Nematolysosomes (Elongate Lysosomes) in Rat Hepatocytes: Their Distribution, Microtubule Dependence, and Role in Endocytic Transport Pathway

Abstract

The distribution of lysosomes in rat hepatocytes was examined by three-dimensional electron microscopy combined with acid phosphatase (ACPase) cytochemistry. In the 2-μm-thick sections observed under 200- or 1000-kV TEM, it was apparent that ACPase activity localized on elongate lysosomes (we refer to them as nematolysosomes) with a diameter of 70-100 nm and lengths of several micrometers, as well as spherical lysosomes and trans-Golgi cisternae. Though most spherical lysosomes were located within the pericanalicular region, nematolysosomes were widely distributed throughout the hepatocytes. Typically, it appeared that the nematolysosomes elongated from the subsinusoidal region to the pericanalicular-Golgi complex area and they frequently formed a network at the cell periphery along the sinusoidal front. Furthermore, the formation of nematolysosomes was independent of new protein synthesis, but highly dependent on the integrity of microtubules. After a 6-8 h colchicine treatment, nematolysosomes were shrunk and/or fragmented, becoming roughly spherical lysosomes scattered throughout the cells. Nematolysosomes recovered their normal profiles after 24 h due to the reversible effect of the drug on microtubules. When the hepatocytes were exposed to horseradish peroxidase (HRP) in vitro or in vivo, HRP was quickly sequestered in nematolysosome-like structures via pinocytosis from the sinusoidal surface and transported to an area near the Golgi complex. These findings raise the possibility that the nematolysosomes engage in microtubule-dependent transport of macromolecules from the sinusoidal circulation to the Golgi complex area.