Optic Vesicle And Optic

As the neural folds progressively fuse in a cranial direction, dilation of the closed neural tube occurs to form the "brain vesicles." By 3 weeks, these vesicles undergo neural segmentation and form the specific parts of the brain, that is, forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) (see Fig. 1-7). Surface ectoderm covers the outside of the forebrain, and neural ectoderm lines the inner or facing surfaces of the paired forebrain vesicles from which the eyes develop (Figs. 1-8, 1-9). The optic sulci develop as bilateral evaginations of neural ectoderm on the facing surfaces of the

FIGURE 1-9A-B. (A) Drawing of a cross section through forebrain and optic sulci of 23- to 26-day-old embryo, during the period of neural tube closure. The optic sulci are lined by neural ectoderm (shaded cells); the surface of the forebrain is covered with surface ectoderm (clear white cells). As the optic sulci (neural ectoderm) evaginate towards the surface ectoderm (hollow arrows), the edges of the brain vesicles move together to fuse, thus closing the neural tube (solid arrows). (B) Drawing of a cross section through a 26-day-old embryo at the level of the optic vesicle. The neural tube has closed, the surface ectoderm now covers the exterior of the forebrain, and the neural ectoderm is completely internalized. The surface ectoderm cells overlying the optic vesicles thicken to form the early lens placode. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;III:1215-1227, with permission).

forebrain vesicles. Expansion of the optic sulci toward the surface ectoderm and fusion of the forebrain vesicles create the optic vesicles (Figs. 1-9, 1-10) by approximately day 25 to 26 (embryo size, 3 mm). Closure of the neural tube and expansion of the optic vesicles occur through the mechanical influences of the cytoskeletal and extracellular matrix and localized proliferation and cell growth.91

The mesencephalic neural crest cells populate the region around the optic vesicle and ultimately give rise to nearly all the connective tissue structures of the avian eye, and the same can be presumed for the mammalian eye (see Table 1-1).55,64 An external bulge indicating the presence of the invaginating optic vesicle can be seen at approximately 25 days human gestation (see Fig. 1-9). The optic vesicle appears to play a significant role in the induction and size determination of the palpebral fissure and orbital and periocular structures.56

At approximately 27 days gestation, the surface ectoderm that is in contact with the optic vesicle thickens to form the lens placode (Figs. 1-9, 1-10, 1-11). The lens placode and underlying neural ectoderm invaginate through differential growth (Fig. 1-10). The invaginating neural ectoderm folds onto itself as the optic vesicle collapses, creating a double layer of neural

FIGURE 1-10. Drawing of a transection through a 28-day-old embryo shows invaginating lens placode and optic vesicle (arrows), thus creating the optic cup. Note the orientation of the eyes 180° from each other; this corresponds to the SEM view shown in Figure 1-12C.

Lens vesicle

FIGURE 1-11A,B. Drawings show the formation of the lens vesicle and optic cup. Note that the optic fissure is present as the optic cup is not yet fused inferiorly. Mesenchyme (M) surrounds the invaginating lens placode. The optic stalk is continuous with the forebrain. Note that the optic cup and optic stalk are neural ectoderm. RPE, retinal pigment epithelium.

ectoderm, the optic cup (Fig. 1-11). The optic cup will eventually differentiate into neurosensory retina (inner layer) and retinal pigment epithelium (RPE) (outer layer) (Fig. 1-11). Local apical contraction112 and physiological cell death91 have been identified during invagination of the lens placode and formation of the optic cup. In the mouse embryo, Msx2, a homeobox-containing transcription factor, is expressed only in the cells of the optic cup that are destined to become neural retina. In vitro Msx2 has been shown to suppress RPE differentiation and may be involved in the initial patterning of the optic cup.48 Abnormal differentiation of the outer layer of the optic cup to form aberrant neural retina has been demonstrated in several mutant mouse strains.21,26,109 The area of future retinal differentiation demonstrates the greatest concentration of vimentin (a cytoskeletal protein) in the optic cup.53 Regionally, within the optic cup, spatial orientation is predicted by expression of the transcription factor, vax2, which defines the ventral region (area of the optic fissure).10 The PAX6 gene has been demonstrated within cells of neural ectodermal origin (optic cup and, later, in the ciliary body and retina), surface ectoderm (lens), and neural crest (cornea).74 The widespread distribution of this gene supports its involvement in many stages of ocular morphogenesis.

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 neu-rosensory 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^

Anterior ^^

lens epithelium

Lid bud

Lid bud

Muscle

Primary vitreous

Muscle

Secondary vitreous

Neurosensory retina

Hyaloid artery

Optic nerve

Mesenchyme

Muscle

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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