Eye: Ocular Embryological Development
Embryological development of the ocular system is the formation of the eye during pregnancy. While the process continues throughout gestation, it is not complete even at birth. The newly formed eyes are unable to see the same as those of an adult. Babies have much lower numbers of photoreceptors and the ones present are still immature, resulting in poor vision, especially at distances (myopia). It takes another two to three years before the child's eyes are fully functional.
Ocular tissues are derived from all embryonic layers: ectoderm—surface ectoderm (lens, all eye epithelia, epidermis of the eyelids, lacrimal gland) and neural ectoderm (retina, retinal pigment epithelium, optic nerve), mesoderm (blood vessel endothelium) and endoderm.
The human eye begins to develop in the second week of pregnancy with formation of the optic primordium. This develops into the optic vesicle at 22 days, which soon forms easily recognizable optic cup. In the part distal from the surface ectoderm it is possible to identify the developing retina. Simultaneously at the surface ectoderm, the lens begins to form and becomes fully identifiable within five days (32 days). After separation from the surface ectoderm during the fifth week, the lens vesicle forms reaching the definite size at eight weeks. However, the lens development does not stop after birth, but continues during one's lifetime.
An important connection between the optic cup and brain happens at 36 days when the optic stalk connects the cup to the forebrain. During this early eye development, a hyaloid artery comes into the space in the optic cup between the forming lens and retina supplying the nutrients.
Parallel to the development of the retinal layer and lens, the vitreous is formed, which will fill the chamber between the two structures. Development of the retina is a very long process. It takes approximately four months to form the photoreceptor precursors and the retinal vessels. The retinal layers are fully developed at six months and the central retinal artery is fully functional by eight months, having developed from the hyaloid artery, which itself regresses from the vitreous. The macular area, responsible for color vision starts forming at four to five months, but does not mature until after birth at approximately six months.
Development of the anterior part of the eye happens very early during embryogenesis as the ocular structures develop and differentiate sequentially from the optic cup tissues. The cornea forms initially as a single cell layer soon after the lens at 33 days, and during the next two months, transforms into outer (epithelial) and inner (stromal) layers. However, it becomes innervated at five months, at the same time as the Bowman's membrane covering the cornea develops. Moreover, the cornea remains non-transparent due to high hydration until tissue maturation is complete. The iris starts to develop from the rim of the optic cup at 30-35 days. Once the iris is formed, the anterior eye chamber develops from seven weeks. The muscles regulating the iris (sphincter and dilator), however, do not develop till five to six months and pigmentation occurs after birth.
The developing eyes need protection, and the eyelids start to form at two months and fuse together allowing the development of additional structures such as muscles, connective tissues and tarsal plates. Starting from the fifth month, the eyelids begin to separate and the process is complete in two months.
Functionally important parts of the eye are the extraocular muscles, which allow eye movement. These start forming at four weeks, at the same time as the orbits form, their size being predetermined by the size of the optic cup. Depending on the postnatal growth of the globe, the orbits might mature at anytime from two to sixteen years postnatally.
Lacrimal glands that will provide tears, an important part of eye protection, start forming at six weeks, but the maturation of the epithelial cords to form a hollow channel happens postnatally. Moreover, the tears are not formed in the glands until the third month.
As the back of the eye develops and becomes innervated, a protective layer of sclera forms in the anterior part of the eye at seven weeks, and within five more weeks, surrounds the optic nerve at the posterior part of the eye.
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