QUANTITATIVE HISTOLOGICAL ANALYSIS OF THE HUMAN SINOATRIAL NODE DURING GROWTH AND AGING

Background. Fibrosis or fatty infiltration of the human sinoatrial (SA) node is generally believed to represent replacement of the SA nodal cells by connective tissue. Quantitative analysis, however, has not been performed precisely to validate the interpretation of such histological changes. Methods and Results. The actual volume of the SA node and its components were calculated according to the sum of the pixel number representing the colors of SA nodal cells and connective tissue in serial sections using a digital color image analyzer. Average volume occupied by the total SA nodal cells in adolescents and adults (n=7) was 3.55+/-0.45 mm3, which was 2.4 times greater than that in infants (n=3). The rate of increase was smaller than that of the total SA node (4.2 times, 16.68+/-2.56 mm3 in adolescents and adults). The considerable discrepancy in the growth ratio between the SA nodal cells and the total SA node resulted from an increase in the volume of connective tissue (7.4 times). In the elderly (n=9), the volume of total SA node and SA nodal cells actually decreased (13.10+/-1.85 mm3 and 2.18+/-0.44 mm3), whereas that of fibrous connective tissue remained unchanged. Constant DNA ploidy patterns of SA nodal cells determined by cytofluorometry indicated that SA nodal cells never synthesize DNA during growth. Conclusions. Until adulthood, the actual volume of SA nodal cells does not decrease, although the increase in volume ratio of the interstitial tissue to the total SA node has merely given a false impression of involution of SA nodal cells. Atrophy of SA nodal cells, however, occurs during aging together with reduction of the SA node and/or infiltration of fatty tissue.