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ANAT D502 – Basic Histology
The Eye
Revised 12.4.15
Reading assignment: Chapter 24: Eye; pay particular attention to Boxes 24.1, 24.2, and 24.3
Outline
I.
II.
III.
IV.
V.
VI.
VII.
Introduction
Structure of the eye
Development of the eye
Corneosclera
Uvea
Lens
Retina
VIII.
Accessory organs
I. Introduction
Sensory systems convey information to the organism from both the inside and outside world. Physical
stimuli are converted by receptors into neural signals which are received by the brain and processed into
perceptions.
Sensory systems are divided into somatic and special. The somatic system has receptors distributed
throughout the body and processes many types of stimuli covering 4 modalities: Touch, proprioceptive,
pain, and thermal sensations. In contrast, each of the special sensory systems conveys a single modality
and is spatially restricted.
Humans, as craniates, potentially posses 7 special senses each with its own modality. The primary
receptors are all clustered in the head (cranium), a reflection of our ancestral heritage as active foragers:
a.
taste
b.
olfaction
c.
vomerolfaction – thought to be only transiently present during embryogenesis in humans
d.
vision (sight)
e.
hearing (sound)
f.
balance
g.
electroreception – lost a long time ago in humans but still retained by many mammals
(e.g., platypus)
Vision is the perception of light and the formation of images. It requires both a sensing organ (eye) and
processing organ (brain). The organ of light perception is the eye. It collects light of various intensities
and color and converts this information into neural signals. These neural signals are processed by the
brain to form an image. Extant mammals, such as humans, have retained only the two lateral eyes of the
ancestral three eyes. The third eye (parietal eye, pineal eye) has been lost in mammalian evolution and
its endocrine function has been assumed by the lateral eyes.
Functions of the eye include:
1. special sense – light perceiving organ for vision
2. hormonal function – both endocrine and paracrine
a. endocrine - diurnal changes in light levels are conveyed to the pineal body (epiphysis
cerebri) to modulate melatonin secretion and thereby entrain circadian rhythms.
b. paracrine - retinal release of melatonin is part of “night vision” adaptation
II. Structure of the eye
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The eye consists of a spherical wall surrounding a lens and 3 fluid filled chambers. The wall of the eye is
formed by 3 concentric tissue coats (tunics), from external to internal :
1. corneosclera (tunica fibrosa bulbi) – the outermost layer is composed of the fibrous, opaque
sclera posteriorly and the transparent cornea anteriorly.
2. uvea (tunica vasculosa bulbi) – the vascular middle layer is composed of the iris and ciliary
body anteriorly and the choroid posteriorly
3. retina (tunica interna bulbi) – a bi-laminar epithelium, the posterior 2/3 forms the optic part
(visual) and the anterior 1/3 is comprised of the ciliary and iridial parts (non-visual).
[The astute student will note by reading below that the ciliary body and iris are in fact compound
structures in that they are formed by two of the tunics, the uvea and the non-visual retina. Thus,
technically, the uvea forms the (1) choroid (in its entirety), (2) the stroma of the ciliary body (i.e.,
not the entire ciliary body), and (3) the stroma of the iris (similarly, not the entire iris). However,
this is usually simplified to the convention seen in the previous paragraph.]
[Note also that since the eye is a spherical object, structures closest to the center of the sphere
are described as internal and those further from the sphere are external. Here, the terms internal
(or inner) and external (or outer) are entirely relative and not absolute (i.e., they are not
synonymous with medial (closer to the midline of the body) and lateral (further from the midline of
the body).]
The lens is suspended from the ciliary body and separates two liquid filled cavities. The vitreous chamber
lies between the lens and retina and is filled by the gel-like vitreous body. The vitreous body is composed
of vitreous humor which is primarily water, collagen, hyaluronic acid and salts. Anteriorly, the cavity
between the lens and cornea is subdivided by the iris into anterior and posterior chambers, both of these
chambers are filled with aqueous humor
The cornea, aqueous humor, lens, and vitreous body form the refractile media. These transparent
elements refract (deflect) light onto the photoreceptors of the retina.
III. Development of the eye
Development of the eye begins with an outgrowth from the brain (neuroectoderm) called the optic vesicle.
The optic vesicle induces formation of the lens placode from the overlying ectoderm. Subsequently, both
the optic vesicle and lens placode invaginate. Invagination of the optic vesicle forms the bi-laminar optic
cup. The posterior portion of the cup will form the visual retina (comprising the retinal pigment epithelium
and neural retina) while the anterior portions will form the bilaminar non-visual retina (the epithelia of the
ciliary body and iris). Invagination of most of the lens placode forms the lens vesicle which pinches off to
form the lens. The remainder of the placode remains superficial and forms the cornea, conjunctiva and
lacrimal gland. The mesodermal portions of the eye (see Text table 24.1) arise from migrations of
mesenchymal cells between and around the epithelial structures.
IV. Corneosclera
A. Scerla
The sclera forms the “white of the eyes”. This tough, protective, non-transparent layer covers the
posterior 4/5 of the eye and is continuous with the dura mater of the meninges. The sclera serves as an
attachment for the extra-ocular muscles (see below). It is comprised of collagen and elastic fibers and
fibroblasts.
B. Cornea
The cornea is the transparent, avascular layer that covers the anterior 1/5 of the eye. It is the major site
of light refraction. Its transparency is produced by the uniform arrangement of its collagen fibers. Lacking
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blood vessels, it obtains its nutrients from the aqueous humor of the anterior chamber. Nutrients within
the aqueous humor are absorbed by the corneal endothelial cells and released internally to diffuse
throughout the cornea.
The cornea consists of 5 layers (from external to internal):
1.corneal epithelium – a stratified squamous epithelium richly endowed with abundant afferent
nerve endings sensitive to pain (think Oedipus).
2. basement membrane of the cornel epithelium (Bowman’s membrane)– a thick [12 microns],
non-regenerative, acellular layer of mainly collagen fibers; it provides a passive immune
barrier.
3. corneal stroma (substantia propria) - forms the bulk of the cornea; comprised of orthogonallly
stacked collagen lamellae with interspersed fibroblasts
4. basement membrane of the corneal endothelium (Descemet’s membrane) - the thickened
basement membrane of the endothelial layer (below)
5. corneal endothelium – a simple squamous epithelium specialized for metabolic exchange;
nutrients for the entire cornea are taken up and transported through this endothelium.
C. Limbus (corneoscleral junction)
The limbus marks the abrupt transition from the clear collagen coat of the cornea to the opaque collagen
coat of the sclera. Externally, the corneal epithelium transitions to the conjunctiva which is the mucosa of
the external eye. Internally, adjacent to the iridocorneal angle, the outflow network for the aqueous humor
is found. Within the stroma of the sclera, endothelial lined channels called the trabeculae meshwork (or
space of Fontana) collect aqueous humor and merge to form the scleral venous sinus (= canal of
Schlemm) that encircles the eye. From this sinus the aqueous humor drains via aqueous veins to the
systemic (caval) venous system.
V. Uvea
The uvea or vascular coat (tunic) is, logically enough, the vascular layer of the eye and forms, completely
or in part, three structures: Choroid, ciliary body and iris.
A. Choroid
The choroid is the posterior and largest portion of the uvea and lies between the sclera and retina. Four
layers can be defined (from external to internal):
1. suprachoroid – attached to the sclera; a loose CT with melanocytes, fibroblasts and
macrophages.
2. vessel (vascular) layer – similar to the suprachoroid but with numerous arterioles and venules.
3. choriocapillary layer – similar to above but with highly fenestrated capillaries that supply the
external retina.
4. basal complex (Bruch’s membrane) – this is the thickened basement membrane of the retinal
pigment epithelium (RPE.
B. Ciliary Body
The ciliary body forms the portion of the uvea between the ora serrata and iris. Along its anterior 1/3
internal margin it forms ridges called ciliary processes to which the zonular fibers attach (see below). The
ciliary body is comprised of (1) a stroma and (2) double layered epithelium. The outer portion of the
stroma consists of the ciliary muscle and the inner portion is essentially a continuation of the choroid
consisting of a vascular loose connective tissue with melanocytes.
The double layered epithelium (ciliary epithelium or ciliary retina) consists of an outer pigmented layer
and inner non-pigmented layer. The ciliary epithelium has 2 principle functions
1. secretion of aqueous humor
2. secretion and anchoring of zonular fibers that form the suspensory ligament of the lens
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Aqueous humor is produced by the ciliary epithelium in the posterior chamber. This plasma filtrate
supplies the metabolic demands of the avascular lens and cornea. It passes from the posterior chamber
to the anterior chamber via the pupillary aperture and drains to the systemic venous (caval) system at the
limbus (corneoscleral junction) as described above.
The ciliary muscle is a complex arrangement of smooth muscle fibers whose actions
1. promote aqueous humor flow, and
2. relax tension on the lens for accommodation
C. Iris
The iris is the anterior-most portion of the uvea and extends from the ciliary body to its free margin, the
pupil. The pupil is the central aperture of the iris whose changes in diameter regulate the amount of light
passing to the retina. Like the ciliary body the iris is also comprised of (1) a stroma and (2) doublelayered epithelium (iridial epithelium or iridial retina). The stroma consists of an exposed vascular loose
connective tissue with interspersed melanocytes. The distribution of melanocytes within the stroma
determines eye color. Both layers of the bi-laminar iridial epithelium (iridial retina) in the iris are heavily
pigmented and consist of:
1. anterior epithelium – outer layer; composed of pigmented myoepithelial cells; continuous with
the pigmented epithelium of ciliary body
2. posterior epithelium – inner layer; continuous with non-pigmented epithelium of ciliary body.
The diameter of the pupil can be regulated by two sets of contractile tissues. The sphincter pupillae
muscle is a circularly arranged band of smooth muscle close to the free margin of the iris.
Parasympathetic stimulation results in constriction of the pupil. The dilator pupillae is the radially
arranged muscular processes of the myoepithelial anterior epithelium; sympathetic stimulation causes the
pupil to open (dilate).
VI. Lens
The lens is a transparent, elastic, avascular, biconvex epithelial structure that forms part of the refractile
media; the lens changes shape to alter the plane of focus. The lens is suspended by the suspensory
ligament of the lens that lies between the iris and vitreous body; the ligament is formed by coalesced
zonular fibers of the ciliary body. The lens is comprised of 3 components:
1. lens capsule – the thickened basement membrane of the subcapsular epithelium; this
membrane surrounds the entire lens
2. subcapsular epithelium – a simple cuboidal epithelium found only on the anterior surface
3. lens fibers – formed by cells of the subcapsular epithelium in the germinal zone along the
equator of the lens; these cells detach from the basement membrane and undergo
anoikis, shedding their organelles and becoming filled with proteins called crystallins that
produce the refractive index of the lens
A. Accommodation
Contraction of the ciliary muscle under parasympathetic stimulation causes the suspensory ligament to
loosen allowing the lens to assume a more round shape; this permits focusing on near objects
(accommodation). Relaxation of the ciliary muscle tightens the ligaments and flattens the lens allowing
focusing on far objects. [For reasons known only to the gods (probably stinkin’ physiologists), only lens
relaxation is referred to as accommodation.]
VII.
Retina
The retina is the innermost coat (tunic) of the eye wall and is derived from the inner and outer epithelium
of the optic cup. It is divided into 3 parts (2 non-visual and 1 visual):
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anterior to the ora serrata are the non-visual ciliary and iridial parts; here the two layers form the
bi-laminar epithelium of the ciliary body and iris (i.e., the ciliary and iridial epithelia or
retinas)
posterior to the ora serrata is the photosensitive optic part
In the optic part the outer epithelium forms the retinal pigment epithelium and inner epithelium forms
neural retina. The potential space between the outer and inner epithelium is site of retinal detachment.
The optic part is comprised of 10 layers containing 2 areas of specialization:
optic disc – is the site of exit of optic “nerve” (CN II) and entry of central retinal artery and vein;
this area is devoid of photoreceptors and thus the optic disc creates a blind spot in each
eye’s visual field
macula lutea – literally a yellow spot (due to the pigment xanthopyll) lateral to the optic disc and
lying in the center of the eye’s visual field. This area is devoid of retinal arteries and at its
center lies the fovea centralis. The fovea is comprised entirely of cone cells and is the
most visually acute region of the retina owing to (1) the absence of blood vessels and (2)
displacement of the cells and their processes superficial (internal) to the photoreceptors.
Thus, here photons have an unimpeded path to the light receptors.
A. Rod and cone cells
The rod and cone cells are the site of visual signal transduction, i.e., the conversion of stimuli (specifically
photons) to electrical signals (specifically hyper-polarization of the cell membrane). In the human retina
there approximately 130 million such cells in a ratio of 17:1 (rods:cones). Rod cells are more sensitive to
light and function under low light intensity to produce black and white images; cone cells detect color.
Both type of cells consist of a (1) rod fiber, (2) inner segment and (3) outer segment. The rod fiber
contains the nucleus (retinal layer 4) and synaptic terminals (retinal layer 5). The inner segment contains
the cytoplasmic organelles and shows a profile typical of protein synthesis (which would be what?). The
outer segment is the site of photo-transduction and it shape is cylindrical in rods and conical in cone cells.
Each outer segment (basically highly specialized cilia) contains a tall stack of membranous disks.
The disks contain the visual pigments, rhodopsin in rods and iodopsin in cones. All visual pigments
consist of two parts: (1) A membrane bound protein called opsin (scotopsin in rods, photopsins in cones)
and (2) a light absorbing chromophore consisting of retinal, the aldehyde form of vitamin A. [Thus, your
mother was right about eating your carrots; a dietary deficiency in vitamin A can lead to “night blindness.”
Thought question: Given retinal is found in both visual pigments, why is vitamin A deficiency first
manifest in the rod component of vision?]
The opsins influence the spectrum and sensitivity of the retinal chromophore, thus there are three
varieties of photopsin for red, green and blue vision. Regardless of type, photons reaching the outer
segment produce conformational changes in the visual pigments releasing an internal cascade of events.
However, the effect of this cascade of events is counter-intuitive. Basically, light (photons) striking a
photoreceptor will cause hyper-polarization of the photoreceptor cell membrane resulting in reduction of
neurotransmitter release to the bipolar cell (i.e, the photoreceptor cell is turned “off”.). [Thus, a decrease
in activity of the photoreceptors is interpreted by the brain as “light”.] Conversely, in the dark (no
photons), the photoreceptor cell membrane becomes hypo-polarized resulting in continuous
neurotransmitter release (and increasing the metabolic demands of the cell; i.e., the cell is turned “on”).
Weird, huh? However, given the ancestral/original function of photoreceptors (i.e., to detect light and turn
off melatonin release), this makes perfect sense.
B. Layers (1-10) of the optic part
1. Retinal pigment epithelium (RPE) forms the outermost layer. The RPE is a complex, pigmented,
simple cuboidal epithelium. Its basement membrane forms the basal complex (Bruch’s
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membrane) of the choroid and its apical projections interdigitate with the photoreceptors. The
RPE performs multiple functions including:
a. reduction of glare by pigmental absorption of light passing through the photosensitive layer.
b. formation of the blood-retina barrier by means of tight junctions.
c. recycling of visual pigments via phagocytosis of shed membranous disks from the
photoreceptor cells.
2. Layer of rods and cones consists of the outer segments of the rod and cone cells.
3. External limiting membrane – not a true membrane but the site of attachments (zonula adherens) of the
apical ends of Muller cells. Muller cells are slender, support (glial) cells that extend nearly the
thickness of the retina. Their apical attachments create a metabolic barrier to the outermost
layers of the retina (i.e., the RPE and photoreceptors).
4. Outer nuclear layer consists of the nuclei of the rod and cone cells.
5. Outer plexiform layer is formed by synaptic terminals of the rods and cones and corresponding
processes of horizontal cells and bipolar cells:
Bipolar cells connect the photoreceptors to the ganglion cells;
Horizontal cells interconnect the photoreceptors in an integrative function.
6. Inner nuclear layer contains the nuclei (cell bodies) of the bipolar, horizontal, Muller and amacrine cells:
Amacrine cells form synapses with bipolar and ganglion cells and other amacrine cells,
presumably providing an additional level of integration to the visual signal.
7. Inner plexiform layer is formed by cell processes of the amacrine, bipolar and ganglion cells.
8. Ganglion cell layer is formed by the nerve bodies of the ganglion cells.
9. Layer of the optic nerve fibers is composed of the unmyelinated axons of the ganglion cells which form
the optic “nerve”, cranial nerve II (CN II); these fibers exit the retina at the optic disk.
10. Inner limiting membrane is formed by the basement membrane of the Muller cells.
C. Vasculature of the retina
Despite an epithelial origin, portions of the visual retina are vascularized. Branches of the central retinal
artery (and vein) ramify in the nerve fiber layer and smaller arterioles (and venules) penetrate to the inner
nuclear layer. Capillaries are also limited to the inner layers of the retina and exhibit the tight junctions
necessary for maintaining the blood-brain barrier. Thus, the outer-most layers of the visual retina (the
RPE through the outer plexiform, layers 1-5) are avascular and receive their nutrients (including 02) via
long distance diffusion, principally from the choroidal vessels.
VIII.
Accessory organs
A.
Conjunctiva
The conjunctiva is a mucous membrane that lines the exposed portions of the sclera (bulbar conjunctiva)
and reflects onto the internal surface of the eyelids (palpebral conjunctiva). It is continuous with the
corneal epithelium and epidermis of the palpebra. The conjunctiva consists of a varying stratified
epithelium containing goblet cells overlaying a subconjunctival layer (lamina propria) of loose CT.
Secretions from the goblet cells assist in lubrication of the conjunctiva.
B.
Palpebrae (eyelids)
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The palpebrae (upper and lower) are folds of skin that serve to protect the eye from dehydration and
abrasion. They are covered externally by thin skin and internally by palpebral conjunctiva. Associated
with the skin are eccrine sweat glands and short stiffened hairs (eye lashes) arranged into rows found
along the free margin. Deep to skin is orbicularis oculi, a facial muscle which closes the lids. The tarsal
plate is dense fibroelastic band that stiffens the lid and lies deep to the muscle.
A surprisingly large number of glands are associated with the free margin of the lid:
1. tarsal (Meibom) glands within the tarsal plate (or tarsus) are specialized sebaceous glands
whose ducts opens onto the free edge of eye eyelid
2. sebaceous glands of the eyelashes (glands of Zeis)
3. apocrine glands of the eyelashes (glands of Moll)
C.
Lacrimal glands
The lacrimal glands are found bilaterally beneath the conjunctiva on the upper lateral side of the orbit.
These are serous glands whose ducts open to the conjuctival reflection (fornix). Secretions from these
glands (tears) drain at the medial canthus of the eye and eventually find their way to the nasal cavity via a
complicated series of canals. Occasionally accessory lacrimal glands can be found within upper eyelid.
Tears are a complex mixture of lacrimal gland, tarsal gland and goblet cell secretions. Secretion form
these glands is coordinated by parasympathetic innervation. Tears serve multiple functions including:
1. moistening and lubricating the conjunctival and corneal epithelium
2. washing away debris
3. anti-bacterial actions (contains lysozymes)
4. communication (e.g., distress over a comprehensive final exam)
D. Extra-ocular muscles
Six pairs of skeletal muscles (4 recti: medial, lateral, superior, and inferior), 2 obliques (superior and
inferior) form the extra-ocular or extrinsic eye muscles. These muscles arise from the margin of bony
orbit and attach to the sclera. The brain coordinates the motor activity of these muscles so that the eyes
move in parallel (conjugate gaze). Their innervation can be remembered by the formula [SO4LR6]3.
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