Fine structural examination of the single heart tube in the fifteen day ferret embryo.

The different segments of the embryonic heart tube of the ferret were examined with light and transmission electron microscopy. The cells of bulbus cordis, bulboventricular junction, primitive ventricle, atrioventricular junction, and primitive atria were in the process of differentiating into myocardial cells. The ventricular muscle cells were the most developed cells; the least mature muscle cells were those located at the arterial and venous ends of the heart tube. The cells between the ventricle and the two ends of the heart tube showed a spectrum of developmental stages, especially with respect to the morphological development of the myofibrils. Other organelles as well as surface specializations did not permit a distinction between cells of the different regions of the heart tube. There was no striking difference in the size and shape of the developing muscle cells of the atrioventricular and bulboventricular junctions compared to the developing ventricular muscle cells. Morphologically there was no evidence to suggest that the tissue of the atrioventricular and bulboventricular rings was specialized or different from any of the other segments of the single heart tube of the embryonic ferret.