Fig. 6

 Histological analysis of adult pitx2^M64* mutant phenotype. H&E transverse sections of wild-type (A, D) and pitx2^M64* mutant (B, C, E, F) heads and corneas. Gross mutant phenotypes range from mild thickening of the corneal epithelium (C) to complete disorganization of the entire optic globe (B). The most prevalent phenotype includes the absence of both the anterior and posterior chambers as well as a severe reduction or a complete absence of the iris (B, C). In the cornea, the wild-type displays a typical stratified squamous epithelium, a uniformly thick stroma, and a single-layer endothelium (D). Mutant corneal epithelium is thicker, and a subset of cells takes on larger columnar morphology resembling that of epidermal stratified epithelium (black arrow in E). Mutant corneal stroma appears thinner, wavier, and sometimes interrupted (gray arrowhead in E). The analogous monolayer corneal endothelium region in the mutant is a noticeably thickened, disorganized layer of cells of unknown origin (black asterisk in E, F). Aberrant pigmentation is also typically observed in the mutant corneal epithelium and endothelium (black arrowheads in E, F), as well as cartilaginous layers (ca) in the anterior segment of mutant eyes (E, F). Some eyes that appeared opaque or hidden by gross analysis were found to be covered by skin and cartilage structures growing together over the eye globe (B, C, E, F). PAS staining of 1-mpf (G, H, M, N) and adult (I–L, O–Q) transverse sections. In wild-type adults 1-mpf and older, mucin-secreting goblet cells are present in the eye periphery (black arrows in G, H, J) but absent from the central corneal epithelium (G–I, K, L). In contrast, age-matched mutants develop goblet cells throughout the entire corneal epithelium (M–Q; black arrows in N and Q). Aberrant pigment deposits (black arrowheads in P, Q) and disorganized tissue next to the lens (asterisks in P, Q) are also evident. ac, anterior chamber; al, annular ligament; c, cornea; ca, cartilage; ir, iris; l, lens; re, retina; on, optic nerve; pc, posterior chamber.