16.1.      Development of cornea

The development of cornea is a highly coordinated, multistep process.

The ectoderm layer overlying the lens is known as corneal epithelium.  This corneal epithelium secretes collagen-rich extracellular matrix.

Neural crest-derived mesenchymal cells start migration towards this primary stroma. Later migration of the second wave of neural crest-derived mesenchymal cells is also observed.

The high level of thyroxine hormone in posterior stroma cell triggers the dehydration and compaction of the posterior stroma cell.

Ultimately this leads to the formation of the mature, transparent cornea. The ciliary body and iris are derived from the distal tip of the optic cup at the point where the inner and outer optic cup layers meet. The Otx1, an orthodenticle related transcription factor plays an essential role in retina development, regulate the lens polarization and regulate the of retinal differentiation.

The ocular lens of eye differs from all other organs because it is a vascular tissue without any innervations. Lens cells are of ectodermal origin and ultimately differentiate into either lens fiber or the lens epithelium.

When the lens formed, it induces other tissues. The inducer becomes the induced (reciprocal inductions), then optic vesicle becomes optic cup and wall of the optic cup differentiates into the pigmented retina and the neural retina layers.

The lens induces the corneal ectodermal cells collagen to secrete. After collagen secretion, the corneal ectodermal cells differentiate into the cornea. The third signal as thyroxin hormone that dehydrates the tissue and makes it transparent.

The molecular signalling mechanism for eye lens induction and differentiation has cell fate decisions and differentiation steps occur during lens induction and differentiation involving bone morphogenetic, fibroblast growth factors and Wnt signalling pathways as well as inhibition of some specific pathways. The process of induction as shown in the figure below

The inductive events reach at completion with the formation of Pax6 common progenitor cells in pre placodal ectoderm that ultimately produces distinct neuronal and non-neuronal cell types by  BMP signalling and its transitional inhibition (αBMP). In addition to the contribution of BMP, FGF, Wnt signalling, somatostatin and nociceptin from the anterior mesendoderm also play a surprising role in controlling lens and olfactory placode development by stimulating Pax6 expression.

Pax6 in the ectoderm is essential for lens placode development. Once the optic vesicle makes contact with the head ectoderm the Sox2 and Sox3 genes are activated in the ectoderm and initiate synthesis of the encoded proteins. This region of the head ectoderm expressing SOX2 and Pax6 together gives rise to the lens placode and activates the d1-crystallin gene.

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