The Optic Fissure

Invagination of the optic cup occurs in an eccentric manner with formation of a seam, the optic fissure, inferiorly (Figs. 1-11, 1-12). The optic fissure is also known as the embryonic fissure or choroidal fissure. Mesenchymal tissue (of primarily neural crest origin) surrounds and is within the optic fissure and optic cup, and at 5 weeks the hyaloid artery develops from mesenchyme in the optic fissure. This artery courses from the optic stalk (precursor to the optic nerve) through the optic fissure to the developing lens (Fig. 1-12). The lens vesicle separates from the surface ectoderm at approximately 6 weeks, the same time as closure of the optic fissure. Closure of the optic cup occurs initially at the equator with progression anteriorly and posteriorly.

Once the fissure has closed, secretion of primitive aqueous fluid by the primitive ciliary epithelium establishes intraocular pressure (IOP), which contributes to expansion of the optic cup.15,29 Experimental studies have shown that placement of a capillary tube into the vitreous cavity of a chick eye reduces the IOP and markedly slows growth of the eye.29 Histological

Optic cup Intraretinal space

Optic cup Intraretinal space

FIGURE 1-12. Drawing of cross section at approximately 5 weeks gestation through optic cup and optic fissure. The lens vesicle is separated from the surface ectoderm. Mesenchyme (M) surrounds the developing lens vesicle; the hyaloid artery is seen within the optic fissure. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;111:1215-1227, with permission.)

FIGURE 1-12. Drawing of cross section at approximately 5 weeks gestation through optic cup and optic fissure. The lens vesicle is separated from the surface ectoderm. Mesenchyme (M) surrounds the developing lens vesicle; the hyaloid artery is seen within the optic fissure. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;111:1215-1227, with permission.)

examination of these intubated eyes demonstrated proportional reduction in size of all the ocular tissues except the neural retina and the lens, which were normal in size for the age of the eye. The retina in these eyes was highly convoluted and filled the small posterior segment. Thus, it may be concluded that growth of the neural retina occurs independently of that of the other ocular tissues. Experimental removal of the lens in the eye does not alter retinal growth.30 Growth of the choroid and sclera appear to be dependent upon IOP, as is folding of the ciliary epithelium.12 Failure or late closure of the optic fissure prevents the establishment of normal fetal IOP and can therefore result in microphthalmia associated with colobomas, that is, colobo-matous microphthalmia (see Ocular Dysgenesis later in this chapter).

Figure 1-13 shows a diagram of the eye at the end of the seventh week and after optic fissure closure. At this stage, the neurosensory retina and pigment epithelium are in apposition, the optic nerve is developing, and the lens has separated from the cornea, thus forming the anterior chamber. Mesenchymal tissue

Anterior chamber

Lens fibers"

Cornea

Anterior lens epithelium

Lid bud

Lid bud

Mesenchyme

Primary vitreous Muscle

/ Secondary vitreous RPE

Neurosensory retina

Hyaloid artery

Optic nerve

Mesenchyme

FIGURE 1-13. Overview at the 7th week of gestation. The developing eye is surrounded by mesenchyme of neural crest origin. (From Sulik KK, Schoenwolf GC. Scanning Electron Microsc 1985;IV: 1735-1752, with permission.)

(neural crest cell origin) around the primitive retina develops into the choroid and sclera. Peripheral to the developing globe are linear accumulations of myoblasts (mesodermal origin) that are anlagen of the extraocular muscles. The eyelids are small buds above and below the developing eye. The hyaloid vasculature courses from the primitive optic nerve to the posterior lens capsule.

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