Fig. S1.

Lens development and structure. During embryogenesis, the lens arises from the surface ectoderm adjacent to forebrain, which thickens to form the lens placode and then invaginates to form the lens pit (A, E10.5). The lens pit finally separates from the ectoderm to form the lens vesicle (B, E11.5). Posterior epithelial cells of the lens vesicle start to elongate towards the anterior surface and finally fill the lumen of the lens vesicle (E12.5). These elongating cells are called primary lens fibre cells and constitute the central region of mature lens. Anterior epithelial cells of lens vesicle continue to proliferate as the lens expands whilst those near to the lens equator stop dividing and begin their program of differentiation. At the equator, these cells start to elongate and differentiate into secondary fibre cells (C). Newly formed fibre cells are added around the central mass of primary lens fibre cells. The secondary lens fibre cells are also polarised, with the basal membranes attached to the posterior lens capsule and the apical region contacting the apical membranes of the anterior lens epithelial cells (D). At their apical ends, neighbouring lens fibre cells are inter-connected by apical cell junctions, which are continuous to lens epithelial cell junctions.