TUBULAR LYSOSOMES IN EXOCRINE ACINAR-CELLS - A REVIEW

Recently, attention has been focused on a distinct type of secondary lysosome, the tubular lysosome. These lysosomes have been reported in a number of different cell types. In most instances, the tubular lysosomes have been identified as a distinct population of lysosomes solely on the basis of their morphology. Thus, the question remains whether or not the tubular lysosomes are a separate population. In order to address this question, we have examined the tubular lysosomes in exocrine acinar cells. These cells are highly polarized, with the tubular lysosomes being concentrated near the basal surface of the cells and the more typical secondary lysosomes located near the Golgi apparatus. The tubular lysosomes often form an anastomosing network, are intercalated between cisternae of rough endoplasmic reticulum and are frequently associated with mitochondria and microtubules. Cytochemically, these lysosomes are unique. In contrast to the typical secondary lysosomes, they appear to lack acid phosphatase, but they do react for trimetaphosphatase, aryl sulfatase B, and nonspecific esterase. The tubular lysosomes in the exocrine acinar cells can also be distinguished on the basis of their participation in the endocytic process. When exocrine acinar cells are exposed in vivo to intravenously injected horseradish peroxidase, the tracer is sequestered in the tubular lysosomes within 5-10 min after injection, while the peroxidase does not appear in the secondary lysosomes for 2-3 hr. In addition, the tubular lysosomes can be separated from the secondary lysosomes on Percoll density gradients. The presence of the tubular lysosomes is also related to the state of differentiation of the acinar cells. In undifferentiated AR42J cells, a pancreatic acinar cell line, tubular lysosomes are extremely rare, but when the cells are stimulated to differentiate with dexamethasone, tubular lysosomes are seen adjacent to the plasma membrane. These results confirm the fact that, at least in exocrine acinar cells, the tubular lysosomes are distinct from the typical spherical secondary lysosomes. © 1990, JAPAN SOCIETY OF HISTOCHEMISTRY AND CYTOCHEMISTRY. All rights reserved.