William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
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:
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
- a rigid supporting structure,
- a light-excluding lining,
- a moveable control or stop for the light allowed to enter,
- a lens to focus that light on
- a light-sensitive sheet, and
- protective devices.
A ANTERIOR EYE
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
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.
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
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
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.
- Is a biconvex, elastic, transparent, protein structure
- thick `elastic' glycoprotein outer capsule tending to give it a
round form, under which lies
- 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
- 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
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
3 The hyaloid canal extends anteroposteriorly through it.
2 Neural retina
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
- Curved membrane terminating its receptor function as an irregular
line at the ora serrata/ora terminalis.
- 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.
- In most regions of the retina, light has to pass through the inner
structures to reach the outer ones that are actually photosensitive.
- 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
3 Retinal layers details:
4 Retinal modifications
- (a) 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.
- (b) Photoreceptors
- (i) An outer segment of stacked, infolded cell membrane,
incorporating the light-sensitive chemical, is connected via a cilium-like
- (ii) an inner segment with mitochondria, Golgi body and ER for
the continual replacement of outer segment materials.
- (iii) Then comes a dilation with the nucleus, and further inward the cell
narrows to become an inner fibre before synapsing with the dendrites of
bipolar nerve cells.
- (iv) The nerve and photoreceptor cells are separated by processes of the
special glial cells, Müller cells.
- (v) 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).
- 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
- (d) Outer nuclear layer - nuclei of photoreceptors.
- (e) Outer plexiform layer - synapsing processes: photoreceptors'
spherules or pedicles with bipolar neuron dendrites.
- (f) Inner nuclear layer - nuclei and bodies of bipolar neurons;
Müller cells, horizontal neurons and amacrine cells.
- (g) Inner plexiform layer - axons of bipolar neurons synapsing with
dendrites of ganglion neurons.
- (h) Ganglion cell layer - somas of ganglion neurons, whose
axons pass over the internal surface of the retina as the
- (i) 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.
- (j) Inner limiting membrane - a basal lamina separates the inner
processes of Müller cells from the vitreous.
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
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
Posterior part of the uvea - the eyeball's middle tunic - acts as a
light-dense, nutritive backing for the retina with:
- Bruch's membrane supporting the retina.
- Choriocapillaris - a plexus of large capillaries.
- 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.)
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
C ACCESSORY STRUCTURES (ADNEXA)
l Eyelids protect and lubricate the eye's anterior surface.
- Fine skin with a loose dermis faces outward.
- Palpebral conjunctiva (stratified columnar epithelium with goblet
cells on a lamina propria) faces inward.
- Orbicularis oculi skeletal muscles (served by VIIth nerve) close
- Levator palpebrae muscle raises the upper lids.
- Tarsal plates of dense CT have imbedded in them
- Meibomian sebaceous glands to make protective secretions.
- Eyelash hair follicles are separated by the
- sweat glands of Moll and sebaceous glands of Zeiss.
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
- Optical refractive agents: cornea, lens, aqueous humour, and
vitreous humour; form a small, inverted, real image.
- Receptor and neural tissues: retina and optic nerve.
- Sustaining and light-excluding tissue: vascular uvea
comprising the pigmented choroid coat, ciliary body and iris.
- Form- and rigidity-endowing tissues: cornea, sclera and
- Oculomotor system: sclera and three pairs of muscles.
- 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
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.