Chapter 20 – Development of the Eye
Introduction
• The eye is a specialized organ for vision, and its development involves complex interactions
among:
o Neuroectoderm of the forebrain.
o Surface ectoderm.
o Surrounding mesoderm.
• The process begins in Week 3 (Day 22) and continues through fetal life.
• Key steps:
o Formation of the optic vesicle from the forebrain.
o Induction of the lens placode from surface ectoderm.
o Invagination to form the optic cup and subsequent differentiation into all ocular
structures.
Major Stages of Eye Development
1. Formation of Optic Vesicles
• The optic grooves first appear on the lateral walls of the forebrain during Week 3.
• These grooves extend outward to form optic vesicles as the neural tube closes.
• Optic vesicles grow laterally and come into contact with surface ectoderm, initiating lens
development through inductive signalling.
2. Induction of the Lens Placode
• When the optic vesicle contacts the surface ectoderm:
o The ectoderm thickens, forming the lens placode.
o This structure is critical for further eye development, including:
§ Lens formation.
§ Optic cup invagination.
o Without proper lens placode induction, optic cup development fails.
3. Formation of the Optic Cup
• The optic vesicle invaginates to form the optic cup, a double-walled structure:
o Outer wall → Forms the pigmented epithelium of the retina.
o Inner wall → Forms the neural retina, which contains photoreceptors and
associated neurons.
• The invagination also creates the choroid fissure on the ventral side:
, o This allows the hyaloid artery to reach the developing retina and lens.
4. Development of the Retina
1. Outer Layer of the Optic Cup:
o Forms the pigmented layer of the retina.
o Contains melanin granules that prevent light scattering and provide support to
photoreceptor cells.
2. Inner Layer of the Optic Cup:
o Forms the neural retina, which is divided into:
§ Pars optica retinae (posterior 4/5th):
§ Contains photoreceptors (rods and cones), bipolar cells, ganglion
cells, and supporting cells.
§ Layers of the pars optica retinae:
1. Photoreceptor Layer: Rods and cones detect light.
2. Outer Nuclear Layer: Nuclei of rods and cones.
3. Inner Nuclear Layer: Nuclei of bipolar and supporting cells.
4. Ganglion Cell Layer: Transmits signals to the optic nerve.
§ Pars ceca retinae (anterior 1/5th):
§ Pars iridica retinae: Inner layer of the iris.
§ Pars ciliaris retinae: Forms the ciliary body.
5. Formation of the Lens
1. Lens Placode:
o Thickened ectoderm overlying the optic vesicle.
o Invaginates to form the lens pit.
2. Lens Vesicle:
o Completely detaches from the surface ectoderm and resides within the optic cup.
3. Lens Differentiation:
o Primary Lens Fibers:
§ Derived from the posterior wall of the lens vesicle.
§ Elongate to fill the vesicle by Week 7.
o Secondary Lens Fibers:
§ Continuously added throughout life, increasing lens size.
o The lens is avascular; it depends on the aqueous humor and vitreous body for
nutrients.
Introduction
• The eye is a specialized organ for vision, and its development involves complex interactions
among:
o Neuroectoderm of the forebrain.
o Surface ectoderm.
o Surrounding mesoderm.
• The process begins in Week 3 (Day 22) and continues through fetal life.
• Key steps:
o Formation of the optic vesicle from the forebrain.
o Induction of the lens placode from surface ectoderm.
o Invagination to form the optic cup and subsequent differentiation into all ocular
structures.
Major Stages of Eye Development
1. Formation of Optic Vesicles
• The optic grooves first appear on the lateral walls of the forebrain during Week 3.
• These grooves extend outward to form optic vesicles as the neural tube closes.
• Optic vesicles grow laterally and come into contact with surface ectoderm, initiating lens
development through inductive signalling.
2. Induction of the Lens Placode
• When the optic vesicle contacts the surface ectoderm:
o The ectoderm thickens, forming the lens placode.
o This structure is critical for further eye development, including:
§ Lens formation.
§ Optic cup invagination.
o Without proper lens placode induction, optic cup development fails.
3. Formation of the Optic Cup
• The optic vesicle invaginates to form the optic cup, a double-walled structure:
o Outer wall → Forms the pigmented epithelium of the retina.
o Inner wall → Forms the neural retina, which contains photoreceptors and
associated neurons.
• The invagination also creates the choroid fissure on the ventral side:
, o This allows the hyaloid artery to reach the developing retina and lens.
4. Development of the Retina
1. Outer Layer of the Optic Cup:
o Forms the pigmented layer of the retina.
o Contains melanin granules that prevent light scattering and provide support to
photoreceptor cells.
2. Inner Layer of the Optic Cup:
o Forms the neural retina, which is divided into:
§ Pars optica retinae (posterior 4/5th):
§ Contains photoreceptors (rods and cones), bipolar cells, ganglion
cells, and supporting cells.
§ Layers of the pars optica retinae:
1. Photoreceptor Layer: Rods and cones detect light.
2. Outer Nuclear Layer: Nuclei of rods and cones.
3. Inner Nuclear Layer: Nuclei of bipolar and supporting cells.
4. Ganglion Cell Layer: Transmits signals to the optic nerve.
§ Pars ceca retinae (anterior 1/5th):
§ Pars iridica retinae: Inner layer of the iris.
§ Pars ciliaris retinae: Forms the ciliary body.
5. Formation of the Lens
1. Lens Placode:
o Thickened ectoderm overlying the optic vesicle.
o Invaginates to form the lens pit.
2. Lens Vesicle:
o Completely detaches from the surface ectoderm and resides within the optic cup.
3. Lens Differentiation:
o Primary Lens Fibers:
§ Derived from the posterior wall of the lens vesicle.
§ Elongate to fill the vesicle by Week 7.
o Secondary Lens Fibers:
§ Continuously added throughout life, increasing lens size.
o The lens is avascular; it depends on the aqueous humor and vitreous body for
nutrients.
