Development Of Other Eye Structures see Table 14

A. Sclera. The sclera develops from mesoderm surrounding the optic cup. It forms an outer fibrous layer that is continuous with the dura mater posteriorly and the cornea anteriorly.

B. Choroid. The choroid develops from mesoderm surrounding the optic cup. It forms a vascular layer which is continuous with the pia/arachnoid posteriorly and the iris/ciliary body anteriorly.

C. Anterior chamber. This chamber develops from mesoderm over the anterior aspect of the eye, which is continuous with the sclera and undergoes vacuolization to form a chamber. The anterior chamber essentially splits the mesoderm into two layers:

1. The mesoderm posterior to the anterior chamber is called the iridopupillary membrane, which is normally resorbed before birth.

2. The mesoderm anterior to the anterior chamber develops into the substantia propria of the cornea and the corneal endothelium.

D. Cornea. The cornea develops from both surface ectoderm and mesoderm lying anterior to the anterior chamber. The surface ectoderm forms the anterior epithelium of the cornea. The mesoderm forms the substantia propria of the cornea and corneal endothelium.

E. Lens. The lens develops from surface ectoderm, which forms the lens placode. The

Stroma

Stroma

Dilator Pupillae Muscles

Choroid

Outer pigment layer Inner nuclea' layer

Two layers of A columnar cells

Ciliary body

Choroid

Outer pigment layer Inner nuclea' layer

Sphincter pupillae muscle

Dilator pupillae muscle

Ciliary muscle

Dilator pupillae muscle

Ciliary muscle

Sphincter Pupillae

Suspensory fibers

Ciliary processes

Suspensory fibers

Ciliary processes

Two layers of A columnar cells

Figure 14-2. (A, B) Sagittal sections through the developing iris and ciliary body. The iris and ciliary body form from the outer pigment layer and inner neural layer of the optic cup. In the adult, this embryologic origin is reflected histologically by two layers of columnar epithelium that line both the iris and the ciliary body. (Adapted from Dudek RW, Fix JD: DRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 116.)

lens placode invaginates to form the lens vesicle. The cells of the posterior wall of the lens vesicle elongate, lose their nuclei, and form the lens fibers of the adult lens.

F. Vitreous body. The vitreous body develops from mesoderm that migrates through the choroid fissure and forms a transparent gelatinous substance between the lens and the retina. It contains the hyaloid artery, which later obliterates to form the hyaloid canal of the adult eye.

G. Canal of Schlemm. The canal of Schlemm is found at the sclerocorneal junction called the limbus. This canal drains the aqueous humor into the venous circulation. Obstruction results in increased intraocular pressure (glaucoma).

H. Extraocular muscles. These muscles develop from mesoderm surrounding the optic cup that has been called preotic myotomes, although in humans the origin is still controversial.

A. Coloboma iridis (Figure 14^3A) is a cleft in the iris caused by failure of the choroid fissure to close in week 7 of development. This cleft may extend into the ciliary body, retina, choroid, or optic nerve. Palpebral coloboma, a notch in the eyelid, results from a defect in the developing eyelid.

B. Congenital cataracts (see Figure 14-3B) are opacities of the lens and are usually bilateral. They are common and may result from rubella virus infection, toxoplasmosis, congenital syphilis, Down syndrome (trisomy 21), or galactosemia (an inborn error of metabolism).

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