The cells which will form the smooth muscle tunica media are derived embryologically from the cardiac mesenchyme referred to as endocardial cushion tissue. These progenitor smooth muscle cells, however, are derived primarily from aortic arch mesenchyme as opposed to the endocardium in the prevalvular areas. The matrical microenvironments of these cells begin to change at approximately 5 days of development when the progenitor smooth muscle cells acquire a more fibrillar matrix. The prospective adventitia and valvular areas still maintain an environment rich in hyaluronate and proteoglycans. By Day 6, the 110Ao microfibril, characteristic of smooth muscle cells, appears in the matrix well in advance of the amorphous elastin component seen at Days 8 and 9. The orientation of the collagenous microfibrils and elastic microfibrils is non-random with respect to the layers of cells, and this precludes a simple substrate alignment model in establishing the characteristic laminarity of this tissue.