Embryology 19 eye

EMBRYLOGY BY

DR. THAAER MAOHAMMED DAHER ALSAADM.B.Ch.B. (MBBS) F.I.B.M.S. (PhD)

SPECIALIST IN GENERAL SURGERYSENIOR LECTURER

IMS MSU

Eye

DEVELOPMENT

Optic Cup and Lens Vesicle 1/2

• The developing eye appears in the 22-day embryo as a pair of shallow grooves on the sides of the forebrain.

• With closure of the neural tube, these grooves form outpocketings of the forebrain, the optic vesicles.

• The optic vesicle begins to invaginate and forms the double-walled optic cup.

• 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.• 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.

C. Transverse sectionthrough the forebrain of a 5-mm embryo showing invagination of the optic vesicle andthe lens placode

A. Transverse section through the forebrain of a 22-day embryo (approximately14 somites), showing the optic grooves

B. Transverse section through the forebrainof a 4-week embryo showing the optic vesicles in contact with the surface ectoderm.Note the slight thickening of the ectoderm (lens placode).

E. Scanning electron micrograph of amouse embryo during formation of the optic vesicle. The embryo has been cut sagittallyto reveal the inside of the brain vesicles and outpocketing of the optic vesicle(arrow) from the forebrain. H, hindbrain; and M, midbrain.

D. Scanning electron micrograph showing a frontal view of a mouse

H

M

Scanning electron micrographs of sections through the eyes of mouse embryos at stages equivalent to (A) 6 weeks (b) 7 weeks

A. Theforming lens vesicle, not entirely closed; the two layers of the optic cup; and the lumen(L) of the optic stalk.

(B) 7 weeks in the human B. Lens fibers (Lf ) and neural (N) andpigment layers (arrow) forming

Development of the iris and ciliary body. The rim of the optic cup is coveredby mesenchyme, in which the sphincter and dilator pupillae develop from the underlying ectoderm

Section through the eye of a 15-week fetus showing the anterior chamber,iridopupillary membrane, inner and outer vascular layers, choroid, and sclera

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

A. Sixth week (9 mm).. B. Seventh week (15 mm). C. Ninth week

B. Persistence of the iridopupillary membrane.

A. Coloboma iris.

• During the seventh week,• The lips of the choroid fissure fuse,• The mouth of the optic cup becomes a round opening, the

future pupil.• The cells of the surface ectoderm begin to elongate and

form the lens placode.• This placode subsequently invaginates and develops into

the lens vesicle. • During the fifth week, the lens vesicle loses contact with

the surface ectoderm and lies in the mouth of the optic cup.

Optic Cup and Lens Vesicle 2/2

Retina, Iris, and Ciliary Body 1/4

• The outer layer of the optic cup is known as the pigmented layer of the retina.

• Development of the inner (neural) layer of the optic cup is more complicated. The posterior four-fifths, the pars optica retinae, contains cells bordering the intraretinal space that differentiate into light-receptive elements, rods and cones.

• Adjacent to this photoreceptive layer is the mantle layer, which, as in the brain, gives rise to neurons and supporting cells, including the outer nuclear layer, inner nuclear layer, and ganglion cell layer.

Development of the iris and ciliary body. The rim of the optic cup is coveredby mesenchyme, in which the sphincter and dilator pupillae develop from the underlying ectoderm

• The optic stalk develops into the optic nerve. • The anterior fifth of the inner layer, the pars ceca

retinae, remains one cell layer thick. • It later divides into the pars iridica retinae, which

forms the inner layer of the iris, and the pars ciliaris retinae, which participates in formation of the ciliary body.

• The region between the optic cup and the overlying surface epithelium is filled with loose mesenchyme.


• 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.


• The pars ciliaris retinae is easily recognized by its marked folding.

• Externally it is covered by a layer of mesenchyme that forms the ciliary muscle; on the inside it is connected to the lens by a network of elastic fibers, the suspensory ligament or zonula.

• Contraction of the ciliary muscle changes tension in the ligament and controls curvature of the lens.


Lens

• Shortly after formation of the lens vesicle, cells of the posterior wall begin to elongate anteriorly and form long fibers that gradually fill the lumen of the vesicle.

• By the end of the seventh week, these primary lens fibers reach the anterior wall of the lens vesicle.

• Growth of the lens is not finished at this stage,• New (secondary) lens fibers are continuously added

to the central core.

Choroid, Sclera, and Cornea 1/2

• At the end of the fifth week, • The eye primordium is completely surrounded by loose

mesenchyme.• This tissue 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.

• Differentiation of mesenchymal layers overlying the anterior aspect of the eye is different.

• The anterior chamber forms through vacuolization and splits the mesenchyme into;

• Inner layer in front of the lens and iris, the iridopupillary membrane, • Outer layer continuous with the sclera, the substantia propria of the cornea.• The anterior chamber itself is lined by flattened mesenchymal cells.• The cornea is formed by;

1. An epithelial layer derived from the surface ectoderm,2. The substantia propria or stroma, which is continuous with the sclera,3. An epithelial layer, which borders the anterior chamber. The iridopupillary membrane in front of the lens disappears

completely, providing communication between the anterior and posterior eye chambers.

Choroid, Sclera, and Cornea 2/2

Vitreous Body

• Mesenchyme not only surrounds the eye primordium from the outside but also invades the inside of the optic cup by way of the choroid fissure.

• Here it forms the hyaloid vessels, which during intrauterine life supply the lens and form the vascular layer on the inner surface of the retina.

• In addition, it forms a delicate network of fibers between the lens and retina.

• The interstitial spaces of this network later fill with a transparent gelatinous substance, forming the vitreous body.

• The hyaloid vessels in this region are obliterated and disappear during fetal life, leaving behind the hyaloid canal.

Optic Nerve• The optic cup is connected to the brain by the optic stalk, which has a groove, the

choroid fissure, on its ventral surface.• In this groove are the hyaloid vessels. • The nerve fibers of the retina returning to the brain lie among cells of the inner wall of

the stalk.• During the seventh week,• The choroid fissure closes, and a narrow tunnel forms inside the optic stalk.• As a result of the continuously increasing number of nerve fibers, the inner wall of the

stalk grows, and the inside and outside walls of the stalk fuse.• Cells of the inner layer provide a network of neuroglia that support the optic nerve

fibers.• The optic stalk is thus transformed into the optic nerve.• Its center contains a portion of the hyaloid artery, later called the 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 in the optic nerve.

A. Sixth week (9 mm).. B. Seventh week (15 mm). C. Ninth week

C L I N I C A L C O R R E L A T E SEye Abnormalities 1/2

1. 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. 2. The iridopupillary membrane (Fig. 17.11B) may persist instead of

being resorbed during formation of the anterior chamber.3. 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)

4. The hyaloid artery may persist to form a cord or cyst.5. In microphthalmia the eye is too small. (results from intrauterine infections

such as cytomegalovirus).

6. Anophthalmia is absence of the eye.7. Congenital aphakia (absence of the lens) and aniridia (absence of the

iris) are rare anomalies.8. Cyclopia (single eye) and synophthalmia (fusion of the eyes) (comprise a

spectrum of defects in which the eyes are partially or completely fused), (these defects are invariably associated with cranial defects, such as holoprosencephaly, in which the cerebral hemispheres are partially or completely merged into a single telencephalic vesicle).

9. Blue sclera. (thin sclera through which the pigment of choroid can be seen).10. Anomalies of pigmentation/ albinism.

C L I N I C A L C O R R E L A T E SEye Abnormalities 2/2

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 part of the eye ball.

Hypophysis Cerebri, Pineal, Adrenal

• Hypophysis Cerebri;• Developed from two parts;

1. The anterior and intermediate parts develop from an ectodermal diverticulum (Rathke’s pouch).

2. The pars nervosa and stalk develop from a downgrowth from the floor of the third ventricle (diencephalon).

Notes; Craniopharyngioma is a peculiar tumor of the remnant of the

Rathke’s pouch. Accessory anterior lobe may be seen in relation to the posterior wall

of the pharynx. Rarely hypophysis agenesis or hypoplasia (failure to develop).

Pineal Gland

• The pineal gland or the pineal body arises from the roof of the diencephalon.

• The specific cells of the pineal body are believed to be modified neurological cells.

• For long considered vestigeal structure of no importance.

• The pineal gland secretes a number of hormones that have a regulatory influence on many other endocrine glands.

Adrenal Gland• The adrenal gland consists of a superficial cortex and a deeper

medulla.• The cells of the cortex arise from coelomic epithelium (mesoderm).• The cells of the medulla are derived from the neural crest (ectoderm).• Chromaffin tissue is made up of cells similar to those of the adrenal

medulla and derived from the cells of the neural crest. (this tissue is related to the abdominal aorta, also seen in relation to sympathetic ganglia and plexuses and along the splanchnic nerves).

• The cells of the coelomic epithelium may be divided into two groups:1. Fetal cortex – disappear after birth.2. Defenitive cortex.

Anomalies of Adrenal Gland

1. Ectopic adrenal cortical tissue.2. Ectopic Adrenal gland.3. Congenital hyperplasia;

A. In male it leads to Adrenogenital syndrome.B. In female it leads to pseudohermaphroditism.

Embryology 19 eye

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