Embryology of Human Eye

By: Sumayya NaseemOptometrist

Research Officer, School Eye Health Programme, Pakistan.CHEF International

Embryo & Embryogenesis…. A short Introduction

In humans, an embryo is the name given to a form until about eight weeks after 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, the result 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 a multicellular 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 (4th 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“cup

Lens pit—invaginates to formlens vesicle

The cells of the surface ectoderm begin to elongate and form the lens placode.This placode subsequently invaginates and develops into the lens 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 —sulcus on ventral aspect optic cup/stalk

Invagination is not restricted to the central portion of the cup but also involves a part of the inferior surface that forms the choroid fissure.

Formation of this fissure allows the hyaloid artery to reach the inner chamber 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.

Transformation of the optic stalk into the optic nerve. (Note the central artery of the retina)

A. Sixth week (9 mm).

B. Seventh week (15 mm). C. Ninth week

VISUAL REFLEXES

Pupillary Light Reflexes: 30wks gestation Constriction (parasympathetic) Dilatation (sympathetic) Accommodation (4 months = well developed)

Accommodation and natural lens.mp4Accommodation.mp4

C L I N I C A L C O R R E L A T E S 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.

C L I N I C A L C O R R E L A T E S 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.

C L I N I C A L C O R R E L A T E S 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)

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.

C L I N I C A L C O R R E L A T E S Eye Abnormalities

C L I N I C A L C O R R E L A T E S 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.

C L I N I C A L C O R R E L A T E S Eye

Abnormalities

Retinal detachment—between inner and outer portions of the optic cup derivatives

•congenital—failure of fusion •acquired—trauma

C L I N I C A L C O R R E L A T E S

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.

Part Derived from

Lens Surface ectoderm

Retina Neuroectoderm (optic cup)

Vitreous Mesoderm

Choroid Mesoderm (infiltrated by neural crest cells)

Ciliary body Mesoderm

Ciliary muscles Mesenchymal cells covering the developing ciliary body (neural crest)

Iris Mesoderm

Muscles 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 crest

Conjunctiva Surface ectoderm

Blood vessels mesoderm

Optic nerve Neuroectoderm. Its covering (pia, arachnoid and dura) are derived from mesoderm

Summary of various parts of the eye ball