The effects of long-term hypoxia on epicardium and myocardium in developing chick embryo hearts

The consequences of long-term O2 deprivation on heart development were analyzed morphometrically and ultrastructurally, utilizing the hearts of chicken embryos developed under hypoxia from the 3rd to the 18th incubation day. The results indicate that embryos kept under low O2 blood tension do not show disturbances in heart morphohistogenesis, but are characterized by a thicker epicardium and a thinner myocardium than the controls; moreover, both the number and calibre of the heart microvessels are increased. The thickening of the epicardium is due to hyperplasia of the mesothelial cells, increment in calibre of the submesothelial vessels, and to conspicuous perivascular infiltration of blood-derived cells. The thinning of the cardiac muscle seems to be dependent on myocardiocyte hypotrophy and myofibril reduction. The increase in the volume density of myocardium vessels, due to their dilatation and proliferation, may be considered expression of a vascular adaptive reaction to low oxygen tissue concentration.