Transcript Histology of the Eye II

Histology of the Eye II
David L. McWhorter, Ph.D.
Tunica Interna (Retina)
•
Inner layer of eye that consists of
two portions separated by ora
serrata (serrated extremity of
optic part of retina)
1. Posterior portion is
photosensitive
2. Anterior portion is
nonphotosensitive
• forms inner lining of ciliary body
and posterior part of iris
•
Posterior part of retina is further
divisible into two layers, based on
structure, function, and embryonic
origin:
1. Pigmented layer
2. Neural layer
1
2
Pigmented Epithelium
• Melanin-rich simple cuboidal-tocolumnar epithelium
• Basal surface adheres firmly to Bruch's
membrane of choroid
• Apical microvilli envelop outer
segments of rods and cones in overlying
neural retina:
– Not anatomically joined to photoreceptors,
which can lead to detachment of the
retina:
• Head trauma or other conditions can cause two
layers to separate
• Photoreceptor cells no longer have access to
metabolic support from pigmented layer and
choroid and will eventually die
• Reattachment with laser surgery is an effective
treatment
• Diverse functions include:
– Serve as part of blood-retina barrier
– Absorb light passing through to prevent its
reflection
– Phagocytose shed components from
photoreceptors
– Remove free radicals
– Isomerize and regenerate retinoids used as
chromophores by rods and cones
The posterior photosensitive part of retina (neural
retina) has three major layers of neurons that
receive, integrate, and transmit visual signals to
the brain as nerve impulses
1. Outer layer of
photoreceptors:
3
– rods
– cones
2
2. Intermediate layer of:
– bipolar cells that connects to
rod and cone cells
1
3. Internal layer of:
– ganglion cells that synapse
with bipolar cells and send out
axons that converge to form the
optic nerve
1
Rods and cones are polarized
photoreceptors (receptor sensitive to light)
located next to pigmented epithelium
•
Named for the shape of
their outer segments:
–
one pole is a single
photosensitive dendrite
other pole are synapses
with bipolar cells
–
•
•
Light must penetrate more
superficial layers of retina to
reach and stimulate them
Each cell has two
structurally different
segments:
1.
2.
Outer segment or
specialized dendritic
(receptive) part
Inner segment
2
1
Two Segments of Rods and Cones
1.
Outer segment or specialized
dendritic (receptive) part contain
stacks of membrane-limited
vesicles/saccules with visual pigment
Inner segment is rich in
polyribosomes and glycogen and is
separated from the outer segment by a
constriction
2.
–
–
Basal bodies near constriction anchor
one or two intracytoplasmic cilia that
extend through constriction into outer
segment
Mitochondria near constriction provide
energy for the visual process
2
1
Rod Cells are thin, elongated cells (50
x 3 µm) composed of two portions
1. Outer segment is photosensitive and
composed mainly of numerous (600–1,000)
flattened membranous disks stacked up like
coins:
–
contain proteins, including the pigment visual
purple, or rhodopsin
•
bleached by light and initiates the visual stimulus
2. Inner segment contains metabolic
machinery necessary for cell’s biosynthetic
and energy-producing processes (e.g.,
glycogen, mitochondria, and polyribosomes)
• Human retina has approximately 120 million
rods
• Extremely sensitive to light and are
considered to be receptors used when low
levels of light are encountered (e.g., dusk or
nighttime)
Cone Cells are also
elongated (60 x 1.5 µm)
neurons
• Each human retina has about 6-7 million
cone cells
• Structure similar to rods (elongated with
outer and inner segments), but differ:
– outer segments shorter and conical
– stacked membranous disks remain
continuous invaginations of the plasma
membrane along one side
• Less sensitive to low light
• Specialized for color vision in bright light
• Three functional types of cones
(indistinguishable morphologically)
contain variations of visual pigment,
iodopsin:
– maximum sensitivity is in the red, green,
or blue region of the visible spectrum
• Are believed to permit better visual
acuity than do rods
Color blindness is associated with
functional deficits in what structures?
• Color blindness is associated with the
absence of the red-, green-, or bluesensitive photopigments in the cones
Phototransduction is a process
by which light is converted into
electrical signals in the rod and
cone cells of the retina
• Main parts of process are similar in both rods
and cones
• Phototransduction begins when light hits the
stacked membranous discs (next slide):
– Membranes of the stacked membranous discs
are densely packed with proteins:
• Rhodopsin in rods (~ 1 billion molecules in each
rod)
• One of the Iodopsin proteins in cones
• Each visual pigment (rhodopsin and iodopsin)
contains:
– a transmembrane protein, opsin
– with a bound molecule of retinal, the lightsensitive chromophore
• Rhodopsin and each of the three iodopsins:
– absorb light most effectively at different
specific wavelengths in the visible spectrum
In darkness, rhodopsin is inactive
• Intracellular concentration
of the second messenger,
cyclic GMP (cGMP) is high
• One effect of cGMP is:
– to keep open the abundant
cation channels in the cell
membrane, and therefore the
cell is depolarized:
• continuously releasing its
neurotransmitter (glutamate) at
the synapse with the bipolar
neurons
When photons of light are absorbed by the retinal
of rhodopsin, the molecule isomerizes from 11—
cis—retinal to all—trans—retinal
• This change activates opsin, which in turn
activates:
– An adjacent peripheral membrane protein,
transducin, a trimeric G protein:
• allowing it to release its α subunit, which moves
laterally and stimulates another membrane protein,
phosphodiesterase:
– hydrolyzes cGMP
• With less cGMP:
– many sodium channels close, producing
hyperpolarization of the cell which:
• decreases the release of neurotransmitter at the
synapses
• This change at the synapse:
– depolarizes sets of bipolar neurons which
then:
• send action potentials to the various ganglion cells
of the optic nerve:
– will allow the brain to produce an image
Conformation change induced by light in
retinal, which initiates cascade of events
producing neural activity, also causes…
• Retinal, the chromophore, to dissociate
from rhodopsin, leaving a more pale—
colored (bleached) opsin
• The free retinal all—trans-retinal diffuses
into the surrounding pigmented
epithelium, where it is converted back to
11-cis-retinal
• 11-cis-retinal is then transported back
into a rod or cone cell to combine with
opsin for another round of
phototransduction
• Cycle of retinal regeneration and
rhodopsin recovery from bleaching may
take a minute or more:
– Is part of adaptation of eyes that occurs
Bipolar cells lie in middle of neural
• Comprise two
retina
populations of
interneurons that relay
visual signals from
photoreceptors to
ganglion cells:
1. Each diffuse bipolar
cell synapses with:
• ganglion cells
• two or more photoreceptors
(mostly rods)
2. Each monosynaptic
bipolar cell synapses
with:
• a single cone
• a single ganglion cell
•
accounting for cones'
greater visual acuity
1 2
Ganglion Cells lie near globe's inner
surface and have large cell bodies and
nuclei
• Their dendrites make
synaptic contact with bipolar
cells
• Axons of ganglion layer
project to a specific region of
the retina, where they come
together to form optic
nerve:
– This region is devoid of
receptors and is called blind
spot of the retina (optic disc,
optic papilla or optic nerve
head)
Other Cell Types in Neural Retina
•
Two minor populations of
neurons whose function is
thought to integrate visual
signals before they reach the
brain:
1.
Horizontal cells processes
terminate near synapses
between photoreceptor and
bipolar cells
Amacrine cells processes
terminate near synapses
between bipolar and
ganglion cells
2.
•
Muller cells are large, highly
branched cells that span
neural retina's entire width
and embrace processes of
retinal neurons:
–
–
Heinrich, German anatomist
functionally analogous to
neuroglia:
•
support, nourish, and insulate
retinal neurons and fibers
2
1
Fovea (L., pit) or fovea centralis is
a specialized area of the retina
• Shallow depression on the
temporal side of the optic disc at
the posterior pole of the optical
axis
• Contains retina's thinnest region,
lies directly opposite the lens
center:
– Having only cone cells at its center
with ganglion and bipolar cells at
periphery of depression:
• Cone cells in the fovea are long
and narrow, resembling rod cells
– permits closer packing of cones,
increasing visual acuity
• Light falls directly on cones in
central part of fovea
– helps account for extremely precise
visual acuity of this region
Macula lutea (L., spot + yellow) or
Macula surrounds the fovea centralis
• All layers of retina are
present
• Two plexiform layers are
rich in various
carotenoids:
– Give this area its yellowish color
– have antioxidant properties
and filter potentially damaging
short-wavelength light:
• helping to protect the cone cells
of the fovea
Age-related macular degeneration is
a leading cause of blindness in elderly
individuals
•
•
Causes blindness in the center of the visual
field
Degenerative changes in the retina around
the macula include:
– Depigmentation of the posterior epithelium
– Focal thickening of the adjacent Bruch’s
membrane
– Major changes and blood loss in the capillaries in
the choroid and retina
– Eventual loss of photoreceptor cells producing
blind spots
•
•
Appears to be a genetic predisposition to the
disorder, along with environmental triggers
(e.g., excessive exposure to ultraviolet
radiation)
Progression of the disease can be slowed by
laser surgery to destroy abnormal and
excessive retinal capillaries
Visual field of Age-related macular
degeneration
Fundoscopic or Ophthalmoscopic View
1. Fovea ___
2. Macula ___
3. Optic disc ___
A
C
B
Vitreous Body is transparent, gel-like
connective tissue (mostly water and
hyaluronan) that fills the space between
lens and retina (vitreous chamber)
• Provide structural support
to eyeball while offering a
clear unobstructed path
for light to reach the retina
• Contained within a
vitreous membrane:
– Composed of type IV
collagen and other proteins
of external laminae
• Contains a few
macrophages and
hyalocytes:
– stellate cells with oval
nuclei that produce the
collagen and hyaluronate
Eye Floaters (vitreous opacities)
• Age-related changes with the gel-like
vitreous body becoming more liquid
cause debris within it to clump:
– as light passes through the vitreous body,
the debris forms shadows on the retina:
• appear as black spots, streaks, strings,
spider webs, etc.
• move about in the visual fields
– These are called eye floaters that can
become an annoyance:
• however, most people learn to disregard
them
– Usually, they resolve in a few weeks or so
• When eye floaters are accompanied by
bright flashes of light or fuzzy vision:
– an ophthalmologist should be consulted:
• may indicate a tear in the retina
Lens is a transparent, elastic, biconvex
structure immediately behind the iris
• Used to focus light on the
retina
• Nourished by aqueous
humor:
– it has neither blood nor nerve
supply
• Suspended by the zonule of
the ciliary body behind the
pupil
• Highly elastic, a feature lost
with age
• Ciliary muscle contraction
changes curvature of lens to
enable focus on objects near
or far:
– a process called
accommodation
Lens is a transparent, elastic tissue that
focuses light on the retina
•
•
•
•
•
•
Surrounding the entire lens, the lens capsule
(LC) is a thick, homogenous external lamina
formed by the epithelial cells and fibers
Anterior surface of the lens, beneath the
capsule, is covered by a simple columnar lens
epithelium (LE)
At the equator of the lens, near the ciliary
zonule, the epithelial cells proliferate and give
rise to cells that align parallel to the epithelium
and become the lens fibers
Differentiating lens fibers (DLF) still have
their nuclei, but are greatly elongating and
filling their cytoplasm with proteins called
crystallins
Mature lens fibers (MLF) have lost their nuclei
and become densely packed to produce a
unique transparent structure
Lens is difficult to process histologically and
sections usually have cracks or blebs among
the lens fibers
Lens Components
1. Lens capsule is an elastic and transparent basal lamina that covers entire lens and prevents
wandering cells from penetrating it
–
–
Insertion of zonule fibers from ciliary processes
consists mainly of type-IV collagen fibrils and proteoglycans
2. Subcapsular epithelium is low cuboidal epithelium on the anterior lens surface, but increases
to columnar near the lens equator, where cell division occurs
–
Its cells contain few organelles and form the lens fibers
3. Lens fibers are long, narrow, hexagonal, specialized epithelial cells derived from subcapsular
epithelial cells that make up most of lens
–
During differentiation, they lose their nuclei, fill with proteins called crystallins, and develop a variety of
specialized plasma membrane components, including junctional complexes and ridgelike processes
•
Crystallins belong to the family of heat-shock proteins
1
2
3
Accessory Structures of the Eye
1. Conjunctiva
2. Eyelids (palpebrae)
3. Lacrimal apparatus
Conjunctiva is a thin transparent
membrane comprised of two parts
1. Bulbar conjunctiva:
–
–
nonkeratinized stratified squamous epithelium
covers eye's anterior surface (sclera) to the cornea
2. Palpebral conjunctiva:
–
–
stratified columnar epithelium with goblet cells
covers inner eyelid surface
Conjunctivitis (Pinkeye) is an
inflammation of the conjunctiva
producing red sclera and discharge
• It may be caused by
bacteria, viruses,
parasites, and allergens
• Some forms are
contagious and may cause
blindness if left untreated
Infections near an opening of the
tarsal gland are called styes
• “eyelid bumps”
• Generally caused by
Staphylococcus
aureus
• Most common in
infants, but can occur
at any age
• Can be painful
• Can occur in periods
of
immunosuppression
caused by:
– Poor nutrition
– stress
Eyelid
• Pliable tissue with skin (S) covering its
external surface and smooth conjunctiva
(C) lining its inner surface
• At the outer rim of the eyelid are a series of
large hair follicles (F) for the eyelashes
• Associated with these hair follicles are small
sebaceous glands and modified apocrine
sweat glands
• Internally eyelids contain fascicles of
striated muscle (M) comprising the
orbicularis oculi muscle and closer to the
conjunctiva a thick plate of fibroelastic
connective tissue called the tarsus (T)
• Tarsal plate provides structural support for
the eyelid and surrounds a series of large
sebaceous glands, the tarsal glands (TG)
(aka Meibomian glands), with acini secreting
into long central ducts (D) that empty at the
free edge of the eyelids
Eyelid at higher magnification
• Only the inner aspect of the
eyelid is seen:
– shows that the conjunctiva is a
mucous membrane consisting of:
• a stratified columnar epithelium with
small cells resembling goblet cells
and resting on a thin lamina propria
(LP)
• Large cells undergoing typical
holocrine secretion are shown in
the tarsal gland acini (TG), and
the fibrous connective tissue in
the tarsus (T) surrounding the
acini:
– Sebum from these glands is
added to the tear film and helps
lubricate the ocular surface
Lacrimal Apparatus is a system of
glands and ducts, providing tears to
lubricate and protect the eyes
•
•
•
•
•
•
Lacrimal glands are tear-secreting
compound tubuloalveolar glands located
superolaterally in the bony orbits
Tears are released behind the upper
eyelid and flow over the eye's anterior
surface
Excess tears drain through the lacrimal
puncta (one small hole in the free
margin of each lid near the medial
palpebral angle)
Tears enter by capillary attraction and,
with the aid of a pumping action provided
by the orbicularis oculi muscles, follow
the lacrimal canaliculi into a short duct
that empties into the lacrimal sac
Lacrimal sac delivers tears to the
nasolacrimal duct with the aid of gravity
Nasolacrimal duct empties through a
bony canal into the nasal cavity through
the inferior meatus