The present ultrastructural study on developing and regenerating feathers of chick and zebrafinch describes the ultrastructural changes that occur during the differentiation of barb cells that leads to the formation of the ramus of barbs. Differently from barbule and barb cortical cells that accumulate feather keratin, barb medullary cells undergo to lipid degeneration. Eventually, lipids disappear and medullary cells become empty cavities in the central part of the ramus. In barb medullary cells feather keratin is accumulated in few peripheral bundles that merge with those of cortical cells to fom the wall of the ramus. The latter is joined with branching barbules. The process that controls the transition from keratin-synthesizing to lipid-producing barb cells remains unknown. The accumulation of lipids among keratin bundles confirms the capability of beta-keratin cells to undergo an intense lipidogenesis under specific conditions