Embryology of the eye by Sumayya Naseem Optometrist
Embryology of Human EyeBy: Sumayya NaseemOptometristResearch Officer, School Eye Health Programme, Pakistan.CHEF International
Embryo & Embryogenesis…. A short IntroductionIn humans, an embryo is the name given to a form until about eight weeksafter fertilization and from then it is instead called a fetus.The development of the embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm fertilizes an egg cell, theresult is a cell called the zygote that has half of the DNA of each of two parents.In plants, animals, the zygote will begin to divide by mitosis to produce amulticellular organism. The result of this process is an embryo.
Optic Cup and Lens Vesicle The developing eye appears in the 22-day embryo as a pair of shallow grooves on the sides of the forebrain. These grooves form outpocketings of the forebrain, the optic vesicles (4 th week). The optic vesicle begins to invaginate and forms the double-walled optic cup (formation of Optic Cup). (Transverse section through the forebrain of embryo)
Optic cup —invagination of distaloptic vesicle to form double walled“cupLens pit—invaginates to formlens vesicleThe cells of the surface ectodermbegin to elongate and form the lensplacode.This placode subsequentlyinvaginates and develops into thelens vesicle.
The inner and outer layers of this cup are initially separated by a lumen,the intraretinal space,This lumen disappears, and the two layers appose each other.
Optic (choroid) fissure —sulcuson ventral aspect optic cup/stalkInvagination is not restricted to thecentral portion of the cup but alsoinvolves a part of the inferior surfacethat forms the choroid fissure. Formation of this fissure allows thehyaloid artery to reach the innerchamber of the eye.
Iris Muscles , Pupil The sphincter and dilator pupillae muscles form in this tissue. 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.
Choroid & Sclera At the end of the fifth week, tissues soon differentiates into an inner layer comparable with the pia mater of the brain and an outer layer comparable with the dura mater. The inner layer later forms a highly vascularized pigmented layer known as the choroid. The outer layer develops into the sclera and is continuous with the dura mater around the optic nerve.
Optic Nerve The optic stalk is thus transformed into the optic nerve. Its center contains a portion of central artery of the retina. On the outside, a continuation of the choroid and sclera, the pia arachnoid and dura layer of the nerve, respectively, surround the optic nerve.
B. Seventh week (15 mm). C. Ninth weekA. Sixth week (9mm). Transformation of the optic stalk into the optic nerve. (Note the central artery of the retina)
CLINICAL CORRELATES Eye Abnormalities Coloboma may occur if the choroid fissure fails to close. (Normally this fissure closes during the seventh week of development). Coloboma iridis. Coloboma is a common eye abnormality frequently associated with other eye defects. Colobomas (clefts) of the eyelids may also occur.
CLINICAL CORRELATES Eye Abnormalities The iridopupillary membrane in front of the lens disappears completely, providing communication between the anterior and posterior eye chambers. The iridopupillary membrane may persist instead of being resorbed during formation of the anterior chamber.
CLINICAL CORRELATES Eye Abnormalities In congenital cataracts the lens becomes opaque during intrauterine life. Although this anomaly is usually genetically determined, many children of mothers who have had German measles (rubella) between the fourth and seventh weeks of pregnancy have cataracts. (If the mother is infected after the seventh week of pregnancy, the lens escapes damage, but the child may be deaf as a result of abnormalities of the cochlea)
CLINICAL CORRELATES Eye Abnormalities The hyaloid artery may persist to form a cord or cyst. In microphthalmia the eye is too small. (results from intrauterine infections such as cytomegalovirus). Anophthalmia is absence of the eye. Congenital aphakia (absence of the lens) and aniridia (absence of the iris) are rare anomalies.
CLINICAL CORRELATES Eye Abnormalities Cyclopia (single eye) and synophthalmia (fusion of the eyes) (comprise a spectrum of defects in which the eyes are partially or completely fused) Blue sclera. (thin sclera through which the pigment of choroid can be seen). Anomalies of pigmentation/ albinism.
CLINICAL CORRELATES Eye Abnormalities Retinal detachment—between inner and outer portions of the optic cup derivatives •congenital—failure of fusion •acquired—trauma
CLINICAL CORRELATES Eye Abnormalities Extraocular Muscles… Innervated via CN III, IV, & VI--- Coordinate movements between the two eyes Extraocular Muscle Anomalies (congenital): Agenesis (single muscle usually) Anomalous Attachments (misplaced additional attachments) Adherence & Fibrosis Syndromes **Failure to align visual axes (strabismus), thus potentially resulting in diplopia (double-vision) and Amblyopia—reduced/absent visual ability in one eye “lazy” eye.
Summary of various parts of the eye ballPart Derived fromLens Surface ectodermRetina Neuroectoderm (optic cup)Vitreous MesodermChoroid Mesoderm (infiltrated by neural crest cells)Ciliary body MesodermCiliary muscles Mesenchymal cells covering the developing ciliary body (neural crest)Iris MesodermMuscles of the iris Neuroectoderm (from optic cup)Sclera Mesoderm (infiltrated by neural crest cells?)Cornea Surface epithelium by ectoderm, substantia propria and inner epithelium by neural crestConjunctiva Surface ectodermBlood vessels mesodermOptic nerve Neuroectoderm. Its covering (pia, arachnoid and dura) are derived from mesoderm