Cranial Nerve III

The third cranial nerve or oculomotor nerve controls most of the eye movements and also provides fibers for constriction of the pupil, accomodation, and elevating the eyelid .
The oculomotor nerve arises the ocular motor nucleus in the midbrain on the anterior aspect in floor of the cerebral aqueduct. The third nerve passes forward thru the tegmentum, red nucleus, substantia nigra and emerge on the medial aspect of the cerebral peductal. Figure 1 : Plan of oculomotor nerve.

The nucleus of the oculomotor nerve is composed of subnuclei, which are arranged into anterior and posterior groups. The posterior group nubmers six. The anterior group consists of two subnuclei, an antero-medial and an antero-lateral . Fibers for pupillary constriction and accomodation arise from the Edinger-Westfall nucleus.

The course of the third nerve. It is critical to review the circle of Willis (Figure below). The third nerve passes (#3 in figure) between posterior cerebral arteries (#11 in figure) and superior cerebellar (#12 in Figure) and runs parallel to the posterior communicating artery (#10 in Figure), which joins the internal carotid artery (#9 in Figure) and the posterior cerebral artery. In fact the 3rd nerve is fed by branches of the posterior communicating artery. Of extreme clinical significance is the fact that most aneurysms producing a pupil involving 3rd nerve palsy affect the posterior communicating artery at this point. The 3rd nerve penetrates the dura mater in front of and lateral to the posterior clinoid process, runs along the lateral wall of the cavernous sinus. After division into two branches, the nerve enters the orbit through the superior orbital fissure, between the two heads of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.
The superior ramus, the smaller, passes medialward over the optic nerve, and supplies the Rectus superior and Levator palpebræ superioris. The inferior ramus, the larger, divides into three branches. One passes beneath the optic nerve to the Rectus medialis; another, to the Rectus inferior; the third and longest runs forward between the Recti inferior and lateralis to the Obliquus inferior. From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the inferior oblique muscle, which enters the muscle at its posterior border.
Reference: Gray's Anatomy

Optic Nerve-Cranial Nerve II

The optic nerve is composed of retinal ganglion cell axons and support cells. It leaves the orbit (eye) via the optic canal, running postero-medially towards the optic chiasm where there is a partial decussation (crossing) of fibers from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral geniculate nucleus from where information is relayed to the visual cortex. Its diameter increases from about 1.6 mm within the eye, to 3.5 mm in the orbit to 4.5 mm within the cranial space. (The precise dimensions of the optic nerve head are 1.5 mm (H) x 1.75 mm (V). The optic nerve component lengths are 1mm in the globe, 25mm in the orbit, 9mm in the optic canal and 16mm in the cranial space before joining the optic chiasm. There, partial decussation occurs and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body.
From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. More specifically, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly.

Sixth Cranial Nerve

Cranial nerve VI, the abducens nerve, innervates the lateral rectus muscle which abducts the eye (move away from the midline). The abducens nerve emerges from the ipsilateral abducens nucleus between the caudal pons beneath the floor of the fourth ventricle and the medulla (the pontomedullary junction). The abducens nerve exits the skull through the superior orbital fissure (one of the holes in the skull behind the eye).
Looking for a 6th nerve palsy is a good screening sign in children with suspected meningitis. As the abducens emerges near the bottom of the brain, it is often the first nerve compressed when there is any rise in intracranial pressure. The lateral rectus muscle of the eye that the abducens nerve innervates is opposed by the action of the medial rectus muscle. Damage to the abducens nerve causes medial strabismus as the individual is no longer able to control lateral eye movement via the lateral rectus motor neurons.

A sixth nerve palsy can often be localized by the accompanying symptoms. The patient with an inability to move either eye to the affected side may have a lesion in the abducens nucleus. This results from loss of motor neurons or interneurons that project to the contralateral medial longitudnal fasciculus.
In the ventral pons a lesion in the fasciculus of the 6th nerve may also involve an adjacent pyramidal tract to produce a contralateral hemiplegia.
Increased intracranial pressure may stretch the 6th nerve in the subarachnoid portion that exits in the brainstem and travels upward to the dural attachment at the clivus. Cerebral edema for example may cause a downward displacement of the brainstem and may damage the nerve or simple swelling may compress the nerve. Nausea and vomiting associated with a sixth nerve palsy should prompt one to think of this mechanism.
Within the cavernous sinus the sixth nerve travels with sympathetic fibers and may produce a post ganglionic Horner's syndrome to produce anisocoria. The opposite pupil will seem dilated. Alternatively the 3rd nerve lesion may accompany a 6th nerve lesion in the cavernous sinus in such things as Tolosa Hunt syndrome (inflammation in the cavernous sinus associated with ophthalmoplegia).

OPTIC NERVE- Cranial nerve II

The optic nerve extends from the back of the eye about 3 mm medial and slightly superior to the true posterior pole of the eye. The optic nerve varies in length from 35 to 55 mm. The optic nerve is divided into 4 segments along its course: intraocular, orbital, intracanalicular and intracranial.

1. Intraocular optic nerve is subdivided into retinal choroidal and scleral portions or alternatively into: a. optic disc b. prelaminar c. laminar and d. retrolaminar portions.

The intraocular portion is about 1 mm in length and is thinnest at the oval shaped disc before nerve fibers have myelinated and measures about 1.5 mm ( H) x 1.75 mm (V) diameter. It is conical in shape as myelinated fibers are accumulated. The blood supply includes both retinal and posterior ciliary arteries.

2. The orbital portion of the optic nerve is about 25-30 mm in length and about 3.5 mm in diameter. There is slack in the optic nerve of about 7-12 mm so the eye will not be constrained upon turning. The blood supply includes the intraneural branches of the central retinal artery, which enters the eye about 6-12 mm posterior to the sclera, as well as the pial network of the nerve derived from the adjacent branches of the ophthalmic artery.

3. The intracanalicular portion of the optic nerve measures 11.4 mm and enters the canal with the outer portion of the dura fusing to the bone. The optic canal measures about 3.6 (H) mm x 4.8 mm (V).These attachments constrain the expansion of the optic nerve during infections resulting in a compartment syndrome and may also account for pain of eye movement in optic neuritis. The blood supply is derived from the ophthalmic artery.

4. The intracranial portion of the optic nerve includes that from the end of the optic canal to the optic chiasm and measures about 10 mm in length. The blood supply is derived from the ophthalmic and internal carotid arteries.

Macula and Fovea

The macula, area centralis, has a diameter of about 5.5 mm and is defined histologically by the presence of more than 1 layer of ganglion cells. Clinically one uses the presence of yellow carotenoid pigments that are present in Henle's layer to identify the macula. The correlation is not perfect however than the clinical definition is not as precise as the histologic one.

The fovea is a depression in the center of the macula that measures about 1.5 mm in diameter. The center of the fovea (foveola) resides usually about 4.0 mm temporal and 0.8 mm inferior to the optic nerve head. There are no ganglion cells in the fovea but only glial cells and Muller cells in the area of the fovea referred to as the foveola which measures about 350 microns in diameter. There is a foveal avascular zone or capillary-free zone that is about 400 microns in diameter.

Retinal Blood Supply

Ultimately the retina derives its blood supply from the ophthalmic artery. The outer retina is supplied from the choriocapillaries. The inner retina is supplied from the distribution of the central retinal artery.
The central retinal artery is a branch of the ophthalmic artery that begins about 4 mm posterior to the optic nerve head. The branch enters the optic nerve to become central within the optic nerve and divides to 2 main branches in the optic nerve. The vessels further divide to supply each quadrant of the eye with an artery and a vein. In about 20 of individuals there are cilioretinal arteries derived from the choroidal circulation that in some individual may supply the macula alone.

The central retinal artery is a small muscular artery with fenestrated elastic lamina. retinal arterioles, do not have an internal elastic lamina and upon penetration of the retina do not have smooth muscle cells.

Retinal blood vessels maintain the blood-retinal barrier (similar to the blood-brain barrier). The endothelium is non-fenestrated and do not permit leakage of fluorescein.

Tear Proteins

The major tear proteins in relative concentration or mass.
•Lysozyme ~300 micromolar (~4.6 mg/ml)
•Tear Lipocalin 74-85 micromolar (~1.5 mg/ml)
•Lactoferrin ~24 micromolar (~2 mg/ml)
•Lipophilin ~3 micromolar (45-100 ug/ml)
•IgA (5.9 ug/ml )

Functions

• Lysozyme- antibacterial action by cleaving cell wall constituents
• Tear Lipocalin- scavenges lipids from the cornea surface, role in tear film stability, antifungal activity by binding to fungal siderophores, endonuclease activity
• Lactoferrin- antimicrobial action by competing for iron with microbes.

Aqueous and Aqueous Outflow Tract

The anterior chamber has a volume of about 1.5 ml. Aqueous humor is produced at estimated 1.5 ml/minute with a diurnal variation. The anterior chamber is bordered anteriorly by the corneal endothelium, posteriorly by the iris and lens (over the pupil) and peripherally by the trabecular meshwork, ciliary body face and scleral spur. The anterior chamber is shallower in women than men. It is shallower in hyperopes than others. The anterior chamber is shallower in older patients. The average depth is about 3.5 mm in males.

Gonioscopically one encounters all of the structures lining the lens and the classic landmarks moving anterior to posterior are: 1. Schwalbe's line (termination of Descemet's membrane); it appears as a white line in the gonioscopic view. See diagram 2. The trabecular meshwork, which is tan and characteristically trabeculated, extends to the iris. Underneath the trabecular meshwork one may see a white band which is the scleral spur. The trabecular meshwork empties into Schlemm's canal which is a venous channel in the internal scleral sulcus. The canal froms a ring 36 mm in circumerence. Multiple collector vessel arise from the canal and join the intrascleral venous plexus and eventually into the aqueous veins.

Spiral of Tillaux

Rectus Muscle-Insertion Distance from Limbus (mm)

• Medial - 5.5 *
• Inferior - 6.6-6.9
• Lateral - 6.9*
• Superior - 7.0-7.7

There is of course a range to these values. When the range is omitted in other texts, figures are usually taken from Wolff's Anatomy of the Eye and Orbit. The actual numbers have far greater variation in recent measurements. [IOVS]

CT Scan Images of Orbital Anatomy

Click on images for larger view


1. Maxillary sinus
2.Temporalis Fossa (the arrow points to the zygoma which of course is missing on the opposite side). Notice the internal auditory canal which, combined with the nasal septum and maxillary sinus, gives you a fairly accurate plane of sectioning.
3. Nasal septum










4. Zygoma
5. Inferior orbital fissure (note it narrows posteriorly and medially)
6. Greater wing of sphenoid (one location for meningioma, Langerhans histiocytoma and plasmacytomas of the orbit)
7. Beginning of the opening for the nasolacrimal duct in the lacrimal fossa
8. Inferior rectus muscle




It may help you greatly to study the dissected view of a skull in the radiologic plane. Click to open in a new window the image from the University of Utah.

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CHOROID

The choroid is that part of the uveal tract extending from the edge of the optic nerve head to the ora serrata. The choroid underlies the retinal pigment epithelium and is continuous anteriorly at the ora serrata with the ciliary body.

ATTACHMENTS:

The choroid is part of the uveal tract which includes the iris and ciliary body as well. The uveal tract is attached at 3 sites:

1. the scleral spur

2. internal scleral exit channels of the vortex veins

3. optic nerve

The usually heavily pigmented posterior or choroidal portion of the uveal tract is loosely adherent to the overlying sclera. This plane of loose attachment is a zone of potential separation known as the suprachoroidal space that is common to both the choroidal and ciliary portions of the uveal tract. The suprachoroidal space has relevance for massive choroidal edema, clinically referred to as "choroidal detachments" or in local parlance as "choroidals". Attachment of the longitudinal (meridional) ciliary muscle to the scleral spur limits the space . The enlargement is limited posteriorly by attachment of the choroid to the sclera, augmented by the outward passage of the vortex veins, by the perforating short posterior ciliary arteries and by border tissue at the scleral aperture for the optic nerve.

VASCULATURE: The choroid is richly vascular and provides nutrients for the outer portion of the retina including the photoreceptors and the retinal pigment epithelium . It has an extremely rapid blood flow which is provided by the choriocapillaris, inner vascular layer and outer vascular layer. The capillary layer of the choroid, the choriocapillaris, lies directly under Bruch’s membrane and is critical to supply retinal photoreceptors. The larger arteries are found most readily in the outer layers of the posterior choroidal stroma. The long posterior ciliary arteries and their corresponding long ciliary nerves lie within the suprachoroidal space in the horizontal plane, encased by collagenous tissue. Branches from the nerves to the adjacent choroid form small net-like arrangements where large ganglion cells may be observed. The venous drainage system is seen as four vortex systems each located in a posterior quadrant. Each system converges to form a single vestibule, the ampulla, which then exits through the sclera by a vortex vein.
STROMA: The choroid contains flattened or interconnecting collagen lamellae that give the melanocytes a spindle shaped appearance. These melanocytes have a stellate shape and contain pigment granules. The cells contain small oval nucleoli. They are usually accompanied by the fibrovascular stroma of the choroid. The melanocytic cells cells are considered to be the source for the most common primary malignant neoplasm of the eye- melanoma.

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REFERENCES

1. Jakobiec FA. Ocular anatomy, embryology, and teratology. Philadelphia: Harper & Row, 1982
2. Hogan MJ, Alvarado JA, Weddell JE. Histology of the human eye. Philadelphia: W.B. Saunders, 1971.
3. Last RJ. Eugene Wolff’s anatomy of the eye and orbit. Philadelphia: W.B. Saunders, 1961.
4. Fine BS, Yanoff M. Ocular histology, a text and atlas. New York: Harper and Row, 1972
5. Strenstrom S. Untersuchungen uber die variation unk kovariation der optishen elemente des menshlickhen auges. Acta Ophthalmol 1946;26:1.
6. Duke-Elder WS. The anatomy of the visual system. In: System of ophthalmology. St. Louis: CV Mosby, 1961;2:410-413
7. Greiner JV, Covington HI, Allansmith MR. Surface morphology of the human upper tarsa conjuctiva. Am J Ophthalmol 1977;83:892-905.
8. Dark AJ, Durrant TE, McGinty F, Shortland JR, et al. Tarsal conjuctiva of the upper eyelid. Am J Ophthalmol 1974;77:555-564.
9. Hogan MJ, Alvarado JA, Weddell JE. Histology of the human eye. Philadelphia: W.B. Saunders, 1971.
10. Blumcke S, Morgenroth K Jr. The stereo ultrastructure of the external and internal surface of the cornea. J Ultrastruct Res 1967;18:502.
11. Hogan et al., Histology of the human eye, 202-255.
12. Glasgow BJ. Intraocular fine needle aspiration of coronal adenomas. Diagn Cytopathol 1991;7:239-242.
13. Tolentino FI, Schepens CL, Freeman HM. Vitreoretinal disorders, diagnosis and management. Philadelphia: W.B. Saunders, 1976;1-43.
14. Sebag J, Balazs EA. Morphology and ultrastructure of human vitreous fibers. Invest Ophthalmol Vis Sci 1989;30:1867-1871.
15. Foos RY. Vitreoretinal juncture: topographical variations. Invest Ophthalmol Vis Sci 1972;10:801-808.
16. Foos RY. Anatomic and pathologic aspects of the vitreous body. Trans Am Acad Ophthalmol Otolaryngol 1973;77:OP171-OP183.
17. Gartner J. Histologische Beobachtugen uber physiologische vitreovaskulare. Adharenzen Klin Mbl Augen 1962;141:530-545.
18. Foos RY. Vitreous base, retinal tufts, and retinal tears: pathogenic relationships. In: Pruett RC and Regan CCJ, ed. Retina Congress. New York: Appleton-Century-Crofts, 1974.

RPE

RETINAL PIGMENT EPITHELIUM

The retinal pigment epithelium is a monolayer that lies between photoreceptor outer segments and Bruch’s membrane. The estimated number of retinal pigment epithelial cells is about 4-6 million cells per eye. This epithelium has many functions, including matrix production for photoreceptors, phagocytosis or outer segments, barrier protection, and active transport. These cells are large; are polygonal in shape; and contain abundant cytoplasm, round nuclei, and single nucleoli. The cytoplasm contains large distinctive ovoid and elliptical pigment granules. The retinal pigment epithelium has a remarkable potential to proliferate and undergo metaplastic transformation. RPE cells in the macula (in a 4 mm diameter region) are taller and more pigmented than elsewhere in the retina. At the ora serrata RPE cells are flatter and wider.

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RETINA

The sensory neuroepithelium of the eye is the retina, which is composed of many layers (Figure 2-18). These include the layer of outer and inner segments of the photoreceptor cells (rods and cones), the outer nuclear layer (cell bodies of photoreceptor cells), the outer plexiform layer, the inner nuclear layer, the inner plexiform layer, the ganglion cell layer, the nerve fiber layer, and the inner limiting lamina (membrane). The photoreceptors are composed laminated discs that are connected to cilia with a 9+0 motif. The discs of the cone are connected to the cell membrane whereas rod disks are not. The cilia of both rods and cones connect to mitochondrial rich ellipsoids that are contiguous with myoids. The 120 million rods and 6 million cones synapse with horizontal cells, bipolar cells and other rods and cones. Bipolar cells synapse with ganglion cells, amacrine cells and of course rods and cones. The axons of the ganglion cells form the nerve fiber layer and coalsesce to become the 1 million fibers of the optic nerve. These fibers travel posteriorly to synapse in the lateral geniculate body. Muller cells are supportive cells that extend fibers from the internal limiting membrane to the "external limiting membrane of the retina. The retina is loosely attached to the pigment epithelium, which is separated from the choroid by Bruch’s membrane.
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VITREOUS

The vitreous cavity is simply an expanded extracellular space that normally contains 4.0 ml of clear gelatinous substance that is composed largely of water, hyaluronic acid, and collagen. The vitreous normally contains anteroposterior oriented collagen fibrils and occasional macrophages or hyalocytes. The presence of even small numbers of acute or chronic inflammatory cells within the vitreous is distinctly abnormal.

click on photo for a larger image

Above -dark pigmentation on the anterior border of the vitreous base (VB). The serrated border of the retina is the ora serrata. The ora measures about 5.75 mm from the limbus nasally and about 6.5 mm temporally.

Below- transillumination highlights the pigmentation of the vitreous base as it straddles the ora serrata.

The vitreous has distinct attachments to ocular structures. It is attached anteriorly in a circumferential band extending from the posterior pars plana to a few millimeters behind the ora serrata in what has been termed the vitreous base. Traction exerted by the vitreous body at the base results in hyperpigmentation of the underlying pigment epithelium and is evident grossly. The vitreous is also attached to the retina over retinal blood vessels and at the optic disc. These attachments are important to understanding vitreous traction, retinal tears, and retinal detachment, for which vitrectomies are sometimes performed.

The dentate processes of the retina form a serrated border to the retina and ciliary body called the ora serrata (see photo above). The upper nasal region contains more dentate processes and ora bays than do the temporal or inferior regions. There is some variability in the number of dentates in normal eyes however.

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CILIARY BODY

The ciliary body is composed of the ciliary processes, ciliary muscle, and ciliary epithelium. About 70 radially arranged ciliary processes form the pars plicata anteriorly and are joined posteriorly with the smooth portion of the ciliary body, the pars plana. The pars plana joins the retina and choroid at the ora serrata. The ciliary body is covered by two nonpigmented layer and an outer pigmented layer. Under normal circumstances, ciliary body structures will not appear in vitrectomy specimens. However, ciliary epithelium may be sampled by fine needle aspiration of adjacent tumors. It is important to recognize the two-layered structure of the epithelium with abundant cytoplasm and large pigmented granules.

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LENS

What is the overall composition and size of the lens? The lens is a transparent soft encapsulated biconvex structure composed of crystallins. The lens is suspended by thin zonules that are attached to the ciliary body. The anterior surface of the lens has a radius of curvature that is greater than the posterior surface. The average anterior-posterior human lens thickness at birth is 3.5 to 4 mm and 4.5 to 5 mm after 65 years of age. The diameter of the human lens at birth is 6.0 to 6.5 mm and 9.0 to 9.5 mm after 65 years of age.

What is the lens capsule? The lens is completely enveloped by the thickest basement membrane in the body the capsule (#1 in photomicrograph ) which is 10-20 µm thick of hyaline material containing type IV collagen.

What is the lens epithelium? There is layer of large cuboidal epithelial cells, (the lens epithelium) beneath the anterior capsule (#2 in photomicrograph). The lens epithelium is located on the internal surface of the capsule. The cells of the lens epithelium continue to divide with age; thus, the lens grows in anterior-posterior thickness.

What is the structure of the lens cortex and nucleus? In the center (#3 in photomicrograph) tightly packed cells have lost their nuclei and become packed by special transparent proteins (crystallins) to form so-called lens fibers. New lens cells are added to the margin of the lens throughout life from the lens epithelium, but the cells at the cortex and nucleus (center) of the lens do not undergo turnover or replacement and are therefore the oldest cells in the body of an adult. The lens is avascular and nourished by diffusion from the aqueous and vitreous. The radius of curvature of the anterior surface averages 10 mm, but it is subject to marked changes during accommodation. Because the lens nucleus is formed by increasing density of cortical cells, cortex and nucleus are considered together. Both are made up exclusively of cells derived from the lens epithelium. The common designation of “lens fiber” for the cortical cells is a misnomer for the elongated cells of the lens substance. Lens cortical cells are elongated and on cross-section, appear hexagonal in shape. The cortex resembles the cut surface of a honeycomb. In light microscopy, the transition from cortex to nucleus is characterized by less distinct lamellae. The posterior capsule (#5 in the photomicrographs does not have any epithelium associated with it as the epithelium "migrates" anterior from the lens equator. Below is a photograph at the lens bow. Note the epithelium abruptly stops where the posterior capsule begins. Click to enlarge the photo and then use the back key to return.

IRIS

What is the iris? The iris is the pigmented diaphragm separating the anterior and posterior chambers. The iris is a component of the uveal tract. The iris stroma contains melanocytes, fibroblasts, and blood vessels arranged in a loose network.
What are the boundaries of the iris? It is joined to the ciliary body at the iris root. The anterior surface, also known as the anterior border layer, is composed of a condensed layer of fibroblasts, melanocytes, and collagen fibrils and borders the anterior chamber.The posterior surface of the iris is composed of two pigmented epithelial layers and borders the posterior chamber.
What are the layers of the iris? The iris contains pigmented cells and muscle and is composed of four layers: the anterior border layer, the stroma, the dilator muscle layer and the posterior epithelium. (Click to enlarge photo)
What is the structure of the anterior border layer? The anterior border layer consists of a dense packing of pigmented or nonpigmented cells similar in appearance to the cells present throughout the remainder of the stroma. Absence of cells produces the so called crypts in the border layer. What is histology of the stroma?
(The iris stroma is a loose fibrocollagenous support issue associated with spindle- shaped fibroblasts (stromal cells), blood vessels, nerves and macrophages (clump cells of Koganei) containing phagocytosed melanin pigment; at the pupil margin is the circumferentially arranged smooth muscle of the sphincter muscle of the pupil.
What is the composition of the posterior epithelium of the iris? The posterior epithelium is composed of two layers of cells which are densely pigmented with melanin. The posterior boundary of the iris stroma, peripheral to the sphincter muscle, is demarcated by another sheet of smooth muscle, the dilator muscle. The fibers of the dilator muscle are derived from, and remain in continuity with, the cuboidal pigmented cell bodies which make up the anterior layer of iris pigment epithelium.

What is the blood supply to the iris? The iris is supplied from the major arterial circle that is located in the ciliary body. Rupture of this vessels with ciliary muscle tears is frequently the injury that cause hyphemas. The blood vessels of the iris run in a radial direction. The anterior border layer contains very few vessels. Vessels on the surface of the border layer are abnormal and suggest ischemia in the eye such as in diabetic retinopathy, branch vein occlusions, neoplasms. Iris blood vessels appeared sheathed and have a characteristic loose appearance. They are nonfenestrated and have pericytes.

What is the innervation to the iris? The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion. They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the blood vessels of the choroid. After reaching the iris they form a plexus around its attached margin; from this are derived non-medullated fibers which end in the Sphincter and Dilator pupillæ. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.
What determines individual eye color? Eye color is determined by the relative number of melanocytes in the stroma. Few cells give a blue color, whereas many melanin- containing cells produce a dark brown color; gray and green are the intermediate colors.

What is the function of the pupil? It has a circular aperture (pupil) that can be opened and closed by the action of groups of smooth muscle. Contraction of the pupil reduces the amount of light entering the eye and thereby reduces the glare from light scattered from the periphery of the lens.
The dilator muscle layer is composed of the contractile processes of the myoepithelial cells of the inner layer of the posterior epithelium; it extends from the base of the iris to the sphincter muscle.

What is the appearance of the iris in cytology preparations? Normal iris may appear in intraocular washings from incidental ocutome cutting of the iris in an attempt to remove vitreous or lens fragments in the anterior chamber. Normal iris also may appear in fine needle aspiration specimens of iris neoplasms. In general, normal iris epithelium is so densely pigmented that cellular details are obscured. Iris stroma is characterized by the fine reticular meshwork of very cohesive and vascularized stroma.

(Text on Mission for Vision>modified from Ocular Cytopathology Glasgow & Foos)

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CONJUNCTIVA

The conjunctiva covers the posterior surface of the eyelids (palpebral conjunctiva), curves anteriorly at the fornix to reflect onto the anterior surface of the eye as the bulbar conjunctiva. There are subtle histologic differences in the conjunctiva of the lid margins, tarsus, fornix, and bulbar conjunctivae. The conjunctiva covering the lid margin and bulbar conjunctiva is a modified nonkeratinized, stratified squamous epithelium. The tarsal and fornix conjunctiva is covered by stratified cuboidal to columnar epithelium of varying thickness. This epithelium is unusual because it retains some squamoid features, such as numerous desmosomes, yet has a microvillus surface architecture. Goblet cells are abundant over the tarsus, fornix, and specialized areas such as the plica semilunaris. Goblet cells are scarce near the lid margin and adjacent to the cornea at the limbus. Test your ability to identify the various areas of the conjunctiva by identifying the type of conjunctiva and its histologic characteristics in the pictures below.




















Click on the photo to enlarge and identify the structures that are numbered. Click here to link to the answers.

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CORNEA

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The cornea is the transparent disk-like anterior portion of the eye. The cornea is more curved than the eye and protrudes anteriorly. The adult cornea has a horizontal diameter of 11.5 to 12.6 mm and a vertical diameter of 10.5 to 11.7 mm. Because the posterior surface of the cornea is more spherical than the anterior surface, the central cornea is thinner (0.52 mm average thickness) than the periphery (0.65 mm average thickness). Microcornea is when the greatest corneal diameter is less than 11 mm. Megalocornea is when the greatest corneal diameter is greater than 13 mm. The cornea has five layers.
Corneal epithelium- The non-keratinizing squamous epithelium with a basal cell layer gives rise to five to six superficial layers with a total thickness of about 50 µm. Numerous free nerve endings terminate in this epithelium and are the afferent part of the blink (ciliary) reflex, which is mediated through the sensory part of the fifth cranial nerve. The basal layer is composed of cuboidal to columnar cells (~ 18 μ in height). The basal cells are smaller and have a higher nuclear-to-cytoplasmic ratio than the other epithelial cells in the cornea. As the basal cells undergo mitotic division, they force the wing cells (shaped like a wing) upward. There are two to three layers of wing cells with interdigitating cytoplasmic processes connected by desmosomes to other wing cells. These attachments may explain why corneal epithelium tends to be removed in sheets. Beyond layers with wing cells become more flattened and more dense) with loss of cell organelles The two top layers are flattened, superficial cells with small, round nuclei and inconspicuous nucleoli. The superficial epithelial cells are normally uniform in size and shape and have many microvilli that form a microplical complex on the external surface of the cornea. Epithelial cells are attached to a basement membrane, beneath which lies Bowman’s layer, a specialized layer of collagen that does not regenerate after injury.
Bowman’s membrane -This is composed of fine collagen fibrils and is about 10 µm thick. It is acellular. It is limited anteriorly by the basement membrane of the corneal epithelium. The layer is composed of collagen fibrils arranged in random distribution, Where it joins underlying lamellar stroma, Bowman layer changes in a narrow transition zone to collagen fibrils that are obliquely arranged, collagenous lamellas of the superficial corneal stroma.

Corneal stroma- The main layer of the cornea is composed of 60-70 successive layers (lamellae of obliquely oriented tightly bound, collagen fibers (corneal lamellae) embedded in an extracellular matrix composed mainly of sulfated glycosaminoglycans. Each of 2 micron thick lamellar sheets of collagen are arranged perpendicularly which provides maximum mechanical strength as the direction of the collagen fibers differs in each layer. Between the lamellae are sparse fibrocytes (keratocytes). As there are no blood vessels in the cornea, the regular parallel arrangement of the collagen and the paucity of cells render the cornea translucent and allow it to transmit light. The clarity of the cornea was explained by Maurice in 1957, who theorized that the orderly arrangement of the collagen fibrils eliminated light scatter by destructive interference. Because the corneal fibrils form a sort of 3 dimensional array of diffraction gratings of lamellae, scattered light is eliminated by destructive interference. The proportion of the stroma that contains hydrated fibrils appears to be an important feature as well. The fibrils however must be separated from each other by less than ½ of a wavelength of light to remain transparent. Hence the irregularity of Bowman’s layer can be compensated by this arrangement.
Descemet’s membrane which closely resembles the lens capsule, is a true PAS-positive membrane. Produced by the endothelium, the membrane is thin in infancy, increases in thickness to ~ 5 μ in childhood, and then to ~ 8 to 10 μ adulthood.
Corneal endothelial cells the posterior surface of the cornea and forming the anterior boundary of the anterior aqueous chamber is a single layer (~ 5 to 6 μ thick) of flattened hexagonally arranged cells generally known as the corneal endothelium.
Endothelial cells possess numerous mitochondria, are linked together by both desmosomal and occluding junctions, and pump fluid from the corneal stroma, thereby preventing excessive hydration of the extracellular matrix, which would result in opacification of the cornea as the separation between lamellae exceeds ½ of a wavelength.

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Normal Anatomic and Cytologic Features

Accurate interpretation of ocular pathology specimens requires a fundamental knowledge of normal ocular anatomy and histology. A general overview is presented here. The average adult eye measures about 25 mm horizontally, 23 mm vertically, and 21 to 26 mm anterior-posteriorly. The eye has an external approximate volume of 7.6 milliliters (ml), the aqueous has a volume of about ~250 microliters (decreasing with age to ~160 microliters), and the vitreous a volume of 4.0 ml. The eye is contained in the pear-shaped orbit that has dimensions of about 35 mm vertically, 45 mm horizontally, and 40-45 mm anteroposteriorly. The orbital volume is ~30 ml in the adult. The lacrimal gland is located superolaterally in the orbit and is divided by the orbital septum.

References

Toris CB, Yablonski ME, Wang YL, Camras CB. Aqueous humor dynamics in the aging human eye. Am J Ophthalmol. 1999 Apr;127(4):407-12. [Pubmed]

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Anatomy of the Eye- A study guide for residents.

Basic Science Study List for first year ophthalmology residents.

This list serves to guide you in the reading of ocular anatomy. This is a work in progress. The format is designed to take advantage of the proven study technique known as SQ5R. You can attempt to recite the answers and then click on the link to read to find the right answer. Use it as a self test and for board preparation. Some answers are linked to information that has been published. This list is under continuous revision and should improve in time.

Ophthalmology Question Database for Board Review

Table of Contents- ANATOMY OF THE EYE, ORBIT AND ADNEXAE

ORBIT

1. Name the precise dimensions and volume of the orbit.
2. Name the bones that form the orbit and their relations.
3. Which are the thinnest 2 bones of the orbit (the most susceptible to blow out fracture)
4. Describe relationships of paranasal sinuses to each other, and the orbital bones and eyes.
5. Name the foramina, ducts, fissures, canals and their contents in all orbital bones
6. The microanatomy of the optic canal. The position of the optic nerve relative to other important structures after passing through the optic canal.
7. Identify the structures on a CT scan, e.g. zygoma, frontal bone, superior ophthalmic vein, superior and inferior orbital fissures, optic canal, lacrimal gland, extraocular muscles, maxillary, ethmoid, sphenoid sinuses, optic nerve, and lens.
8. CT scan: identify maxillary sinus, zygoma bone, temporalis fossa, ethmoid sinus, superior and inferior orbital fissues, optic canal, superior ophthalmic vein, superior oblique muscle, lacrimal gland, frontal sinus, anterior and posterior clinoids, cavernous sinus, sella turcica.
9. What is the ciliary ganglion and what is its precise location?

CRANIAL NERVE PATHWAYS

1. The complete pathway of cranial nerves II,III,IV,V,VI,VII from its nucleus to final distribution.
2. Anatomic relationships and accompanying important structures at each point along the pathway for cranial nerves. Cranial Nerve #: III, IV, V, VI
3. Organization of fibers at the optic chiasm. What is Wilbrant's knee?
4. Representation of the visual field on the visual cortex and eye sensation on the sensory cortex.
5. The subnuclei of CN III.
6. Know the divisions of the oculomotor nerve and the location of parasympathetic fibers around the nerve.
7. Describe in detail the relationship between CN III and the Circle of Willis
8. Be able to diagram the pathways for pupillary reflexes, both input and output in detail and the adjacent structures, which may be affected by pathology .
9. Detailed anatomic pathway for the IV nerve.
10. Be able to trace the sensory pathway from the iris/cornea back to the brainstem.
11. What is the anatomic basis of Hutchinson sign?
12. Location and contents of the Cavernous Sinus and the relationship to the ophthalmic veins.

EXTRAOCULAR MUSCLES

1. Extraocular muscles and tendons, origin, insertion, blood supply and size, length and width for each.
2. Spiral of Tillaux precise distances.
3. Pulleys composition and relationship to extraocular movement.

EYELIDS

1. Eyelid anatomy, layers in various areas, microscopic anatomy.
2. Details of levator aponeurosis, origin insertion and path, position location and particularly insertion of canthal tendons, composition types and histology of the various glands, attachments and location of the orbital septum, orbicularis, levator, Mueller’s muscle, and tarsus, vascular supply and drainage, lymphatic drainage.
3. Which of the muscles of the eyelid are composed of smooth muscle and which striated muscle?
4. What are the origins and insertions of the eyelid muscles?

LACRIMAL GLANDS AND TEARS

1. Lacrimal gland: Describe the relationships to the orbital bones, levator, etc., and
2. Lacrimal gland histology
3. Lacrimal excretory system, anatomy, histology, important surgical landmarks.
4. The dimensions of the human eye, average, AP, at birth, 3 years, and adulthood.
5. Layers of tear film, source and function
6. What are the major proteins in the tear film?

THE EYE

1. Describe the conjunctival anatomy, histology at different sites, e.g. bulbar, marginal fornices, caruncle, and plica semilunaris.
2. Landmarks of the limbus
3. What are Tenons capsule, sclera, and episclera?
4. Scleral thickness at equator, macula, etc. where is it the thinnest and thickest
5. Dimensions of the cornea horizontal and vertical.
6. Which layers of the cornea are not restored after injury?
7. Determinants of corneal transparency.
8. Describe the anatomy as related to the gonioscopic appearance of the anterior chamber angle.
9. The attachment sites of the uveal tract.
10. Histology of the eye in an overall way. Identification from a glass slide.
11. Layers of the iris, be complete.
12. Innervation of the iris dilator and sphincter muscles, know their pathways
13. Parts of the ciliary body and location
14. Ciliary muscle: innervation, function
15. Dimensions of the lens and how these change throughout life
16. Regional differences in the lens epithelium
17. Regional differences in the RPE and retina
18. Neuronal elements and their connections in the retina
19. Characteristics of retinal blood vessels
20. Differences between the central retinal artery and retinal arterioles.

    Know the location and dimensions of the macula, fovea, foveola, and FAZ
21. Differences between the nasal and temporal ora serrata and relationship to vitreous base and distances from the limbus.
22. What is the blood supply and drainage for the choroid?
23. Attachments of the vitreous.
24. Regional differences dimensions and blood supply of the optic nerve
25. Blood supply to the optic nerve.
26. What are the precise horizontal and vertical dimensions of the optic nerve head?

INTRODUCTION

This eBook facilitates the study efforts of students, residents, and physicians learning about eye pathology. Dr. Glasgow will publish his study guide one chapter at a time. Topics covered will encompass the basic science and pathology of disease affecting the cornea & conjunctiva, uvea, lens, retina/vitreous, orbit, adnexa, optic nerve, anterior chamber & trabecular meshwork, and sclera. Please come back or pick up the Atom Feed for updates on new chapters and learning materials.

Andrew Doan, MD, PhD

Ocular Pathology Study Guide

Ocular Pathology Study Guide For Ophthalmology Residents

Ophthalmology Question Database for Board Review

Ben Glasgow, MD

Professor of Ophthalmology and Pathology

Jules Stein Eye Institute

University of California, Los Angeles

Copyright © 2005 Ben Glasgow, MD

Published and distributed by MedRounds Publications, Inc.

All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Published in The United States of America.

DISCLAIMER

The following material is intended for licensed physicians trained to practice medicine. The practice of medicine has associated risks and complications. The Authors and MedRounds Publications, Inc. do not intend for this material to replace proper medical and surgical training, and we shall not be liable to any user of our materials or any third person as a result of use of our educational materials.

Although the published material has been reviewed by licensed physicians for accuracy at the time of publication, medicine and the standard of care may change quickly. Physicians are reminded, therefore, that guidelines for care can change and opinions can be controversial. Neither MedRounds Publications, Inc., the sponsors nor contributing institutions, nor the individual authors and editors are responsible for deletions or inaccuracies in information or for claims of injury resulting from any such deletions or inaccuracies. We advise physicians to consult the primary research literature before implementing any new treatments.

The author has no financial interests in the commerical products discussed in this publication.

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