Sclera
This article may be too technical for most readers to understand.(February 2020) |
This article about biology may be excessively human-centric. |
The sclera,[note 1] also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber.[2]
In the development of the embryo, the sclera is derived from the neural crest.[3] 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. People with dark skin can have naturally darkened sclerae, the result of melanin pigmentation.[4]
In
Structure
The sclera forms the posterior five-sixths of the
In many
The eyes of all non-human primates have been thought to be dark with small, barely visible sclera, but recent research has suggested that white sclera are not uncommon in chimpanzees, and are also present in other mammals.[8]
Histology
Histologically speaking, it is characterized as dense connective tissue made primarily of type 1 collagen fibers.[9] The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are:
- episclera
- stroma
- lamina fusca
- endothelium
The sclera is opaque due to the irregularity of the Type I[10] collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils.
The cornea, unlike the sclera, has six layers. The middle (third in number; provided the first layer being the anterior and outermost and the sixth layer being the posterior and the inner most), 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.[6]
Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.
Function
Human eyes are somewhat distinctive in the
Another hypothesis states that the distinctiveness of the human's sclera is due to genetic drift and sexual selection. A visibly white sclera is perceived as a sign of good health and youthfullness,[13] this could have been a criteria of selection when looking for a mate. In this case, humans' ability to communicate with their eyes (glancing, cluing at others) would only be a consequence of a very visible sclera.[14]
Trauma
The bony area that makes up the human eye socket provides exceptional protection to the sclera. However, if the sclera is ruptured by a blunt force or is penetrated by a sharp object, the recovery of full former vision is usually rare. If pressure is applied slowly, the eye is actually very elastic. However, most ruptures involve objects moving at some velocity. The cushion of orbital fat protects the sclera from head-on blunt forces, but damage from oblique forces striking the eye from the side is not prevented by this cushion. Hemorrhaging and a dramatic drop in intraocular pressure are common, along with a reduction in visual perception to only broad hand movements and the presence or absence of light. However, a low-velocity injury which does not puncture and penetrate the sclera requires only superficial treatment and the removal of the object. Sufficiently small objects which become embedded and which are subsequently left untreated may eventually become surrounded by a benign cyst, causing no other damage or discomfort.[15]
Thermal trauma
The sclera is rarely damaged by brief exposure to heat: the eyelids provide exceptional protection, and the fact that the sclera is covered in layers of moist tissue means that these tissues are able to cause much of the offending heat to become dissipated as steam before the sclera itself is damaged. Even relatively low-temperature molten metals when splashed against an open eye have been shown to cause very little damage to the sclera, even while creating detailed casts of the surrounding eyelashes. Prolonged exposure, however—on the order of 30 seconds—at temperatures above 45 °C (113 °F) will begin to cause scarring, and above 55 °C (131 °F) will cause extreme changes in the sclera and surrounding tissue. Such long exposures even in industrial settings are virtually nonexistent.[15]
Chemical injury
The sclera is highly resistant to injury from brief exposure to toxic chemicals. The reflexive production of tears at the onset of chemical exposure tends to quickly wash away such irritants, preventing further harm. Acids with a pH below 2.5 are the source of greatest acidic burn risk, with sulfuric acid, the kind present in car batteries and therefore commonly available, being among the most dangerous in this regard. However, acid burns, even severe ones, seldom result in loss of the eye.[15]
Alkali burns, on the other hand, such as those resulting from exposure to
Abnormal coloring
Redness of the sclera is typically caused by
Yellowing or a light green color of the sclera is a visual symptom of jaundice.
In cases of
In those with
In very rare but severe cases of kidney failure and liver failure, the sclera may turn black.[citation needed]
See also
Notes
- ^ The word sclera (/ˈsklɛərə/ or /ˈsklɪərə/; both are common), plural sclerae (/ˈsklɛəri/ or /ˈsklɪəri/) or scleras, is from the Greek skleros, meaning hard.[1]
References
- ISBN 978-0815161134.
- ISBN 978-0937404331.
- ^ Hermann D. Schubert. Anatomy of the Orbit "New York Eye and Ear Infirmary of Mount Sinai - New York City - NYEE" (PDF). Archived from the original (PDF) on 2008-10-08. Retrieved 2008-05-19.
- ^ Mukamal, Reena (30 July 2020). "Why Are the Whites of My Eyes Discolored?". American Academy of Ophthalmology. Retrieved 11 December 2020.
- S2CID 256314941.
- ^ Encyclopædia Britannica 2006 Ultimate Reference Suite DVD2009
- ISBN 978-0-03-910284-5.
- PMID 36706647.
- PMID 31335063, retrieved 2023-11-11
- PMID 6519194.
- ^ Michael Tomasello, Brian Hare, Hagen Lehmann, Josep Call. "Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis" http://www.chrisknight.co.uk/wp-content/uploads/2008/06/eyes-cooperation.pdf
- BBC2.
- ^ Russell, R., Sweda, J. R., Porcheron, A., & Mauger, E. (2014). Sclera color changes with age and is a cue for perceiving age, health, and beauty. Psychology and Aging, 29(3), 626–635. https://doi.org/10.1037/a0036142
- ^ Caspar, K.R., Biggemann, M., Geissmann, T. et al. Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions. Sci Rep 11, 12994 (2021). https://doi.org/10.1038/s41598-021-92348-z
- ^ ISBN 978-1-907816-07-9.
- ^ Asanad, S., Bayomi, M., Brown, D., Buzzard, J., Lai, E., Ling, C., Miglani, T., Mohammed, T., Tsai, J., Uddin, O., & Singman, E. (2022). Ehlers-Danlos syndromes and their manifestations in the visual system. Frontiers in Medicine, 9. https://doi.org/10.3389/fmed.2022.996458
External links
- Histology image: 08008loa – Histology Learning System at Boston University
- Atlas image: eye_1 at the University of Michigan Health System—"Sagittal Section Through the Eyeball"
- MedlinePlus Encyclopedia: 002295