Ординатура / Офтальмология / Английские материалы / Comprehensive Ophthalmology_Khurana_2007

Section-I

ANATOMY,

PHYSIOLOGY

AND

DISEASES

OF THE EYE

ANATOMY OF THE EYE

zThe eyeball

zVisual pathway

zOrbit, extraocular muscles and appendages of the eye

DEVELOPMENT OF THE EYE

zFormation of optic vesicle and optic stalk

zFormation of lens vesicle

zFormation of optic cup

zChanges in the associated mesoderm

zDevelopment of various ocular structures

zStructures derived from the embryonic layers

zImportant milestones in the development of the eye

ANATOMY OF THE EYE

This chapter gives only a brief account of the anatomy of eyeball and its related structures. The detailed anatomy of different structures is described in the relevant chapters.

THE EYEBALL

Each eyeball (Fig. 1.1) is a cystic structure kept distended by the pressure inside it. Although, generally referred to as a globe, the eyeball is not a sphere but an ablate spheroid. The central point on the maximal convexities of the anterior and posterior curvatures of the eyeball is called the anterior and posterior pole, respectively. The equator of the eyeball lies at the mid plane between the two poles (Fig.1.2).

Coats of the eyeball

The eyeball comprises three coats: outer (fibrous coat), middle (vascular coat) and inner (nervous coat).

1.Fibrous coat. It is a dense strong wall which protects the intraocular contents. Anterior 1/6th of this fibrous coat is transparent and is called cornea. Posterior 5/6th opaque part is called sclera. Cornea is set into sclera like a watch glass. Junction of the cornea and sclera is called limbus. Conjunctiva is firmly attached at the limbus.

2.Vascular coat (uveal tissue). It supplies nutrition to the various structures of the eyeball. It consists of three parts which from anterior to posterior are : iris, ciliary body and choroid.

3.Nervous coat (retina). It is concerned with visual functions.

Segments and chambers of the eyeball

The eyeball can be divided into two segments: anterior and posterior.

1. Anterior segment. It includes crystalline lens (which is suspended from the ciliary body by zonules), and structures anterior to it, viz., iris, cornea and two aqueous humour-filled spaces : anterior and posterior chambers.

zAnterior chamber. It is bounded anteriorly by the back of cornea, and posteriorly by the iris and part of ciliary body. The anterior chamber is about 2.5 mm deep in the centre in normal adults. It is shallower in hypermetropes and deeper in myopes, but is almost equal in the two eyes of the same individual. It contains about 0.25 ml of the aqueous humour.

zPosterior chamber. It is a triangular space containing 0.06 ml of aqueous humour. It is bounded anteriorly by the posterior surface of iris and part of ciliary body, posteriorly by the crystalline lens and its zonules, and laterally by the ciliary body.

2. Posterior segment. It includes the structures posterior to lens, viz., vitreous humour (a gel like material which fills the space behind the lens), retina, choroid and optic disc.

VISUAL PATHWAY

Each eyeball acts as a camera; it perceives the images and relays the sensations to the brain (occipital cortex) via visual pathway which comprises optic nerves, optic chiasma, optic tracts, geniculate bodies and optic radiations (Fig. 1.3).

ORBIT, EXTRAOCULAR MUSCLES AND APPENDAGES OF THE EYE (FIG. 1.4)

Fig. 1.2. Poles and equators of the eyeball.

Each eyeball is suspended by extraocular muscles and fascial sheaths in a quadrilateral pyramid-shaped

bony cavity called orbit (Fig. 1.4). Each eyeball is located in the anterior orbit, nearer to the roof and lateral wall than to the floor and medial wall. Each eye is protected anteriorly by two shutters called the eyelids. The anterior part of the sclera and posterior surface of lids are lined by a thin membrane called conjunctiva. For smooth functioning, the cornea and conjunctiva are to be kept moist by tears which are produced by lacrimal gland and drained by the lacrimal passages. These structures (eyelids, eyebrows, conjunctiva and lacrimal apparatus) are collectively called ‘the appendages of the eye’.

z Visceral mesoderm of maxillary process. Before going into the development of individual

structures, it will be helpful to understand the formation of optic vesicle, lens placode, optic cup and changes in the surrounding mesenchyme, which play a major role in the development of the eye and its related structures.

FORMATION OF OPTIC VESICLE AND OPTIC STALK

The area of neural plate (Fig. 1.5A) which forms the prosencepholon develops a linear thickened area on either side (Fig. 1.5B), which soon becomes depressed to form the optic sulcus (Fig. 1.5C). Meanwhile the neural plate gets converted into prosencephalic vesicle. As the optic sulcus deepens, the walls of the prosencepholon overlying the sulcus bulge out to form the optic vesicle (Figs. 1.5D, E&F). The proximal part of the optic vesicle becomes constricted and elongated to form the optic stalk (Figs. 1.5G&H).

FORMATION OF LENS VESICLE

The optic vesicle grows laterally and comes in contact with the surface ectoderm. The surface ectoderm, overlying the optic vesicle becomes thickened to form the lens placode (Fig. 1.6A) which sinks below the surface and is converted into the lens vesicle (Figs. 1.6 B&C). It is soon separated from the surface ectoderm at 33rd day of gestation (Fig. 1.6D).

FORMATION OF OPTIC CUP

The optic vesicle is converted into a double-layered optic cup. It appears from Fig. 1.6 that this has happened because the developing lens has invaginated itself into the optic vesicle. In fact conversion of the optic vesicle to the optic cup is due to differential growth of the walls of the vesicle. The margins of optic cup grow over the upper and lateral sides of the lens to enclose it. However, such a growth does not take place over the inferior part of the lens, and therefore, the walls of the cup show deficiency in this part. This deficiency extends to

Fig. 1.5. Formation of the optic vesicle and optic stalk.

Fig. 1.7. Optic cup and stalk seen from below to show

CHANGES IN THE ASSOCIATED MESENCHYME

The developing neural tube (from which central nervous system develops) is surrounded by mesenchyme, which subsequently condenses to form meninges. An extension of this mesenchyme also covers the optic vesicle. Later, this mesenchyme differentiates to form a superficial fibrous layer (corresponding to dura) and a deeper vascular layer (corresponding to pia-arachnoid) (Fig. 1.8).

With the formation of optic cup, part of the inner vascular layer of mesenchyme is carried into the cup through the choroidal fissure. With the closure of this fissure, the portion of mesenchyme which has made its way into the eye is cut off from the surrounding mesenchyme and gives rise to the hyaloid system of the vessels (Fig. 1.9).

The fibrous layer of mesenchyme surrounding the anterior part of optic cup forms the cornea. The corresponding vascular layer of mesenchyme becomes the iridopupillary membrane, which in the peripheral region attaches to the anterior part of the optic cup to form the iris. The central part of this lamina is pupillary membrane which also forms the tunica vasculosa lentis (Fig. 1.9).

Fig. 1.8. Developing optic cup surrounded by mesenchyme.

Fig. 1.9. Derivation of various structures of the eyeball.

Fig. 1.10. Development of the retina.

zMatrix cell layer. Cells of this layer form the rods and cones.

zMantle layer. Cells of this layer form the bipolar cells, ganglion cells, other neurons of retina and the supporting tissue.

zMarginal layer. This layer forms the ganglion cells, axons of which form the nerve fibre layer.

(b) Outer pigment epithelial layer. Cells of the outer wall of the optic cup become pigmented. Its posterior part forms the pigmented epithelium of retina and the anterior part continues forward in ciliary body and iris as their anterior pigmented epithelium.

Optic nerve

It develops in the framework of optic stalk as below:

zFibres from the nerve fibre layer of retina grow into optic stalk by passing through the choroidal fissure and form the optic nerve fibres.

zThe neuroectodermal cells forming the walls of optic stalk develop into glial system of the nerve.

zThe fibrous septa of the optic nerve are developed from the vascular layer of mesenchyme which invades the nerve at 3rd fetal month.

zSheaths of optic nerve are formed from the layers of mesenchyme like meninges of other parts of central nervous system.

zMyelination of nerve fibres takes place from brain distally and reaches the lamina cribrosa just before birth and stops there. In some cases, this extends up to around the optic disc and presents as congenital opaque nerve fibres. These develop after birth.

Crystalline lens

The crystalline lens is developed from the surface ectoderm as below :

Lens placode and lens vesicle formation (see page 5, 6 and Fig. 1.6 .

Primary lens fibres. The cells of posterior wall of lens vesicle elongate rapidly to form the primary lens fibres which obliterate the cavity of lens vesicle. The primary lens fibres are formed upto 3rd month of gestation and are preserved as the compact core of lens, known as embryonic nucleus (Fig. 1.11).

Secondary lens fibres are formed from equatorial cells of anterior epithelium which remain active through out life. Since the secondary lens fibres are laid down concentrically, the lens on section has a laminated appearance. Depending upon the period of development, the secondary lens fibres are named as below :

zFetal nucleus (3rd to 8th month),

zInfantile nucleus (last weeks of fetal life to puberty),

zAdult nucleus (after puberty), and

zCortex (superficial lens fibres of adult lens) Lens capsule is a true basement membrane produced by the lens epithelium on its external aspect.

Cornea (Fig. 1.9)

1.Epithelium is formed from the surface ectoderm.

2.Other layers viz. endothelium, Descemet's membrane, stroma and Bowman's layer are derived from the fibrous layer of mesenchyme lying anterior to the optic cup (Fig. 1.9).

Sclera

Sclera is developed from the fibrous layer of mesenchyme surrounding the optic cup (corresponding to dura of CNS) (Fig. 1.9).

Choroid

It is derived from the inner vascular layer of mesenchyme that surrounds the optic cup (Fig. 1.9).

Ciliary body

zThe two layers of epithelium of ciliary body develop from the anterior part of the two layers of optic cup (neuroectodermal).

zStroma of ciliary body, ciliary muscle and blood vessels are developed from the vascular layer of mesenchyme surrounding the optic cup (Fig. 1.9).

Fig. 1.11. Development of the crystalline lens.

Iris

zBoth layers of epithelium are derived from the marginal region of optic cup (neuroectodermal) (Fig. 1.9).

zSphincter and dilator pupillae muscles are derived from the anterior epithelium (neuroectodermal).

zStroma and blood vessels of the iris develop from the vascular mesenchyme present anterior to the optic cup.

Fig. 1.12. Development of the eyelids, conjunctiva and lacrimal gland.