Download History of Corneal Transplantation and Eye Banking

History of Corneal Transplantation and Eye Banking
The idea of corneal transplantation was first thought of in the late 1700’s, and the first man-toman corneal transplant was attempted in the late 1800’s. These first attempts were not
successful. In 1906, however, the first successful corneal transplant was performed in humans.
During the 1930-1960’s, there were many innovative technical and surgical changes, which
improved the feasibility of corneal transplantation. The first Eye Bank, in New York City, was
started in 1944, and the Eye Bank Association of America (EBAA) was founded in 1961. The
EBAA provides the eye banking community with Medical Standards and is the certifying agency
for its members.
The present day surgeon has microsurgical techniques, fine instruments and sutures,
improvement in eye banking techniques, and corneal storage media. With these improvements
corneal transplantation now boast a success rate of over 90% for favorable cases.
Corneal storage has progressed immensely since the mid 1970’s. The ideal storage solution
would maintain endothelial viability and compact stroma, would be easy to use and transplant,
and would be inexpensive. Originally, eyes were simply stored in a moist chamber at 4 C, and
had to be used within 36-48 hours of death. This method was in use until McKarey-Kaufman
(MK) media was developed in 1974. This media also was stored at 4 C, but prolonged storage
time of the cornea 3-5 days. Other storage methods, in very limited use due to impracticability
and cost, were organ culture and cryopresevation. Both were attempts to extend storage time,
but are not widespread in use today. In the early 1980’s, chondroitin sulfate media was
introduced, which provided less tissue swelling, and a greatly prolonged storage time of 7-10
days. Further modification of the media produced what is now in use, Optisol GS, which
provides thinner, clearer tissue, greater endothelial preservation, and again, a greater storage time
of 7-10 days.
Ocular Anatomy
The Eyes
The eyes are the receptor organs for the senses of sight. The term optic means pertaining to the
eye or sight.
Ocular also pertains to the eye.
Extraocular – means on the outside of the eye.
Intraocular – means within the eye
The adnexa of the eye include the orbit, the muscles of the eye, the eyelids, the conjunctiva and
the lacrimal apparatus. (Adnexa refer to the appendages or accessory structures of an organ.)
The Orbit
The orbit is the bony cavity of the skull that contains and protects the eyeball and its associated
muscles, blood vessels and nerves.
The Eyelids
Each eye has a pair of eyelids that protect the eyeball from foreign matter, excessive light and
impact.
Blephar/o is the combining form that means eyelid.
Canthus is the angle where the upper and lower eyelids meet.
Epicanthus is a vertical fold of skin on either side of he nose, sometimes covering the inner
canthus.
The Conjunctiva
The conjunctiva is the mucous membrane that lines the underside of each eyelid and continues to
from a protective covering to the exposed surface of the eyeball.
The Structure of the Eye
The Eyeball
The eyeball is a sphere about 1 inch in diameter. It is a hollow structure with walls made up of
three layers: the sclera, the choroids and the retina.
The Sclera and Cornea
Sclera, also known as the white of the eyes, is a fibrous tissue that maintains the shape of the eye
and protects the delicate inner layers of tissue.
The Cornea is the clear “window” in the front of the eye, made of collagen fibers arranged so as
to transmit light. It is avascular.
The choroids, Pupil and Iris
The Choroid also known as the choroids layer or choroids coat, is the middle layer of the
eyeball. The choroid is opaque and contains many vessels. It provides the blood supply to the
outer portion or the retina.
The Pupil is the circular opening in the front of the choroids.
The Iris is the colored portion of the eye. It controls light entry into the eye.
The amount of light that is permitted to enter the eye is controlled by two sets of muscles within
the iris that changes the shape of the pupil. To decrease the amount of light, the muscles contract
the pupil (making it smaller). To increase the amount of light, the muscles dilate the pupil
(making it larger).
The Lens and Related Structures
The Lens is the flexible and curved structure located behind the iris of the pupil.
The Ciliary Muscles, located within the Ciliary Body of the choroids, adjust the shape and
thickness of the lens.
Chambers of the Eye
The Ocular Chamber lies in front of the lens. This chamber is subdivided by the iris into the
anterior and posterior chambers. The ocular chamber is filled with a watery substance called
aqueous humor. (A humor is any clear body liquid or semi-fluid substance.)
The Vitreous Chamber is the larger region behind the lens. It is filled with a jelly-like material,
the vitreous humor.
The Retina
The Retina is the sensitive inner nerve layer of the eye. It is located between the posterior
chamber and the choroids layer at the back of the eye.
The Retina contains specialized light-sensitive cells called rods and cones the initiate nerve
impulses that travel from the eye to the brain via the optic nerve.
The Optic Disk, also known as the blind spot, is the region in the eye where nerve endings of the
retina gather to from the optic nerve. It is called the blind spot because it does not contain any
rods or cones.
The Macula Lutea is the yellow spot in the center of the retina.
The Fovea Centralis is the pit located within the macula lutea. This section of the retina is the
area of sharpest vision.
The Action of the Eye
Light rays enter the eye through he cornea, pass through the lens and travels to the retina. The
image focused there is transmitted to the brain.
Refraction is the ability of the lens to bend the light rays to help them focus on the retina.
Accommodation is the process whereby the eyes make adjustments for seeing objects at various
distances. Accommodation includes constriction and dilation of the pupil, movement of the eyes
and changes in the shape of the lens.
Anatomy and Physiology of the Cornea
The Cornea is composed of five layers:
1. Epithelium – the outer most layer of the cornea. It is 5 – 6 cell layers deep and protects
against infection and excessive hydration.
Corneal Epithelial – Cell Types
Basal Cells are the deepest layer, they are columnar and are active in metabolism and cell
division.
Wing Cells from the middle layer.
Squamous Cells are the superficial layers. They are flat non-keratinized cells that slough when
dead. They have microville to hold tear film, this is important for corneal survival.
2. Bowmans Membrane – It is beneath the epithelium, is a very thin compact layer of collagen
tissue. It provides structure and a strong barrier to infection.
3.Stroma – makes up the bulk of corneal thickness. The stroma is made of collagen tissue
(keratocytes) and is 75-80% water in a state of hydration, which is maintained by the
endothelium.
Ground substance of the stroma:
 Keratan sulfate
 Chondroitin
 Chondroitin sulfate A
4. Descemet’s Membrane – is the basement membrane of the endothelium. The descemet’s
membrane is a product of the endothelium, it is highly elastic and provides the final barrier to
infection.
5. Endothelium – is the innermost cell layer. The endothelium is made up of a single layer of
cells. The cell density at birth is 3500-4000 cells/sq. mm, but decreases with age, disease, or
injury due to lack of regenerative power of the endothelium. This layer works as a pumping
system to maintain the steady state hydration of the cornea.
Because the cornea is avascular, it is nourished externally from tear film and internally from the
aqueous humor.