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Eye and its Adnexa

Anterior Eye
  1. Cornea
    1. Stratified squamous epithelium roughly five cells thick. Cells are held together by desmosomes, and supported on
    2. Bowman’s membrane: collagen fibrils in an amorphous matrix, viewed as a limiting condensation of the wide
    3. Comeal stroma: orderly lamellae of collagen fibrils of uniform diameter, and keratocytes/fibroblasts with plenty of chondroitin and keratan sulphates; no blood vessels or lymphatics; takes up to 90 per cent of the comeal thickness.
    4. Descemet’s membrane: thick, distinct basal lamina with collagen fibrils in orderly array.
    5. Endothelium: single layer of pavement/squamous cells, working to control the water content of the cornea. Comeal functions: refraction, transparency, protection, and sensitivity (from intraepithelial free axons) for protective reflexes.
  2. Anterior chamber
    Limited by the posterior surface of the cornea and anterior surfaces of the iris and lens. It is filled to turgor with aqueous humour resembling serum, but very low in protein, and produced in the posterior chamber. To define this, some structures off the optical axis must be discussed.
  3. Angle of the iris/anterior-chamber angle
    Limbus forms the boundary between the cornea and the sclera which, although collagenous, is not transparent because of the disorder of its collagen fibres, its deficiency of sulphated ground substances, and its greater water content than the cornea.
    Where Descemet’s membrane terminates is a comeo-scleral trabecular meshwork/pectinate ligament enclosing the spaces of Fontana. These drain the aqueous humour towards Schlemm’s canal, from which it passes to the episcleral or aqueous veins for venous return. The meshwork lies in the drainage angle between the sclera and the scleral spur.
  4. Iris
    1. Rings the pupil and controls, by dilation or constriction, the light entering and the depth of focus.
    2. Stroma: loose vascular CT with a variable proportion of pigment cells/melanophores.
    3. Posterior surface is covered by a pigmented cuboidal epithelium forming the inner layer of the iridial retina.
    4. Sphincter smooth muscle near the pupillary margin receives parasympathetic fibres, eliciting a contraction in response to increased light intensity.
    5. Dilator muscle is a less substantial myoepithelial structure lying peripherally and posteriorly as the outer layer of the iridial retina, with fibres oriented radially and under sympathetic autonomic control.
  5. Lens
    1. Is a biconvex, elastic, transparent, protein structure with:
    2. thick ‘elastic’ glycoprotein outer capsule tending to give it a round form, under which lies
    3. an inner layer of cuboidal epithelial cells which peel off, elongate and insinuate themselves into the inner substance as lens fibres at the lens bow, thereby adding to lens crystallins - the main lens proteins.
    4. The lens is held out of the round shape of its own inclination by its attaching zonule/suspensory ligament running to the smooth muscle ciliary body, which is itself firmly attached to the CT sclera. Lens nutrition is indirect by the aqueous humour.
  6. Ciliary body
    1. Circular smooth muscle (Müller’s ciliaris muscle): innervated by para-sympathetic fibres from the ciliary ganglion to contract, reducing tension in the zonule thus allowing the lens to become round and accommodate to near vision.
    2. Radial and meridional muscle fibres (Brucke’s muscle): function and innervation are uncertain.
    3. Covered by a double layer of cuboidal epithelial cells (ciliary retina), with the outer ones heavily pigmented.
    4. Gives off a number of projections, ciliary processes, covered by the two-layered epithelium and enclosing fenestrated blood capillaries, which produce the aqueous humour in a manner similar to the production of CSF by the choroid plexus.
  7. Posterior chamber
    1. Is limited by the posterior surface of the iris, the zonule and parts of the lens and ciliary body;
    2. from the last of which comes the aqueous humour that fills it and passes out via the pupil to the anterior chamber.
Posterior Eye
Here the three tunics of the wall - sclera, uvea, and retina - are most clearly recognized.
  1. Neural retina
    1. Curved membrane terminating its receptor function as an irregular line at the ora serrata/ora terminalis.
    2. Contains a pigment cell layer of cuboidal cells, light-sensitive photoreceptors, and nerve cells arranged in layers to partially integrate the nervous information and transmit it out of the eye to the brain.
    3. In most regions of the retina, light has to pass through the inner structures to reach the outer ones that are actually photosensitive.
    4. Retinal layers in brief:
      1. Pigment-cell layer
      2. Photoreceptors
      3. External limiting membrane
      4. Outer nuclear layer
      5. Outer plexiform layer
      6. Inner nuclear layer
      7. Inner plexiform layer
      8. Ganglion cell layer
      9. Nerve-fibre layer
      10. Inner limiting membrane
  1. Retinal layers details:
    1. Pigmented epithelium, simple cuboidal, lying on Bruch’s wide membrane of basal laminae reinforced by collagen and elastic fibrils. Pigment cells absorb light, and destroy the used-up tips of the rods.
    2. Photoreceptors
      1. An outer segment of stacked, infolded cell membrane, incorporating the light-sensitive chemical, is connected via a cilium-like neck to
      2. An inner segment with mitochondria, Golgi body and ER for the continual replacement of outer segment materials.
      3. Then comes dilation with the nucleus, and further inward the cell narrows to become an inner fibre before synapsing with the dendrites of bipolar nerve cells.
      4. The nerve and photoreceptor cells are separated by processes of the special gtial cells, Müller cells.
      5. Photoreceptors are classified by shape as: (a) rods (rhodopsin as the visual pigment and converging neural connections give them high sensitivity, but poor acuity and no colour discrimination); or (b) cones (varieties of iodopsin and less convergence in connections provide for colour vision and fine acuity).
    3. External limiting ‘membrane’ is formed of outer terminal processes of Müller cells and lies at the level of the photoreceptors’ inner segments, to which the Müller cells tightly attach by junctional complexes.
    4. Outer nuclear layer-nuclei of photoreceptors.
    5. Outer plexiform layer - synapsing processes: photoreceptors’ spherules or pedicles with bipolar neuron dendrites.
    6. Inner nuclear layer - nuclei and bodies of bipolar neurons; Müller cells, horizontal neurons and amacrine cells.
    7. Inner plexifonn layer - axons of bipolar neurons synapsing with dendrites of ganglion neurons.
    8. Ganglion cell layer - somas of ganglion neurons, whose axons pass over the internal surface of the retina as the
    9. Nerve fibre layer to converge on the optic papilla, where they pass out unmyelinated through the eye’s other two coats to form the optic nerve.
    10. Inner limiting membrane - a basal lamina separates the inner processes of Müller cells from the vitreous.
  2. Retinal modifications
    1. Macula lutea with fovea centralis - on the visual axis is a yellow-ringed depression, from which the inner layers have been displaced to a peripheral hump so that : (a) the light can fall directly on the photoreceptors, that (b) are all tightly packed cones with straight-through neural connections, for high acuity.
    2. Optic papilla/nerve head, where optic nerve fibres leave the eye (no receptors, therefore a blind spot), and where retinal blood vessels leave and enter for widespread retinal distribution. The condition of these vessels is a crucial part of the ophthalmoscopic examination.
    3. Optic nerve - the ganglion cells’ fibres acquire myelin sheaths, then run centrally with accompanying glial cells and a meningeal sheath as a CNS tract. The retinal artery and vein run centrally in the intraorbital section of the nerve.
  3. Choroid
    1. Posterior part of the uvea - the eyeball’s middle tunic - acts as a light-dense, nutritive backing for the retina with:
    2. Bruch’s membrane supporting the retina.
    3. Choriocapillaris – a plexus of large capillaries.
    4. Choroid and outermost epichoroid/suprachoroid - highly vascular, loose stroma of collagen and elastic fibres, fibroblasts and pigmented melanophores.
  4. Sclera
    Dense, tough outer tunic of collagenous fibrous tissue. It has some regional variations:
    1. At the lamina cribrosa, where its fibres interweave with bundles of optic nerve fibres leaving the eye.
    2. At the limbus, where it is more vascular, related to Schlemm’s canal and the ciliary body.
    3. Near to the limbus are the insertions for the oculomotor skeletal muscles moving the eye.
    4. Throughout, its innermost layer (lamina fusca) also has melanophores and elastic fibres.
Accessory Structures (Adnexa)
  1. Eyelids protect and lubricate the eye’s anterior surface.
    1. Fine skin with a loose dermis faces outward.
    2. Palpebral conjunctiva (stratified columnar epithelium with goblet cells on a lamina propria) faces inward.
    3. Orbicularis oculi skeletal muscles (served by VIIth nerve) close lids.
    4. Levator palpebrae muscle raises the upper lids.
    5. Tarsal plates of dense CT have imbedded in them
    6. Meibomian sebaceous glands to make protective secretions.
    7. Eyelash hair follicles are separated by the
    8. Sweat glands of Moll and sebaceous glands of Zeiss.
  2. Conjunctiva
    1. Palpebral conjunctiva lines the eyelids, and bulbar covers the eyeball’s sclera, with the fomices as the angle of reflection.
    2. Stratified columnar epithelium has goblet and Langerhans cells, with many lymphocytes in the loose lamina propria.
    3. Epithelium changes at the limbus (to corneal) and at the lid margin (to skin). Conjunctival epithelium is a source of cells to repair damaged comeal epithelium.
    4. Plica semilunaris is a small conjunctival fold in the medial margin of the eye above the
    5. Caruncle, with its sebaceous glands.

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