These structures are of the variable thickness, in humans up to 0. At the center of the human cornea, there is approximately lamellar thickness. The packing density is higher in the anterior lamellae than in the posterior stroma. These anterior lamellae are highly interwoven and most appear to insert into the Bowman's layer. The posterior lamellae in the central cornea are more hydrated. The posterior stromal can swell easily whereas the more interwoven cannot.
Cornea lamellae in the deepest stroma have preferred directions which appear to close to inferior superior and nasal-temporal directions. X-rays scattering also has suggested a network of collagen lamella that enters the cornea close to the inferior, superior, nasal, and temporal which probably originate in the adjacent sclera.
Collagen fibrils in the prepupillary appear to be more closely packed than in the peripheral cornea. Collagen fibrils within the cornea are narrower than in many other connecting tissues, and there is an important factor for transparency, which is a function of diameter.
There are about — triple-helical molecules within the cross section of the fibril. Ultrastructure within the organization of the lamellar appears to vary based on the depth within the stroma. Deeper layers are more strictly organized than superficial layers. This accounts for care of surgical dissection in a particular plane as one approaches the posterior depths of the corneal stroma.
A high refractive index and symmetric curvature are essential for optimal refractive power and minimum astigmatism of the cornea. The smooth curvature at the anterior side is not affected in conditions with increased corneal hydration. As the diameter of the cornea hardly changes, swelling is compensated by large undulations in Descemet's membrane at the posterior side.
The corneal swelling behavior shows a gradual decrease in swelling from posterior to anterior side; this seems to be related to the organization of collagen lamellae and to the presence of different types of proteoglycan.
In the posterior part keratan sulfate, a more hydrophilic proteoglycan is prevalent, whereas in the anterior part dermatan sulfate, a much less hydrophilic proteoglycan is prevalent.
There is also an elastic fiber network within the stroma. These elastic fibers present throughout the stromal depth, are concentrated below the posterior-most keratocyte layer. Cornea stroma is made up of keratocytes and ECM. The glycosaminoglycans constitute keratan sulfate, chondroitin sulfate, and dermatan sulfate. The corneal stroma has keratocytes and about collagen lamellae which are regularly arranged. The glycosaminoglycans are predominantly keratan sulfate.
Less contribution is by dermatan sulfate and chondroitin sulfate. Hyaluronan is seen in infancy. Keratocytes are the major cell type of stroma. They are involved in maintaining the ECM environment. They are able to synthesize collagen molecules and glycosaminoglycans, while also creating matrix metalloproteinases MMPs , all important in maintaining stromal homeostasis.
Most of the keratocytes reside in the anterior stroma. It is made up of Type IV collagen and laminin. Descemet's membrane begins in utero at the 8-week stage. Endothelial cells continuously secrete Descemet's membrane. Descemet's membrane produced after birth is unbanded and has an amorphous ultrastructural texture. The Descemet's membrane is elastic and curls on getting severed. The cells are hexagonal and are metabolically active.
There is an endothelial pump which regulates water content. Endothelium is a monolayer which appears as a honeycomb-like mosaic when viewed from the posterior side. In early embryogenesis, the posterior surface is lined with a neural crest-derived monolayer of orderly arranged cuboidal cells. Adjacent cells share extensive lateral interdigitations and possess gap and tight junctions along the lateral borders. Activity in both these pathways produces the net flux of ions from the stroma to the aqueous humor.
The basal surface of the endothelium contains numerous hemidesmosomes that promote adhesion to Descemet's membrane. Immediately anterior to the flattened layer of endothelial cell is a discontinuous homogeneous acellular layer Descemet's membrane. Descemet's membrane becomes continuous and uniform, fusing peripheral with the trabecular beams. The fusion site, known as Schwalbe line, is a gonioscopic landmark that defines the end of Descemet's membrane and the start of trabecular meshwork.
Endothelial cell density continues to change throughout life, from second to eight decades of life. Human central endothelial cell density decreases at an average of approximately 0. A deep corneal layer has been getting attention recently. It is a well-defined, acellular, strong layer in the pre-Descemet's cornea.
There are no keratocytes in this layer, and it is impervious to air. In donor eyes from eye banks, the bubble can be inflated to pressure about mmHg before it ruptures. It is also found in children. The cornea is one of the most heavily innervated and most sensitive tissues in the body. The sensation is derived from the nasociliary branch of the first division of ophthalmic of the trigeminal nerve.
Thick and straight stromal nerve trunks extend laterally and anteriorly to give rise to plexiform arrangements of progressively thin nerve fibers at several levels within the stroma. The cornea also contains autonomic sympathetic nerve fibers. Normal human cornea is avascular. The aqueous humor is the main source of nutrients to the cornea. Blood supply is by tiny vessels at the outer edge of the cornea as well as components supplied by end branches of the facial and ophthalmic arteries through the aqueous humor and the tear film.
The minute surface irregularities of the corneal surface epithelium are masked by a smooth and regular overlying tear film. The tear film is the first layer of the cornea with which light comes into contact. In produces lubrication and hydration to the ocular surface. The tear film is traditionally told to be composed of three distinct layers.
The most superficial layer is composed of oily secretions of meibomian glands. Because oil is less dense than water, the secretions float to the top of the tear film to form a smooth refractive surface. This oily layer also provides an important barrier against the evaporation of the tear film. The aqueous middle layer of the tear film lies immediately beneath the oily layer. It is produced from the secretion of the lacrimal gland which is located in the superior lateral orbit. The aqueous is secreted onto the ocular surface from ducts in the superior fornix.
There are also numerous scattered accessory lacrimal glands embedded within the conjunctiva stroma which contribute to this aqueous layer. The innermost mucin layer is produced primarily by the conjunctival goblet cells although the epithelial cells of the cornea and the conjunctiva also contribute.
Current understanding is that all the three layers are in the form of a gel on the ocular surface. Conjunctival goblet cells seem to be important in performing the functions of debris removal and immune surveillance. Thoft and Friend first suggested that cornea, conjunctiva, lacrimal glands, and lids work as an integrated unit called ocular surface. Sclera, in contrast to cornea, has collagen fibrils which are more haphazard and random.
The bulk of stroma is mostly acellular, except for outer episcleral layer. Conjunctiva extends from the limbus to the fornices, forming a fold at the fornices of the eyelid and then extends back around onto the posterior surface of the eyelids. Plica semilunaris is a crescent-shaped fold of conjunctiva medially with thickened stroma. The caruncle is located medial to the plica and marks the most medial aspect of the interpalpebral fissure.
The bulbar conjunctiva is fairly loosely adherent to the underlying anterior Tenon's capsule. The retina is made up by millions of specialised cells known as rods and cones, which work together to transform the image into electrical energy, which is sent to the optic disk on the retina and transferred via electrical impulses along the optic nerve to be processed by the brain.
Donate now Modern slavery and human trafficking statement Contact Cookies High contrast. A A A Type size. Commercial Moorfields Private Moorfields Dubai. It covers the pupil the opening at the center of the eye , iris the colored part of the eye , and anterior chamber the fluid-filled inside of the eye.
The cornea is responsible for focusing most of the light that enters the eye. The cornea is composed of proteins and cells. It does not contain blood vessels, unlike most of the tissues in the human body. Blood vessels may cloud the cornea, which may prevent it from refracting light properly and may adversely affect vision. Since there are no nutrient-supplying blood vessels in the cornea, tears and the aqueous humor a watery fluid in the anterior chamber provide the cornea with nutrients. The first layer, the epithelium, is a layer of cells covering the cornea.
It absorbs nutrients and oxygen from tears and conveys it to the rest of the cornea. It contains free nerve endings. It also prevents foreign matter from entering the eye. The cornea tends to repair itself quickly from minor abrasions. However, deeper abrasions may cause scars to form on the cornea, which causes the cornea to lose its transparency, leading to visual impairment. The superior oblique is a fusiform spindle-shaped muscle belonging to the extraocular group of muscles. It originates near the nose.
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