Late coronary development, as seen in 26–30 week fetuses. A, a major development is the deposition of elastic fibers in the major coronary arteries (CA). B, a second characteristic is the accumulation of collagen to the adventitia (Adv) of the major coronary arteries; thus the tunica adventitial becomes thicker than the media. Ao, aorta; RCA & LCA, right & left coronary arteries; Epi, epicardium; Adv, adventitia. A stained for elastin, B stained with Masson trichrome. Magnification bar = 100 μm.