Corneal endothelium

Corneal endothelium
Corneal endothelium
	Vertical section of human cornea from near the margin. (Corneal endothelium is #5, labeled at bottom right.)
Details
Identifiers
Latin	epithelium posterius corneae
MeSH	D004728
TA98	A15.2.02.022
FMA	312267
	Anatomical terminology [ edit on Wikidata]

Last updated November 09, 2023

The corneal endothelium is a single layer of endothelial cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris.

The corneal endothelium are specialized, flattened, mitochondria-rich cells that line the posterior surface of the cornea and face the anterior chamber of the eye. The corneal endothelium governs fluid and solute transport across the posterior surface of the cornea and 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 crest. 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, as the fetal cornea grows in surface area,^[1] achieving a final adult density of approximately 2400 - 3200 cells/mm². The number of endothelial cells in the fully developed cornea decreases with age up until early adulthood, stabilizing around 50 years of age.^[2]

The normal corneal endothelium is a single layer 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. The corneal endothelium is attached to the rest of the cornea through Descemet's membrane, which is an acellular layer composed mostly of collagen IV.^{[ citation needed ]}

Physiology

Hexagonal cells of corneal endothelium visualized by specular microscopy. Cornea endothelium specular.jpg — Hexagonal cells of corneal endothelium visualized by specular microscopy.

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 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 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 anhydrase. Bicarbonate ions formed by the action of carbonic anhydrase are translocated across the cell membrane, allowing water to passively follow.^{[ citation needed ]}

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 of 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 collagen 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.^{[ citation needed ]}

Causes of endothelial disease

Leading causes of endothelial failure include inadvertent endothelial trauma from intraocular surgery (such as cataract surgery) 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.

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.^[3]^[4] 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.

Related Research Articles

<span class="mw-page-title-main">Cornea</span> Transparent front layer of the eye

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK.

A microkeratome is a precision surgical instrument with an oscillating blade designed for creating the corneal flap in LASIK or ALK surgery. The normal human cornea varies from around 500 to 600 micrometres in thickness; and in the LASIK procedure, the microkeratome creates an 83 to 200 micrometre thick flap. This piece of equipment is used all around the world to cut the cornea flap. The microkeratome is also used in Descemet's stripping automated endothelial keratoplasty (DSAEK), where it is used to slice a thin layer from the back of the donor cornea, which is then transplanted into the posterior cornea of the recipient. It was invented by Jose Barraquer and Cesar Carlos Carriazo in the 1950s in Colombia.

Corneal transplantation, also known as corneal grafting, is a surgical procedure where a damaged or diseased cornea is replaced by donated corneal tissue. When the entire cornea is replaced it is known as penetrating keratoplasty and when only part of the cornea is replaced it is known as lamellar keratoplasty. Keratoplasty simply means surgery to the cornea. The graft is taken from a recently deceased individual with no known diseases or other factors that may affect the chance of survival of the donated tissue or the health of the recipient.

Fuchs dystrophy, also referred to as Fuchs endothelial corneal dystrophy (FECD) and Fuchs endothelial dystrophy (FED), is a slowly progressing corneal dystrophy that usually affects both eyes and is slightly more common in women than in men. Although early signs of Fuchs dystrophy are sometimes seen in people in their 30s and 40s, the disease rarely affects vision until people reach their 50s and 60s.

Descemet's membrane is the basement membrane that lies between the corneal proper substance, also called stroma, and the endothelial layer of the cornea. It is composed of different kinds of collagen than the stroma. The endothelial layer is located at the posterior of the cornea. Descemet's membrane, as the basement membrane for the endothelial layer, is secreted by the single layer of squamous epithelial cells that compose the endothelial layer of the cornea.

A corneal ulcer, or ulcerative keratitis, is an inflammatory condition of the cornea involving loss of its outer layer. It is very common in dogs and is sometimes seen in cats. In veterinary medicine, the term corneal ulcer is a generic name for any condition involving the loss of the outer layer of the cornea, and as such is used to describe conditions with both inflammatory and traumatic causes.

Corneal dystrophy is a group of rare hereditary disorders characterised by bilateral abnormal deposition of substances in the transparent front part of the eye called the cornea.

Iridocorneal endothelial (ICE) syndromes are a spectrum of diseases characterized by slowly progressive abnormalities of the corneal endothelium and features including corneal edema, iris distortion, and secondary angle-closure glaucoma. ICE syndromes are predominantly unilateral and nonhereditary. The condition occurs in predominantly middle-aged women.

<span class="mw-page-title-main">Corneal neovascularization</span> Medical condition

Corneal neovascularization (CNV) is the in-growth of new blood vessels from the pericorneal plexus into avascular corneal tissue as a result of oxygen deprivation. Maintaining avascularity of the corneal stroma is an important aspect of corneal pathophysiology as it is required for corneal transparency and optimal vision. A decrease in corneal transparency causes visual acuity deterioration. Corneal tissue is avascular in nature and the presence of vascularization, which can be deep or superficial, is always pathologically related.

Band keratopathy is a corneal disease derived from the appearance of calcium on the central cornea. This is an example of metastatic calcification, which by definition, occurs in the presence of hypercalcemia.

<span class="mw-page-title-main">Bullous keratopathy</span> Medical condition

Bullous keratopathy, also known as pseudophakic bullous keratopathy (PBK), is a pathological condition in which small vesicles, or bullae, are formed in the cornea due to endothelial dysfunction.

Corneal dystrophies are a group of diseases that affect the cornea in dogs.

Posterior polymorphous corneal dystrophy is a type of corneal dystrophy, characterised by changes in Descemet's membrane and endothelial layer. Symptoms mainly consist of decreased vision due to corneal edema. In some cases they are present from birth, other patients are asymptomatic. Histopathological analysis shows that the cells of endothelium have some characteristics of epithelial cells and have become multilayered. The disease was first described in 1916 by Koeppe as keratitis bullosa interna.

Congenital hereditary corneal dystrophy (CHED) is a form of corneal endothelial dystrophy that presents at birth.

Dua's layer, according to a 2013 paper by Harminder Singh Dua's group at the University of Nottingham, is a layer of the cornea that had not been detected previously. It is hypothetically 15 micrometres thick, the fourth caudal layer, and located between the corneal stroma and Descemet's membrane. Despite its thinness, the layer is very strong and impervious to air. It is strong enough to withstand up to 2 bars of pressure. While some scientists welcomed the announcement, other scientists cautioned that time was needed for other researchers to confirm the discovery and its significance. Others have met the claim "with incredulity".

Corneal hydrops is an uncommon complication seen in people with advanced keratoconus or other corneal ectatic disorders, and is characterized by stromal edema due to leakage of aqueous humor through a tear in Descemet's membrane. Although a hydrops usually causes increased scarring of the cornea, occasionally it will benefit a patient by creating a flatter cone, aiding the fitting of contact lenses. Corneal transplantation is not usually indicated during corneal hydrops.

Pre Descemet's endothelial keratoplasty (PDEK) is a kind of endothelial keratoplasty, where the pre descemet's layer (PDL) along with descemet's membrane (DM) and endothelium is transplanted. Conventionally in a corneal transplantation, doctors use a whole cornea or parts of the five layers of the cornea to perform correction surgeries. In May 2013, Dr Harminder Dua discovered a sixth layer between the stroma and the descemet membrane which was named after him as the Dua's layer. In the PDEK technique, doctors take the innermost two layers of the cornea, along with the Dua's layer and graft it in the patient's eye.

In ophthalmology, glued intraocular lens or glued IOL is a surgical technique for implantation, with the use of biological glue, of a posterior chamber IOL in eyes with deficient or absent posterior capsules. A quick-acting surgical fibrin sealant derived from human blood plasma, with both hemostatic and adhesive properties, is used.

Descemet membrane endothelial keratoplasty (DMEK) is a method of corneal transplantation. The DMEK technique involves the removal of a very thin sheet of tissue from the posterior (innermost) side of a person's cornea, replacing it with the two posterior (innermost) layers of corneal tissue from a donor's eyeball. The two corneal layers which are exchanged are the Descemet's membrane and the corneal endothelium. The person's corneal tissue is gently excised, peeled off, and replaced with the donor tissue via small 'clear corneal incisions' (small corneal incisions just anterior to the corneal limbus. The donor tissue is tamponaded against the person's exposed posterior corneal stroma by injecting a small air bubble into the anterior chamber. To ensure the air tamponade is effective, it is necessary for people to strictly maintain such a posture that they are looking up at the ceiling during the recovery period until the air bubble has fully resorbed.

Corneal opacification is a term used when the human cornea loses its transparency. The term corneal opacity is used particularly for the loss of transparency of cornea due to scarring. Transparency of the cornea is dependent on the uniform diameter and the regular spacing and arrangement of the collagen fibrils within the stroma. Alterations in the spacing of collagen fibrils in a variety of conditions including corneal edema, scars, and macular corneal dystrophy is clinically manifested as corneal opacity. The term corneal blindness is commonly used to describe blindness due to corneal opacity.

References

↑ Murphy, C; Alvarado, J; Juster, R; Maglio, M (March 1984). "Prenatal and postnatal cellularity of the human corneal endothelium. A quantitative histologic study". Investigative Ophthalmology & Visual Science. 25 (3): 312–22. PMID 6698749.
↑ Wilson, R S; Roper-Hall, M J (1982). "Effect of age on the endothelial cell count in the normal eye". British Journal of Ophthalmology. 66 (8): 513–515. doi:10.1136/bjo.66.8.513. PMC 1039838 . PMID 7104267.
↑ Parikumar, P (2018). "Human corneal endothelial cell transplantation using nanocomposite gel sheet in bullous keratopathy". Am J Stem Cells. 7 (1): 18–24. PMC 5840311 . PMID 29531856.
↑ Kinoshita, S (2018). "Injection of Cultured Cells with a ROCK Inhibitor for Bullous Keratopathy". N Engl J Med. 378 (11): 995–1003. doi: 10.1056/NEJMoa1712770 . PMID 29539291.