Lysosomal activity in the aging rat liver: II. Morphometry of acid phosphatase positive dense bodies

Hepatocytes from 3-, 12- and 36-month-old rats were studied in situ with cytochemical and morphometrical techniques to obtain information about the intralobular and age-dependent variation in lysosomal volume. An increase with age was found for the lysosomal volume density. This increase is evident throughout the lobule, but occurs at different rates in the centrolobular and peripheral areas. It is most pronounced in the centrolobular area, resulting in a lysosomal volume density of 7.1% in old rats, compared to 3.6% in the peripheral area of the liver lobule. Both the size and the number of individual lysosomes contribute to the overall effect. Most dense bodies, including lipofuscin-containing aggregates in senescent animals, exhibit acid phosphatase activity. Our results suggest that the use of enzyme activity as a marker may yield higher values for lysosomal volume density than morphologic characteristics alone. The alleged homogeneity of the liver as a source of hepatocytes for studies on isolated cells must be questioned when lysosomal functions are concerned.