Eye development

Transverse section of head of chick embryo of forty-eight hours’ incubation
Transverse section of head of chick embryo of fifty-two hours’ incubation, showing the lens and the optic cup

Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week.[1] Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.[2][3][4]

Neuroepithelium forms the retina, ciliary body, iris, and optic nerves. Surface ectoderm forms the lens, corneal epithelium and eyelid. The extracellular mesenchyme forms the sclera, the corneal endothelium and stroma, blood vessels, muscles, and vitreous.

The eye begins to develop as a pair of optic vesicles on each side of the forebrain at the end of the fourth week of pregnancy. Optic vesicles are outgrowings of the brain which make contact with the surface ectoderm and this contact induces changes necessary for further development of the eye. Through a groove at the bottom of the optic vesicle known as choroid fissure the blood vessels enter the eye. Several layers such as the neural tube, neural crest, surface ectoderm, and mesoderm contribute to the development of the eye.[2][3][4]

Eye development is initiated by the master control gene PAX6, a homeobox gene with known homologues in humans (aniridia), mice (small eye), and Drosophila (eyeless). The PAX6 gene locus is a transcription factor for the various genes and growth factors involved in eye formation.[1][5] Eye morphogenesis begins with the evagination, or outgrowth, of the optic grooves or sulci. These two grooves in the neural folds transform into optic vesicles with the closure of the neural tube.[6] The optic vesicles then develop into the optic cup with the inner layer forming the retina and the outer portion forming the retinal pigment epithelium. The middle portion of the optic cup develops into the ciliary body and iris.[7] During the invagination of the optic cup, the ectoderm begins to thicken and form the lens placode, which eventually separates from the ectoderm to form the lens vesicle at the open end of the optic cup.[1][3][4]

Further differentiation and mechanical rearrangement of cells in and around the optic cup gives rise to the fully developed eye.

  1. ^ a b c Ort, D., David, H., "Development of the Eye". Retrieved 22 April 2015.
  2. ^ a b Sadler, T.W (1990). Langman's medical embryology (6th ed.). Williams and Wilkins. ISBN 978-0683074932.
  3. ^ a b c Hosseini, Hadi S.; Beebe, David C.; Taber, Larry A. (2014). "Mechanical effects of the surface ectoderm on optic vesicle morphogenesis in the chick embryo". Journal of Biomechanics. 47 (16): 3837–3846. doi:10.1016/j.jbiomech.2014.10.018. PMC 4261019. PMID 25458577.
  4. ^ a b c Hosseini, Hadi S.; Taber, Larry A. (2018). "How mechanical forces shape the developing eye". Progress in Biophysics and Molecular Biology. 137 (16): 25–36. doi:10.1016/j.pbiomolbio.2018.01.004. PMC 6085168. PMID 29432780.
  5. ^ Keller, A. M. V., "Embryonic Development of the Eye" Archived 2015-04-08 at the Wayback Machine. Retrieved 22 April 2015.
  6. ^ Fuhrmann, S., Levine, E. M. and Reh, T. A. (2000). "Extraocular mesenchyme patterns the optic vesicle during early eye development in the embryonic chick". Development 127, 4599–4609.
  7. ^ LifeMap Science, Inc. "Embryonic and Postnatal Development of the Eye". Retrieved 22 April 2015.