Massachusetts Eye and Ear. Flaum Eye Institute

 Definitions:

Retina: the layer of nerve cells lining the back wall inside the eye that senses light. The retina communicates with your brain to help you see. The retina, the inside surface of the eye, is made up of three layers of cells. The layer closest to the external surface of the eyeball consists of two kinds of photoreceptor cells; the rods and cones. Rods, which get their name from the shape of their outer segment, are responsible for black and white vision when the illumination is dim. In contrast, cones are responsible for high visual acuity (sharpenss) and oclar vision. Cones have a cone shaped outer segment. The next layer contains bipolar cells and the layer closest to the cavity of the eye is composed of ganglion cells. Thus, light must first pass through the ganglion cells and bipolar cells in order to reach the photoreceptors. The rods and cones synapse with the bipoplar cells, and the bipolar cells synapse with the ganglion cells, which transmit impulses to the brain via the optic nerve. Ganglion cells are the only neurons of the retina capable of sending action potentials to the brain. The flow of sensory information in the retina is thus opposite to the path of light through the retina.

Macula: An area in the center of the retina that helps you see objects in front of your clearly.

Cornea: the window in the front of your eye that focuses light.

The cornea is a transparent tissue that works as a “lens” within the eye to focus light onto the retina. Consequently, it must retain its transparency if it is to serve this function. This transparency is maintained by the corneal endothelium, which regulates water flow between the aqueous humor and the corneal stroma by pump-and-leak barrier functions. However, the corneal endothelia cells (CECs) that perform this function have severely limited proliferative capacity, so any severe damage to the corneal endothelium, such as that arising from pathological conditions like Fuchs endothelia corneal dystrophy or from iatrongenic damage during cataract surgery, cuases irreversible cell loss. (Koizumi, “Feasibility of a cryopreservation of cultured human corenal endothelial cells, PLOS One, June 21, 2019).

Cornea endothelium (CE):

The CE plays a critical role in the regulation of corneal hydration, maintaining corneal thickness and keeping the cornea transparent. The human CE has very limited propensity to proliferate in vivo. Accordingly, in order to replace dead of damaged corneal endothelial cells (CECs), the existing cells spread out to maintain functional integrity and sustain corneal deturgenscence. In a situation where an individual experiences accelerated or acute corneal endothelail cell loss due to either accidental or surgical trauma, endothelial dysfunction of the CE layer may occur. This results in their inability to pump fluid out of the stroma, causing stromal and epitheilial edema, loss of corneal clarity and visual acuity and will eventually lead to the clinical condition of bullous keratopathy. (Mehta, “Cultivation of human corneal endothelial cells isolated form paired donor corneas” 2011).

Optic nerve: the nerve in the back of your eye that sends signals to your brain.

Dilation: a procedure in which a doctor uses eye drops to widen your pupil, allowing for a close look inside your eye.

Legal blindness: when vision can not be made better than 20/200, meaning that you have to be 20 feet or lower to see an object that someone with normal vision could see 200 feet away.

Sensory Transduction in Photoreceptors:

The transduction of light energy into nerve impulses follows a sequence that is the opposite of the usual way that sensory stimuli are detected. In the dark, the photoreceptor cells release an inhibitory neurotransmitter that hyperpolirizes the bipolar neurons. This prevents the bipolar neurons from releasing excitatory neurotransmitter to the ganglion cells that signal to the brain. The production of inhibitory neurotransmitter by photoreceptor cells is due to the presence of ligand gated Na+ channels. In the dark, many of these channels are open, allowing an influx of Na+ which depolarizes the membrane of photoreceptor cells. In this state, the cells produce inhibitory neurotrasmitter that hyperpolarizes the membrane of bipolar cells.

In the light, the process works in the opposite way. When a photopigment absorbs light, cis-retinal isomerizes and dissociates from the receptor protein, opsin, in which is known as the bleaching reaction. As a result of this dissociation, the opsin receptor protein changes shape, activating its associated G protein. The activated G protein then activates its effector protein, phosphodiesterase, which cleaves cGMP to GMP. The loss of cGMP causes the cGMP gated Na+ channels to close, reducing the dark current. Each opsin is associated with over 100 regulatory G proteins, which, when activated, release subunits that activate hundreds of molecules of the phosphodiesterase enyme. Each enzyme molecule can ocvert thousands of cGMP to GMP, closing the Na+ channels at a rate of about 1000 per soenc and inhibiting the dark current. The absorption of a single photon of light can block the entry of mroe than a million Na+, whichout changing K+ permeability. The photoreceptor becomes hyperpolirzed and releases less inhibitory neurotransmitter. Freed from inhibition, the bipolar cells release excitatory neurotransmitter to the ganglion cells, which send impulses ot the brain.

Evolution of the Eye:

In the early 19902, biologists studied the development of the eye in both vertebrates and insects. In each case, a gene was discovered that codes for a transcription factor important in lens formation; the mouse gene was called Pax6 and the fly gene was termed eyeless. A mutation in the eless gene led to a lack of production of the transcription factor and compelte absence of eye development, giving the gene its name. When the genes were sequenced, it became apparent that they were highly similar. In fact, insertion of the mouse version of the Pax6 into the genome of a fuit fy created a transgene fly. In this fly, the Pax6 gene was turned on by regulatory factors in the fly’s leg and an eye formed ont he leg of the fly.

Common Vision Disorders

The incidence of vision related disorders increases significantly with advancing age. The most serious age related disorders include cataract, glaucoma, and age-related macular degeneration (AMD). Collectively, these three diseases account for about 65% of new cases of legal blindness in the U.S. 8.4 million office visits and 426k hospitalizations annually are attributable to these and related conditions (WO 95/17673).

Age-Related Macular Degeneration (AMD)

Cataract is associated with an increase in the pressure inside of the eye “intraocular pressure or IOP” which eventually results in the degeneration of a portion or the retina. Once detected, glaucoma usually can be arrested by treatment with pressure lowering drugs. Delayed detection and treatment, however, leads to impaired vision and eventual blindess. Glaucoma can be effectively treated by replacing diseased lens with a plastic intraocular lens or IOL.

The number one reason you have cataracts is age. By 50, we can usually start to see some changes in the transparency of the lens that are an indication of early cataracts.

Corneal endothelia decompensation:

In the past the only treatment for corneal endothelial decompensation was transplantation of a donor corea. However, tissue engineering technology is now receiving increased attention as a way to overcome problems of corneal transplantations, which include a shortage of donor corneas, late graft failure due to continuous cell loss, graft rejection and the learning curve involved in performing corneal transplant procedures. (Koizumi, “Feasibility of a cryopreservation of cultured human corenal endothelial cells, PLOS One, June 21, 2019).

Koizumi, (“Feasibility of a cryopreservation of cultured human corneal endothelial cells, PLOS One, June 21, 2019) disclosed in 2013 cell based therapy involving injection of a suspension of cultured human corneal endothelial cells (HCECs) in combination with a Rho kinase inhibitor, into the anterior chamber. All 11 cases recovered corneal transparency and none experience any severe adverse effects. The HCECs were obtained form donor corneas and expanded in vitro culture. The HCECs were harvested form a culture plate, placed in a tube in the form of a cell suspension and immediately transported to the operating room in the same facility. (Koizumi, “Feasibility of a cryopreservation of cultured human corenal endothelial cells, PLOS One, June 21, 2019).

–Cell Culture of human corneal endothelial cells (HCECs): 

Mehta, (“Cultivation of human corneal endothelial cells isolated form paired donor corneas” 2011) discloses culturing isolated primary hCECs in 4 different medium, expanding the cells for two passages and anlyzing them for their propensity to adhere and proliferate., their expression of characteristic corneal endothelium markers; Na+K+/ATPase and ZO-1 and (3) their cellular morphology. hCECs established in the four media exhibited different proliferation profiles with striking norphological differences. Corneal endothelial cells cultured in M1 and M3 could not be propagated beyond the frist and secone passage, respectively. The hCECs cultured in M2 and M4 were significant more proliferative and epxressed markers characteristic of human coneal endothelium.

–Cryopreservation of human corneal endothelial cells (HCECs): 

Koizumi, “Feasibility of a cryopreservation of cultured human corenal endothelial cells, PLOS One, June 21, 2019) further screened several cyroperservation reagents for their effectiveness in preserving HCECs. Bambanker hRM enabled the cryopreservation of HCECs by maintaing cell viability and cell density.

Glaucomas are a group of neuropathic eye diseases characterized by increased intraocular pressure (IOP) and damage to the optic nerve.  Glaucoma can lead to reduction or loss of vision, and is the second leading cause of blindness. Current treatments involve reduction of intraocular pressure by chemical or mechanical means. For example, the most common treatment is trabeculectomy, a surgical procedure in which part of the trabecular meshwork of the eye is removed to allow drainage of the aqueous humor out of the eye into the conjunctiva, thereby reducing intraocular pressure.  However, trabeculectomy is generally followed by inflammation and fibrosis.

A comprehensive dilated-eye exam is the best way to detect glaucoma. Every adult over 50 should get this test every year, but if you have a fmaily history of glaucoma, you may need the test more often. If glacoma is caught early, it is a very treatable disease. When diagnosied late or not at all, glaucoma is extremely aggressive because the optic nerve is already damaged, and a damanged nerve is more vulnerable. Later-stage glaucoma attacks central vision and can cause irreversible blindess.

Corneal Neovascularization (KNV)

The cornea bears an “angiogenic privilege” and is avascular allowing maximal entry of incident light. This angiogenic privilege is maintained by a fine balance between anti-angiogenic factors and angiogenic factors in the cornea. Insults of chemical, mechanical or infectious nature can trigger inflammatory and immune-mediated pathways which upregulate expression of VEGF (vascular endothelial growth factor), the key player in KNV, and its signalling cascades. Common infectious diseases associated with KNV are the following:

1. Aspergillosis, Candidiasis, Chamydia trachomatis, Fusarium, Herpes simplex keratisis:

Herpes stromal keratitis (recurrent infection on the cornea by herpes simplex virus) is the most common cause of infectious corneal blindness in the western world. In the US an estimated 400,000 persons are affected, with 20,000 new cases occurring annually. Each episode increases the risk of future episodes. Current treatment consists of topical steroids in addiiton to prophylactice oral (acyclovir or valacyclovir) or topical (trifuridine) anti-viral drug therapy. Despite this treatment, patients develop severe corneal scarring due to repeated episodes of the disease, which often require corneal transplantaiton.

2. Onchocerciasis, Pseudomonas, Syphillis:

Common inflammatory conditions assocaited with KNV are graft rejection, acne rosacea, Stevens-Johnson syndrome, GVHD, Pemphigoid and Atopic conjunctivitis.

Diabetic Retinopathy:

Uncontrolled, elevated blood sugar damages blood vessels in the retina, cuasing a leakage of fluid into these tissues, a conditions called diabetic retinopathy. The early stage of the disease can be controlled with lifestyle changes by keeping A1c (a diabetes test result) below 6.5 with diet and exercise, as well as maintaining healthy blood pressue and lipid levels.

Diabetic retinopathy is the top cause of blindness in working age adults in the U.S. The later, more symptomatic stage of DR is called profliferative, in which new, but weeak, blood vessels grow in the retina to compensate for those that were damanged by blood sugar. When caught at this later stage, when symptoms like burriness or glare arise, you may need additional eye injections or even surgery.

Ocular Fibrosis:

Treatments for Ocular Fibrosis:

A proposed method of treatment of ocular fibrosis is inhibition of certain lysyl oxidase-type enzymes which have been found to occur in parallel with the fibrotic damage that can follow trabeculectomy.  A lysyl oxidase-type enzyme refers to a member of a family of proteins that catalyzes formation of aldehydes from lysine residues in collagen and elastin precursors. The aldehyde residues of allysine are reactive and can spontaneously condense with other allysine and lysine residues, resulting in crosslinking of collagen molecules to form collagen fibrils. The first member of this family to be isolated was lysyl oxidase, also known as protein-lysine 6-oxidase, protein-L lysine or LoX. Additional lysyl oxidase type enzymes were subsequently discovered and termed LOX-like or LOXL Although all lysyl oxidase-tyep enzymes share a common catalytic domain, they differ, particularly in their amino terminal regions.

Chemical Injiries of the eye: are opthalmic emergiceis that require immediate evaluation adn management. These in juries often result in significant occular morbidity. Injireis caused by alkali agents are more common and generally more serious than those casued by acids. There are three main goals in manageing a chemcial injury,: enhance recovery of the corneal epithelium; augment collagen synthesis while minimizing collagen breakdown and sterile ulceration and control inflammation. (US 2020/0289580).

Rare Eye Diseases

Purtscher’s retinopathy: is a rare retinal disorder characterized by acute visual loss and retinal findings such as cotton-wool spots, intraretinal hemorrhages and retinal whitening following head or chest trauma. When the etiology is not a trauma, the disease is called Purtscher-like retinopathy. Nuermous conditions such as acute pancreatitis, connetive tissue disorders, autoimmune diseases, pregnancy-related diseases, and thrombotic microangiopathic diseases can cause Purtscher-like retinopathy. It has also been associated with aHUS (Turk J. Ophthalmol 47; 6: 2017).