Cartilage and bone formation in arterial wall. 2. Ultrastructural patterns.

The ultrastructural aspects of cartilaginous and osseous foci developed in aorta of rabbits immunized against rat aorta homogenates was studied. Besides normal and modified smooth muscle cells, various types of transformed mediacytes were observed in and around these foci: some of them resembled connective and young mesenchymatous cells, others had the appearances of cartilaginous and osseous cells. The possible role of the modified (multipotential) smooth muscle cells in aortic chondro- and osteogenesis is considered in some cases. Following histogenetic pathway is suggested: s.m.c. leads to modified s.m.c. lead to young mesenchyme-like cells ("inducible osteogenic precursor cells") leads to "chondro-mediacytes", "osteomediacyte".