Lavender, a chick melanocyte mutant with defective melanosome translocation: a possible role for 10 nm filaments and microfilaments but not microtubules.

Lavender is a mutation of chick neural-crest-derived melanocytes showing dilute feather pigmentation. This defect, previously attributed to a lack of attenuation of dendrites, was found to be due to a defect in melanosome translocation. The mutant phenotype, of melanincongested perikarya and pigmentless dendrites is expressed both in vivo and in vitro. Studies with colcemid and cytochalasin B suggest that the avian melanocyte resembles a dispersing amphibian melanophore in its requirement for microfilaments but not microtubules. Ultrastructural analysis revealed a normal complement of intracellular filaments. Microtubules, however, are scarce. Intermediate (10 nm) filaments surround and are closely associated with intracellular organelles, while microfilaments interconnect all filaments and organelles. While-cell centrifugation at 300 g showed that 10 nm filaments stream behind and appear to attach to mobile membrane-bound organelles including the nucleus, lipid granules and mitochondria, as well as melanosomes. It is suggested that all intracellular filaments, especially microfilaments and intermediate filaments, interconnect forming a network responsible for organelle motility.