Evidence for Golgi Origin of Melanosomes

Abstract

Electron microscopy has provided evidence that the melanosome evolves as a membrane bound structure with a highly complex internal architecture. The premelanosomes are found in close proximity to the golgi apparatus. Therefore, it was generally agreed that the melanosomes originate from the golgi apparatus.Vesicles have been described to pinch off the cysternae of the golgi apparatus. The vesicles would then grow and acquire a dense material. This material is aggregating to form the characteristic helical strands onto which melanin is deposited. Cloned human malignant melanoma lines were used to reinvestigate the problem of melanosome formation. The reconstruction of serial sections revealed the arrangement of premelanosomes and melanosomes in relation to the golgi complex. This study demonstrated that premelanosomes and melanosomes are continuous with the golgi complex by a smooth-surfaced tubular endoplasmic reticulum (SER) (Fig. la-d). The continuity of membranes of the SER and the premelanosome is depicted in Fig. 2. In this early premelanosome the protein strands have not yet coiled up into a helix. Rough-surfaced endoplasmic reticulum (RER) was also observed to be continuous with the golgi apparatus and melanosomes. After melanogenesis has started (Fig. 3) small vesicles appear inside the premelanosomes.