The ultrastructure of organising experimental mural thrombi in the pig aorta.

This ultrastructural study of experimentally produced mural thrombi in the pig aorta has shown that at 3 days there are, amongst the other constituents, two distinct cell populations, macrophages and undifferentiated mesenchymal cells. Over the subsequent 2 wk the mesenchymal cells, which appear to be homogeneous initially throughout the thrombus, gradually differentiate on the surface to form an endothelial lining, in the superficial layers to form myointimal cells and ultimately smooth muscle and, in the deeper layers, to form new capillaries which join with endothelial lined clefts from the surface. The morphological evidence supports the postulate that the early stage of organisation of a mural thrombus in effected by pluripotent precursor cells from the circulating blood. The new endothelium, myointimal cells, capillaries, intimal smooth muscle and the extracellular tissues characteristic of an organising thrombus all appear to derive from this common source. In this study we saw no evidence to suggest that, at this stage, there was a contribution from the cells native to the vessel wall which, however, may be involved in the later stages of organisation.