The eye is essentially a highly specialized extension of the brain. It  is a sensory organ assembled of different tissues for optical, light sensing, transmission and display. The eye development starts with the bilateral evagination of diencephalon in the early neurula stage of development. However in the mammals the formation of optic pit is hallmark of eye development. whereas in fish and amphibians a bulging of the optic primordia is observed.

Optic vesicles are formed by the continuous  evagination of the optic primordial. These extend towards the overlying, non-neural surface ectoderm. The overlying ectoderm ultimately give rise to the lens and cornea. The mesenchyme is present between the optic vesicle and the surface ectoderm. During eye development the mesenchyme is displaced  as the two tissues come into close physical  contact.

The physical contact between optical vesicle and the surface ectoderm is the key event in eye development. The inductive signals between the optic vesicle and the surface ectoderm leads to differentiation of outer ectodermal cells into lens placode.

The first step of lens morphogenesis is thickening of the head surface ectoderm to form the lens placodes with lens progenitor cells. The invagination of progenitor cells establishes the lens vesicle in which anterior cells forms lens epithelium and posterior cells elongates to form primary lens fibers that makes the lens nucleus. The proliferation of lens epithelial cells produces secondary lens fiber cells to form the shells of lens and contributes to lens growth throughout life.

The lens placode start expressing the crystalline protein. Crystalline protein helps in generating and maintaining the lens transparency. Later the lens placode show coordinated invagination with optical vesicle.  After the invagination process the lens placode is called as lens vesicle and optic vesicle form a double layer  optic cup. Optic cup provides the first indication of the final shape of the eye. The inner layer of the optic cup (facing the lens) forms the neural retina, while the outer layer of the optic cup gives rise to the retinal pigment epithelium (RPE).

A groove like structure is formed at the ventral extremity of the optic vesicle. This groove is formed because of the invagination. This groove runs continuously from the ventral-most region of the neural retina and  along the ventral aspect of the optic stalk to the junction with the neural tube. The point at which the laterally growing edges of the optic cup fuse is known as the choroidal (or optic) fissure. Optic fissure provides a channel for blood vessels within the eye and an exit route for projecting axons.

The lens is a polar structure. the posterior cell of the lens start the expression of  fiber cell specific crystalline protein  and convert the posterior cell of lens ( those facing the optic cup) into primary lens fiber cell mass. Thus the polarity in lens is established by the formation of a primary lens fiber cell mass.

At the anterior end of lens a monolayer of proliferating epithelial cells are present. Secondary lens fiber cells originate from the equatorial region or transitional zone of the lens in a process that continues throughout adulthood. During Fiber cell differentiation  process the cell start loosing its organelles including the nucleus, mitochondria and endoplasmic reticulum.

16.1.      Development of cornea

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

  1. The ectoderm layer overlying the lens is known as corneal epithelium.  This corneal epithelium secretes collagen-rich extracellular matrix.
  2. Neural crest–derived mesenchymal cells  starts migration towards this primary stroma. Later migration of a 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  play 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 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 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 produce 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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