30.7B: Development of Vision
The eye forms from the neural tube, epidermis, and periocular mesenchyme, with sequential inductions of tissue during development.
- Describe the development of vision
Key Points
- Organogenesis of the eye is an example of a developmental cascade of inductions.
- Development of the optic vesicles starts in the three-week embryo from a progressively deepening groove in the neural plate called the optic sulcus.
- The optic vesicles come into contact with the epithelium and induce the epidermis to thicken to form the lens placode.
- The periocular mesenchyme migrates in during the formation of the optic cup and is critical for the induction of the retinal pigment epithelium and the optic nerve.
Key Terms
- optic cup : During embryonic development of the eye, the outer wall of the bulb of the optic vesicles becomes thickened and invaginated, converting it to a cup consisting of two strata of cells. These strata are continuous with each other at the cup margin, which ultimately overlaps the front of the lens and reaches as far forward as the future aperture of the pupil.
- optic sulcus : A progressively-deepening groove in the neural plate from which the optic vesicles develop.
- lens placode : A thickened portion of ectoderm which serves as the precursor to the lens.
The eye develops from the neural tube, the epidermis, and the periocular mesenchyme, which receives contributions from both the neural crest and mesoderm lineages. The organogenesis of the eye is an example of a developmental cascade of inductions, with three different tissues contributing to its differentiations.
Neural Tube
Chick embryo head with optic vesicle : The eyes make their appearance before the closure of the anterior end of the neural tube. After the closure of the tube they are known as the optic vesicles.
First, an outpocketing of the neural tube occurs, creating optic vesicles. Development of the optic vesicles starts in the three-week embryo from a progressively deepening groove in the neural plate called the optic sulcus. As this expands, the rostral neuropore (the exit of the brain cavity out of the embryo) closes and the optic sulcus and the neural plate becomes the optic vesicle.
Epidermis
The optic vesicles come into contact with the epithelium and induce the epidermis. The epithelium thickens to form the lens placode. The lens differentiates and invaginates until it detaches from the epithelium. The lens then acts as an inducer back to the optic vesicle to transform it into the optic cup and back to the epidermis to transform it into the cornea. The optic cup then delaminates into two layers: the neural retina and the retinal pigment epithelium.
Periocular Mesenchyme
The Optic Stalk and Optic Cup : During embryonic development of the eye, the outer wall of the bulb of the optic vesicles becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup.
The periocular mesenchyme migrates inward during the formation of the optic cup and is critical for the induction of the retinal pigment epithelium and the optic nerve. The mesenchyme contributes to the cornea, iris, ciliary body, sclera, and blood vessels of the eye.
Chordamesoderm
Chordamesoderm induces the anterior portion of the neural tube to form the precursors of the synapomorphic tripartite brain of vertebrates, a bulge called the diencephalon. Further induction by the chordamesoderm forms a protrusion: the optic vesicle. This vesicle is subsequently invaginated by further inductions from the chordamesoderm, and induces the ectoderm that thickens (lens placode) and further invaginates to a point that detaches from the ectoderm and forms a neurogenic placode. The lens placode is triggered by the chordamesoderm to invaginate and form the optic cup, composed of an outer layer of neural retina and inner layer of pigmented retina that unite and form the optic stalk. The pigmented retina is formed by rods and cones and composed of small cilia typical of the ependymal epithelium of the neural tube. Some cells in the lens vesicle will form the cornea and the lens vesicle will develop completely to form the definitive lens. Iris is formed from the optic cup cells.