Retina Iris and Ciliary Body

The outer layer of the optic cup, which is characterized by small pigment granules, is known as the pigmented layer of the retina (Figs. 17.3, 17.4, and 17.7). Development of the inner (neural) layer of the optic cup is more complicated. The posterior four-fifths, the pars optica retinae, contains cells bordering the in-traretinal space (Fig. 17.3) that differentiate into light-receptive elements, rods and cones (Fig. 17.5). Adjacent to this photoreceptive layer is the mantle layer, which, as in the brain, gives rise to neurons and supporting cells, including the

Figure 17.2 A. Ventrolateral view of the optic cup and optic stalk of a 6-week embryo. The choroid fissure on the undersurface of the optic stalk gradually tapers off. B. Transverse section through the optic stalk as indicated in A, showing the hyaloid artery in the choroid fissure. C. Section through the lens vesicle, the optic cup, and optic stalk at the plane of the choroid fissure.

Figure 17.3 Section through the eye of a 7-week embryo. The eye primordium is completely embedded in mesenchyme. Fibers of the neural retina converge toward the optic nerve.

Vesicle Retina

Figure 17.4 Scanning electron micrographs of sections through the eyes of mouse embryos at stages equivalent to (A) 6 weeks and (B) 7 weeks in the human. A. The forming lens vesicle, not entirely closed; the two layers of the optic cup; and the lumen (L) of the optic stalk. (Compare with Fig. 17.2C.) B. Lens fibers (Lf) and neural (N) and pigment layers (arrow) forming. (Compare with Fig. 17.3.)

Figure 17.4 Scanning electron micrographs of sections through the eyes of mouse embryos at stages equivalent to (A) 6 weeks and (B) 7 weeks in the human. A. The forming lens vesicle, not entirely closed; the two layers of the optic cup; and the lumen (L) of the optic stalk. (Compare with Fig. 17.2C.) B. Lens fibers (Lf) and neural (N) and pigment layers (arrow) forming. (Compare with Fig. 17.3.)

outer nuclear layer, inner nuclear layer, and ganglion cell layer (Fig. 17.5). On the surface is a fibrous layer that contains axons of nerve cells of the deeper layers. Nerve fibers in this zone converge toward the optic stalk, which develops into the optic nerve (Figs. 17.3 and 17.5). Hence, light impulses pass through most layers of the retina before they reach the rods and cones.

The anterior fifth of the inner layer, the pars ceca retinae, remains one cell layer thick. It later divides into the pars iridica retinae, which forms the inner layer of the iris, and the pars ciliaris retinae, which participates in formation of the ciliary body (Fig. 17.6 and 17.7).

Meanwhile, the region between the optic cup and the overlying surface epithelium is filled with loose mesenchyme (Figs. 17.3, 17.4, and 17.7). The sphincter and dilator pupillae muscles form in this tissue (Fig. 17.6). These muscles develop from the underlying ectoderm of the optic cup. In the adult, the iris is formed by the pigment-containing external layer, the unpigmented internal layer of the optic cup, and a layer of richly vascularized connective tissue that contains the pupillary muscles (Fig. 17.6).

The pars ciliaris retinae is easily recognized by its marked folding (Figs. 17.6B and 17.7). Externally it is covered by a layer of mesenchyme that forms the ciliary muscle; on the inside it is connected to the lens by a network of elastic fibers, the suspensory ligament or zonula (Fig. 17.7). Contraction of the ciliary muscle changes tension in the ligament and controls curvature of the lens.

Figure 17.5 Various layers of the pars optica retinae in a fetus of approximately 25 weeks.

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