Development of the embryonic chick's tectorial membrane.

The nascent tectorial membrane (TM) is identifiable as early as stage 33 (7th day) as thin, wispy material. By stage 37 (11th day), the dense mesh of the immature TM and fibrous webs (subtectorial threads) that attach the TM to the basilar papilla are distinct but scanty. The TM condenses slightly in its upper face. The growth of the columnar cells and basilar papilla during the following days pulls the TM, lifting it upward, and resembling the cables on a suspension bridge in cross-section. As a result, a large hollow wedge forms. During stages 40-44 (14th-18th days), the columnar cells secrete large amounts of fibrous material, which fills the hollow wedge and condenses into the dense meshes. The honeycombed patterns appear at this time. The supporting cells secrete the fibrous webs. Their secretory activity closely corresponds to that of the columnar cells. The secretory material from both cell types remains attached to the apical ends of their respective cells after secretory activity ends. By hatching (stage 46-21 days), the columnar cells have filled with fibrous material and their cytoplasmic organelles are restricted to the apices. The cytoplasm of supporting cells is relatively clear, with few cytoplasmic remnants of their intense secretory activity earlier.