paths to connect and build pial collaterals.…”

“…To test this hypothesis, Cx40CreER; Rosa26 TdTomato ; Tie2:: eGFP::Claudin5 mice were used to label artery ECs with TdTomato and all endothelial cells with eGFP. To activate TdTomato, Tamoxifen was administered at e13.5, when both MCA and ACA are present and would be labelled in 2 days, 9,10 by e15.5. Imaging of endothelial cells derived from arteries was performed at e16-e17.…”

“…We propose that as artery tips extend and get connected in the watershed, microvascular cells are “trapped” within pial collaterals. This could generate a mixed lineage distribution 10 within pial collaterals—both in embryos and adults. Together, multiple mechanisms could be actively building pial vasculature during embryogenesis.…”