The uvea (also called uveal tract)—comprising the ciliary body, iris, and choroid—develops from a combination of mesoderm and neural crest cells. The corresponding epithelial layers of the ciliary body and iris are derived from the neuroectoderm and are not considered part of the uvea, the pigmented vascular layer of the eye. The uvea obtains its dark color from neural crest–derived melanocytes residing within it. Its blood vessels and ciliary muscles are derived from the mesoderm.
Ciliary body and iris
At the anterior aspect of the optic cup, the surrounding mesoderm proliferates, pushing the neuroectoderm inward and centrally between the corneal endothelium and the anterior lens surface, giving rise to the ciliary body and iris epithelium (Fig 4-13).
Figure 4-11 Development of the vitreous. The mesoderm gives rise to the hyaloid artery, which is contained within the primary vitreous. This vascular system supplies the tunica vasculosa lentis. The secondary vitreous forms from hyalocytes as the primary vitreous regresses. The zonular fibers develop from the tertiary vitreous.
(Modified with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E. The Eye: Basic Sciences in Practice. 4th ed. Edinburgh: Elsevier; 2016:123.)
Figure 4-12 Illustration and corresponding histologic image showing the relationship between the optic cup, primary vitreous, and lens. The ciliary body and iris are absent. Note the cornea and eyelids developing from surface ectoderm.
(Reproduced with permission from Spalton D, Hitchings R, Hunter P. Atlas of Clinical Ophthalmology. 3rd ed. New York: Elsevier/Mosby; 2005:398.)
Figure 4-13 Development of the iris and ciliary body in a human fetal eye from 12 to 22 weeks. A, Arrows indicate proliferating vascular mesoderm behind the neuroectoderm at the optic cup margin (OCM). B, Continued growth of the mesoderm, with infolding of the neuroectoderm and development of a ciliary process (CP). Note the inner pigmented and outer nonpigmented layers of the ciliary epithelium. C, Anteriorly, the neuroectoderm forms the epithelial layers of the iris (I). At this stage, the angle recess is present, with developing trabecular meshwork (TM) and ciliary muscle (CM) and intervening scleral spur (SS). D, Developing CPs and iris. Note that the posterior nonpigmented epithelium of the iris (PNPE) is continuous with the nonpigmented ciliary epithelium. The PNPE will acquire pigment as the iris develops. CB = ciliary body; PM = pupillary membrane; R = retina; SC = Schlemm canal.
(Modified with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E. The Eye: Basic Sciences in Practice. 4th ed. Edinburgh: Elsevier; 2016:125.)
Figure 4-14 Development of the iris. The iris sphincter and dilator muscles are derived from neuroectoderm. The dilator muscle arises directly from the anterior iris epithelium.
(Modified with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E. The Eye: Basic Sciences in Practice. 4th ed. Edinburgh: Elsevier; 2016:126.)
The mesodermal proliferation results in formation of the ciliary muscle and leads to infolding of the neuroectoderm. These folds give rise to the ciliary processes, which are lined by 2 layers of epithelium: an inner pigmented layer and an outer nonpigmented layer. The outer nonpigmented layer of the ciliary body is continuous with the retina posteriorly and the nonpigmented posterior epithelium of the iris anteriorly. The latter acquires pigment over the course of development, starting at the pupil margin and progressing radially to the iris root, leading to the posterior iris pigment epithelium found in the adult eye. Pigmentation does not occur in the anterior epithelial layer of the iris.
Anteriorly, the neuroectoderm incorporates surrounding mesenchymal elements from the tunica vasculosa lentis. The subsequent anterior component, of mesodermal origin, gives rise to the iris stroma and vasculature. Posteriorly, the neuroectoderm continues as the epithelial layers of the iris and gives rise to the sphincter and dilator muscles. The dilator muscles are a direct extension of the anterior iris epithelium (Fig 4-14).
Condensation of neural crest cells and mesoderm surrounding the optic cup produces the choroid on the inner aspect of the cup and the sclera and cornea on its outer aspect (see Fig 4-6D, E). A layer of small blood vessels, the choriocapillaris, forms first and is fenestrated. This is followed by development of an outer layer of larger vessels, which gives rise to the vortex veins and branches of the posterior ciliary circulation. Subsequently, a middle layer of arterioles forms between the choriocapillaris and the outer layer of larger vessels. Melanocytes develop in the choroid later in gestation.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.