*Article* **Collateral Vessels Have Unique Endothelial and Smooth Muscle Cell Phenotypes**

### **Hua Zhang, Dan Chalothorn and James E Faber \***

Department of Cell Biology and Physiology, Curriculum in Neuroscience, McAllister Heart Institute, University of North Carolina, Chapel Hill, NC 27599-7545, USA

**\*** Correspondence: jefaber@med.unc.edu; Tel.: +919-966-0327; Fax: +919-966-6927

Received: 29 May 2019; Accepted: 19 July 2019; Published: 24 July 2019

**Abstract:** Collaterals are unique blood vessels present in the microcirculation of most tissues that, by cross-connecting a small fraction of the outer branches of adjacent arterial trees, provide alternate routes of perfusion. However, collaterals are especially susceptible to rarefaction caused by aging, other vascular risk factors, and mouse models of Alzheimer's disease—a vulnerability attributed to the disturbed hemodynamic environment in the watershed regions where they reside. We examined the hypothesis that endothelial and smooth muscle cells (ECs and SMCs, respectively) of collaterals have specializations, distinct from those of similarly-sized nearby distal-most arterioles (DMAs) that maintain collateral integrity despite their continuous exposure to low and oscillatory/disturbed shear stress, high wall stress, and low blood oxygen. Examination of mouse brain revealed the following: Unlike the pro-inflammatory cobble-stoned morphology of ECs exposed to low/oscillatory shear stress elsewhere in the vasculature, collateral ECs are aligned with the vessel axis. Primary cilia, which sense shear stress, are present, unexpectedly, on ECs of collaterals and DMAs but are less abundant on collaterals. Unlike DMAs, collaterals are continuously invested with SMCs, have increased expression of *Pycard, Ki67, Pdgfb, Angpt2, Dll4, Ephrinb2*, and eNOS, and maintain expression of *Klf2*/*4*. Collaterals lack tortuosity when first formed during development, but tortuosity becomes evident within days after birth, progresses through middle age, and then declines—results consistent with the concept that collateral wall cells have a higher turnover rate than DMAs that favors proliferative senescence and collateral rarefaction. In conclusion, endothelial and SMCs of collaterals have morphologic and functional differences from those of nearby similarly sized arterioles. Future studies are required to determine if they represent specializations that counterbalance the disturbed hemodynamic, pro-inflammatory, and pro-proliferative environment in which collaterals reside and thus mitigate their risk factor-induced rarefaction.

**Keywords:** collateral circulation; cerebral collaterals; endothelial cells; ischemic stroke; primary cilia
