HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 13 EYE AND ITS ADNEXA

The eyeball is one of a pair of roughly spherical, rigid structures sensitive to precise light stimuli and movable in coordination with its fellow. The camera performs a similar task, and the camera and the eye have in common:
  1. a rigid supporting structure,
  2. a light-excluding lining,
  3. a moveable control or stop for the light allowed to enter,
  4. a lens to focus that light on
  5. a light-sensitive sheet, and
  6. protective devices.
Before the histology is considered the overall anatomy should be briefly reviewed. Then the various structures will be taken in order as they are met on the light path. After that the accessory structures or adnexa will be dealt with, to lead to a final classification of all the structures along functional lines.

A ANTERIOR EYE

l Cornea
l 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 Corneal 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 corneal 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.

Corneal 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 corneo-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
l 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 rounder 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
l Circular smooth muscle (Müller's muscle): innervated by para-sympathetic fibres from the ciliary ganglion to contract, reducing tension in the zonule thus allowing the lens to become rounder and accommodate to near vision.
3 Radial and meridional muscle fibres (Brücke'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
l 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.

B POSTERIOR EYE

Here the three tunics of the wall - sclera, uvea, retina - are most clearly recognized.

l Vitreous body
l Viscid and transparent fluid which, although mainly water, contains proteoglycans, hyaluronic acid, and collagen.
2 It fills the space bounded by the lens, zonule, pars plana and neural retina.
3 The hyaloid canal extends anteroposteriorly through it.

2 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, 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:
    .. (i) Pigment-cell layer
    .. (ii) Photoreceptors
    .. (iii) External limiting membrane
    .. (iv) Outer nuclear layer
    .. (v) Outer plexiform layer
    .. (vi) Inner nuclear layer
    .. (vii) Inner plexiform layer
    .. (viii) Ganglion cell layer
    .. (ix) Nerve-fibre layer
    .. (x) Inner limiting membrane
When looking at slides of the posterior eye, resist the temptation to view the neural retina as an epithelium facing a lumen. The reference point for 'inner' and 'outer' is the unseen vitreous, not the BL on which the pigment cells sit.

3 Retinal layers details:

4 Retinal modifications
l 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.

5 Choroid
Posterior part of the uvea - the eyeball's middle tunic - acts as a light-dense, nutritive backing for the retina with:

  1. Bruch's membrane supporting the retina.
  2. Choriocapillaris - a plexus of large capillaries.
  3. Choroid and outermost epichoroid/suprachoroid - highly vascular, loose stroma of collagen and elastic fibres, fibroblasts and pigmented melanophores. (The pigment is static inside mammalian melanophores, which are thus unlike those of lower vertebrates.)

6 Sclera
Dense, tough outer tunic of collagenous fibrous tissue. It has some regional variations:
l 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.

C ACCESSORY STRUCTURES (ADNEXA)

l 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
l Palpebral conjunctiva lines the eyelids, and bulbar covers the eyeball's sclera, with the fornices 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 corneal epithelium.
4 Plica semilunaris is a small conjunctival fold in the medial margin of the eye above the
5 caruncle, with its sebaceous glands.

3 Lachrymal glands
l In upper, lateral orbit, opening via ducts to the conjunctiva.
2 Compound, tubulo-acinar, serous gland with many myoepithelial cells. Mucous cells also are present.
3 Tears drain through the lachrymal punctum via lachrymal canaliculi into the lachrymal sac. Then they pass via the nasolachrymal duct to the lateral side of the inferior meatus of the nose.
4 Tear fluid is chemically complex. Tears have water, salts, glycoproteins, and bactericidal factors, e.g., lysozyme.

4 Other orbital structures
l Tenon's CT capsule.
2 Extraocular skeletal muscles (fine-fibred).
3 Adipose tissue.
4 Ciliary ganglion.

D FUNCTIONAL CLASSIFICATION OF THE EYE AND ADNEXA

  1. Optical refractive agents: cornea, lens, aqueous humour, and vitreous humour; form a small, inverted, real image.
  2. Receptor and neural tissues: retina and optic nerve.
  3. Sustaining and light-excluding tissue: vascular uvea comprising the pigmented choroid coat, ciliary body and iris.
  4. Form- and rigidity-endowing tissues: cornea, sclera and intraocular fluid.
  5. Oculomotor system: sclera and three pairs of muscles.
  6. Protective tissues: lids, conjunctiva, cornea, lachrymal, Meibomian and other glands, and the orbital bone.

E DEVELOPMENT OF THE EYE

l Forebrain grows out as the hollow optic vesicle, whose
2 proximal part constricts to become an optic stalk, later the optic nerve.
3 Superficial ectoderm over the optic vesicle thickens, then separates to become the lens vesicle.
4 Meanwhile, the anterior wall of the optic vesicle invaginates into the posterior producing a two-layered cup that becomes the retina with its posterior pigment epithelium.
5 Mesectoderm gives the corneal stroma, uvea and sclera.
6 Ectoderm provides the corneal and conjunctival epithelia.

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159