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Histology D502
11.22.04
The Eye
Outline:
I. Introduction
II. Structure of the eye
III. Genesis of the eye
IV. Sclera
V. Uvea
VI. Lens
VII. Retina
VIII. Conjunctiva, Eyelids and Glands associated with the eye
I. Introduction:
a) organ collects light of various intensity and color
b) translates this information into neural signals
c) signals processed to give image
d) depth of field conferred via binocular vision
II. Structure of the Eye (tunics and others):
a) Sclera (corneosclera):
i) fibrous coat over most of the eye
ii) cornea over anterior eye
b) Uvea (vascular layer):
i) choroid
ii) ciliary body
iv) iris
c) Retina:
i) pigmented epithelium (retinal pigmented epithelium)
ii) photosensitive retina proper (neural retina)
d) Lens
e) Internal, liquid filled, chambers:
i) anterior chamber: between cornea and iris
ii) posterior chamber: between iris and lens
iii) vitreous body: between lens and retina, 99% water, remainder salts, collagen and hyaluronic
acid (very hydrated)
III. Genesis of the eye:
a) obvious development by day 22 in the embryo and made of neuroectoderm, surface ectoderm,
and mesoderm
b) neuroectoderm develops into brain and forebrain outgrowth occurs giving the optic stalk and
optic vesicle, invagination of vesicle occurs forming the double layered optic cup
i) inner layer develops into the neural retina
ii) outer layer develops into the retinal pigmented epithelium, pigmented epithelium of iris and
ciliary body, and dilator and sphincter muscles for the pupil (within iris)
c) surface ectoderm invaginates within the optic cup:
i) this initially forms the lens vesicle that pinches off and develops into the lens
ii) the contiguous surface ectoderm develops into the corneal epithelium and the lining of the
eyelids
d) the mesoderm gives rise to:
i) stroma of sclera and cornea
ii) uvea: stroma of choroid, ciliary body, and iris
IV. Sclera (outermost layer)
a) posterior 5/6 covered with opaque connective tissue capsule containing flat collagen bundles
and fibroblasts (sclera proper) that approximates 3 layers:
i) episclera: outer most layer in contact with eye socket made of loose connective tissue
ii) sclera proper (Tenon’s capsule): middle layer, dense network of collagen fibers, tendons of
extraocular muscles attach to Tenon’s capsule
iii) lamina fuscia: inner layer, adjacent to choroid, made of collagen and elastic fibers and
contains pigmented cells
b) anterior 1/6 covered with the cornea
i) 5 layers thick: (outer to inner)
- epithelium: strat. sq. epith, non-keratinized, 5 - 6
cell layers thick:
- rich in sensory nerves
- high regenerative capacity
- cells turnover is 7 days
- Bowman’s membrane: 7 -12 micron thick
acellular layer of collagen fibers with little ground
substance, provides barrier preventing infection but
does not regenerate
- stroma: about 60 layers of parallel collagen
bundles that cross at right angles giving
transparency to the cornea, flattened fibroblasts
between layers, ground substance of chondroitin and karatan sulfate, thickest layer of cornea,
- Descemet’s membrane: 5 - 10 micron thick basal lamina of corneal endothelial cells, can
regenerate and increases in thickness with age
- Endothelium: simple squamous epithelia with directional ion transport properties
ii) cornea is avascular so receives nutrients via transport across the endothelium from anterior
chamber and oxygen via diffusion
iii) corneal transparency results from uniform arrangement of collagen fibers and their low light
scattering
iv) junction between cornea and sclera (corneoscleral junction) called limbus:
- transition from clear to opaque collagen coat
- highly vascular region
- location of the canal of Schlemm
V. Uvea (vascular or middle layer)
a) 3 parts:
i) choroid
ii) ciliary body
iii) iris
b) choroid: largest component, 3 layers (out to in)
i) vessel layer: medium sized arteries and veins, loose CT and melanocytes
ii) chorocapillary layer: capillaries arranged in one plane, fenestrated type
iii) Bruch’s membrane: 3 - 4 micron thick amorphous hyaline membrane that the retinal
pigmented epithelia rests upon
c) ciliary body: expansion of the stroma of choroid near the lens
i) contacts three regions: vitreous body, sclera, posterior chamber/lens
ii) contains 2 layers of stroma; a vascular loose CT layer with melanocytes, and a smooth muscle
(ciliary muscle) layer
- loose CT stroma lined with two layers of columnar cells with the basal layer being pigmented
- two bundles of smooth muscle
- one stretches the choroid
- contraction of the other relaxes tension on lens
iii) has projections called ciliary processes
- project toward lens
- fibers composed of oxytalin fibers extend from these projections and attach to the lens
- called zonule fibers
- lined with two layers of columnar cells as body
- cells joined by tight junctions and desmosomes and actively transport ions from the plasma into
the posterior chamber forming the aqueous humor of the internal eye
- much lower protein (0.1%) than plasma
- intraocular pressure slightly higher than intracranial pressure helping to maintain the shape of
the eye and the corresponding arrangement of retractile elements of the eye
iv) trabecular meshwork within ciliary body near limbus
- aqueous humor produced by the ciliary processes passes from the posterior chamber into the
anterior chamber (between iris and lens)
- aqueous humor drained from anterior chamber via the trabecular meshwork
- from here, the humor passes into the canal of Schlemm that drains into the venous system
- there is no direct connection between the canal and trabecula, the humor percolates through the
tissue into the canal
d) iris: covers lens, regulates amount of light reaching retina (pupil diameter)
i) anterior aspect made of vascular, loose CT with interspersed melanocytes, number of
melanocytes in this layer determines eye color
ii) posterior surface lined with a double layer of pigmented epithelium
- absorbs light
iii) two muscle masses rest upon the pigmented epithelium and regulate iris opening (pupil
diameter)
- radially arranged myoepithelial cells form the dilator pupillae muscle between the vascular and
pigment layer
- sympathetic innervation
- concentric smooth muscle bundles at the pupil margin (inner aspect of iris) form the sphincter
pupillae muscle
- parasympathetic innervation
VI. Lens: avascular structure that facilitates image focus, made of 3 components:
a) lens capsule: the basement membrane of the epithelial cells that is an homogenous translucent
CT matrix rich in glycoprotein
i) zonule fibers attach to this around the periphery of the lens
b) subcapsular epithelium: single layer of cuboidal cells
c) Lens fibers: elongated cells derived from the subcapsular epithelia near the equator of the lens
i) as the cells grow and are pushed to the optical axis of the lens, they loose their nuclei
ii) at the optical axis, the cells are hexagonal and pack in a highly organized fashion with little
intercellular space
iii) cells contain few organelles but are high in protein (60 -70%)
- major protein is crystallins
- function to increase refractive index of cytosol
d) accommodation of lens:
i) lens thinner when focused on distant objects, relaxed ciliary muscles
ii) lens thicker when focusing on near objects, ciliary muscles contract, relaxing tension on
zonule fibers thus thickening lens (anterior directed movement of ciliary body)
VII. Retina: location of photosensitive cells and neural networks
a) vascularized cellular layer
b) essentially 4 cell (nuclei) layers (out to in)
i) retinal pigmented epithelium
ii) photosensitive layer
iii) intermediate layer
iv) internal layer
c) pigment epithelium:
i) rest upon Bruch’s membrane of choroid
ii) rich in melanin
iii) send processes into photosensitive layer
- processes contain melanin but the photosensitive cells do not bite-off the melanosomes
iv) pigment prevents light scattering after passing through photosensitive layer
v) cells involved in recycling vitamin A of
photopigments via phagocytosis of shed membrane
from photoreceptor cells
d) photosensitive layer: 2 cell types
i) Rod cells: rod-shaped, light intensity sensors, dark
accommodation
- elongate cells with inner and outer segment
- outer segment contains stacks of membrane
containing vit. A bound to rhodopsin
- inner segment contains nucleus, cell organelles and synapse with bipolar cells
- photons cause isomerization of retinal from 11-cis to all trans while bound to rhodopsin (a
trimeric GTP-binding protein linked receptor)
- isomerization results in a cascade of events (st) mediated by transducin (the G-protein trimer
associated with rhodopsin activation):
- activated rhodopsin binds to transducin and weakens the binding of the alpha subunit to the
trimeric G protein
- the alpha subunit dissociates, binds to cyclic GMP (cGMP) phosphodiesterase and increases the
activity of this enzyme
- enzyme activation increases the hydrolysis of cGMP, thus decreasing the cellular cGMP
- Na+/Ca2+ channels within the membrane with bound cGMP are open, without it is closed
- the decreased cGMP results in cGMP dissociating from the cGMP-dependent Na+/Ca2+
channel and close the channel
- decreased Na+ permeability results in membrane hyperpolarization thus generating an electrical
signal (not depolarization) that is sent to the brain
- decreased Ca2+ permeability results in decreased cytosolic Ca2+, Ca2+ dissociates from a
protein called recoverin, without Ca2+, recoverin can bind guanyl cyclase and increase the
activity of this enzyme
- increased guanyl cyclase activity “recovers” the lowered cGMP levels induced by the photons
- the increased cytosolic cGMP results in cGMP binding to the cGMP-dependent Na/Ca2+
channel thus re-opening the channel
- trans retinal released from rhodopsin
- pigment epithelium recycles vit. A
ii) Cone cells: cone-shaped outer segment, similar inner segment to rod cells
- color sensitive cells
- pigment protein is iodopsins
- thought to be three different forms for blue, green and red
- photochemistry similar to rhodopsin but wavelength specific
e) intermediate layer: bipolar cells
i) synapse with inner segment of photosensitive cells
ii) synapse with ganglion cells of inner layer
f) internal layer: ganglion cells
i) form synapse with bipolar cells
ii) fuse together to form optic nerve and carry information to brain
g) additional cells of retina:
i) horizontal cells: connect photoreceptor cells, integrative function
ii) amacrine cells: contact ganglion cells, conducting cells
iii) supporting cell:Muller cells
- Muller cells: ramify through the retinal layer (extend the full its full thickness) and form a
basement membrane adjacent to the vitreous humor, support function
h) specialized structures of retina:
i) fovea: depression in which retina is very thin
- bipolar and ganglion cells at periphery giving thinness
- region devoid of rod cells
- region of increased visual acuity at the optical axis
ii) optic papilla:
- off optical axis
- devoid of photosensitive cells
- location of exit of optic nerve from eye
VIII. The Conjunctiva, Eyelid, and Glands of the eye:
a) conjunctiva: a mucous membrane covering the lateral margins of the cornea, the anterior
aspect of the sclera and the internal
surface of the eyelid:
i) stratified squamous (near cornea) to
columnar epithelia (other regions) with
goblet cells
ii) subdivided into:
- ocular conjunctiva (that over the orbit)
- palpebral conjunctiva (that lining the
interior of the eyelid)
b) Eyelid: protects the eye
i) thin skin on exterior
ii) eyelashes at the palpebral (lid to lid)
junction with 2 to 3 rows of hair follicles
iii) orbicularis oculi skeletal muscle
iv) tarsal plate: fibroelastic tissue
v) conjunctiva
vi) glands within the lids
- sebaceous glands for lashes (Glands of Zeis)
- sweat glands for epidermis (Glands of Moll)
- tear-contributing glands (Meibomian or tarsal glands) see below
c) Non-integumental glands of the eye: function to secrete liquids to protect eye surface from
dehydration and bug growth (lysozyme)
i) tear film: 3 layers
- outer lipid layer to prevent evaporation (tarsal glands)
- middle aqueous layer (lacrimal gland)
- inner mucous layer (goblet cells of conjunctiva)
ii) two main glands involved:
- tarsal (Meibomian) gland:
- located within eyelids on the interior aspect and within the tarsal plate
- 20 to 30 per lid
- specialized sebaceous gland that secretes oily product onto eye (does not secrete onto hair
follicle)
- lacrimal gland:
- located in the anterior, superior, temporal
region of eye socket
- tubuloalveolar serous gland
- tear secreting gland, lubricates and protects
eye epithelium