Corneal endothelium
Encyclopedia
The corneal endothelium is a single layer of cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris.
It is a monolayer of specialized, flattened, mitochondria-rich cell
s that lines the posterior surface of the cornea
and faces the anterior chamber
of the eye
. The corneal endothelium governs fluid and solute transport across the posterior surface of the cornea and actively maintains the cornea in the slightly dehydrated state that is required for optical transparency.
. The postnatal total endothelial cellularity of the cornea (approximately 300,000 cells per cornea) is achieved as early as the second trimester of gestation. Thereafter the endothelial cell density (but not the absolute number of cells) rapidly declines in direct proportion to the areal growth of the fetal cornea, achieving a final adult density of approximately 2400 - 3200 cells/mm². The normal corneal endothelium is a monolayer of uniformly sized cells with a predominantly hexagonal shape. This honeycomb
tiling scheme yields the greatest efficiency, in terms of total perimeter, of packing the posterior corneal surface with cells of a given area.
. Bicarbonate ions formed by the action of carbonic anhydrase are translocated across the cell membrane, allowing water to passively follow.
fibrils, creating light scatter. In addition, excessive corneal hydration can result in edema of the corneal epithelial layer, which creates irregularity at the optically critical tear film-air interface. Both stromal light scatter and surface epithelial irregularity contribute to degraded optical performance of the cornea and can compromise visual acuity.
) and Fuchs' dystrophy
. Surgical causes of endothelial failure include both acute intraoperative trauma as well as chronic postoperative trauma, such as from a malpositioned intraocular lens
or retained nuclear fragment in the anterior chamber. Other risk factors include narrow-angle glaucoma
, aging, and iritis
.
A rare disease called X-linked endothelial corneal dystrophy
was described in 2006.
Historically, penetrating keratoplasty, or full thickness corneal transplantation, was the treatment of choice for irreversible endothelial failure. More recently, new corneal transplant techniques have been developed to enable more selective replacement of the diseased corneal endothelium. This approach, termed endokeratoplasty, is most appropriate for disease processes that exclusively or predominantly involve the corneal endothelium. Penetrating keratoplasty is preferred when the disease process involves irreversible damage not just to the corneal endothelium, but to other layers of the cornea as well. Compared to full-thickness keratoplasty, endokeratoplasty techniques are associated with shorter recovery times, improved visual results, and greater resistance to wound rupture. Although instrumentation and surgical techniques for endokeratoplasty are still in evolution, one commonly performed form of endokeratoplasty at present is Descemet's Stripping (Automated) Endothelial Keratoplasty (DSEK [or DSAEK]). In this form of endokeratoplasty, the diseased host endothelium and associated Descemet's membrane are removed from the central cornea, and in their place a specially harvested layer of healthy donor tissue is grafted. This layer consists of posterior stroma, Descemet's membrane, and endothelium that has been dissected from cadaveric donor corneal tissue, typically using a mechanized (or "automated") instrument.
Investigational methods of corneal endothelial surgical replacement include Descemet's Membrane Endothelial Keratoplasty (DMEK), in which the donor tissue consists only of Descemet's membrane and endothelium, and corneal endothelial cell replacement therapy, in which in vitro cultivated endothelial cells are transplanted. These techniques, although still in an early developmental stage, aim to improve the selectivity of the transplantation approach by eliminating the presence of posterior stromal tissue from the grafted tissue.
It is a monolayer of specialized, flattened, mitochondria-rich cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s that lines the posterior surface of the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
and faces the anterior chamber
Anterior chamber
The anterior chamber is the fluid-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. Aqueous humor is the fluid that fills the anterior chamber. Hyphema and glaucoma are two main pathologies in this area. In hyphema, blood fills the anterior chamber...
of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. The corneal endothelium governs fluid and solute transport across the posterior surface of the cornea and actively maintains the cornea in the slightly dehydrated state that is required for optical transparency.
Embryology and anatomy
The corneal endothelium is embryologically derived from the neural crestNeural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....
. The postnatal total endothelial cellularity of the cornea (approximately 300,000 cells per cornea) is achieved as early as the second trimester of gestation. Thereafter the endothelial cell density (but not the absolute number of cells) rapidly declines in direct proportion to the areal growth of the fetal cornea, achieving a final adult density of approximately 2400 - 3200 cells/mm². The normal corneal endothelium is a monolayer of uniformly sized cells with a predominantly hexagonal shape. This honeycomb
Honeycomb
A honeycomb is a mass of hexagonal waxcells built by honey bees in their nests to contain their larvae and stores of honey and pollen.Beekeepers may remove the entire honeycomb to harvest honey...
tiling scheme yields the greatest efficiency, in terms of total perimeter, of packing the posterior corneal surface with cells of a given area.
Physiology
The principal physiological function of the corneal endothelium is to allow leakage of solutes and nutrients from the aqueous humor to the more superficial layers of the cornea while at the same time actively pumping water in the opposite direction, from the stroma to the aqueous. This dual function of the corneal endothelium is described by the "pump-leak hypothesis." Since the cornea is avascular, which renders it optimally transparent, the nutrition of the corneal epithelium, stromal keratocytes, and corneal endothelium must occur via diffusion of glucose and other solutes from the aqueous humor, across the corneal endothelium. The corneal endothelium then actively transports water from the stromal-facing surface to the aqueous-facing surface by an interrelated series of active and passive ion exchangers. Critical to this energy-driven process is the role of Na+/K+ATPase and carbonic anhydraseCarbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
. Bicarbonate ions formed by the action of carbonic anhydrase are translocated across the cell membrane, allowing water to passively follow.
Mechanisms of corneal edema
Corneal endothelial cells are post-mitotic and divide rarely, if at all, in the post-natal human cornea. Wounding of the corneal endothelium, as from trauma or other insults, prompts healing of the endothelial monolayer by sliding and enlargement of adjacent endothelial cells, rather than mitosis. Endothelial cell loss, if sufficiently severe, can cause endothelial cell density to fall below the threshold level needed to maintain corneal deturgescence. This threshold endothelial cell density varies considerably amongst individuals, but is typically in the range of 500 - 1000 cells/mm². Typically, loss of endothelial cell density is accompanied by increases in cell size variability (polymegathism) and cell shape variation (polymorphism). Corneal edema can also occur as the result of compromised endothelial function due to intraocular inflammation or other causes. Excess hydration of the corneal stroma disrupts the normally uniform periodic spacing of Type I collagenCOL1A1
Collagen, type I, alpha 1, also known as COL1A1, is a human gene that encodes the major component of type I collagen, the fibrillar collagen found in most connective tissues, including cartilage....
fibrils, creating light scatter. In addition, excessive corneal hydration can result in edema of the corneal epithelial layer, which creates irregularity at the optically critical tear film-air interface. Both stromal light scatter and surface epithelial irregularity contribute to degraded optical performance of the cornea and can compromise visual acuity.
Causes of endothelial disease
Leading causes of endothelial failure include inadvertent endothelial trauma from intraocular surgery (such as cataract surgeryCataract surgery
Cataract surgery is the removal of the natural lens of the eye that has developed an opacification, which is referred to as a cataract. Metabolic changes of the crystalline lens fibers over time lead to the development of the cataract and loss of transparency, causing impairment or loss of vision...
) and Fuchs' dystrophy
Fuchs' dystrophy
Fuchs' dystrophy, also known as Fuchs' endothelial dystrophy, is a slowly progressing corneal disease that usually affects both eyes and is slightly more common in women than in men...
. Surgical causes of endothelial failure include both acute intraoperative trauma as well as chronic postoperative trauma, such as from a malpositioned intraocular lens
Intraocular lens
An intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...
or retained nuclear fragment in the anterior chamber. Other risk factors include narrow-angle glaucoma
Glaucoma
Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...
, aging, and iritis
Iritis
Iritis is a form of anterior uveitis and refers to the inflammation of the iris of the eye.-Types:There are two main types of iritis: acute and chronic. They differ in numerous ways....
.
A rare disease called X-linked endothelial corneal dystrophy
X-linked endothelial corneal dystrophy
X-linked endothelial corneal dystrophy is a rare form of human corneal dystrophy described first in 2006. It is manifested as corneal clouding in the form of a ground glass, milky corneal tissue, and moon crater-like changes of corneal endothelium....
was described in 2006.
Treatment for endothelial disease
There is no medical treatment that can promote wound healing or regeneration of the corneal endothelium. In early stages of corneal edema, symptoms of blurred vision and episodic ocular pain predominate, due to edema and blistering (bullae) of the corneal epithelium. Partial palliation of these symptoms can sometimes be obtained through the instillation of topical hypertonic saline drops, use of bandage soft contact lenses, and/or application of anterior stromal micropuncture. In cases in which irreversible corneal endothelial failure develops, severe corneal edema ensues, and the only effective remedy is replacement of the diseased corneal endothelium through the surgical approach of corneal transplantation.Historically, penetrating keratoplasty, or full thickness corneal transplantation, was the treatment of choice for irreversible endothelial failure. More recently, new corneal transplant techniques have been developed to enable more selective replacement of the diseased corneal endothelium. This approach, termed endokeratoplasty, is most appropriate for disease processes that exclusively or predominantly involve the corneal endothelium. Penetrating keratoplasty is preferred when the disease process involves irreversible damage not just to the corneal endothelium, but to other layers of the cornea as well. Compared to full-thickness keratoplasty, endokeratoplasty techniques are associated with shorter recovery times, improved visual results, and greater resistance to wound rupture. Although instrumentation and surgical techniques for endokeratoplasty are still in evolution, one commonly performed form of endokeratoplasty at present is Descemet's Stripping (Automated) Endothelial Keratoplasty (DSEK [or DSAEK]). In this form of endokeratoplasty, the diseased host endothelium and associated Descemet's membrane are removed from the central cornea, and in their place a specially harvested layer of healthy donor tissue is grafted. This layer consists of posterior stroma, Descemet's membrane, and endothelium that has been dissected from cadaveric donor corneal tissue, typically using a mechanized (or "automated") instrument.
Investigational methods of corneal endothelial surgical replacement include Descemet's Membrane Endothelial Keratoplasty (DMEK), in which the donor tissue consists only of Descemet's membrane and endothelium, and corneal endothelial cell replacement therapy, in which in vitro cultivated endothelial cells are transplanted. These techniques, although still in an early developmental stage, aim to improve the selectivity of the transplantation approach by eliminating the presence of posterior stromal tissue from the grafted tissue.