Sclera
Overview
 
The sclera also known as the white or white of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer 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...

 containing collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 and elastic fiber. In the development of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

, the sclera is derived from the neural crest
Neural 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....

. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue.
Encyclopedia
The sclera also known as the white or white of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer 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...

 containing collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 and elastic fiber. In the development of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

, the sclera is derived from the neural crest
Neural 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....

. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow.

Human eyes are somewhat distinctive in the animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 kingdom in that the sclera is very plainly visible whenever the eye is open. This is not just due to the white color of the human sclera, which many other species share, but also to the fact that the human iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 is relatively small and comprises a significantly smaller portion of the exposed eye surface compared to other animals. It is theorized that this adaptation evolved because of our social
Social animal
A social animal is a loosely defined term for an organism that is highly interactive with other members of its species to the point of having a recognizable and distinct society.All mammals are social to the extent that mothers and offspring bond...

 nature as the eye became a useful communication
Animal communication
Animal communication is any behavior on the part of one animal that has an effect on the current or future behaviour of another animal. The study of animal communication, is sometimes called Zoosemiotics has played an important part in the...

 tool in addition to a sensory organ. It is believed that the conspicuous sclera of the human eye makes it easier for one individual to infer where another individual is looking, increasing the efficacy of this particular form of nonverbal communication
Nonverbal communication
Nonverbal communication is usually understood as the process of communication through sending and receiving wordless messages. Messages can be communicated through gestures and touch , by body language or posture, by facial expression and eye contact...

. Animal researchers have also found that, in the course of their domestication, dogs have also developed the ability to pick up visual cues from the eyes of humans, making them one of only two species known to seek visual cues from another individual's eyes. Dogs do not seem to use this form of communication with one another and only look for visual information from the eyes of humans.

Structure

The sclera forms the posterior five-sixths of the connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 coat of the globe
Globe (human eye)
The globe of the eye, or bulbus oculi, is the eyeball apart from its appendages. A hollow structure, the bulbus oculi is composed of a wall enclosing a cavity filled with fluid with three coats: the Sclera, Choroid, and the Retina. Normally, the bulbus oculi is bulb-like structure. However, the...

. It is continuous with the dura mater
Dura mater
The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from Mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for...

 and 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 maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions. The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

. At the optic disc
Optic disc
The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...

 the outer two-thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve. The inner third joins with some choroid
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...

al tissue to form a plate (lamina cribrosa
Lamina cribrosa sclerae
The nerve fibers forming the optic nerve exit the eye posteriorly through a hole in the sclera that is occupied by a mesh-like structure called the lamina cribrosa. It is formed by a multilayered network of collagen fibers that insert into the scleral canal wall. The nerve fibers that comprise the...

) across the optic nerve with perforations through which the optic fibers (fasciculi
Nerve fascicle
A small bundle of nerve fibers, enclosed by the perineurium, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are collected together into larger bundles or nerve fascicles, which are bound together in a common membranous...

) pass. The thickness of the sclera varies from 1mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. The sclera's blood vessels are mainly on the surface. Along with the vessels of the conjunctiva
Conjunctiva
The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...

 (which is a thin layer covering the sclera), those in the episclera
Episclera
The episclera is the outermost layer of the sclera. It is composed of loose, fibrous, elastic tissue and attaches to Tenon's capsule.A vascular plexus is found between the conjunctiva and the sclera consisting of two layers of vessels, the superficial episcleral vessels and the deep episcleral...

 render the inflamed eye bright red.

In many vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s, the sclera is reinforced with plates of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 or bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

, together forming a circular structure called the scleral ring. In primitive fish, this ring consists of four plates, but the number is lower in many living ray-finned fishes, and much higher in lobe-finned fishes, various reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s. The ring has disappeared in many groups, including living amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, some reptiles and fish, and all mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s.

The eyes of all non-human primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s are dark with small, barely visible sclera. See Cooperative eye hypothesis
Cooperative eye hypothesis
The cooperative eye hypothesis is a proposed explanation for the appearance of the human eye. It suggests that the eye's distinctive visible characteristics evolved to make it easier for humans to follow another's gaze while communicating or while working together on tasks.-Differences in primate...


Histology

The collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 of the sclera is continuous with 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...

. From outer to innermost, the four layers of the sclera are:
  • episclera
    Episclera
    The episclera is the outermost layer of the sclera. It is composed of loose, fibrous, elastic tissue and attaches to Tenon's capsule.A vascular plexus is found between the conjunctiva and the sclera consisting of two layers of vessels, the superficial episcleral vessels and the deep episcleral...

  • stroma
    Stroma of iris
    The stroma of the iris consists of fibers and cells.The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement delicate meshes in which the vessels...

  • lamina fusca
    Suprachoroid lamina
    The choroid consists mainly of a dense capillary plexus, and of small arteries and veins carrying blood to and returning it from this plexus.On its external surface is a thin membrane, the suprachoroid lamina, composed of delicate non-vascular lamellae—each lamella consisting of a network of fine...

  • endothelium
    Corneal endothelium
    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....



The sclera is opaque due to the irregularity of the collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide
Glycosaminoglycan
Glycosaminoglycans or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit consists of a hexose or a hexuronic acid, linked to a hexosamine .-Production:Protein cores made in the rough endoplasmic reticulum are posttranslationally...

 (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils.

The cornea, unlike the sclera, has 5 layers. The middle, thickest layer is also called the stroma. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells.

Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.

Additional images

External links

- "Sagittal Section Through the Eyeball"
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