Ultrastructure and function of hepatic fat-storing and pit cells.

The present paper reviews the literature on the ultrastructure and function of sinusoidal fat-storing cells and pit cells in the mammalian liver. Ultrastructurally, fat-storing cells are characterized by the presence of cytoplasmic fat droplets, well developed rough endoplasmic reticulum; a Golgi complex; multivesicular bodies; one or two centrioles; and few, rather small, lysosomes. These lysosomes are sometimes associated with fat droplets. Fat-storing cells may bear a cilium and project characteristic cytoplasmic processes into the space of Disse. These processes contain microtubules and filaments. Fat-storing cells are the main storage site of retinol esters in the mammalian body. Moreover, these cells have the potential of synthesizing several connective tissue components including the collagens type I, III, and IV; fibronectin; laminin; heparan sulfate; chondroitin sulfate; and dermatan sulfate. Pit cells are polarized cells, with most organelles localized at one site of the nucleus near the cytocentre. They are characterized electron microscopically by the presence of dense cytoplasmic granules with a specific ultrastructure, by rod-cored vesicles, and by multivesicular bodies. It has recently been shown that pit cells have natural killer activity to certain tumor cells and have many features in common with large granular lymphocytes. They therefore may act in the liver as a first line of defense against neoplasia, metastasis, and viral infections.