Fig. 8. A model of organ of Corti development. Starting between E12.5 and E13.5, a prosensory domain can be recognized in the cochlear duct as a zone of non-proliferating cells (ZNPC, bracket) that fail to incorporate BrdU (blue) into their nuclei and which express the CDK inhibitor p27^Kip1 (green). Between E13.5 and E14.5, the cells of the ZNPC continue to express p27^Kip1 and Math1 expression begins in the base of the cochlea in cells at the medial edge of the ZNPC (yellow). Math1-positive cells appear as columns of nascent hair cells (Cells 1-3) that span the depth of the sensory epithelium. As differentiation proceeds between E14.5 and E15.5, MyosinVIIa (red), an early marker of hair cell differentiation appears in the base of the cochlea, and the hair cells are displaced in a longitudinal direction by a process of radial intercalation as the columns of nascent hair cells take their place in the mature bilayer of the organ of Corti (Cells 1-3). It is this process of convergent extension that is proposed to partly underlie the postmitotic longitudinal (Z axis) growth of the organ of Corti between E13.5 and E17.5 (see Fig. 1).