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Comparative Anatomy and Histology
proportionately smaller than it is in humans. The
vitreous exterior surface is termed the anterior
hyaloid face and is behind the lens, whereas the
posterior hyaloid face lies just anterior to the retina.
The vitreous is firmly attached at the vitreous
base located at the ora serrata. Additional firm
attachments exist at the optic nerve head, overlying
retinal vessels, and near the fovea, which is the region
corresponding to highest visual acuity in humans.
FIGURE 24 The mouse lens equator. The capsule is thickest
anteriorly (arrow) where the lens epithelial nuclei bend
inward. The histologic appearance of the lens is similar to
that of the human lens. The clefting present in the lens is a
fixation artifact.
FIGURE 25 Posterior mouse lens. The posterior capsule is
thinner (arrow), and normally no nuclei are present.
CP
S
PE
ONH
R
0.5 mm
FIGURE 26 Cross section (5 µm) through a mouse eye stained with Richardson’s stain. Clearly visible are the optic nerve head
(ONH), pigment epithelium (PE), choroidal plexus (CP), and sclera (S). Unlike the human retina, the mouse retina (R) does
not show any regional specialization such as a fovea; however, the nerve fiber and ganglion cell layers become thin peripherally.
Source: Figure provided by Dan Possin.
C h a p t e r 2 1 Special Senses: Eye
Retina
Gross Anatomy
The retina is a complex tissue with multiple layers
of cells responsible for absorption of light and
transmission of this signal to the brain. The retina
has a dual vascular supply that is similar between
the two species. The central retinal artery—a
branch of the ophthalmic artery—and its branches
409
supply the anterior two-thirds of the neurosensory
retina. The outer one-third of the retina is
supplied by the choriocapillaris (described
previously). In humans, the central retinal artery
exits the optic nerve and branches into four
arterial arcades near the anterior surface of the
retina. In mice, there are usually four to six retinal
arterioles, but this is highly variable. The venous
drainage follows a similar vascular pattern.
l Need-to-know
n The outer nuclear layer is
usually twice as thick as the
inner layer in normal mice.
FIGURE 27 Cross section (5 µm) of a mouse retina stained with Richardson’s stain. Three cellular layers and two synaptic
layers of the retina are apparent. Cellular layers are the ganglion cell layer (GCL), inner nucleus layer (INL), and outer
nuclear layer (ONL). Synaptic layers are the inner plexiform layer (IPL) and outer plexiform layer (OPL). Photoreceptor
inner and outer nerve fibers (NF) are indicated. Sclera (S), pigment epithelium (PE), choroid plexus (CP), and photoreceptor
inner (IS) and outer (OS) segments are indicated. Source: Figure provided by Dan Possin.
410
Comparative Anatomy and Histology
Histology
The layers of the neurosensory retina from inner
(vitreous side) to outer are as follows: internal
limiting membrane, nerve fiber layer, ganglion cell
layer, inner plexiform layer, inner nuclear layer, outer
plexiform layer, outer nuclear layer, external limiting
membrane, and photoreceptor layer (Figures 26–30).
External to the neurosensory retina is the RPE, which
is separated from the choroid by Bruch’s membrane.
Bruch’s membrane consists of the basement
membranes of the RPE and the choriocapillaris.
Between these two basement membranes are layers
of collagen surrounding an elastic tissue. Bruch’s
membrane is less prominent in mice than in humans.
The internal limiting membrane is a PAS-positive
basement membrane formed by the footplates
of glial cells that span the normal retina and
whose cell bodies lie within the inner nuclear
layer (Müller cells). The nerve fiber layer is a
collection of unmyelinated ganglion cell axons.
In mice, this layer thickens toward the optic
nerve and can be indistinct peripherally. These
axons become myelinated as they exit the globe
through the lamina cribrosa. The average human
has 1–1.2 million ganglion cell axons; mice have
approximately one-tenth this number, but it is
highly variable between inbred strains. The most
striking difference between mouse and human
retina is that human retina utilizes a specialized
area called the macula, which is responsible for
central vision. The macula is defined histologically
as the region of retina with more than one cell
layer of ganglion cell bodies within the ganglion
cell layer. In humans, this region is located
within the temporal retinal vascular arcades. A
depression at the center of the macula marks the
location of the fovea. At the center of the fovea,
the retina is avascular and consists primarily of
photoreceptors (Figure 30). In mice and in the
human nonmacular peripheral ganglia cell layer,
there is one layer of cell bodies. The highest
density of ganglion cells in mice is temporal to
the optic nerve with decreasing density toward the
dorsal and peripheral retina. The inner plexiform
layer consists of synapses between bipolar and
ganglion cells as well as amacrine and bipolar
cells. The inner nuclear layer contains the nuclei
of bipolar, Müller, horizontal, and amacrine
cells and is usually 6–9 layers thick. The overall
human retinal thickness is variable depending
on its relationship to the fovea. The retinal layers
up to the inner third of the inner nuclear layer
are nourished by the retinal arteries, whereas the
outer retinal layers, beginning at the outer twothirds of the inner nuclear layer, are nourished by
the choroidal vessels. The outer plexiform layer
has synaptic processes between the horizontal and
bipolar cells. In the macula, the outer plexiform
layer is thick, and the axons are oriented
obliquely. This region of the outer plexiform layer
is known as Henle’s layer. The outer nuclear layer
has photoreceptor (cone and rod) nuclei. In mice,
this layer is 10–12 layers thick. Rods account for
approximately 95% of photoreceptors in both
species. Humans have three types of cones, each
specialized to detect short, medium, or long
wavelengths of light, whereas mice have only two
types of cones. The external limiting membrane
is not a true basement membrane but is formed
by the tight junctions between Müller cells and
photoreceptors. It serves as an anatomic landmark
that separates the photoreceptor nuclei from
the photoreceptor inner and outer segments.
These segments have differential staining: the
photoreceptor broad inner segments are more
densely packed and stain more intensely than the
thin outer segments, which are modified cilia.
Connecting the two segments is a thin cilium
visualized by electron microscopy.
C h a p t e r 2 1 Special Senses: Eye
411
C
ONL
OPL
H
B
INL
IPL
GCL
G
NF
Vitreous
10µm
FIGURE 28 Mouse retina. The major cell types of the mouse retina are visualized by a combination of transgenic cell-labeling
methods and immunostaining procedures. Shown here is a vibratome section (60 µm) of paraformaldehyde-fixed retina from
an adult mouse. Cone photoreceptors (C) are immunoreactive for cone arrestin; horizontal cells (H), a subset of amacrine
cells, and ganglion cells (G) are immunoreactive for calbindin. One subtype of retinal bipolar cell (B) is labeled in this retina
from a transgenic mouse in which the metabotropic glutamate receptor 6 promoter drives expression of green fluorescent
protein. The retinal layers (see Figure 27 legend) are indicated. Reproduced from Morgan JL, Dhingra A, Vardi N, Wong RO
(2006). Nat. Neurosci. 9:85–92 (cover).
412
Comparative Anatomy and Histology
Retinal Pigment
Epithelium
Histology
S
FIGURE 29 Mouse retina. Note the lack of fovea and the
relatively thin, heavily pigmented choroid (arrowheads)
adjacent to the posterior sclera (S).
l Need-to-know
n Mice have two lacrimal glands: intra- and exorbital.
The RPE is a monolayer of cells that lies subjacent
to the retina and extends from the optic disc to
the ora serrata, where it is contiguous with the
pigmented epithelium of the ciliary body. The
apical aspect of RPE cells abuts the photoreceptor
outer segments, whereas the RPE basement
membrane forms the inner lamella of Bruch’s
membrane (Figures 26 and 27). RPE cells have
intercellular junctional complexes (zonulae
occludentes) that make up the outer blood–retina
barrier. Numerous round pigment granules
(melanosomes) are visible within the RPE
cytoplasm, particularly in pigmented individuals.
Albino mice have unpigmented melanosomes.
In albino humans, there is a variable amount
of pigmentation depending on the subtype of
oculocutaneous or ocular albinism. At the human
fovea, RPE cells are tall and thin with numerous
melanosomes. In the periphery, the RPE cells are
l Need-to-know
n The macula is the center of visual acuity in humans and
is lacking in mice.
Albino mice and humans with Hermansky-Pudlak
syndrome do not have pigmented melanosomes.
n
FIGURE 30 Human retina. The center of the macula marks the location of the fovea, which is the region corresponding to
highest visual acuity in humans. At the fovea center (foveola), the retina is avascular and consists primarily of photoreceptors.
The thick choroid is well-vascularized with prominent stroma (arrowhead).
C h a p t e r 2 1 Special Senses: Eye
413
shorter, broader, and less pigmented. The RPE
has numerous functions: it metabolizes vitamin A,
forms the outer blood-retina barrier, phagocytoses
photoreceptor outer segments, absorbs light, and
actively transports fluid out of the subretinal space
to maintain adhesion of the neurosensory retina.
Optic Nerve
Gross Anatomy
The human optic nerve is on average 40 mm
in length and can be divided into intraocular
(~1 mm), intraorbital (25 mm), intracanalicular
(4–10 mm), and intracranial (10 mm) sections.
The optic nerve diameter within the human eye is
1.5 mm. The mouse optic nerve is divided into the
optic nerve head, the lamina cribosa region, and
unmyelinated and myelinated regions. Posterior
to the fenestrated portion of the sclera (lamina
cribosa), the optic nerve becomes myelinated,
which increases the diameter. Upon exiting
the posterior globe, the optic nerve becomes
enveloped in a sheath (meninges) consisting
of three layers: dura mater (outer), arachnoid
(center), and pia mater (inner) (Figure 31).
The intraorbital optic nerve in both species is
surrounded by connective tissue, fat, and the rectus
muscles. In mice, the Harderian gland also covers
the optic nerve. The optic nerve is a continuation
of the optic tract, covered in meninges with direct
extension to the central nervous system.
Intraocular Optic
Nerve
Histology
In humans, the optic nerve consists of the 1–1.2
million axons that originate in the ganglion cell
layer and terminate in the lateral geniculate
nucleus of the thalamus. The portion of the optic
nerve visible by ophthalmoscopy is termed the
optic disc. In humans, the optic disc measures
approximately 1.5 mm horizontally and 1.75 mm
FIGURE 31 Human optic nerve.
vertically. A normal optic disc has a temporallydisplaced depression termed the optic cup. The
central retinal artery and vein traverse through
the center of the cup. As ganglion cell axons enter
the optic disc, they are divided into fascicles by
intervening astrocytic glial cells. When the optic
nerve sustains physiologic damage, supporting
glial cells may be lost; this may manifest as an
enlargement of the optic cup. Immediately
posterior to the optic nerve head is the lamina
cribrosa (Figure 32), a collection of connective
tissue plates composed of collagen, elastin,
laminin, and fibronectin, with pores that transmit
the optic nerve axons. Posterior to the lamina
cribrosa, the optic nerve becomes myelinated by
oligodendrocytes.
Extraocular
Muscles
Gross Anatomy
The extraocular muscles are similar between the
two species; however, in mice, a retractor bulbi
muscle is internal to the four rectus muscles and
surrounds the optic nerve. The Harderian gland
surrounds the extraocular muscles (Figure 33).
The inferior oblique may be used for orientation
of the globe in histologic sections of enucleated
eyes as it inserts on the posterior sclera. Details
on the insertions and innervations of the mouse
414
Comparative Anatomy and Histology
S
l Need-to-know
n The optic nerve in mice has 10-fold fewer axons than
that of humans.
Axon number varies greatly between inbred strains.
n
The retractor bulbi muscle is not present in humans.
n
A
FIGURE 32 Human lamina cribosa. The lamina cribosa is a fenestrated collection of horizontal connective tissue plates
(arrows) with pores that transmit the optic nerve axons (A). In mice, the lamina cribosa is also easily identified. Sclera (S) is
indicated.
extraocular muscles may be found in Smith et al.
(see Further Reading).
M
Histology
M
The extraocular muscles of both mice and humans
are skeletal (Figure 34).
H
M
M
ON
H
Eyelids
Gross Anatomy
Both human and mouse eyelids consist of four
layers (from outer to inner): skin, skeletal muscle,
tarsus, and palpebral conjunctiva. The eyelid is
often divided into two leaflets termed the anterior
and posterior lamellae. The anterior lamella
consists of skin and concentric skeletal muscle
that functions in eyelid closure (orbicularis
oculi). The posterior lamella consists of tarsus
and palpebral conjunctiva. Eyelid skin is the
thinnest skin on the human body. At the eyelid
margin, the eyelashes (cilia) project through the
skin anteriorly. In humans, a “gray line” can be
visualized at the lid margin posterior to the cilia
that represents the muscle of Riolan, an extension
of the orbicularis oculi. This structure is not seen
in mice. Posterior to the gray line, both mice and
humans have Meibomian gland orifices, which are
M
FIGURE 33 Mouse posterior orbit. The orbital muscles (M)
are surrounded by the Harderian gland (H). The optic nerve
(ON) is indicated.
S
M
FIGURE 34 Mouse posterior orbit. The extraorbital muscles
are typical skeletal muscle (M) and attach to the sclera (S). The
Harderian gland (H) is indicated. The retina is artifactually
detached from the retinal pigmented epithelium (arrow).
C h a p t e r 2 1 Special Senses: Eye
415
FIGURE 35 Mouse eyelid. In both species, the external eyelid is covered by a thin keratinizing (“cornified”) stratified
squamous epithelium (arrow) that transitions to the noncornified palpebral mucosa at the mucocutaneous junction (thin
arrow) located anterior to the sebaceous glands.
*
openings of sebaceous glands embedded within
the tarsus. In humans, the skin transitions from
keratinizing stratified squamous epithelium to
nonkeratinizing conjunctiva at a location termed
the mucocutaneous junction, posterior to the
Meibomian glands. In mice, the mucocutaneous
junction occurs anterior to the Meibomian gland
orifices, so these glands open onto conjunctiva.
The medial aspects of both superior and inferior
lids contain the lacrimal puncta, which are a
conduit for tear drainage.
Histology
FIGURE 36 Human eyelid. Note the prominent tarsal
plate (arrow) with embedded sebaceous glands. The gray
line, the muscle of Riolan (asterisk), is absent in mice. The
mucocutaneous junction (thin arrow) is indicated.
In both humans and mice, the external eyelid
is covered by a thin keratinizing (“cornified”)
stratified squamous epithelium (Figures 35
and 36). Deep to the epidermis and dermis of the
anterior eyelid are the skeletal muscle fibers of
the orbicularis oculi. Deep to the orbicularis of
the upper lid lies the levator palpebrae superioris,
a muscle that inserts onto the superior anterior
margin of the tarsus to elevate the eyelid. Posterior
to these skeletal muscles is a dense platelike
collection of fibrous connective tissue known as
the tarsal plate (Figures 37 and 38). Within the
416
Comparative Anatomy and Histology
M
MG
C
FIGURE 37 Mouse Meibomian glands (MG) within the tarsal plate. The glands are embedded within a dense connective
tissue. Note the transition from palpebral noncornified epithelium to the conjunctiva with numerous goblet cells (arrow).
Cornea (C) and palpebral muscles (M) are indicated.
MG
l Need-to-know
n In mice, the mucocutaneous junction
occurs anterior to the Meibomian
gland orifices, so the Meibomian
glands open onto the conjunctiva.
FIGURE 38 Human sebaceous (Meibomian) glands (MG) within the tarsal plate. Notice that—as with other human tissues—
the connective tissue stroma is much more prominent than in mice.
tarsal plate are multiple Meibomian glands that
empty at the lid margin to contribute a lipid layer
to the tear film. The Meibomian glands are more
numerous in the upper lid than in the lower lid.
Additional sebaceous glands of Zeis exist more
anteriorly; these connect to the cilia at the eyelid
margin. Mice have Meibomian and glands of
Zeis; they lack eccrine sweat and lacrimal glands
within the palpebra. Deep to the tarsal plates is
the palpebral conjunctiva. As described previously,
the conjunctival epithelium has goblet cells that
provide the mucin layer to the tear film.
C h a p t e r 2 1 Special Senses: Eye
417
L
H
FIGURE 39 Mouse intraorbital lacrimal gland (L) and
Harderian glands (H). Mice have an additional exorbital
lacrimal gland that is adjacent to the salivary glands. Mice
have two lacrimal glands: intra- and exorbital.
FIGURE 40 Human lacrimal gland is an eccrine gland.
The cuboidal acinar cells are arranged in lobules and have a
characteristic basophilic granular cytoplasm. The clear areas
are adipose tissue. The human lacrimal gland is divided into
two lobes; orbital and palpebral.
Lacrimal Gland and
Drainage System
sac passes down the nasolacrimal duct before
emptying into the inferior meatus of the nose in
both species.
Gross Anatomy
The lacrimal glands are responsible for the bulk
of aqueous tear production. Mice have two paired
lacrimal glands. The smaller is the intraorbital
gland located superficially at the lateral canthus
where both the lacrimal and Harderian gland
ducts open. The larger exorbital gland is often
seen in histologic sections of salivary glands
due to its location at the anteroventral base of the
ear adjacent to the parotid salivary gland
(see Chapter 11). In humans, the lacrimal gland
is located at the superotemporal aspect of the
orbit and consists of two contiguous lobes: a larger
and more superficially located orbital lobe and a
smaller palpebral lobe. The lacrimal gland ducts
from the larger orbital lobe pass through and
join with ducts from the palpebral lobe prior to
emptying into the superior fornix. In both species,
the tears flow across the ocular surface and drain
via punctal openings at the medial lid margin.
In humans, the puncta are 2 mm in length and
connect to the 8-mm-long canaliculi, which
merge to form the common canaliculus in
90% of the population. The remaining 10%
have individual canaliculi connecting to the
nasolacrimal sac. The fluid within the nasolacrimal
Histology
The human lacrimal gland is an eccrine gland
that consists of acinar cells and myoepithelial
cells (Figures 39 and 40). The cuboidal acinar
cells are arranged in lobules that line the lumen
of the gland, whereas the myoepithelial cells
contain flattened nuclei and surround the
acinar cells and the ducts; the mouse lacrimal
gland is similarly structured. Human acinar
cells have a characteristic basophilic granular
cytoplasm. Mouse acinar cells have basophilic
basilar cytoplasm with pale-staining apical regions
and moderate nuclear pleomorphism that may
increase with age.
The lacrimal drainage systems of humans and
mice are similar and are both located inferonasal
to the globe. The lacrimal puncta and canaliculi
are lined by nonkeratinizing stratified squamous
epithelium. The substantia propria consists
of both collagenous and elastic tissue. The
lacrimal sac is lined by pseudostratified columnar
epithelium with goblet cells and occasional cilia.
The lacrimal sac wall is highly vascularized.