Resident sinusoidal macrophages in the liver of the brown bullhead (Ictalurus nebulosus): An ultrastructural, functional and cytochemical study

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Abstract
Ultrastructural, functional, and cytochemical characteristics of resident sinusoidal macrophages (RSM) in brown bullhead (Ictalurus nebulosus) liver were examined. Following perfusion fixation of the hepatic vascular bed, light micrographs revealed RSM that possessed multiple elongate cytoplasmic processes and frequently contained erythrocytes in various stages of degradation. Following brief perfusion fixation, light microscope examination of vibratome sections of bullhead liver reacted for peroxidase revealed intensely positive RSM. By transmission electron microscopy, peroxidase activity was localized to the nuclear envelope and cytoplasmic granules of RSM and in endothelial and perisinusoidal fat‐storing cells. In cryostat sections of fresh‐frozen liver, glucose‐6‐phosphate dehydrogenase (G‐6‐PDH) was uniformly distributed over hepatocytes, whereas intensely positive punctate staining for G‐6‐PDH was localized over RSM. To test for phagocytosis by RSM, latex beads (0.81 μ) were injected into a tributary of the hepatic portal vein 2 min prior to perfusion fixation. Latex beads appeared either singly or in dense aggregates within RSM. Ultrastructurally, RSM were characterized by an irregularly shaped, eccentrically located nucleus, electron‐dense vacuoles, small patches of granular endoplasmic reticulum, a well‐developed Golgi apparatus, elongated mitochondria, desmosomes or desmosome‐like densities that served as a source of attachment to endothelial cells, and a centriole with radiating microtubules. Invaginations of the plasma membrane (vermiform processes) characteristic of mammalian Kupffer cells were not observed in bullhead RSM. The results indicated a resident cell population of sinusoidal macrophages in the bullhead liver with properties that protially resembled mammalian Kupffer cells. These results are important for the identification of the normal resident cells in the bullhead liver.