Analysis of chick (Gallus gallus) middle ear columella formation

The columella, which arises from proximal ectomesenchyme of the second pharyngeal arch, is induced and patterned in a dynamic multi-step process. From the footplate, which inserts into the inner ear oval window, the shaft spans the pneumatic middle ear cavity, and the extracolumella inserts into the tympanic membrane. Through marker gene and immunolabeling analysis, we have determined the onset of each stage in the columella's development, from condensation to ossification. Significantly, a single condensation with the putative shaft and extracolumella arms already distinguishable is observed shortly before initiation of five separate chondrogenic centers within these structures. Ossification begins later, with periosteum formation in the shaft and, unexpectedly, a separate periosteum in the footplate.The data presented in this study document the spatiotemporal events leading to morphogenesis of the columella and middle ear structures and provide the first gene expression data for this region. These data identify candidate genes and facilitate future functional studies and elucidation of the molecular mechanisms of columella formation.The columella consists of a persistent cartilaginous extracolumella laterally (comprised of three processes: supracolumella, extracolumella, and infracolumella arms inserting into the tympanic membrane) and an ossified shaft and footplate (also called the columella), which inserts into the inner ear oval window [1]. During columella endochondral ossification, bone replaces cartilage (replacement cartilage), unlike extracolumella (persistent cartilage), which remains throughout life. Identifying the mechanism of columella induction and patterning requires detailed knowledge of the morphological timing and developmental gene expression patterns. This study investigates the temporal events--including mesenchymal condensation, chondrogenesis, ossification of the medial footplate and shaft, and joint formation--between persistent and replac