Page 2 of 4 PREVIOUS | NEXT

Print this Page Print this Chapter ( K)

II. EMBRYOLOGY OF THE EYE

The eye is derived from three of the primitive embryonic layers: surface ectoderm, including its derivative the neural crest; neural ectoderm; and mesoderm. Endoderm does not enter into the formation of the eye. Mesenchyme is the term for embryonic connective tissue. Ocular and adnexal connective tissues previously were thought to be derived from mesoderm, but it has now been shown that most of the mesenchyme of all of the head and neck region is derived from the cranial neural crest.

The surface ectoderm gives rise to the lens, the lacrimal gland, the epithelium of the cornea, conjunctiva, and adnexal glands, and the epidermis of the eyelids.

The neural crest, which arises from the surface ectoderm in the region immediately adjacent to the neural folds of neural ectoderm, is responsible for formation of the corneal keratocytes, the endothelium of the cornea and the trabecular meshwork, the stroma of the iris and choroid, the ciliary muscle, the fibroblasts of the sclera, the vitreous, and the optic nerve meninges. It is also involved in formation of the orbital cartilage and bone, the orbital connective tissues and nerves, the extraocular muscles, and the subepidermal layers of the eyelids.

The neural ectoderm gives rise to the optic vesicle and optic cup and is thus responsible for the formation of the retina and retinal pigment epithelium, the pigmented and nonpigmented layers of ciliary epithelium, the posterior epithelium, the dilator and sphincter muscles of the iris, and the optic nerve fibers and glia.

The mesoderm is now thought to contribute only to the extraocular muscles and the orbital and ocular vascular endothelium.

Optic Vesicle Stage

The embryonic plate is the earliest stage in fetal development during which ocular structures can be differentiated. At the 2.5 mm (2 week) stage, the edges of the neural groove thicken to form the neural folds (Figure 1-28). The folds then fuse to form the neural tube, which sinks into the underlying mesoderm and detaches itself from the surface epithelium. The site of the optic groove or optic sulcus is in the cephalic neural folds on either side of and parallel to the neural groove. This occurs when neural folds begin to close at 3 weeks.

At the 9 mm (4 week) stage, just before the anterior portion of the neural tube closes completely, neural ectoderm grows outward and toward the surface ectoderm on either side to form the spherical optic vesicles. The optic vesicles are connected to the forebrain by the optic stalks. At this stage also, a thickening of the surface ectoderm (lens plate) begins to form opposite the ends of the optic vesicles.

Optic Cup Stage

As the optic vesicle invaginates to produce the optic cup, the original outer wall of the vesicle approaches its inner wall. The invagination of the ventral surface of the optic stalk and of the optic vesicle occurs simultaneously and creates a groove, the optic (embryonic) fissure. The margins of the optic cup then grow around the optic fissure. At the same time, the lens plate invaginates to form first a cup and then a hollow sphere known as the lens vesicle. By the 9 mm (4 week) stage, the lens vesicle separates from the surface ectoderm and lies free in the rim of the optic cup.

The optic fissure allows the vascular mesoderm to enter the optic stalk and eventually to form the hyaloid system of the vitreous cavity. As invagination is completed, the optic fissure narrows and closes during the 13 mm (6 week) stage, leaving one small permanent opening at the anterior end of the optic stalk through which the hyaloid artery passes. At the 100 mm (4 month) stage, the retinal artery and vein pass through this opening. At this stage also, the ultimate general structure of the eye has been determined.

Further development of the eye consists in differentiation of the individual optic structures. In general, differentiation of the optic structures occurs more rapidly in the posterior than in the anterior segment of the eye during the early stages and more rapidly in the anterior segment during the later stages of gestation.

EMBRYOLOGY OF SPECIFIC STRUCTURES

Lids & Lacrimal Apparatus

The lids develop from mesenchyme except for the epidermis of the skin and the epithelium of the conjunctiva, which are derivatives of surface ectoderm. The lid buds are first seen at 16 mm (6 weeks) growing in front of the eye, where they meet and fuse at the 37 mm (8 week) stage. They separate during the fifth month. The lashes and meibomian and other lid glands develop as downgrowths from the epidermis.

The lacrimal and accessory lacrimal glands develop from the conjunctival epithelium. The lacrimal drainage system (canaliculi, lacrimal sac, and nasolacrimal duct) are also surface ectodermal derivatives, which develop from a solid epithelial cord that becomes buried between the maxillary and nasal processes of the developing facial structures. This cord canalizes just before birth.

Sclera & Extraocular Muscles

The sclera and extraocular muscles are formed from condensations of mesenchyme encircling the optic cup and are first identifiable at the 20 mm (7 week) stage. Development of these structures is well advanced by the fourth month. Tenon's capsule appears about the insertions of the rectus muscles at the 80 mm (12 week) stage and is complete at 5 months.

Anterior Segment

The anterior segment of the globe is formed by invasion of neural crest cells into the space between the surface ectoderm, which develops into the corneal epithelium, and the lens vesicle, which has become separated from it. The invasion of neural crest cells occurs in three stages: The first is responsible for formation of the corneal endothelium, the second for formation of the corneal stroma, and the third for formation of the iris stroma. The anterior chamber angle is formed from a residual condensation of mesenchyme at the anterior rim of the optic cup. The mechanism of formation of the anterior chamber itself-and hence the angle structures-is still debated but certainly seems to involve patterns of migration of neural crest cells and subsequent changes in their structure rather than cleavage of mesodermal tissue as previously thought.

The corneal epithelium and endothelium are first apparent at the 12 mm (5 week) stage. Descemet's membrane is secreted by the flattened endothelial cells by the 75 mm (13 week) stage. The stroma slowly thickens and forms an anterior condensation just under the epithelium that is recognizable at 100 mm (4 months) as Bowman's layer. A definite corneoscleral junction is present at 4 months.

The double row of posterior iris epithelium is a forward extension of the anterior rim of the optic cup. This grows forward during the third month (50 mm stage) to lie posterior to the neural crest cells that form the iris stroma. These two epithelial layers become pigmented in the iris, whereas only the outer layer is pigmented in the ciliary body. By the fifth month (150 mm) stage, the sphincter muscle of the pupil is developing from a bud of nonpigmented epithelium derived from the anterior epithelial layer of the iris near the pupillary margin. Soon after the sixth month, the dilator muscle appears in the anterior epithelial layer near the ciliary body.

The anterior chamber of the eye first appears at 20 mm (7 weeks) and remains very shallow until birth. At 65 mm (9-10 weeks), Schlemm's canal appears as a vascular channel at the level of the recess of the angle and gradually assumes a relatively more anterior location as the angle recess develops. The iris, which in the early stages of development is quite anterior, gradually lies relatively more posteriorly as the chamber angle recess develops, most likely because of the difference in rate of growth of the anterior segment structures. The trabecular meshwork develops from the loose vascular mesenchymal tissue lying originally at the margin of the optic cup. The aqueous drainage system is ready to function before birth.

Lens

Soon after the lens vesicle lies free in the rim of the optic cap (13 mm or 6 week stage), the cells of its posterior wall elongate, encroach on the empty cavity, and finally fill it in (26 mm or 7 week stage). At about this stage (13 mm or 6 week), a hyaline capsule is secreted by the lens cells. Secondary lens fibers elongate from the equatorial region and grow forward under the subcapsular epithelium, which remains as a single layer of cuboidal epithelial cells, and backward under the lens capsule. These fibers meet to form the lens sutures (upright Y anteriorly and inverted Y posteriorly), which are complete by the seventh month. (This growth and proliferation of secondary lens fibers continues at a decreasing rate throughout life; the lens therefore continues to enlarge slowly, causing compression of the lens fibers.)

Ciliary Body & Choroid

The ciliary epithelium is formed from the same anterior extension of the optic cup that is responsible for the posterior iris epithelium. Only the outer layer becomes pigmented. The ciliary muscle and blood vessels are derived from mesenchyme.

At the 6 mm (31/2 week) stage, a network of capillaries encircles the optic cup and develops into the choroid. By the third month, the intermediate and large venous channels of the choroid are developed and drain into the vortex veins to exit from the eye.

Retina

The outer layer of the optic cup remains as a single layer and becomes the pigment epithelium of the retina. Pigmentation begins at the 10 mm (5 week) stage. Secretion of the inner layer of Bruch's membrane occurs by the 13 mm (6 week) stage. The inner layer of the optic cup undergoes a complicated differentiation into the other nine layers of the retina. This occurs slowly throughout gestation. By the seventh month, the outermost cell layer (consisting of the nuclei of the rods and cones) is present as well as the bipolar, amacrine, and ganglion cells and nerve fibers. The macular region is thicker than the rest of the retina until the eighth month, when macular depression begins to develop. Macular development is not complete in anatomic terms until 6 months after birth.

Vitreous

A. First Stage:

(Primary vitreous, 4.5 to 13 mm or 3 to 6 week stage.) At about the 4.5 mm stage, mesenchymal cells and fibroblasts derived from mesenchyme at the rim of the optic cup or associated with the hyaloid vascular system, together with minor contributions from the embryonic lens and the inner layer of the optic vesicle, form the vitreous fibrils of the primary vitreous. Ultimately, the primary vitreous comes to lie just behind the posterior pole of the lens in association with remnants of the hyaloid vessels (Cloquet's canal).

B. Second Stage:

(Secondary vitreous, 13 to 65 mm or 6 to 10 week stage.) The fibrils and cells (hyalocytes) of the secondary vitreous are thought to originate from the vascular primary vitreous. Anteriorly, the firm attachment of the secondary vitreous to the internal limiting membrane of the retina constitutes the early stages of formation of the vitreous base. The hyaloid system develops a set of vitreous vessels as well as vessels on the lens capsule surface (tunica vasculosa lentis). The hyaloid system is at its height at 40 mm and then atrophies from posterior to anterior.

C. Third Stage:

(Tertiary vitreous, 65 mm or 10 weeks on.) During the third month, the marginal bundle of Drualt is forming. This consists of vitreous fibrillar condensations extending from the future ciliary epithelium of the optic cup to the equator of the lens. Condensations then form the suspensory ligament of the lens, which is well developed by the 100 mm or 4 month stage. The hyaloid system atrophies completely during this stage.

Optic Nerve

The axons of the ganglion cells of the retina form the nerve fiber layer. The fibers slowly form the optic stalk and then the optic nerve (26 mm stage). Mesenchymal elements enter the surrounding tissue to form the vascular septa of the nerve. Medullation extends from the brain peripherally down the optic nerve, and at birth has reached the lamina cribrosa. Medullation is completed by age 3 months.

Blood Vessels

Long ciliary arteries bud off from the hyaloid at the 16 mm (6 week) stage and anastomose around the optic cup margin with the major circle of the iris by the 30 mm (7 week) stage.

The hyaloid system (see Vitreous, above) atrophies completely by the eighth month. The hyaloid artery gives rise to the central retinal artery and its branches (100 mm or 4 month stage). Buds begin to grow into the retina and develop the retinal circulation, which reaches the ora serrata at 8 months. The branches of the central retinal vein develop simultaneously.

10.1036/1535-8860.ch1

Customer Privacy Policy Copyright ©2002-2003 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Notice. Additional credits and copyright information. For further information about this site contact tech_support@accessmedicine.com. Last modified: October 12, 2002 .