The amphibian Kupffer cells build and demolish melanosomes: an ultrastructural point of view.

This ultrastructural research was carried out to investigate the nature of the liver pigment cells of anuran and caudate amphibians, the pattern of melanosome ontogenesis, and the demolition processes of old melanosomes. We demonstrate that these liver pigment cells are able to internalize zymosan particles and latex beads; therefore, being professional phagocytes, they, as liver resident macrophages, can be classified as Kupffer cells (KCs). They show "melanosomogenesis centers" in which several maturation stages of premelanosomes are visible; the premelanosomes are formed by two principal components: a filamentous structure that will constitute the "inner" area of the melanosome and a vesicular component, budding from the Trans Golgi Network and that carries enzymes, which will constitute the "cortical area" of the melanosome. Thus the KCs, thanks to the presence of the "melanosomogenesis centers," are also melanosome producing cells. They are also able to demolish melanosomes by heterophagocytosis and, probably, also by autophagocytosis. In conclusion, we propose a classification of vertebrate pigment cells.