Eye
Anatomy-Histology Correlate
By: Michael Lu, Class of ‘07
- The superficial features of the eye and eyelids are
detailed here. The palpebral fissure (not labeled )is
the opening of the eye itself. The lateral and medial
commissures (canthus or angles) form the lateral and
medial corners of the eyes. The medial commissure
contains the plica semilunaris (semilunar fold) and
the lacrimal caruncle.
- The conjunctiva can be divided into palpebral and
bulbar portions which cover the inner surface of the
eyelid and the eyeball, respectively. The conjunctival
sac reflects at the superior and inferior conjunctival
fornices, which are potential spaces filled with tears.
- The lacrimal gland secretes tears directly into the
superior fornix. Blinking the eyelids spreads the tears
across the eyeball to keep it lubricated. The tears are
collected at the lacrimal lake and then drained by the
superior and inferior lacrimal papillae. The lacrimal
caruncle prevents the papillae from getting stuck to
each other.
- The superior and inferior tarsal plates are dense
fibrous plates that give support and form to the eyelids.
Tarsal glands produce oily secretions to prevent tears
from leaking out, while the eyelids stay dry.
- The obicularis oculi muscle contains both palpebral
and orbital parts and acts to close the eyelids.
- The levator palpebrae superioris muscle elevates
the upper eyelid. It is a skeletal muscle under voluntary
control. Another muscle, the superior tarsal muscle, is
a smooth muscle that attaches to the superior tarsus.
- Some parts of the lacrimal apparatus were described earlier. The
lacrimal gland is located superior and lateral to the eyeball. It is
divided into orbital and palpebral parts by the levator palpebrae
superioris muscle. The gland secretes tears directly into the
superior conjunctival fornix. The tears are distributed across the
eye by blinking, collected in the lacrimal lake, and drained by the
superior and inferior lacrimal papillae through puncta (pores).
The tears are drained via the canaliculi into the lacrimal sac.
There is a suction action when the lids are closed due to
attachments of the orbicularis oculi muscle into the lateral wall of
the sac. From there, they drain via the nasolacrimal duct into the
nasopharynx.
- The lacrimal gland (bottom left) secretes tears, which contain
lysozyme and electrolytes similar to plasma. They look similar to
salivary glands, with the typical appearance of serous cells.
- The conjunctival epithelium (bottom right) is stratified columnar
with many goblet cells. The mucous secretions contribute to the
protective layer of the exposed eye and allow the eyelids to move
freely over the eye.
- Note the various parts of the anterior segment of the
eyeball.
- Most of the eyeball is occupied by clear, hyaluronanrich vitreous body. The posterior chamber is located
between the vitreous body and the iris. It contains the
lens and zonular fibers (suspensory ligaments) and
is filled with aqueous humor.
- Anterior to the iris and posterior to the cornea is the
anterior chamber.
- Note also the ciliary body and processes, which will be
covered later.
- The opening of the iris, or the pupil, is controlled by
the sphincter (parasympathetic; constricts the pupil)
and dilator (sympathetic; dilates the pupil) pupillae
muscles.
- The lens is an avascular structure that is composed of
highly arranged crystallins to let light pass through.
- The ocular surface of the cornea is lined with nonkeratinized stratified squamous epithelium (bottom
panel).
- This epithelium sits on Bowman’s membrane. Below
that, the corneal stroma consists of highly ordered
arrays of collagen with specific types of proteoglycan.
- The inner aspect of the corneal endothelium sits
upon the Descemet’s membrane. The endothelium
plays a very important role in maintaining a clear
cornea by regulating the state of hydration within the
corneal stroma.
- The ciliary body contains ciliary muscle that is
composed of smooth muscle. Contraction and
relaxation of the ciliary muscles change the tension of
the zonular fibers, or suspensory ligaments, of the lens.
This allows the lens to change shape, a process known
as accommodation.
- The ciliary processes are folds of connective tissue
that are covered by two layers of epithelium. There is
also a complex vasculature that cannot be seen easily.
Fluid from these vessels is processed and transported
by the epithelial cells to the posterior chamber as
aqueous humor. The epithelial cells constitute the
blood-aqueous barrier.
- The aqueous humor enters the anterior chamber
through the pupil as it flows between the lens and the
iris.
- Aqueous humor leaves the anterior chamber through
the trabecular meshwork and into the canal of
Schlemm. This is an endothelial lines, circumferentially
arranged vessel that communicates with veins in the
sclera and returns the aqueous humor back to the
general circulation.
- Obstruction of the trabecular meshwork and canals of
Schlemm are thought to be the major cause of elevated
intraocular pressure, which could then lead to
glaucoma.
- The iris is detailed here in higher magnification. Note
the anterior and posterior chambers to help orient
yourself.
- The anterior surface of the iris contains loose, variably
pigmented stroma. It is open to the circulating aqueous
humor within the anterior chamber.
- Two layers of heavily pigmented epithelium cover the
posterior surface of the iris.
- Note that the sphincter pupillae muscle can be easily
seen near the pupil margin. It is smooth muscle
controlled by parasympathetics. The dilator pupillae
muscle is more difficult to identify, but it dilates the pupil
upon sympathetic innervation.
- Now we will look at the other portions of the eyeball.
The eye can essentially be divided into 3 layers:
- 1) sclera – tough connective tissue that
continues anteriorly as the cornea.
- 2) choroid or uveal layer – vascular, loose
connective tissue that includes the ciliary body
and the iris anteriorly.
- 3) retina – contains photosensory cells in the
posterior portion and a non-sensory two-layered
epithelium in the anterior portion. The boundary is
marked by the ora serrata.
- Note how the photosensory retina is interrupted by
nerve fibers and blood vessels leaving the eye at the
optic nerve.
- The fovea centralis in the macula is a focal region of
thinner retinal epithelium important for visual acuity.
- The ora serrata is shown at higher magnification in
the bottom panel. Note the transition from the tall, multilayered photosensory epithelium into the two-layered
pigmented epithelium.
- The layer adjacent to the vitreous body and aqueous
humor is unpigmented, but the bottom layer remains
pigmented.
- Note the other two layers of the eye – the choroid and
the sclera – as labeled.
- The 3 layers of the eye are shown here again. Note
the retina, the choroid, and the sclera. The top panel
also indicates an extraocular muscle and some orbital
fat.
- Note the 10 distinct layers of the retina. Light travels
through all layers of the retina and strikes the
photoreceptor cells – rods (more sensitive in dim light)
and cones (more sensitive to bright light and color).
Impulses are then sent back up through the integrative
neurons to the ganglion cells, which finally bring the
impulses to the optic nerve.
- The photoreceptor cells are embedded in the retinal
pigment epithelium, which is the weak spot and
susceptible to retinal detachment.
- The top panel shows the retina as viewed through the ophthalmoscope (with
much practice). The numbered areas are magnified in the histological slides.
- In the “generic” retina (#1, bottom left), note the multiple layers. There is a
small vein in the ganglion cell layer and some small capillaries from retinal
vessels in the inner nuclear layer (arrows). Rods and cones receive
nourishment from the underlying network of capillaries in the choroid.
- The fovea centralis (#2, bottom middle) in the macula lutea is the region of
greatest visual acuity. Note the characteristic thinning of the epithelium. In the
top left panel, the macula lutea is indicated by the circle, and the fovea
centralis within the macula lutea is indicated by the arrow. The fovea contains
no rod cells; it is composed of exclusively cone cells.
- The optic disk (#3, bottom right) is the region where the retinal nerve fibers
exit the eye as the optic nerve. It is also the region where blood vessels enter.
Note discontinuity in the retinal epithelium in this area, creating the “blind spot”.
NOTE:
- The extraocular muscles include the following:
- levator palpebrae superioris muscle –
elevates the upper eyelid.
- superior rectus muscle – elevates and adducts
the eyeball.
- medial rectus muscle – adducts the eyeball.
- inferior rectus muscle – depresses and
adducts the eyeball.
- inferior oblique muscle – elevates and abducts
the eyeball.
- superior oblique muscle – depresses and
abducts the eyeball.
- lateral rectus muscle – abducts the eyeball.
Inf. oblique
Sup. rectus
Sup. rectus
Inf. oblique
Lat.
rectus
Lat.
rectus
Med.
rectus
Sup. oblique
Inf. rectus
RIGHT EYE
Med.
rectus
Inf. rectus
Sup. oblique
LEFT EYE
- The eyeball itself is directly connected with, or part of,
the optic nerve (CN II).
- The extraocular muscles receive motor innervations
primarily from cranial nerves III, IV, and VI.
- Oculomotor nerve (III) – The superior division
innervates the levator palpebrae superioris and
superior rectus muscles. The inferior division
innervates the medial rectus, inferior rectus, and
inferior oblique muscles. It also supplies
parasympathetic innervation to the ciliary muscle
and sphincter pupillae muscle via the ciliary
ganglion and short ciliary nerves. The short
ciliary nerves also carry sympathetics to the
dilator pupillae muscle.
- Trochlear nerve (IV) – innervates the superior
oblique muscle.
- Abducens nerve (VI) – innervates the lateral
rectus muscle.
- The ophthalmic division of the trigeminal nerve
(CN V1) carries mainly sensory innervation along with
parasympathetics. Its branches include frontal nerve
that splits into the supraorbital and supratrochlear
nerves, which are the most superior structures in the
orbit. The lacrimal nerve carries both sensory and
parasympathetic impulses to the lacrimal gland. The
nasociliary nerve gives off multiple branches, all
sensory, which include the infratrochlear, long ciliary,
and anterior and posterior ethmoidal nerves.
- The arterial supply to the eyeball, orbit, and eyelid
comes primarily from the ophthalmic artery, which
branches off the internal carotid artery.
- Note the central artery that first branches off the
ophthalmic artery and pierces the optic nerve. It is the
only blood supply to the retina.
- The supraorbital and supratrochlear arteries also
branch off the ophthalmic artery, supplying the muscles,
skin, and fascia of the forehead.
- The anterior and posterior ethmoidal arteries
supply the ethmoid and frontal sinuses and the nasal
mucosa.
- The lacrimal artery carries blood to the lacrimal gland
and eyelids laterally.
- The superior ophthalmic vein drains the eyeball, the
superior portion of the orbit, the ethmoid sinuses, and
the forehead into the cavernous sinus.
- Note the other veins that drain the orbit, such as the
supratrochlear, supraorbital, angular, and even the
maxillary and retromandibular veins.