Feathers derive from a complex process of morphogenesis inside embryonic feather filaments that begins with the formation of barb ridges and terminates with the formation of barbs, free in plumulaceous feathers and regularly joined to arachis in pennaceous feathers (Lucas & Stettenheim, 1972; Chuong & Widelitz, 1999; Prum & Dyck, 2003; Sawyer & Knapp, 2003). Histologically, barbs are formed by an mid-axial part called ramus from which barbules branch laterally. Figs. 1 and 7 illustrate the general process of feather morphogenesis in order to show the process of formation of barbs. The mid-inner cell area of each barb ridge forms the ramus of the future barb while the two external and outer parts of the barb ridge, termed alar plates, form barbules (Figs. 1 and 2). The latter join to the centrally located ramus at different levels with a branching patterns that resemble plant ramifications (Fig. 1E; Alibardi, 2005a,b; Alibardi & Sawyer, 2006). Barb cells pile-up to form the ramus that eventually form a medullated part where large air cavities are present surrounded by a cortical part made of resistant feather keratin that sustain the whole barb.
The formation of the extended cavities in the medulla of rami derives from the degeneration of the central group of barb cells (medullary) that instead of accumulate the resistant feather keratin like the other barb cells (cortical) and barbule cells undergo a process of vacuolization (Matulionis, 1970; Alibardi, 2005a). Despite numerous studies on feather structure, the cytological details of the differentiation of barb medullary cells have been briefly described only in developing downfeathers of the chick (Matulionis; Alibardi, 2005a). The present study aims to deepen the ultrastructural description of the cell process of ramus differentiation, extending the study also to another specie of bird, the passeraceous zebrafinch. This observation aim to extend the knowledge on the histogenesis of barbs in feathers which is essential for further molecular studies on feather morphogenesis. The study is part of a large ultrastructural survey on feather morphogenesis aimed to clarify ans simplify the knowledged of all cell types involved in the formation of feathers.