ANATOMY OF THE HUMAN EYE

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2006-04-05T11:34:00.000-07:00

Photograph of a human eye that has been bisected in the coronal plane to show the view of the anterior segment from a posterior perspective (as though you are looking from the retina). The crystalline lens is suspended by delicate fibers called the zonule. The ciliary body (CB) is composed of about 72 processes that make up the pars plicata and a flat area called the pars plana. The ora serrata (ora) is the place where the retina joins the ciliary body.

Below is a cross section of an eye with the various structures numbered. Click on the photograph to get higher magnification and then after you have identified the structure, verify by checking the key below.

1. Epithelium (cornea)
2.Stroma (cornea)
3. Descemet's membrane and endothelium (cornea)
4. Anterior chamber
5. Iris
6. Lens
7. ciliary body 8. sclera

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Ocular Anatomy- Overview

2006-04-04T18:42:00.000-07:00

What is the choroid?

2006-03-06T06:27:00.000-08:00

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.

REFERENCES

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

What is the thickness of the sclera?

2006-03-03T08:28:00.000-08:00

The sclera is the white tunic that covers and protects the eye. At the insertion of the rectus muscles (posteriorly) it is thinnest 0.3mm. At the equator it measure 0.4-0.5 mm and at the posterior pole in measures 1.0 mm. The thickness of the sclera is relevant to areas prone to rupture. Blunt trauma most frequently results in rupture of the eye at the thinnest site, behind the insertions of the rectus muscles.

At the entry site of the optic nerve, the sclera is perforated in a sieve like structure to admit optic nerve bundles, axons of ganglion cells. This sieve of sclera is called the lamina cribosa. In addition there are other channels through the sclera called emissaria. Posteriorly there are aperatures around the optic nerve through which the long and short posterior ciliary arteries and nerves pass. About 4 mm posterior to the equator apertures permit the passage of the vortex veins. Anteriorly, anterior ciliary vessels, branches of vessels to the rectus muscle, and nerves pass.

Recurrent nerve loop of Axenfeld- Described in 1895 by T. Axenfeld was a nerve that makes a loop in the sclera (arrow 1) anteriorly. The nerves travel through the sclera usually originating from the long ciliary nerves (arrow 2) and some may approach the surface of the sclera about 1-6 mm from the limbus (arrow 1). The nerves bend 180 degrees creating a mushroomed loop at the surface (arrow 3). The nerve loops have been described to produce symptoms of irritation and tenderness. A study by Stevenson showed that about 12% of eyes have nerve loops and that the nerve loops are bilateral in 1.0%. Topographically, nerve loops were not found in the horizontal plane anterior to the medial or lateral rectus muscles. Most (70%) are found inferiorly. 62% are found anteriorly to the vertical rectus muscle insertions (44% inferiorly + 18% superiorly). 38% are found in the quadrants particularly in the inferonasal quadrant (18%). The nerves are accompanied by pigment and may produce an elevated nodule on the sclera surrounded by a ring of pigment. The ignorant well meaning resident may foolishly attempt biopsy, exquisitely painful for the patient. For more information click this link.

The sclera is composed of fibrous tissue ranging in thick from 10-140 microns. The sclera is rich in elastic fibers and contains smooth muscle.

References:
Axenfeld, T Heidelberg, Kongressber. 1895 p.122
Stevenson, TC, Intrascleral nerve loops, AJO 1963, 55:935.

What is the blood supply to the choroid?

2006-03-03T08:19:00.000-08:00

The blood supply to the choroid comes ultimately from the ophthalmic artery (#1 in figure). There are variations but quite posterior branches will become the central retinal artery (#2 in Figure), and ciliary arteries (#3 in Figure) on each side of the optic nerve. These vessels divide into 2 long posterior ciliary arteries(#4 in Figure) and ~20 short posterior ciliary arteries (only one on each side is shown in the diagram #5 in Figure) that enter the eye immediately adjacent and around the optic nerve. The short posterior ciliary arteries directly supply the choroid and the long posterior ciliary arteries travel in the suprachoroidal space anteriorly (#6 in Figure) then supply the choroid anteriorly via recurrent branches. The ophthalmic artery (#1 in Figure) continues to provide branches for the posterior (#7 in Figure) and anterior (#8 in Figure) ethmoidal vessels. The superior oblique muscle is shown for orientation ( #9 in Figure).
Blood in the choroid circulates through the choriocapillaries and larger vessels of the choroid to drain into 4-6 vortex veins. In the photograph of a transilluminated portion of the eye in which the retina has been cut away the whorled vortices of the choroidal veins are evident as they coalesce and drain into a single vortex vein. (Also see #1 in Figure drawing). The vortex veins emerge just posterior to the equator (#2 in Figure) in quadrants. The superotemporal and superonasal (#1 and 2 in Figure) vortex veins will drain into the superior ophthalmic vein. The inferonasal and inferotemporal vortex veins will drain into the inferior ophthalmic vein. These vessels will eventually exit via the cavernous sinus (#5 in Figure). The vortex veins anastomose with the anterior ciliary veins (not shown in the figures). The anterior ciliary veins normally carry blood only from the anterior ciliary muscle, but if the vortex veins are occluded (e.g. from a poorly placed scleral buckle) the anterior ciliary veins may become quite dilated.
The superotemporal ophthalmic vein usually exits the eye directly adjacent to or underneath the superior oblique tendon. This has clinical implications for approaches to surgery in which the superior oblique tendon is recessed. Note that the superior ophthalmic vein moves inferiorly and temporally to exit the eye in the superior orbital fissure. This anatomic feature is important when evaluating radiologic studies for carotid cavernous fistula in which the superior ophthalmic vein is enlarged. The inferior ophthalmic vein is said to pass thru the inferior orbital fissure in section 2 of the BCSC. This may not be correct. Most texts show that it either communicates with the pterygoid plexus in the inferior orbital fissure but then either joins the superior ophthalmic vein or exits separately through the superior orbital fissue.

The circulation of the anterior portion of the eye features an intricately anastomotic system that essentially join the anterior ciliary circulation and the long posterior ciliary circulation in 3 interconnected arterial circles, an episcleral circle where episcleral vessels join, an intramuscular ciliary circle and the major arterial iris circle (circumferential vessels in the ciliary body). The anterior ciliary venous system joins conjunctival and episcleral vessels at the limbus.

This ends the required information for UCLA students taking the one week required clerkship in Ophthalmology. All others please continue. UCLA OP-264.01 students and Jules Stein residents please continue with the ocular pathology tutorial.

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Extraocular Muscles

2006-03-03T07:43:00.000-08:00

The Four Recti Muscles. The four recti muscles arise from a short funnel-shaped tendinous ring called the annulus of Zinn. The annulus of Zinn encloses the optic foramen and a part of the medial end of the superior orbital fissure. There are 2 tendons.
The Lower Tendon (of Zinn) is attached to the inferior root of the lesser wing of the sphenoid between the optic foramen and the superior orbital fissure. The lower tendon gives origin to part of the medial and lateral recti and all of the inferior rectus. The Upper Tendon (of Lockwood) arises from the body of the sphenoid, and gives origin to part of the medial and lateral recti and all of the superior rectus muscle. The superior and medial recti muscles are much more closely attached to the dural sheath of the optic nerve. This fact may be responsible for the characteristic pain which accompanies extreme eye movements in retro-bulbar neuritis.

Medial Rectus: (4 in the figure) The medial rectus is the largest of the ocular muscles and stronger than the lateral.
Origin- The medial rectus muscle, (number 2 in the picture) arises from the annulus of Zinn. It has a wide origin to the medial side of and below the optic foramen from both parts of the common tendon, and from the sheath of the optic nerve.
Insertion – The medial rectus inserts medially, in the horizontal meridian about 5.5 mm from the limbus.
Blood supply – The medial rectus is supplied by the inferior muscular branch of ophthalmic artery and 2 anterior ciliary arteries.
Size – The medial rectus muscle is 40.8 mm long; tendon is 3.7 mm long and 10.3 mm wide.
Relationships– Above the medial rectus lies the superior oblique. The ophthalmic artery and its anterior and posterior ethmoidal branches and the posterior ethmoidal, anterior ethmoidal and infratrochlear nerves run between the medial rectus and superior oblique muscles. Below the medial rectus is the orbital floor. Medial to the rectus is orbital fat, separating it from the orbital plate of the ethmoid (ethmoid air cells). Laterally is the central orbital fat.
Innervation– The inferior division of the 3rd nerve innervates the medial rectus on its lateral surface at about the junction of its middle and posterior thirds.
Action. – The medial rectus is a pure adductor.

Inferior Rectus (7 in the figure): The inferior rectus is the shortest of the recti muscles.
Origin–It arises below the optic foramen, from the middle slip of the lower common tendon of the annulus of Zinn at the apex of the orbit.
Insertion– inserted inferiorly, in vertical meridian about 6.5 mm from the limbus. The inferior rectus is also attached to the lower lid by means of the fascial expansion of its sheath.
Blood supply – the inferior muscular branch of ophthalmic artery and infraorbital artery, 2 anterior ciliary vessels
Size – 40 mm long; tendon is 5.5 mm long and 9.8 mm wide
Relations– Inferior division of the 3rd nerve lies above the muscle, and the optic nerve is separated by orbital fat, and the globe of the eye. Lateral – The nerve to the inferior oblique runs in front of the lateral border of the inferior rectus between it and the lateral rectus. Below is the floor of the orbit, roofing the maxillary sinus. The muscle is in contact with the orbital process of the palatine bone, but more anteriorly it is separated by orbital fat from the orbital plate of the maxilla.
Innervation– The inferior rectus is supplied by the inferior division of the 3rd nerve, which enters it on its upper aspect at about the junction of the middle and posterior thirds.
Actions – The inferior rectus makes the eye look downwards or medially or wheel-rotates it laterally (extorsion). By means of its fascial expansion it also depresses the lower lid.The principal action is depression which increases as the eye is turned out and is nil when the eye is adducted. The inferior rectus is the only depressor in the abducted position of the eye.
Lateral Rectus: (5 in the figure)
Origin – arises from the annulus of Zinn and spans the superior orbital fissure (#8 and #9 in the figure).
Insertion – inserted laterally, in horizontal meridian 6.9 mm from the limbus
Blood supply – the lacrimal artery (the only rectus muscle with a single blood supply a common board question!)
Size – 40.6 mm long; tendon is 8 mm long and 9.2 mm wide. The lateral or external rectus arises from both the lower and upper parts of the common tendon from those portions which bridge the superior orbital (sphenoidal) fissure.The origin is said to assume form of the letter U placed so that the opening faces the optic foramen, the limbs of the U being referred to as the upper and lower heads of the muscle.
Relations– The structures which go through the two heads of the lateral rectus, within the cone of muscles or within the annulus of Zinn, have been referred to as the oculomotor foramen.These structures from above downwards are the upper division of the 3rd nerve, the naso-ciliary, and a branch from the sympathetic, then the lower division of the 3rd, then the 6th, and then sometimes the ophthalmic vein or veins.The 6th nerve is actually passing from being below the lower division of the 3rd to lie lateral and in between the two divisions.
Innervation– The 6th nerve (abducens) enters it on its medial aspect, just behind its middle.
Actions– The lateral rectus is a pure abductor – that is, makes the eye look directly laterally in the horizontal plane.
Superior Rectus: (2 in the figure)
Origin – The superior rectus arises from the upper part of the annulus of Zinn above and to the lateral side of the optic foramen and from the sheath of the optic nerve. This origin lies below that of the levator, and is continuous on the medial side with the medial rectus and on the lateral with the lateral rectus.
Insertion – inserted superiorly, in vertical meridian 7.7 mm from limbus
Blood supply – Superior muscular branch of ophthalmic artery and 2 anterior ciliary a.
Size – 41.8 mm long; tendon is 5.8 mm long and 10.6 mm wide
Relations– Above the superior rectus is the levator and the frontal nerve, which separate it from the roof of the orbit. Below is the optic nerve, but separated by orbital fat, the ophthalmic artery, and the naso-ciliary nerve. Farther forwards the reflected tendon of the superior oblique passes beneath the superior rectus to reach its insertion. Laterally, in the angle between superior and lateral recti, are found the lacrimal artery and nerve.Medially,the ophthalmic artery and naso-ciliary nerve lie in the angle between the superior rectus and the medial rectus and superior oblique muscles.
Innervation – The superior rectus is supplied by the superior division of the oculomotor (3rd cranial), which enters the under-surface of the muscle at the junction of the middle and posterior thirds.
Actions – The superior rectus makes the eye look upwards or medially or wheel-rotates it medially (intorts). It also helps the levator to lift the upper lid.

Superior Oblique: (3 in the figure)The superior oblique is the longest and thinnest eye muscle.
Origin – arises above and medial to the optic foramen by a narrow tendon which partially overlaps the origin of the levator.
Insertion – inserted to trochlea at orbital rim, on the medial wall of the antero-superior-medial orbit on the frontal bone. The muscle stops just before the trochlea and then proceeds as tendon under superior rectus posterior to insert on the temporal aspect of the eye behind the equator.
Blood supply – the superior muscular branch of ophthalmic artery supply blood
Size – 40 mm long; tendon is 20 mm long and 10.8 mm wide.
Trochlea- The trochlea consists of a U-shaped piece of fibro-cartilage. The cartilage merges imperceptibly above with fibrous tissue, and is attached to the fovea or spina trochlearis on the frontal bone a few millimeters behind the orbital margin on the medial wall of the orbit. Immediately before entering the pulley striated muscle joins the tendon, which is enclosed in a synovial sheath, beyond which a strong fibrous sheath accompanies the tendon to the eye.
Actions – The superior oblique moves the eye downwards or laterally or (wheel-) rotates it inwards (i.e. makes twelve o’clock on the cornea move towards the nose).The principal is the depression, and this increases as the eye is adducted. The superior oblique is the only muscle which can depress in the adducted position. Its action is practically nil when the eye is abducted.The abduction and intorsion are the subsidiary actions, and increase as the eye turns out.The superior oblique acts with the inferior rectus to make the eye look directly down. The abductor component of the action of the oblique muscles is due to their being inserted behind the equator of the globe.
Innervation – The superior oblique is supplied by the 4th or trochlear nerve which, having divided into three or four branches, enters the muscle on the upper-surface near its lateral border; the most anterior branch at the junction of the posterior and middle thirds, the most posterior about 8 mm. from its origin.

Inferior Oblique: (6 in the figure)
Origin – The inferior oblique is the only extrinsic muscle to take origin from the front of the orbit; arises from a rounded tendon in a depression on orbital floor near orbital rim (maxilla), just behind the orbital margin and lateral to orifice of the naso-lacrimal duct. Some of its fibres may arise from the fascia covering the lacrimal sac.
Insertion – inserted posterior inferior temporal quadrant at level of macula
Blood supply – the inferior branch of ophthalmic artery and infraorbital artery
Size – 37 mm long; the shortest tendon of insertion ( essentially no tendon) and it is 9.6 mm wide at insertion.
Relations – Near its origin the lower surface of the muscle contacts the periosteum of the orbital floor, laterally it is separated from the floor by fat. Just before the insertion of the muscle, this surface which now faces laterally is covered by the lateral rectus and Tenon's capsule. The upper aspect contacts fat, then the inferior rectus, then finally spreading out and becoming concave it moulds itself on the eye.
Innervation– the inferior division of the oculomotor nerve, crosses above the posterior border to enter the muscle on its upper-surface at about the middle of the muscle.
Blood-supply comes from the infraorbital artery and the inferior muscular branch of the ophthalmic artery.
Actions– The inferior oblique makes the eye look upwards or laterally or wheel-rotates it laterally (extorts). The principal action is the elevation which increases as the eye is turned in and is nil in abduction. The inferior oblique is the only elevator in the adducted position.

Levator Palpebrae Superioris Muscle: (1 in the figure) striated muscle to elevate the eyelid.The levator palpebrae superioris arises from the under-surface of the lesser wing of the sphenoid above and in front of the optic foramen by a short tendon which is blended with the underlying origin of the superior rectus.The flat ribbon-like muscle belly 40 mm in length passes forwards below the roof of the orbit and on the superior rectus to about 1 cm. behind the orbital septum (at the upper fornix or a few millimeters in front of the equator of the eye), where it ends in a membranous expansion or aponeurosis. The tendon is about 10-15 mm in length and extend from the equator forward. This spreads out in a fan-shaped manner, so as to occupy the whole breadth of the orbit and thus gives the whole muscle tendon complex the approximate form of an isosceles triangle.
Attachments. – (a) The main insertion of the levator is to the skin of the upper lid at and below the upper palpebral sulcus. It reaches this by intercalating with the fibres of the orbicularis.(b) To the Tarsal Plate. – Some of the fibres of the aponeurosis are attached to the front and lower part of the tarsal plate, but the main attachment of the levator here is via the smooth superior palpebral muscle of Muller. This is continuous with the fleshy part of the levator, and is attached to the upper border of the tarsus.
Relations– Above the levator and between it and the roof of the orbit are the 4th and frontal nerves and the supraorbital vessels. The 4th nerve crosses the muscle close to its origin from lateral to medial to reach the superior oblique. The supraorbital artery is above the muscle in its anterior half only. The frontal nerve crosses the muscle obliquely from the lateral to the medial side. Below the levator is the medial part of the superior rectus.
Innervation– The superior division of the 3rd nerve reaches the muscle either by piercing the medial edge of the superior rectus or curving around its medial border.
Action – The levator raises the upper eyelid, thus uncovering the cornea and a portion of the sclera, and deepens the superior palpebral fold. Its antagonist is the palpebral portion of the orbicularis.

Muller's muscle- Also known as the superior palpebral muscle is a smooth muscle that acts as an eyelid elevator.
Origin- arises from the inferior or bulbar aspect of the levator palpebrae behind the fornix.
Insertion-upper edge of the tarsal plate
Action- eyelid elevator
Size- 15-20 mm at its origin, 10 mm in vertical length, slightly wider at its insertion
Relations-lies between the tendon of the levator and the conjunctiva in the eyelid. Muller's muscle begins after the levator muscle has become exclusively tendons.
Innervation- sympathetic fibers

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What are the layers of the eyelid?

2006-02-26T10:49:00.000-08:00

The human eyelids protect, nourish and sustain the cornea. The basic structure is divided arbitrarily by surgeons into 2 lamellae, anterior and posterior. It is more realistic to consider the individual structures that comprise the lids and their function. In the photograph to the left, a low magnification, to show the cornea (1) and the lens (2) for orientation. Click on the photo to enlarge it and see the structures more clearly. The inner lining of the eyelids is conjunctival epithelium, which contains a variable number of goblet cells. The goblet cells are high in density in the fornix (3) and less so near the marginal conjunctiva (4). The upper eyelids also contain accessory lacrimal glands (5) known as the glands of Krauss that are scattered over the entire measure of the lids and supply constant lubrication. They empty directly into the fornix. The eyelids get much of their form from the tarsus, a specialized fibrous layer (6) that contains numerous clear sebaceous glands called Meibomian glands. The lower lid is similar to the upper lid but notice the shortened tarsal plate. The upper lid has more Meibomian glands which may account for the greater incidence of sebaceous carcinoma in the upper eyelids.

A pictorial rendition from Grey's anatomy is more graphic as to the numerous structures in a cross section of the eyelid even though the resolution is low. The levator aponeurosis(1 Fig left) , one of the elevators of the eyelid enters between the orbicularis oculi muscle and the conjunctival surface. Accessory lacrimal glands of Wolfring are shown (2 Fig left ). The meibomian glands (3 Fig left) of the tarsal plate produce the lipid that will line the layer of the tear film. The Meibomian lids empty into ducts that dot the marginal surface of the eyelid and can be seen emanating droplets of oil for the tears. The orbicularis muscle (4 Fig left ) is striated muscle that is responsible for blinking and squeezing eyelids shut. The cilia or eyelashes (6) emanate from the lid immediately adjacent to apocrine glands of Moll (7). The skin surface (8) of the eyelid is the thinned epidermis in the body and contains hair and adnexa.
The color image of the eyelid section provides an excellent opportunity to test knowledge of the eyelid structures. Click to enlarge the photograph and then see if you have correctly identified by the structures by using the back function key and correlating your answers with the numbers below.
1- skin surface; 2 fat and fascia; 3 orbicularis oculi; 4 levator aponeurosis; 5 tarsal plate; 6 conjunctival surface; 7 Meibomian ducts; 8 glands of Moll; 9 accessory lacrimal gland of Wolfring

Lymphatic drainage: The upper eyelid drains to the parotid lymph nodes and the lower eyelid drains to the submandibular lymph nodes. This has ramications for tumors that have predilections for specific eyelids.

What orbital bones are prone to fracture and infection?

2006-02-26T10:07:00.000-08:00

The medial wall of the orbit is in areas paper thin (0.2-0.4 mm in thickness). This area over the ethmoidal air cells (ethmoid bone shown in green #3) and green in the figure below is prone to infection and blow out fractures. Interestingly, the honeycomb structure of the ethmoid air cells may provide additional protection. The floor of the orbit has been said to be thicker than the medial wall (0.5 mm thickness in the thinnest areas). However, in our experience it is quite thin. In the accompanying photograph a cross section of a bone from the floor of the orbit is shown that was submitted after trauma. The scale at the bottom shows divisions in millimiters. One can see that maxillary bone measures less than 0.2 mm in some areas. This may be the reason the floor is more prone to blow-out fractures. The blow-out fracture can occur by a mechanism in which intraorbital pressure is increased from blunt trauma. The floor of the orbit is composed of the maxillary bone, part of the zygoma and the palantine bone. In a blow out fracture the contents of the inferior orbital fissure are susceptible to entrapment and injury. The medial wall of the orbit at the lesser wing of the sphenoid is the thickest area; forming the optic canal (nerve).

Tripod fractures, currently called quadruped fractures result from the inferior-posteriorly displaced zygoma; separating it from articulations with maxillary, frontal and zygomatic arch of the temporal bone. The zygoma may also be separated from the sphenoid bone, a fourth articulation. It is called quadruped because the floor, lateral orbital rim, the lateral wall of the maxillary sinus and zygomatic arch are fractured. The key findings are a depressed malar eminence (zygomatic arch depressed) , diplopia (entrapment of extraocular muscle), hypoesthesia in the distribution of the infraorbital nerve and trismus, pain and difficulty opening the mouth, because of impingement of the zygoma on the coronoid process of the mandible. A direct blow to the body of the zygoma is usually the cause of this type of fracture.

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What are the paraorbital sinuses?

2006-02-26T09:22:00.000-08:00

The paraorbital sinuses include the frontal sinus above the orbit (orange), the ethmoid sinuses (green) in the medial wall of the orbit, the sphenoid sinus (blue) that is posteriorly located and the maxillary sinus (purple-pink).

Below, the sinuses are shown in a sagittal view. The maxillary sinus (red-pink) and the ethmoid sinuses would not be visible in the midline plane that shows the drainage from the sinuses (arrows). It is evident that the maxillary sinus is close to the teeth. Pain from the sinus may be referred to teeth. This occurs in maxillary sinus infections. Occasionally, normal teeth have been removed from patients with life threatening fungal sinus infections because this source of referred pain was unrecognized.

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Bones of the Orbit

2006-02-26T08:51:00.000-08:00

Seven bones make up the orbit. Click on the photograph to enlarge and test yourself by naming the numbered bones. The roof of the orbit is composed of 2 bones, the frontal bone and the sphenoid bone. The frontal bone (#1 in blue) comprises anterior part of the roof of the orbit and the lesser wing of the sphenoid (#2 in tan) surrounds the optic canal and forms the posterior part of the roof.

The medial wall of the orbit is composed of 4 bones: sphenoid, ethmoid, lacrimal and maxillary bone. The lesser wing of the sphenoid (#2 in tan) is most posterior and is joined to the ethmoid bone (#3 in dark green), moving anteriorly to the lacrimal bone (#4 in light red) and then to the maxillary bone (#5 in light green).

The floor of the orbit is composed of 3 bones: maxillary bone (#5 light green); zygoma (#6 in pink) and posteriorly the palantine bone (#8 in bright red). The palantine bone borders on the inferior orbital fissure. Notice that the inferior orbital fissure narrow posteriorly, a useful landmark in CT scans.

The lateral wall of the orbit is composed of the zygoma (#6 in pink) and the greater wing of the sphenoid (#7 in tan). There is an important landmark on the zygoma that every ophthalmologist and emergency physician must know. The lateral orbital tubercle (that dark area above #6 in the figure) is the place where the lateral canthal tendon joins the orbit. The lateral canthal tendon, which is a fibrous extension of the tarsi of both eyelids, constrains the orbital contents. In traumatic injury with orbital hemorrhage the intraorbital pressure and intraocular pressure may rise high enough to snuff out blood flow and vision. In this case the lateral canthal tendon can be lysed at the lateral orbital tubercle. The inferior crux of the lateral canthal tendon can be divided as well. This emergency procedure, lateral canthotomy and cantholysis may prevent permanent vision loss.

Some foramina or openings through the bones of the orbit and their associated contents are shown in this depiction of the boney posterior left orbit. It is important for students interested in ophthalmology to know the contents of these canals. With this knowledge, findings in such disorders as superior orbital fissue syndrome and carotid cavernous fistulas will make much more sense. See how many entering nerves and vessels you can name correctly.

Backlighting of the cranial cavity with the skull-cap removed, exposes some foramina (optic canal, superior orbital fissure and pterygoid canal); light is shining in from the cranial cavity. The inferior orbital fissure does not enter the cranial cavity and therefore is relatively dark in this illustration.

Superior orbital fissure:

1. lacrimal nerve

2. frontal nerve

3. trochlear nerve

4. oculomotor nerve (superior division)

5. nasociliary nerve

6. abducens nerve

7. oculomotor nerve (inferior division)

8. superior ophthalmic vein

blue arrow- sympathetic fibers of ciliary ganglion

Inferior Orbital Fissure:

9. maxillary nerve

10. nerve entering from the pterygoid canal

11. inferior ophthalmic vein

Optic Canal:

12. optic nerve

13. ophthalmic artery

red arrow- sympathetic fibers carotid plexus

Nasolacrimal duct, puncta, lacrimal sac, cannaliculi

2006-02-13T16:12:00.000-08:00

-Click to enlarge the diagram and photomicrographs and identify the structures, type of epithelial lining.
-What is the length of tubing needed to cannulate the entire system from puncta to nose?
-What is the diameter of the normal nasolacrimal duct?
-What prevents mucous from traversing up the nasolacrimal duct with a giant SNEEZE!?
-An infant presents at 3 months of age with tearing. What is the most probable cause and what treatment is indicated?
-What is the source of oncocytomas in this location?

Then Click on the link to retrieve the answers.

Lacrimal Excretory System

2006-02-13T16:11:00.000-08:00

Tears are produced in the lacrimal gland (#1 in diagram to the left)exit the ocular surface via the puncta at the medial portion of the eyelids (#2 Diagram left). Each punctum is a small, round, or transversely oval aperture situated on a slight elevation. The puncta can be seen to be roughly in line with the openings of the Meibomian glands, the nearest of which is only 0.5 to 1 mm away.
Each puncta empties into a canaliculus (Diagram left #3)which has at first a vertical (2 mm length) and then a horizontal (8 mm length) segment. The canaliculi join in the common canaliculus and may dilate to be called the sinus of Maier which then transitions to the lacrimal sac.
In the photo below, the puncta and canaliculi are lined by stratified squamous epithelium (
Photomicrographs below #'s 1 and 2). Click to enlarge the photograph. The puncta are positioned between conjunctiva (Photo below 3) and skin (Photo below #4) at the border of the eyelid.

The lacrimal sac is placed in the lacrimal fossa (formed by the lacrimal bone and the frontal process of the maxilla) which lies in the anterior part of the medial wall of the orbit. The sac is closed above (Diagram above #4) and open below, where it is continuous with the naso-lacrimal duct (Diagram above #5). The upper portion of the lacrimal sac is called the fundus and the lower portion is called the body and the length of these segments, somewhat arbitrarily distinguished are about 3 and 10 mm respectively. The lacrimal sac joins the nasolacrimal duct which measures about 12 mm in length and 3-4 mm in diameter. The lacrimal sac and duct are both lined by two layers of epithelium, the superficial of which is columnar, the deeper flattened. The epithelium of the lacrimal sac "stratified columnar" and may have areas that are ciliated and areas in which the superficial layer contains only Goblet cells and mucous. The bases of the columnar cells pass through the deeper layer to reach the basement membrane. The lacrimal sac has a papillary appearance with numerous infoldings. Occasional oncocytic cells may be evident particularly in lacrimal sacs that are chronically inflamed and are probably the source for oncocytomas that may affect the lacrimal sac. The nasolacrimal duct enters the nose at the inferior nasal meatus. The point of entry is called the lacrimal osteum and it is covered by a fold of nasal mucosa, the plica lacrimalis or if you prefer the valve of Hasner, that prevents mucous from entering the system in a retrograde fashion from the nose (with sneezing or "blowing one's nose against a handkerchief"). The "valve" of Hasner is closed in about 70% of newborns but opens spontaneously by 6-12 months. This is an important clinical point one must consider in an infant with tearing.

By the way there is no end to eponyms for valves in this system if one wants to seem erudite. They are all folds of mucous membranes which have no real valvular function. The valves of Foltz or Bochdalek occur at the junction of the puncta and cannaliculus; the valve of Rosenmuller or Huschke occurs as the common cannaliculus enters the fundus of the lacrimal sac; the valve of Beraud or Krause occur at the junction of the fundus and body of the sac; the valve of Taillefer occurs at the sac-duct junction and of course the valve of Hasner also is called the valve of Cruveilhier or Bianchi.

Human Lens Histology

2006-02-13T12:15:00.000-08:00

The lens is a transparent soft biconvex structure composed of crystallins. The adult lens measures about 9 mm in diameter and is 3.5 mm thick. It 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. There is a layer of large cuboidal epithelial cells, (the lens epithelium) beneath the anterior capsule (#2 in photomicrograph). 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.

Next topic in Eye Pathology

Human Lens-Histology

2006-02-13T11:39:00.000-08:00

Click on the photographs to enlarge and identify the areas in the human lens. Then click on the link to see the answers.

Lacrimal gland-human

2006-02-13T11:03:00.000-08:00

How many lacrimal glands are there in each orbit? Main (orbital and palpebral portions) + Glands of Krause (50) + Glands of Wolfring (5) + caruncle (1) = About 57!
Which lacrimal glands can be identified in the photograph below? This is an anterior posterior section of the orbit cut in the vertical plane.

Main Lacrimal gland- orbital and palpebral portions
Where is the main lacrimal gland? The lacrimal gland consists of an orbital or superior portion; and a small palpebral or inferior portion; which are continuous. Shaped like an almond the main lacrimal gland is situated above and lateral to the eye in the orbit in a shallow depression of the frontal bone. (The lacrimal gland does not lie in the lacrimal fossa or lacrimal bone where the nasolacrimal duct is!). The lacrimal gland secretes tears into ducts in the upper fornix. The lobules of the orbital portion of the lacrimal gland (#1 in photo above) are near the orbital septum but lie under the levator muscle (#4 photo above).
Accessory Lacrimal Glands of Krause
Where is the accessory lacrimal gland of Krause? The accessory lacrimal gland of Krause(#3 photo above) lies immediately adjacent to the fornix of the upper eyelid (#2 photo above). The glands of Krause are accessory lacrimal glands having the same structure as the main gland. They are placed deeply in the substantia propria of the upper fornix between the tarsus and the inferior lacrimal gland, of which they are offshoots. The sclera (5), ciliary body(6) and iris (7) are all identifiable in photograph above.
How many accessory lacrimal glands of Krause are in the upper eyelid?
How many accessory lacrimal glands of Krause are in the lower eyelid?
There are some 42 in the upper fornix and 6 to 8 in the lower fornix. The glands of Krause are found largely on the lateral side of the orbit. Their ducts unite into a rather long duct or sinus which opens into the fornix.
Caruncle-
Are lacrimal glands a normal part of the caruncle? Similar lacrimal glands are found in the caruncle. (Check back for photo).
Accessory Lacrimal glands of Wolfring.
Identify the glands of Wolfring in the photograph.
What structure corresponds to each letter?
Where are the accessory lacrimal glands of Wolfring? The Glands of Wolfring or Ciaccio are also accessory lacrimal glands, but larger than the glands of Krause. There are 2 to 5 in the upper lid and 1-3 in the lower lid, situated in the upper border of the tarsus midway between the end of the tarsal glands. In the photograph of a cross section of the eyelid, A points to one of the sebaceous glands embedded in the tarsus. B is a gland of Wolfring, C is a nerve and D is the orbicularis muscle. The excretory duct for the gland of Wolfring (not shown in the photograph) is large and short and lined by a basal layer of cubical cells and a superficial layer of cylindrical cells like the conjunctiva onto which it opens.
Lacrimal Gland Histology
Identify each of the numbered histologic features below.

Photomicrograph (above) shows a higher magnfication of a human lacrimal gland complete with acinar structures that contain lumens (1) and protein rich acinar cells that secrete lysozyme, tear lipocalin, lactoferrin and IgA. The reddish granules (2) are secretory vesicles replete with protein. Some lumens are filled with protein that is being secreted. Lymphocytes and plasma cells are scattered in the interstitium(3).
The lacrimal gland consists of a lobules and is a tubulo-racemose gland with short branched gland tubules somewhat similar to the parotid. The acini consist of two layers of cells placed on a thin hyaline basement membrane and surrounding a central lumen. The basal cells are myoepithelial in character while the acinar cells are cylindrical, and secrete fluid into a series of ducts of increasing size until becoming the excretory duct.
What species do not have lacrimal glands? The lacrimal gland and its tears exist in animals which live in air. Fish do not have lacrimal glands.
What is the blood supply to the lacrimal glands? The arterial blood supply originates from the ophthalmic artery via the lacrimal artery. The lacrimal artery arises from the ophthalmic artery lateral to the optic nerve and runs forward along the upper border of the lateral rectus muscle in company with the lacrimal nerve. The drainage is to the lacrimal vein which joins the opthalmic vein.
What is the innervation of the lacrimal gland? The lacrimal nerve provides sensory innervation.
Describe the lacrimal reflex arc. The Vth cranial nerve is the afferent pathway from the sensory fibers in the nose and the corneal surface (arrow 1 in the figure below). Corneal fibers traverse the long posterior ciliary nerve in the sclera and exit posteriorly (arrow 2 in the figure below). The fibers pass join the nasociliary nerve as the long sensory root (passing through the ciliary ganglion (arrow 3). The nasociliary nerve exits the orbit thru the superior orbital fissure and enters the cavernous sinus lateral to the internal carotid artery. The nerve passes thru the trigeminal ganglion (also called semilunar or Gasserian ganglion, #4 in the figure below) which is present in Meckel's cave. From the trigeminal ganglion the fibers enter the pons and descend in the ipsilateral spinal trigeminal tract, synapsing in the most ventral portion (synapse 1, see figure- arrow 5). The output from the sensory nucleus is then to the lacrimal and salivatory nuclei (synapse 2, see figure- arrow 6). From here efferents (arrow 7) pass into the seventh nerve (nervus intermedius of the facial nerve) thru the geniculate ganglion (arrow 8), the greater or superficial petrosal nerve. The superficial petrosal nerve enters the pterygoid canal, continues in the pterygopalantine fossa and synapses in the pterygopalantine ganglion (synapse 3, arrow 9). Unmyelinated postganglionic parasympathetic fibers enter the inferior orbital fissure and form a retrobulbar plexus of nerves that include sympathetic fibers from the carotid plexus. These nerves supply the lacrimal gland (arrow 10) as the rami oculares . Tear secretion is mediated by parasympathetic output and vasoactive intestinal polypeptide (VIP) mediated fibers (synapse 4).
Describe the pathway for emotional tearing. Afferents to the lacrimal nucleus from the hypothalamus with cortical input mediate emotional tearing. The pathway through the seventh nerve is identical as described for the reflex lacrimation.
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Lacrimal gland

2006-02-13T10:58:00.000-08:00

Here are some questions for you to keep in mind.
1. Name all the lacrimal glands of the orbit.
2. Approximately how many lacrimal glands are there?
3. Exactly, where are the main lacrimal glands? Do they rest in the lacrimal fossa?
4. What types of accessory lacrimal glands are present around the eye and where are they?
5. What do the lacrimal glands produce?
6. What organisms do not need lacrimal glands?
7. What is the blood supply to the lacrimal gland?
8. What is the innervation to the lacrimal gland?
9. Describe the lacrimal reflex arc for tearing.
Please identify the structures shown in the photographs below. Click on the photos to enlarge them. Then link for the answers.

EYE-MICROSCOPIC-SECTION-LABELED

2006-02-08T19:25:00.000-08:00

CLICK ON THE PHOTO TO ENLARGE AND SEE THE ANSWERS.
This photo is convenient to study the flow of aqueous from the ciliary epithelium of the ciliary body through the posterior chamber and the narrow space between the iris and lens into the anterior chamber and then in the anterior chamber angle and trabecular meshwork, Schlemm's canal and the collector vessels. Anatomic pertubations that may obstruct this flow include apposition of peripheral iris against the cornea (known as angle closure glaucoma), apposition of the iris against the lens because of rotation of the ciliary body leading to (malignant glaucoma), obstruction of the trabecular meshwork (such as in hemorrhage known as ghost cell glaucoma), adherence of the pupillary margin of the iris to the lens as after severe inflammation), and back pressure on the system such as a tumor that obstructs aqueous veins or a carotid cavernous sinus fistula that produces elevation of venous pressure and blood can sometimes be seen in aqueous veins.

Conjunctiva (answers)

2005-11-07T06:33:00.000-08:00

Conjunctiva (named because it conjoins the eyeball to the lids) is a thin, transparent mucous membrane that lines the posterior surface of the lids, and is then reflected forwards on the eye. Conjunctiva is continuous anteriorly with the epithelium of the cornea. Recessed in the eyelids, the conjunctiva forms a cul de sac, which is open in front at the palpebral fissure, and only closed when the eyes are shut. Although all parts of the conjunctiva are continuous each has been given its own name to emphasize anatomic differences. The palpebral portion lines the eyelids; that portion joining the eyeball is the bulbar conjunctiva and that forming the conjunctival sac and reflecting on the eye is called the fornix. The palpebral conjunctiva is subdivided into marginal, tarsal, and orbital zones. The marginal zone transitions between skin and conjunctiva and shows minimal keratinization. The tarsal conjunctiva is a fairly flat layer. The orbital zone shows more numerous Goblet cells.
The regional variation of the conjunctiva start with an overview in a photograph in which the patient is looking up. The limbus (1) is the junction of the conjunctiva and cornea. The bulbar conjunctiva (2) covers the eyeball and extends into the recess created by forniceal conjunctiva (3). The tarsal conjunctiva (4) covers the tarsus. The marginal conjunctiva (6) is at the eyelid margin where the epithelium will begin to be keratinized. The punctum (5) is also shown.
A sagittal or vertical section of both eyelids and the eye are shown to the left. The cornea (1) and lens (2) provide orientation.
The fornix (3) has more redundant conjunctiva. The marginal conjunctiva(4) and tarsal conjunctiva (6) are indicated. The palpebral portion of the lacrimal gland (5) is also shown in this photograph. The composition of each of these regions varies in the Goblet cell density within the epithelium. In addition, note the greater length of the tarsus and higher number of Meibomian glands in the upper eyelid compared to the lower eyelid. This has implications for the origin of sebaceous carcinoma. This photograph also give a view of the cross section of the eyelid and the alert student will notice the skin externally and orbicularis muscle.

The histology of the conjunctiva varies according to its topographic location. The bulbar conjunctiva is relatively less undulating and contains fewer Goblet cells. In this electronmicrograph the microvilli cover the superficial layer of the epithelium (arrow 1) and the mucin granules (2) of the Goblet cell are captured in a plane lacking contiguity with the surface. The Goblet cells produce gel forming mucins called MUC5AC that may be critical to providing lubrication to the ocular surface.

The tarsal conjunctiva shows a stratified squamous epithelium (1) that has few Goblet cells (none seen here) overlying a very dense fibrous stroma, tarsus (2). Meibomian glands are embedded in the tarsus.

This photomicrograph from the fornix shows numerous goblet cells (7) in infoldings of conjunctiva that form the pseudoglands of Henle (8). Striated muscle (9) is seen beneath the substantia propria.

The puncta open on to the marginal portion of the conjunctiva, and through them the conjunctival sac becomes directly continuous with the inferior meatus of the nose via the lacrimal passages. The conjunctiva contains specialized folds or bumps called the plica semilunaris (arrow 10 in the clinical figure) and caruncle (arrow 11). The plica semilunaris lining contains Goblet cells while the caruncle may have hair, sebaceous glands emanating from the surface. The histologic appearance of the caruncle is shown. Transitioning to a stratified squamous epithelium with few goblet cells (area encompassed by arrows at 11), the caruncle may show focal surface keratinization (arrow 14). The caruncle contains hair follicles (arrow 12), sebaceous glands (arrow 13) and adipose tissue (arrow 15).

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Conjunctiva

2005-11-07T06:30:00.000-08:00

Questions:
1. What structures does the conjunctiva cover?
2. What type of epithelium makes up the conjunctiva?
3. How does the type of epithelium vary according to topographic location?
4. Is the conjunctiva normally lined by keratin in the outer layers?
5. What specialized cells are contained in the conjunctiva?
5. What do Goblet cells secrete?
6. Based on the anatomy what types of cancers would you expect to be found in the conjunctiva?
7. In the photo below identify the various topographical areas of conjunctiva.

Click on the photo below to enlarge and identify the structures that are numbered. This is a sagittal section of the orbit to include the eye with the upper and lower eyelid. (Each photo has its own number system that point to different structures so you must check each photo separately. )
1. How does the upper lid tarsus differ from the lower lid tarsus?
2. Which eyelid has more meibomian glands in the figure?

How does the epithelium in this view of tarsal conjunctiva differ from that of closer to the fornix? Specifically what cells are less numerous here?
How is the epithelium different?

The cell (#2) in the electron micrograph corresponds to which cell in the photo above?

Click here to link to the answers for all photos.

Anterior chamber angle labeled

2005-10-30T16:36:00.000-08:00

Click to enlarge and check your answers! Then click the Back key to return to the original photo.

Aqueous fluid is formed by the non pigmented epithelium in the pars plana and ciliary processes and traverses the posterior chamber around the lens and through the opening in the iris called the pupil to reach the anterior chamber. Here aqueous flows through the trabecular meshwork to the canal of Schlemm and finally exists the eye through aqueous veins. Note the iris root is quite thin and clinically prone to rupture with trauma. The number of cells lining the trabecular meshwork has been shown to decrease with age and interestingly in primary open angle glaucoma and pigmentary glaucoma. The cause for the loss of the trabecular lining cells has not been thoroughly investigated.

1. Alvarado J, Ophthalmology 1984;6:564-79

Anterior Chamber Angle

2005-10-30T16:35:00.000-08:00

Click on the photo to enlarge. Identify the numbered structures. Then click the back key to return the original photo. Then click on the link to see the answers.

Optic Nerve Answers

2005-10-30T16:18:00.000-08:00

The optic nerve is the bundle of axons that extend from the cell bodies of ganglion cells in the retina to synapse on the lateral geniculate body in the brain. The optic nerve as a structure begins at the optic disk. A smaller, disc-shaped depression, called the cup, lies slightly temporal to the center of the optic disc. On the surface is circular glial plaque, a developmental remnant. Often there is a nasal and inferiorly located Bergmeister’s papilla. Nerve bundles penetrate the collagen of the sclera through a sieve perforations, termed the lamina cribrosa. As the non-myelinated nerves pass this point the optic nerve becomes myelinated; axons are enveloped by a sheath of doubled plasmalemma, to form the myelin produced by oligodendrocytes. Myelination doubles the cross-sectional thickness of the nerve to a diameter of ~ 3 mm at the posterior surface of the sclera. The glial cells of the central supporting tissue meniscus, are astrocytes. The coverings of the optic nerve in the orbit are similar to other brain tissue. The outer most covering of the optic nerve is a sheath of dura. The thick dura mater, fuses distally with the outer layers of the sclera. Internal to the dura is subarachnoid space, arachnoid, and pia. The pia (photomicrograph left) is most internal and tightly applied to the optic nerve proper. Fibrous septa from the pial layer are prominent (shown in red in the photomicrograph above) and enter the optic nerve and divide the axons into fascicles (the yellow bundles in the photo). The central retinal artery (CRA) and vein (CRV) penetrate the optic nerve from 8 to 15 mm behind the optic nerve scleral exit to enter the center of the nerve. The blood supply to the optic nerve is complex, redundant and topographical. Prelaminar or retinal portion is supplied by the short posterior ciliary and recurrent choroidal arteries. The posterior ciliary arteries are terminal branches creating an area that is vulnerable to ischemia. The laminar portion is provided by short posterior ciliary vessels via anastomoses with the arterial circle of Zinn-Haller in the sclera. The retrolaminar nerve is supplied by pial, short posterior ciliary, central retinal and perhaps recurrent choroidal vessels. The blood supply for the orbital portion of the optic nerve comes mainly from the ophthalmic artery and a pial network around the nerve. The intracanalicular portion of the optic nerve is perfused entirely by the ophthalmic artery. Drainage of both retinal and choroidal layers appears to be in great part via the central retinal vein and its branches. In the photo ILL refers to the inner limiting lamina of the retina. <Next Topic in Anatomy>

Optic Nerve

2005-10-30T15:54:00.000-08:00

Click on the image to enlarge and identify the structure at each number.

Then click on this link for the answers.

Retina-Answers

2005-10-24T18:47:00.000-07:00

The retina is composed of pigment epithelial cells, photoreceptor cells, retinal support cells and nerve cells that are organized into simple but distinct layers starting with photoreceptors and moving inward to the layer that includes modulating interneurons, and synapsing finally on optic nerve cells (the ganglion cells). The vernacular divides this simplicity into a complex anatomic appearance. Click to enlarge the photomicrograph to the left.
Internal limiting lamina, basement membrane at the interface between the vitreous and the retina.
Nerve fiber layer, axons of the ganglion cells en route to the CNS
Ganglion cell layer, cell bodies of the ganglion cells (large nuclei and nucleoli;
Inner plexiform layer, cell processes and synapses of bipolar cells, amacrine cells, interplexiform cells and ganglion cells
Inner nuclear layer, nuclei and cells bodies of bipolar cells, horizontal cells, interplexiform cells and amacrine cells, as well as the nuclei of the Muller’s cells
Outer plexiform layer, synaptic connections between photoreceptor cells, bipolar neurons and horizontal cells;
Outer nuclear layer nuclei and cell bodies of the photoreceptor cells, 8-9 nine rows;
External limiting membrane (Photo the line between 6-7) adherent junctions between Muller’s cells;
Photoreceptors (photo #7), inner and outer segments of rods and cones
Retinal pigment epithelium (9 (RPE)) sits immediately on Bruch's membrane (8).
Above the RPE or on the inner or internal side lies the photoreceptor layer of the retina that has the rods and cones. The RPE are supportive cells and have macrophage function. The photoreceptors called rods are about 2 µm thick and average of 50 µm in length, possessing a roughly cylindrical outer segment. Cones are shorter and thicker, 3-5 µm thick x 40 µm long; their outer segment is conical in shape. The tips of these rods and cones are embedded in the microvilli of RPE, which phagocytose the old photoreceptor disks. Cones, perceive color, are concentrated in the macula and particularly the fovea. Rods, which perceive light intensity but not color, are concentrated at the periphery of the retina. The light- sensitive photoreceptor disks in the photoreceptor cell form huge stacks 600-1000 deep in the outer segment of the cell and originate as deep infolds of the cell membrane. Muller’s cells and astrocytes also called microglia are supporting cells. Muller’s cells extend from the base of the inner segment of the photoreceptor cells, where they link to each other by adherent junctions to form the structure known as the outer limiting membrane, to the retinal surface, where they rest on the internal limiting membrane. Microglia, astrocytes, act as support cells to the nerve cells throughout the retina and are characterized by long dendritic processes that form a scaffold for the delicate processes of nerve cells. Interneurons integrate signals from photoreceptors and include bipolar cells, ganglion cells, horizontal cells, amacrine cells and interplexiform cells. The terminal of a horizontal cell is interposed between the photoreceptor and the bipolar cell to form a structure termed a triad. The horizontal cell can therefore modulate the impulse from the photoreceptors to the bipolar cells and allows integration of signals from adjacent photoreceptors. Bipolar cells connect with the synaptic end of the photoreceptor cells to transmit signals to ganglion cells. Horizontal cells, amacrine cells, and interplexiform cells modulate the impulses from the photoreceptors to the ganglion cells. Their processes interpose the connections between bipolar cells and photoreceptors, and between bipolar cells and ganglion cells.
Specialized regions include the macula lutea which is a small yellow area located lateral (temporal) to the optic disk. The macula is defined anatomically as having multiple ganglion cell layers. At its center the fovea is a thin zone of retina composed exclusively of cones. There are no ganglion cells directly overlying the cones, they are displaced laterally. One cone synapses with one bipolar cell which links to one ganglion cell. These anatomic features enhance resolution, provide color vision and make the fovea the center of vision. In the peripheral retina photoreceptors are triggered by movement and some by bright light. The blood supply of the inner retina comes from the central retinal artery and the outer retina (photoreceptors are supplied by the choriocapillaris #10). Retinal capillaries, which form a dense multilayered plexus thins to one layer peripherally. Retinal capillaries have tight junctions between their constituent endothelial cells to prevent diffusion of substances into the neural retina.

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Retina

2005-10-24T18:28:00.000-07:00

Name the areas of the retina at the numbers. Click on the photo to enlarge or right click to open a new window.
Check your answers by opening this link.