Regenerated Microvascular Networks in Ischemic Skeletal Muscle

Abstract

Skeletal muscle is the largest organ in humans. The viability and performance of this metabolically demanding organ are exquisitely dependent on the integrity of its microcirculation. The architectural and functional attributes of the skeletal muscle microvasculature are acquired during embryonic and early postnatal development. However, peripheral vascular disease in the adult can damage the distal microvasculature, together with damaging the skeletal myofibers. Importantly, adult skeletal muscle has the capacity to regenerate. Understanding the extent to which the microvascular network also reforms, and acquires structural and functional competence, will thus be critical to regenerative medicine efforts for those with peripheral artery disease (PAD). Herein, we discuss recent advances in studying the regenerating microvasculature in the mouse hindlimb following severe ischemic injury. We highlight new insights arising from real-time imaging of the microcirculation. This includes identifying otherwise hidden flaws in both network microarchitecture and function, deficiencies that could underlie the progressive nature of PAD and its refractoriness to therapy. Recognizing and overcoming these vulnerabilities in regenerative angiogenesis will be important for advancing treatment options for PAD.

Keywords: angiogenesis; intravital microscopy; peripheral artery disease; skeletal muscle; smooth muscle cell.

Figure 1

Abnormalities in the regenerated microvascular network in the mouse subjected to ischemic injury. (Top) Intravital microscopy-derived RBC transit maps of the extensor digitorum longus muscle in a C57BL/6 mouse. Maps depict all flow-receiving surface (50 μm deep) vessels over a 15 s period. Custom look-up tables were applied to grayscale image maps for enhanced contrast. A native network is shown in the left panel. Capillaries are running parallel to the skeletal myofibers with ordered anastomoses between parallel capillary segments. A regenerated network 28 days after ischemic injury is shown in the right panel, showing increased and chaotic vascularity. Bar, 100 μm (Bottom) Schematics of native and regenerated (day 28) microcirculatory networks. In the regenerated network the distal arteriole trifurcates. Moreover, one of the three limbs drains directly into a venule rather than branching into a capillary meshwork [See Arpino et al. (2017) for RBC transit maps and videos demonstrating these phenomena].

Figure 2

Conceptual paradigm for regenerative angiogenesis in the mouse hindlimb subjected to ischemic injury. Microvascular regeneration proceeds robustly following severe ischemic injury to skeletal muscle. However, there are structural and functional deficiencies that must be overcome if optimal tissue perfusion is to be restored. TA, terminal arteriole.