Vascular smooth muscle cell contractile proteins as universal markers of vessels of microcirculatory bed

Abstract

Highlights. The use of vascular smooth muscle cell markers, e.g. smooth muscle myosin heavy chain (SM-MHC) and alpha smooth muscle actin (α-SMA) for immunodetection of adventitial and perivascular microvessels (vasa vasorum) is preferrable over endothelial markers (CD31 and VE-cadherin) as it allows to define vascular geometry regardless of sectioning artifacts and provides ideal signal-to-noise ratio.Aside from elastic laminae which discriminate arterioles from venules and capillaries, we were unable to confirm any specific markers of arterial, venous, and capillary differentiation, although KLF2 and PROX1 transcription factors indicated venous specification and HEY1 suggested capillary identity in rat aortas.Aim. To develop an optimal approach to detection of microvessels and to evaluate the techniques for the differential immunostaining of arterioles, venules, and capillaries in human saphenous veins and rat aortas.Methods. Saphenous veins excised during the coronary artery bypass graft surgery were used for the study. Serial cryosections were analyzed by means of haematoxylin and eosin and Russell-Movat’s pentachrome stainings and by immunofluorescent staining for endothelial cell markers (CD31 and VE-cadherin), vascular smooth muscle cell markers (SM-MHC and α-SMA), mechanosensitive transcription factors (KLF2 and KLF4), transcription factors of arterial specification (HES1, HEY1, ERG), transcription factors and markers of venous identity (NR2F2, NRP2), and transcription factors and markers of lymphatic lineage (PROX1, LYVE1, VEGFR3). Samples were visualized by light and confocal microscopy.Results. In comparison with endothelial cell markers (CD31 and VE-cadherin), vascular smooth muscle cell markers (SM-MHC and α-SMA) permitted objective evaluation of vascular geometry and maximized signal-to-noise ratio irrespective of specific marker, microvessel specification or antibody used. Autofluorescence and specific histological  pattern  of  elastic  membranes  at  Russell-Movat’s  pentachrome staining allowed to discriminate arterioles from venules and capillaries. Albeit immunostaining of rat aortas found specific markers of venous endothelial cells (KLF2 and PROX1) and capillary endothelial cells (HEY1), these findings have not been confirmed in saphenous veins. We were unable to find specific markers of human venules and capillaries among the saphenous vein vasa vasorum despite an extensive screening of multiple markers.Conclusion. Immunodetection of microvessels (e.g., vasa vasorum) should be performed by using vascular smooth muscle cell markers (SM-MHC and α-SMA) rather than endothelial cell markers (CD31 and VE-cadherin). Lack of specific markers to discern microvessels of different lineages suggests Russell-Movat’s pentachrome staining as an optimal option for the machine learning of neural networks to analyse the microvessels including vasa vasorum.