Fig. 1.

Corneal development in embryonic chick. Light microscopy of toluidine blue-stained semi-thin sections and corresponding three-dimensional reconstructions from SBF SEM at E3 (A, D), E4 (B, E), E5 (C, F), E6 (G, J), E7 (H, K) and E8 (I, L). At E3, the lens (l) is separated from ectoderm (e) by a thin layer of primary stroma (arrowhead) (A, D). At E4 a multi-layered epithelium (e) overlies a thickened primary stroma with a central undulating fibril condensation (arrow) plus fibrils at large angles to epithelial basement membrane (arrowheads) (B, E). At E5 an endothelial monolayer (en) covers the proximal face of the primary stroma, which shows more parallel arrangement of fibrils; further separation of the lens gives rise to the anterior chamber (a) (C, F). At E6 invading periocular neural crest cells populate the full width of the central region of the swollen primary stroma (G). By E7 cells occupy the whole depth of the stroma and secondary stromal biosynthesis is underway, with only a small extent of sub-epidermal acellular primary stroma remaining (arrow, H). By E8 secondary stroma synthesis increases overall thickness of the stroma and cells appear more flattened (I). In reconstructions (J–L), endothelium is not included in field of view, which shows epithelium (e), sub-epithelial primary stroma and stromal cells. At E7 keratocytes populate the stromal matrix, except for a narrow zone of residual primary stroma below the epithelium. Bar, 50 μm, A - C and G - I; 10 μm, D - F and J - L. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)