Development of the Eye and Orbit
The initial manifestation of eye development in the embryo occurs during the third week of gestation when grooves appear in the cranial aspect of the neural tube; these grooves are termed the optic sulci. As the neural folds fuse during the fourth week, the optic sulci bulge outward from the neural tube toward the surface ectoderm; at this point they are referred to as the optic vesicles. As the optic vesicles extend outward, they remain connected to the developing diencephalon by the hollow optic stalks, which continue to elongate until the optic vesicles contact the surface ectoderm. During this process, neural crest cells from the dorsolateral portion of the neural tube migrate ventrally to surround the developing eye. The neural crest cells differentiate into the skeletal and connective tissue components of the orbit, lids, and conjunctiva. The neural crest cells also give rise to the middle ear bones, teeth, meninges, and endocrine gland components. This common developmental pathway accounts for the association of some developmental ocular lesions with craniofacial, dental, spinal cord, and ear anomalies.
During the fifth week of gestation, the optic vesicles invaginate inferiorly to form the optic cups. Each optic cup is composed of inner and outer layers that are separated by a potential space. A groove termed the choroidal fissure forms along the inferonasal aspect of each optic cup and stalk. The choroidal fissure serves as the pathway for mesenchyme of neural crest cell origin to migrate into the optic cup and stalk. The hyaloid vessels that supply to the developing eye also course through the choroidal fissure. Closure of the choroidal fissure begins in the central aspect of the optic cup during the fifth gestational week and proceeds anteriorly and posteriorly until complete by the end of the seventh week. The optic cups contribute to formation of parts of the retinas, ciliary bodies, and irises.
The optic vesicles induce the overlying surface ectoderm to form the lens placodes. The lens placodes differentiate into the lens pits, which invaginate to form the lens cups and subsequently the lens vesicles. The lens vesicles are hollow spherical structures that are lined by cuboidal epithelium. The lens vesicles eventually detach from the surface ectoderm and come to lie anterior to the developing optic cups. The lens vesicles induce the overlying surface ectoderm to form the corneal epithelium.
Anophthalmia and Microphthalmia
Microphthalmos (microphthalmia) refers to an abnormally small globe, either due to congenital underdevelopment or acquired diminution in size. Simple congenital microphthalmos consists of a small globe without other orbital abnormalities. The deformity is usually unilateral. Congenital complex microphthalmos is most often bilateral and tends to be associated with colobomas and intracranial anomalies.
The most common causes of microphthalmos include posterior coloboma, microphthalmos with cyst, persistent hyperplastic primary vitreous, phthisis bulbi, retinopathy of prematurity, and ...