Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009

Initially the stroma is thick. It then undergoes dehydration of hyaluronic acid and compression of the stromal tissue to attain the normal thickness.

By the 12th week the two-layered endothelial cells become a single layer and secrete the Descemet’s membrane. The Bowman’s membrane develops by the 16th week.

The cornea measures 2 mm by the 12th week and is 9.3 mm by the 35th week.

ANGLE OF THE ANTERIOR CHAMBER (FIG. 2.4)

The angle of the anterior chamber lies at the junction of cornea and the iris and it consists of:

a.The Schwalbe’s line, which is the most prominent portion and is called the anterior annular line.

b.The Schlemm’s canal a venous channel, which resembles a lymphatic. It is lined by non-fenestrated endothelium and has a thin connective tissue wall. The cells have tight junctions. Large vesicles are seen in the cells. When this canal is empty it is invisible but when filled with blood it can be seen clearly just in front of the scleral spur.

c. Trabecular meshwork (ligamentum pectinatum iridis or cribriform ligament of Henderson) this is triangular in shape with apex at the Schwalbe’s line. The base is formed by the scleral spur and the ciliary body. It is present all around between the sinus venosus sclerae and the anterior chamber. It has three parts the uveal portion, corneoscleral meshwork and the juxta canalicular portion. The part in front of an imaginary line drawn from the Schwalbe’s line to the scleral spur is the uveal portion. Pigmentation of the uveal part of trabecular meshwork is more in people with dark irises. The

Fig. 2.4: Angle of anterior chamber

uveal meshwork is larger than the corneoscleral meshwork. The pores in the meshwork are called spaces of Fontana.

The corneoscleral meshwork consists of thin sheets of connective tissue arranged in a laminar pattern. Each fibril is formed by a central core of collagen and elastic fibrils surrounded by a basal lamina and then a single layer of endothelial cells. These cells have many vesicles for pinocytosis.

d.The scleral spur. This is seen as a white line behind the scleral trabeculae and is called the posterior annular line.

e.The iris processes are the true ligamentum pectinatum. They bridge over the angle and are seen as yellowish vertical lines.

f.Anterior border of the ciliary body. The anterior surface of the ciliary body forms a concave recess or sinus. It can be recognized by the fact it is darker in color compared to the iris, which lies next.

g.Iris.

Development

After the in growth of the first wave of endothelial cells and the posterior extension of the second wave which forms the pupillary membrane there is a slit like space in between the two layers of cells. This becomes the angle of the anterior chamber. During the seventh week the angle is occupied by mesenchymal cells derived from the neural crest cells. The trabecular meshwork develops from these cells. Around the 15th week the endothelial cells extend up to the angle of the anterior chamber and meets the anterior surface of the developing iris. Full development of angle develops only after the age of four. As the cornea develops the angle recess moves posteriorly. Around the 14th week the trabecular meshwork becomes perforated.

The Schlemm’s canal develops during the 12th week of gestation from the venous plexus. It is derived from the mesoderm. The circulation of aqueous humor begins at this time and the Schlemm’s canal acts as a conduit for aqueous drainage and does not become a blood vessel.

SCLERA

Sclera is composed of type I collagen and proteoglycans (decorin, biglycan and aggrecan), elastin and glycoproteins such as fibronectin. Fibroblasts lie along the collagen bundles. Long posterior ciliary artery nerves supply the anterior sclera. The intrascleral loop of these nerves sometimes form a nodule over the ciliary body. Deposits of calcium phosphate anterior to the insertion of medial and lateral rectus will cause dehydration of sclera. This will produce thinning of sclera, which will appear blue in color due to the underlying uvea.

Development

The sclera is formed by the mesenchymal cells derived from the neural crest. Just like the choroid also develops from the anterior end. This occurs during the seventh week.

3

The Lens

The human lens is a transparent biconvex structure with a refractive index of 1.39 suspended between the iris and the vitreous by the zonules. The lens is able to refract light as its refractive index is different from the refractive index of the aqueous and vitreous, which surround the lens. The refractive power of the lens is about 15-20D. The diameter of the lens is 9-10 mm at the equator from right to left. Anteroposteriorly it measures about 4 mm in the young but it continuously increases in size as new lens fibers are laid throughout life.

The anterior surface of the lens is less convex (10 mm radius) than the posterior surface (6 mm). Anterior to the lens are the pupil, the anterior chamber, iris, the posterior chamber and the ciliary processes. The anterior pole of the lens lies 3 mm from the cornea.

The posterior surface is separated from the vitreous by a space (Berger’s) which is filled by the primary vitreous. The equator of the lens lies 0.5 mm within the ciliary processes. The indentations formed by the zonules disappear during accommodation.

Parts of the Lens (Fig. 3.1)

Fig. 3.1: Sutures of the lens

Lenticular capsule is not a true capsule but only a thickening of the basement membrane of the epithelial cells. Though it does not have any elastic tissue the fine fibrils of collagen present in the capsule gives it elasticity. It is thicker in

front than behind. The part of the capsule just in front and behind the equator is also slightly thicker than other areas. The posterior capsule is very thin just 2 to 4 micrometers in the center. The capsule is made of type IV collagen.

Lenticular epithelium is formed by a single row of cubical cells present under the capsule, which becomes columnar as they reach the equator. Mitosis is seen more near the equator. These fibers elongate to form the lens fibers.

The lens fibers are 15 micro m/6 micro m in size. The fibers remain parallel except at the sutures where they converge. The central fibers loose their nuclei and other organelles so that light will not get scattered, and become part of the lens nucleus. As newer fibers are laid externally closer to the capsule a laminated structure is formed. An anteroposterior section will show concentric layers. The nuclei lie closer to the equator. The lens fibers start and finish in such a way that anterior and posterior y sutures are formed.

The older fibers lack nuclei have serrated edges and are compactly arranged to form the nucleus. There are around 2000 fibers in an adult lens. The inter digitations between the lens fibers help in movement of the lens fibers during accommodation.

The lens epithelium is metabolically very active. As the cells elongate into lens fibers they loose all the cell organelles so that light is not scattered.

The nucleus is harder than the cortex. Though the lens becomes flatter as age advances the refractive power remains the same due to increase in the refractive index. The color of the lens also becomes more yellow as years pass by and on oblique illumination appears gray. The oldest fibers form the embryonic or fetal nucleus, then the infantile and finally the adult nucleus.

Ciliary zonule (zonule of Zinn or suspensory ligament of the lens). The lens is suspended from the ciliary body in between the iris and the vitreous by a series of fine fibers called zonules. Thus, it divides the eye into a small anterior and a large posterior portion.

The zonules are attached to the equator and parts of the anterior and posterior lens capsule on one side and the ciliary body on the other up to the ora serrata.

The posterior chamber is situated in front and the vitreous behind. The space between the layers of zonule is called the canal of Petit.

The zonules are smooth and inextensible. The ends spread in a fan shaped manner to become continuous with the lens capsule. The zonules have many auxilliary fibers also for strengthening the main fibers.

The main fibers are subdivided into:

Orbiculoposterior capsular fibers which originate from the ora and are attached to the posterior capsule,

Orbiculoanterior capsular fibers which are the thickest, arising from orbiculus ciliaris and attaching itself to the anterior capsule.

Cilioposterior capsular fibers which are the most numerous, arise from the valleys of the ciliary body and inserted into the posterior capsule,

And the cilioequatorial fibers present only in young eyes.

The zonules can be seen in the living through a slit lamp when there is coloboma of iris.

The lens being avascular is nourished by the aqueous humor.

Development of the Lens (Fig. 3.2)

The lens develops from the surface ectoderm in front of the primary optic vesicle. There are four stages, lens placode or plate, lens pit or recess, lens pouch or sac and lens vesicle.

a.Between the 3rd and 4th week of development the surface ectoderm becomes thicker as the cells become columnar. This thickened single layer of cells is called the lens placode.

b.A groove appears in this layer towards the end of fourth week. This is called the lens pit.

c. The groove closes to form a pouch.

d.This is converted into a vesicle between the 4th and 5th week.

Once the lens vesicle forms it moves inwards and gets separated from the

surface ectoderm. At this time the optic vesicle invaginates to form the optic cup.

When the lens vesicle forms, the cells are arranged in such a way that the basement membrane of the anteriorly situated cells face anteriorly. The posterior epithelial cells grow forward towards the cavity of the vesicle. The cells

Fig. 3.2: Development of lens

become columnar. The central fibers elongate first followed by the fibers on either side. These fibers fill the lens vesicle by the later part of 6th week. These fibers extend from the posterior surface to the anterior surface. Once the cells elongate they do not multiply. As these fibers grow forward the nuclei also move forward and meet the nuclei of the anterior epithelium to form the nuclear bow. The cells of the anterior layer of the lens vesicle continue to form lens fibers throughout life. The new fibers are laid concentrically around the fibers formed by the posterior epithelium. This gives a laminated appearance to the lens. The fibers at the equator are arranged radially. The fibers do not make a complete circle but stop short at various levels depending on where they start. The fibers are of equal length. This forms the sutures of the lens (Fig. 3.1).

The lens is almost spherical at birth. It then becomes flattened anteroposteriorly. The capsule is secreted by the epithelium around the sixth week. It is only the basement membrane of the epithelium.

system regresses. The vitreous increases in volume. The Cloquet’s canal represents the remnants of the primary vitreous and hyaloid vessels. The anterior most end of the hyaloid vessel is slightly nasal and below the posterior pole of the lens. Sometimes the degenerated vessel remains attached to the posterior capsule. It can even form an opacity at this position. This is called Mittendorf’s dot. Condensation of the fibrils extends from the rim of the optic cup to the lens equator. This is called the tertiary vitreous. The zonules develop anterior to this condensation from the neurectoderm.

b. Sphincter and dilator pupillae muscles are derived from the neuroectoderm. c. Both layers of the posterior epithelium are derived from the marginal region of the optic cup, i.e. the neuroectoderm. Though this corresponds to the neurosensory retina it is pigmented. The pigmentation starts from the

pupillary margin and goes up to the root of iris.

d.The anterior layers are formed by the neural crest cells. They are designated as mesenchymal cells and are divided into superficial and deep layers.

During the sixth week the annular vessel near the rim of the optic cup gives rise to vessels which grow into the mesenchymal cells in front of the lens. The anterior part of the tunica vasculosa lentis forms the pupillary membrane. At the end of the 12th week both walls of the optic cup grow forward. The sphincter pupillae forms from the anterior epithelial layer during the 12th week. The dilator starts developing during the 24th week and continues to develop even after birth. Due to this it is difficult to dilate an infants eye. The tunica vasculosa lentis is reabsorbed during the 24th week. Parts of the arteriovenous anastomosis remains at the level of the collarette, which is closer to the pupil in the newborn.

Gross Appearance

The iris separates the anterior chamber from the posterior chamber and is situated between the cornea and the lens. It is the anterior most part of the uveal tract. It is bathed on both sides by the aqueous. The diameter is approximately 12 mm. The collarette is the thickest part of the iris while the root of the iris is the thinnest. Hence, during injuries the root of the iris gets torn causing iridodialysis.

The iris is a contractile diaphragm with a central aperture called the pupil. The pupil is situated slightly nasal to the center but still lies in the optical axis. The anterior surface of the iris is slightly convex as it lies on the lens, which is convex. When the lens is removed the iris is flat and often tremulous. Contact between the posterior surface of the iris and the lens causes a relative pupillary block to the flow of aqueous humor, which is most marked during mid dilatation. This, further causes forward bowing of the iris (physiological iris bombe), which in persons with narrow angles may precipitate angle closure

glaucoma.

The anterior surface of the iris is divided into two zones, the central pupillary zone and the peripheral ciliary zone. At the junction of these two zones is a circular ridge called the ‘collarette’. The collarette marks the embryonic site of minor vascular circle of iris from which the embryonic pupillary membrane originates. The ciliary zone has many ridges and crypts, which are unique to each individual just like fingerprints. The pupillary zone is relatively flat.

The superficial mesenchymal layer is shorter than the deeper layer. It extends from the ciliary border to the collarette and gives the ciliary portion of the iris its color. It contains the crypts and trabeculae of the collarette.