Intercalated clear cells or pale cells in the sinus node of canine hearts?An ultrastructural study

Two types of sinus nodal cells were responsible for the main differences in the literature concerning the ultrastructure of the sinuatrial node: the i ntercalated clear cells and pale cells. Canine hearts were arrested by (1) aortic cross clamping, (2) coronary perfusion with the cardioplegic solutio n St. Thomas, and (3) coronary perfusion with the cardioplegic solution HTK (Custodiol(R)). After fixation by immersion or perfusion the sinus node ti ssue was prepared for electron microscopy. Following cardioplegic arrest an d pel fusion fixation, three nodal cell types in the non-ischemic sinuatria l node were observed: typical nodal cells, transitional cells, and intercal ated clear cells. Less than 1% of the non-ischemic sinuatrial cells were in tercalated clear cells, surrounded by typical nodal cells or transitional c ells. The contractile apparatus of the intercalated clear cells was extreme ly poorly developed. Great structural variations in the mitochondria were o bserved in intercalated clear cells, variations that would not appear under conditions of ischemia. In contrast, after 15-25 min of ischemia at 25 deg rees C the appearance of the sinus nodal cells was strikingly different fro m that of the nonischemic sinuatrial cells. More than 10% of the nodal cell s showed typical ischemic alterations, e.g., mitochondrial swelling, clumpi ng of nuclear chromatin, loss of glycogen particles, and cell swelling in v arying degrees. Because they look very pale, these nodal cells have been de scribed as pale cells in the literature. Intercalated clear cells appear ma inly in non-ischemic nodal tissue. Pale cells are ischemically damaged sinu s nodal cells. Anat nec 260:33-41, 2000. (C) 2000 Wiley-Liss, Inc.