ORGAN CULTURE PROVIDES CLUES FOR THE EXISTENCE OF A DOUBLE STEM CELL NICHE IN THE HUMAN ARTERY WALL.

Artery wall is a recipient and source of Mesenchymal Stem Cells (MSCs). These cells, which were first identified in the bone marrow stroma, are considerably plastic and can differentiate along multiple lineages and give rise to cartilage, bone, fat, muscle and vascular tissue. In human adult vascular wall, a “vasculogenic zone” was identified between the smooth muscle and the adventitial layers; in this zone a complete hierarchy of resident stem cells most likely exist serving as a local reservoir of cells for postnatal vasculogenesis1. Recently our group described the presence of resident MSCs (VW-MSCs) in normal human adult thoracic aortas from healthy and heart beating donors with angiogenic ability2 whilst others found multipotent vascular resident MSCs in human internal mammarial artery1, pulmonary artery3 and saphena vein4. In this study we have undertaken a three dimensional ex-vivo culture study to establish the MSC role in the spontaneous vascular wall remodeling without VEGF support. Human fresh femoral artery was collected from a heart-beating donor; after 72 hrs in an antibiotic mixture, the sample was processed for light (LM) and transmission electron microscopy (TEM); the remaining tissue was cut in 3 x 3 mm slices and cultured for up to 70 days in medium containing 5% of FBS. The segments were processed for LM and TEM weekly. After 42 days, ultrastructural analysis showed the presence of several undifferentiated cells with a high N/C ratio and prominent nucleoli; LM revealed that spindle–shaped cells migrated toward the inner and outer surfaces of the arterial wall. Surprisingly we observed that the adventitia was lined with a continuous layer of spindle cells positive to mesenchymal markers and, at the same time, the intimal side was covered with CD31+ CD34+ spindle cells; the remodeling became more evident from the 49th day of culture. Noteworthy, this phenomenon occurred in vitro without specific growth factor stimuli and without any contribute of hematopoietic cells. Indeed, this study demonstrates that human vascular conduits natively contain a resident cell population particularly resistant to mechanical and chemical stresses; this population in our experimental condition origins two morphologically similar but phenotipically different pseudo-intimal layers: one with mesenchymal phenotype at the external border, and the other one with endothelial phenotype at the internal one. We cannot exclude the presence of two different niches of vascular wall resident stem cells both in adventitial and in intimal side which differently contribute to vascular remodeling.