The Corneal Epithelium and Lens Develop Independently From a Common Pool of Precursors

Abstract

Background: The corneal epithelium (CE) overlays a stroma, which is derived from neural crest cells, and appears to be committed during chick development, but appears still labile in adult rabbit. Its specification was hitherto regarded as resolved and dependent upon the lens, although without experimental support. Here, we challenged CE fate by changing its environment at different stages. Results: Recombination with a dermis showed that CE commitment is linked to stroma formation, which results in Pax6 stabilization in both species. Surgical ablation shows that CE specification has already taken place when the lens placode invaginates, while removal of the early lens placode led to lens renewal. To block lens formation, bone morphogenetic protein (BMP) signaling, one of its last inducing factors, was inhibited by over-expression of Gremlin in the ocular ectoderm. This resulted in lens-less embryos which formed a corneal epithelium if they survived 2 weeks. Conclusion: The corneal epithelium and lens share a common pool of precursors. The adoption of the CE fate might be dependent on the loss of a lens placode favoring environment. The corneal fate is definitively stabilized by the migration of Gremlinexpressing neural crest cells in the lens peripheral ectoderm. Developmental Dynamics 242:401-413, 2013. V C 2013 Wiley Periodicals, Inc.Key words: cornea; lens; epidermis; specification; commitment Key findings:Inhibition of BMP signaling prevents lens but not corneal epithelium formation: the specification of the cornea does not require lens formation. A common pool of eye ectodermal precursors arising early in development can give rise to either lens or corneal epithelium with their fate ultimately being determined by whether the environment is supportive of lens placode formation or not. This pool can renew a surgically ablated lens placodal ectoderm as long as the environment is still conducive to the last step of lens induction. The commitment of corneal epithelium correlates with both the loss of its capacity to down-regulate Pax6 as well as with the formation of its associated stroma.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

“…Clonal analysis of cells comprising the aPPR identified a down-regulation of Dlx5 in the lens compared to the olfactory progenitor cells [57], Dlx5 + cells are ultimatelly excluded from the lens altogether [49]. It has also been proposed that some lens-like Pax6 + progenitor cells, namely those in the more peripheral locations, later give rise to the corneal epithelium [58]. In zebrafish, PPR patterning and signaling are similar to that of the chick model [59].…”

Corneal Development

“…1 However, recent studies indicate that the presumptive corneal and lens epithelium is specified in the ectoderm that expresses the paired box protein Pax6. 2 However, BMP signaling from the optic vesicles induces the formation of the lens placode, while the adjacent Pax6-expressing ectoderm forms the presumptive corneal epithelium. Microsurgical ablation of the lens placode at the time when the optic vesicles are signaling results in the induction of the lens vesicle from the regenerating adjacent presumptive epithelium.…”

“…The corneal epithelium is specified by E5, given that it does not lose Pax6 expression when challenged by mesenchyme from the dorsal dermis. 2,25 By E6, the epithelium is three-layers thick and it increases to 4-5 cell layers at E19. 26 The corneal epithelium begins to express the keratin pair (K3/K12) at about E14, 27 and it is fully developed and consists of 6-8 stratified cell layers at the time of hatching (Fig.…”

Derivation of keratinocytes from chicken embryonic stem cells: Establishment and characterization of differentiated proliferative cell populations

“…The other antibodies were kindly provided by Pr Danielle Dhouailly for αK-11E10 (keratin 14) (Collomb et al, 2013), by Dr Furuse for anti-occludin (Furuse et al, 1993) and by Dr Philippe Djian for anti-involucrin (Vanhoutteghem et al, 2008).…”

“…Keratin expression in differentiated cells could be monitored by staining with the αK-11E10 monoclonal antibody (Collomb et al, 2013), an antibody which detects the KRT14 in the chicken skin (Fig. S2).…”