Ophthalmology for Undergraduates PS Girija Devi
INDEX
×
Chapter Notes

Save Clear


1Anatomy and Physiology of the Eye2

Development of the Eye1

Girija Devi PS, Umesan KG
The eye develops from:
  1. The neural ectoderm: This part gives rise to the retina and accessory pigmented structures as well as the fibers of the optic nerve.
  2. Surface ectoderm: It forms the lens, corneal and conjunctival epithelium; and also the tarsal and lacrimal glands.
  3. The neural crest mesenchyme: This part differentiates to form the fibrous coats of the eye, choroid, vitreous and tissues in the anterior segment of the eye.
 
STAGES OF DEVELOPMENT
The development has three stages, which are as follows:
  1. Optic sulcus.
  2. Optic vesicle.
  3. Optic cup.
 
Optic Sulcus
Optic sulcus first appears as a thickening on the diencephalic neural fold called optic sulcus. This occurs by about 29 days postovulation. The optic sulcus appears as transverse sulcus on either side of the cranial end of the neural crest. By the 30th day, the walls of diencephalons evaginate around the optic sulcus and this leads to the formation of the optic vesicle by the 32nd day. The vesicle becomes surrounded by the cranial mesenchyme and cells of neural crest origin.
 
Optic Vesicle
Optic vesicle shows three distinct parts:
  1. Optic stalk attaches the vesicle to the diencephalon.
  2. A flat disk of thickened epithelium close to the surface epithelium, this differentiates into the neural retina.
  3. A layer of epithelial cells lying between the optic stalk and the flat disk of thickened epithelium, which differentiates into the future pigment epithelium of retina.
 
Optic Cup
Optic vesicle undergoes invagination in its lateral part. The invagination results in the formation of the optic cup (Fig. 1.1) with an inner wall and an outer wall. The inner layer becomes the sensory retina and the outer layer forms the retinal pigmented epithelium. The lumen of the original vesicle gets converted to the potential space between the sensory retina and the pigment layer.
 
DEVELOPMENT OF DIFFERENT PARTS OF EYE
 
Choroid Fissure/Optic Fissure
Ventral part of the optic vesicle and the lateral part of the optic stalk invaginate to form the choroid fissure (Figs 1.2A and B). At about the 5th week of intrauterine life (IUL), the hyaloid artery grows into this fissure. Apoptosis at the margins of the optic fissure gradually leads to closure of the fissure.
zoom view
FIGURE 1.1: Development of optic cup and optic fissure
4
zoom view
FIGURES 1.2A and B: Transverse section of forebrain of a 5 mm embryo showing the optic vesicle and lens primordium. A. Optic primordium; B. Optic fissure.
 
Lens Placode
Due to induction by the optic vesicle, the surface ectoderm overlying the optic vesicle thickens and forms the lens placode by the 33rd day (Fig. 1.3). By the 35th day, the lens placode invaginates to form a pit. The invaginated ectoderm gradually detaches from the surface ectoderm to form the lens vesicle. The surface ectoderm at the site of detachment regrows and forms the corneal epithelium.
 
Lens
Lens vesicle migrates toward the optic cup. The cubical cells from its posterior wall elongate to form the primary lens fibers, thus producing the embryonic nucleus (Figs 1.4A and B). Cells at the equator of the vesicle elongate to form the secondary lens fibers. They are added around the embryonic nucleus to get the fetal nucleus. The elongating lamina of lens fibers form a Y-shaped junction anteriorly and a λ-shaped junction posteriorly, they are the lens sutures. The lens capsule is secreted by the epithelial cells (Figs 1.5A to E).
 
Vitreous
During 4th–5th weeks, the space between the lens vesicle and the optic cup get filled by fibrillar material from both the lens vesicle and the neural crest derived mesoderm in the optic cup. This material is the primary vitreous. It is supplied by the hyaloid system of vessels. The hyaloid artery is a branch of the ophthalmic artery. The branches of the artery form the tunica vasculosa lentis on the posterior surface of the lens by the 5th week. The venous return is via the capsulopupillary membrane, which covers the lens from the equator to the pupillary margin. Once the lens capsule is formed, further vitreous is produced only by the neuroectoderm of the optic cup. This constitutes the secondary vitreous. A passage develops for the hyaloid vessels between the primary vitreous and secondary vitreous by the 4th month, the tunica vasculosa lentis regresses and the hyaloid artery atrophies. Remnants on the posterior lens capsule form the Mittendorf's dots.
The tertiary vitreous is formed from the neuroectoderm of the ciliary region at the 12th week. The zonules develop from the tertiary vitreous. The outermost layers of the vitreous body condense to form the hyaloid membrane.
 
Retina
Inner and outer layers of the bilaminar optic cup give rise to the retinal layers (refer Fig. 1.5D). They are separated by the cavity of the optic cup, which becomes the intraretinal space. The outer layer of cubical cells becomes the retinal pigment epithelium. The inner layer multiplies to get three to four layers of cells by mitosis by the 26th day. The outermost layer is called the germinative or ependymal layer. It gets attached to the pigment layer and forms the outer segments of rod and cones by the 4th month. The inner marginal layer forms the nerve fiber layer. During the retinal differentiation, ganglion cells and Muller's cells
zoom view
FIGURE 1.3: Development of lens placode
5
zoom view
FIGURES 1.4A and B: Lens vesicle. A. Elongating posterior lens cells; B. Formation of lens fibers.
develop very early. The transient layer of Chievitz forms the inner plexiform layer by the 10th week. Rods are formed by 22nd week. Bipolar cells and horizontal cells develop by the 25th week. The fovea centralis is fully developed by the 7th month. The ora serrata is developed by the 6th month.
At first, the retina and the optic nerve are avascular since the hyaloid system blood vessel supplies only the vitreous and the lens. Blood vessels of retina grow from the hyaloid vessels from the 4th month of IUL. They reach full growth by the 3rd month after birth. The hyaloid system regresses and the portion within the optic nerve remains as the central retinal artery.
 
Optic Nerve
Closure of the optic (choroid) fissure causes the optic stalk to become a double walled tube, which connects the cavity of the diencephalon with the primary optic vesicle. The cavity of the tube gets filled by the growth of axons from ganglion cells (refer Fig. 1.5E). These axons grow toward the brain and reach it by the 6th week of IUL. The nerve sheaths are formed from the neural crest cells around the optic stalk. Crossing of nasal fibers at the optic chiasma occurs at 9th week. Myelination of optic nerve fibers starts from the distal end at 7th month of IUL and proceeds toward the optic disk. It stops at the level of lamina cribrosa.
 
Bergmeister's Papilla
Bergmeister's papilla is a projection due to glial proliferation at the site of entry of hyaloid vessels into the vitreous. Later this glia disappears due to apoptosis. This leads to the formation of the optic cup.
 
Uveal Tract
Neuroectoderm, neural crest derived mesenchyme and vascular channels together form the uveal tract. Condensation of neural crest cells around the optic cup forms the mesenchyme where blood spaces appear. Vessels close to RPE form the choriocapillaris during 4th–5th week. Cytoplasmic processes extending into the lumen is a feature of the endothelium of these vessels. Pericytes appear by the 6th week. Basal lamina is formed by the 6th week. By the 3rd–5th month of IUL, the outer large vessel layer of Haller's and Sattler's layer appear. The veins drain to the vortex veins. The choroidal stroma is formed from fibroblasts, collagen fibrils, elastic fibers and melanocytes of neural crest origin. The inner layer of collagen condenses to form the Bruch's membrane by the 3rd month.
 
Ciliary Body
Ciliary body develops from the growing rim of the optic cup. The part of the cup that extends beyond the lens is called pars caeca and the part behind is pars optica. The pars caeca, by the 3rd month shows about 75% longitudinal folds. These folds are invaded by a vascular mesenchyme by the 4th month and form the ciliary processes. The double membrane of the optic cup forms the ciliary epithelium lining these folds. By the 12th week, ciliary muscle differentiates from the mesenchyme. Ciliary zonules develop by the 7th month. The outer layer of the ciliary epithelium becomes pigmented.6
 
Iris
Iris is partly neuroectodermal and partly mesodermal in origin. The two posterior layers of epithelium, the sphincter and dilator muscles are neuroectodermal in origin. The blood vessels and the stroma develop from the mesoderm (refer Fig. 1.5E).
zoom view
FIGURES 1.5A to E: Stages of development of human eye. A. Layers of eye in developmental stage; B. Formation of optic stalk and optic cup; C. Differentiation of choroid, sclera and cornea; D. Formation of lens and visual retina;E. Differentiation into eyelid, conjunctiva, iris and other features of a developed eye.
By the 4th month, iris develops from the rim of the optic cup. Both layers of the neuroectoderm become the pigmented iris epithelium. Some cells of the outer layer differentiate into the sphincter and dilator pupillae. The neural crest derived mesenchyme forms the iris stroma and vessels. The pupillary membrane is formed anterior to the tunica vasculosa lentis and is supported 7 he long posterior ciliary arteries and the annular vessel at the anterior margin of the optic cup. During the 6th month the pupillary portion of tunica vasculosa gets regressed and the remnants are phagocytized. By the 9th month the pupillary membrane disappears leaving a ring of vessels around the sphincter. This region becomes the collarette.
 
Cornea
The mesoderm between the surface ectoderm and the lens vesicle splits into two. The anterior part becomes the corneal stroma and the posterior part becomes the iridopupillary membrane. The cavity between the two, lined by mesothelium becomes the anterior chamber. The corneal endothelium formed from neural crest cells by the 5th week. The Bowman's membrane is formed by about the 16th week. The corneal epithelium is differentiated from the surface ectoderm by the 5th month of IUL (refer Figs 1.5D and E).
 
Sclera
Anterior part of sclera is formed from the neural crest derived mesenchyme around the optic cup. The caudal part is from the paraxial mesoderm (refer Figs 1.5C to E).
 
Eyelids
The upper lid is formed from the frontonasal process and the lower lid is from the maxillary process. The surface ectoderm of the corresponding process develops into the eyelid skin externally and the conjunctiva internally. The lids are kept closed between the 10th week and 5th month. Eyelids are totally separated by the 8th month (refer Fig. 1.5E).
 
Tarsal Glands
Ectodermal cells grow into the lid mesoderm as cords by the 10th week. Later they canalize to form the tarsal glands.
 
Lacrimal Glands
By the 8th week, epithelial cords grow upwards from the superolateral aspect of the conjunctival sac with the formation of the levator palpebrae superioris, the gland is divided into the orbital part and palpebral part. The gland starts secreting only by the 3rd month after birth. Conjunctival glands are formed as early as the 6th month of IUL.
 
Lacrimal Pathway
The ectoderm in the groove between the lateral nasal and maxillary process gets buried to form a solid cord of cells. Upper part of this canalizes at 3rd month to form the lacrimal sac. It grows upward the lids to form the canaliculi. The caudal part of the epithelial cord develops into the nasolacrimal duct at about the 6th month.