Eyelash | |
---|---|
Details | |
System | Sensory |
Function | Lines the edges of the eyelids, heightening protection of the eye from dust and debris and triggers the blink reflex |
Identifiers | |
Latin | cilium |
Greek | Bλέφαρον (blépharon) |
MeSH | D005140 |
TA98 | A15.2.07.037 |
TA2 | 7057 |
FMA | 53669 |
Anatomical terminology |
An eyelash (also called lash) (Neo-Latin: cilium, plural cilia) is one of the hairs that grows at the edges of the top and bottom eyelids, spanning outwards and away from the eyes. The lashes grow in up to six layers on each of the upper and lower eyelids. [1] Eyelashes serve to protect the eye from debris, dust, and small particles, and are highly sensitive to touch, thus providing a warning that an object (such as an insect or lint) is near the eye, which then reflexively closes or flutters to rid the area of the object. The eyelid margin from which lashes grow is among the most sensitive parts of the human body, with many nerve endings enveloping the roots of the lashes, giving it sensitivity to very light tactile input even at the tips of the lashes, [2] enabling it to trigger the blink reflex when touched. [3] Eyelashes are also an important component of physical attractiveness, with long prominent lashes giving the illusion of large, gazing eyes, [4] and drawing attention to the eyes.
The word eyelash is a compound consisting of the words eye, referring to the eye which the lashes surround and protect; and lash, referring to the braided cord of a whip or to the act of stroking with a whip. This is in reference to both the individual lashes resembling whips, and the action of blinking the eyes causing the lashes to move in a stroking or whipping motion.
In humans, the eyelashes serve four main functions:
The eyelashes of the human embryo develop from the ectoderm [8] between the 22nd and 26th week of pregnancy. [9] Eyelashes take about seven to eight weeks to grow back if pulled out, but constant pulling may lead to permanent damage. Their color may differ from that of the hair, although they tend to be dark on someone with dark hair and lighter on someone with light hair. Eyelash hair is not androgenic [10] and is therefore not affected by puberty. Lash follicles do not have an arrector pili muscle associated with them, making the lashes static. [3]
Human eyelash length is the subject of extensive studies, both for the effect on their functional efficacy, and for their effect on human physical attractiveness. This is also because lashes do not continually grow to long lengths like scalp hair, and the individual variability in their length is almost solely determined by genetics. [11]
While there is considerable variation in the length of lashes between individuals (3 mm to 10 mm for upper lashes), lashes do not vary in length by sex [12] or ethnicity, [13] with the upper lashes of humans typically 7 to 8 mm in length regardless of sex, [12] and generally do not exceed 10 mm in length. [14] Thus, the lashes of humans are considered to be remarkably long when they are 10 mm or greater in length. [15] Lower lashes average between 5 and 6 mm in length. [12] In trichomegaly, the lashes may grow exceptionally long [16] (occasionally 15 mm for the upper lashes). The longest lashes usually grow from the centre of the eyelid. [13] Human eyelash length, thickness, and darkness decrease significantly with age, [17] hence long and thick lashes are sometimes seen as a mark of fertility and youthfulness.
Eyelash length is closely linked to the width of the eye, with the lashes typically growing to one-third the width of the eye as an evolutionary adaptation to reduce tear film evaporation and dust deposition. Lashes longer or shorter than one-third the width of the eye have been shown to have reduced efficacy in serving their function. [5] The typical width of the human eye is 24.2 mm, [18] resulting in the average human eyelash length of 7 to 8 mm. This evolutionary trait is perhaps why lashes in humans rarely grow beyond 10 mm in length. [14] However, longer lashes offer more protection against ultraviolet radiation than their shorter counterparts. [7]
Lashes grow thick at the roots, and have tapered ends. People of Asian ethnicities have significantly thicker lashes, with an average lash diameter of 71.7 μm, than people of Caucasian ethnicities, with have an average lash diameter of 61.0 μm. [13] However, the number of lashes on the eyelids is fewer in Asians, who typically have 150 lash hairs on the upper lids, than in Caucasians, who typically have 170 lash hairs on the upper lid. [13]
Eyelashes grow outwards from both upper and lower eyelids, with varying degrees of curl. People of Caucasian ethnicities have lashes that are more curled than those of Asian ethnicities. [13] However, the degree of curling varies significantly even within a population or ethnicity, where people of Asian ethnicities can also exhibit naturally highly curled lashes. [19]
The number of individual lashes on the upper eyelid is typically 90 to 160, and on the lower eyelid 75 to 80. [2] The number of follicles and hence lashes cannot be increased after birth because all follicles develop during embryogenesis. [3]
The follicles of eyelashes are associated with a number of glands known as the glands of Zeis and the glands of Moll.
There are a number of diseases or disorders involving the eyelashes:
Eyelash transplant surgeries may help to reconstruct lost or damaged lashes. The procedure may also be employed to increase the number of lash follicles on the eyelid to enhance the prominence and fullness of the lashes. The transplanted hairs usually do not have the tapered structure of real lashes, and continue to grow at the same rate as hairs from the area the follicle was extracted from. As such, they require maintenance by means of trimming.
Prominent, luscious eyelashes have long been considered a sign of beauty in many cultures, almost universally, [23] and as such are highly coveted, particularly by women. [24] Having longer and thicker lashes can also have a positive psychological effect on women. [25] Despite not varying by sex and not being a secondary sex characteristic, long, prominent lashes are frequently considered a feminine trait, with studies showing that women possessing longer lashes are perceived as healthier and more feminine. [26] Nevertheless, long lashes are regarded as an attractive facial feature in both males and females. [4] On the other hand, Hadza women are known to trim their own eyelashes. [27] Nonetheless, an average length eyelash is considered the most attractive across Asian, Indian, Black and White faces. [28]
Ancient Romans considered long eyelashes a component of aesthetically ideal eyes. Pliny the Elder wrote that eyelashes fell out from sexual excess, so women desired long lashes as a symbol of chastity. [29] This may have had medical basis as a symptom of syphilis is often alopecia of the lashes, [30] suggesting that individuals with short or sparse lashes were sexually promiscuous.
The traditional Meitei writing system has two letters (symbols), related to eyelashes. The letter "ꯄ" ("Paa") symbolises the human eyelash, and its letter name "Paa" itself means "eyelash" in Meitei language. It has an additional form of letter, known as "Pa Lonsum" ("ꯞ"). [31] [32] [33]
The lashes and lids play a role in eye contact and nonverbal communication. Voluntarily blinking slowly or a few times in succession quickly is a feminine flirtatious behaviour commonly referred to as to "bat an eyelash", "bat/batting eyelashes", or "flutter/fluttering eyelashes".
Because long eyelashes are aesthetically pleasing and seen as a desirable trait that adds to physical attractiveness, some people seek to enhance their eyelash length artificially, by means of eyelash extensions, false lashes attached to the eyelid, cosmetics, or growth products. Additionally, eyelashes which are curled are also more prominent, being able to better display their length especially when viewed from the front. The enhanced appearance of curled eyelashes can be cosmetically achieved by use of eyelash curlers, or eyelash perms. However, these methods do not physically lengthen the lashes.
Kohl, a black putty (usually antimony sulfide or lead sulfide), has been worn as far back as the Bronze Age to darken the edge of the eyelid (just at the bottom of the eyelashes). In Ancient Egypt, it was used as well by the wealthy and the royal to beautify their eyes. Modern eye makeup includes mascara, eyeliner, eye putty, and eye shadow to emphasize the eyes. The twentieth century saw the beginning of convincing false eyelashes, popular in the 1960s.
Permanent eyelash tints and eyelash extensions have also become popular procedures, even in fairly basic salons. It is also possible to get eyelash transplants, which are similar in nature to hair transplantation often done on the head. Since the hair is transplanted from the hair on the head, the new eyelashes will continue to grow like head hair and will need to be trimmed regularly. [34]
Latisse was introduced in the first quarter of 2009 by Allergan as the first drug to receive FDA approval for eyelash growth. Latisse is a solution of bimatoprost, a prostaglandin analog and the active component of the glaucoma medication Lumigan. According to Allergan, noticeable eyelash growth occurs within 16 weeks. Growth is reported to occur primarily on the upper eyelashes, with as much as a 25% increase in length. [35] In addition, the past decade has seen the rapid increase in the development of eyelash conditioners. These conditioners are designed to increase the health and length of lashes. Many utilize seed extract, minerals, and other chemicals to achieve these results. [36]
Cosmetic companies have recently relied on scientific research of prostaglandins and the Wnt/b-catenin signaling pathways to develop eyelash products. Although bimatoprost is effective in promoting increased growth of healthy eyelashes and adnexal hairs, its effectiveness in patients with eyelash alopecia areata is debatable. [37] Some cosmetic brands have begun using peptides in their formulation rather than prostaglandins because of regulatory rules in places like Canada and California. [38]
Lashes, being hair, are found in mammals. Camels' lashes are remarkably long and thick. Horses and cows feature eyelashes as well. Inherited eyelash problems are common in some breeds of dogs as well as horses.
Eyelashes are an uncommon but not unknown feature in birds. Hornbills have prominent eyelashes (vestigial feathers with no barbs), as do ostriches. Amongst the reptiles, only Eyelash vipers show a set of modified scales over the eyes which look much like eyelashes.
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein, notably alpha-keratin.
Intense pulsed light (IPL) is a technology used by cosmetic and medical practitioners to perform various skin treatments for aesthetic and therapeutic purposes, including hair removal, photorejuvenation as well as to alleviate dermatologic diseases such as acne. IPL is increasingly used in optometry and ophthalmology as well, to treat evaporative dry eye disease due to meibomian gland dysfunction.
An eyebrow is an area of short hairs above each eye that follows the shape of the lower margin of the brow ridges of some mammals. In humans, eyebrows serve two main functions: first, communication through facial expression, and second, prevention of sweat, water, and other debris from falling down into the eye socket. It is common for people to modify their eyebrows by means of hair removal and makeup.
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between hormones, neuropeptides, and immune cells. This complex interaction induces the hair follicle to produce different types of hair as seen on different parts of the body. For example, terminal hairs grow on the scalp and lanugo hairs are seen covering the bodies of fetuses in the uterus and in some newborn babies. The process of hair growth occurs in distinct sequential stages: anagen is the active growth phase, catagen is the regression of the hair follicle phase, telogen is the resting stage, exogen is the active shedding of hair phase and kenogen is the phase between the empty hair follicle and the growth of new hair.
A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Surrounding the female nipple, areolar glands are specialized sebaceous glands for lubricating the nipple. Fordyce spots are benign, visible, sebaceous glands found usually on the lips, gums and inner cheeks, and genitals.
Blepharitis, sometimes known as granulated eyelids, is one of the most common ocular conditions characterized by inflammation, scaling, reddening, and crusting of the eyelid. This condition may also cause swelling, burning, itching, or a grainy sensation when introducing foreign objects or substances to the eye. Although blepharitis by itself is not sight-threatening, it can lead to permanent alterations of the eyelid margin. The primary cause is bacteria and inflammation from congested meibomian oil glands at the base of each eyelash. Other conditions may give rise to blepharitis, whether they be infectious or noninfectious, including, but not limited to, bacterial infections or allergies.
An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. "Palpebral" means relating to the eyelids. Its key function is to regularly spread the tears and other secretions on the eye surface to keep it moist, since the cornea must be continuously moist. They keep the eyes from drying out when asleep. Moreover, the blink reflex protects the eye from foreign bodies. A set of specialized hairs known as lashes grow from the upper and lower eyelid margins to further protect the eye from dust and debris.
The management of hair loss, includes prevention and treatment of alopecia, baldness, and hair thinning, and regrowth of hair.
Entropion is a medical condition in which the eyelid folds inward. It is very uncomfortable, as the eyelashes continuously rub against the cornea causing irritation. Entropion is usually caused by genetic factors. This is different from when an extra fold of skin on the lower eyelid causes lashes to turn in towards the eye (epiblepharon). In epiblepharons, the eyelid margin itself is in the correct position, but the extra fold of skin causes the lashes to be misdirected. Entropion can also create secondary pain of the eye. The upper or lower eyelid can be involved, and one or both eyes may be affected. When entropion occurs in both eyes, this is known as "bilateral entropion". Repeated cases of trachoma infection may cause scarring of the inner eyelid, which may cause entropion. In human cases, this condition is most common to people over 60 years of age.
Blepharoplasty is the plastic surgery operation for correcting defects, deformities, and disfigurations of the eyelids; and for aesthetically modifying the eye region of the face. With the excision and the removal, or the repositioning of excess tissues, such as skin and adipocyte fat, and the reinforcement of the corresponding muscle and tendon tissues, the blepharoplasty procedure resolves functional and cosmetic problems of the periorbita, which is the area from the eyebrow to the upper portion of the cheek. The procedure is more common among women, who accounted for approximately 85% of blepharoplasty procedures in 2014 in the US and 88% of such procedures in the UK.
Hair transplantation is a surgical technique that removes hair follicles from one part of the body, called the 'donor site', to a bald or balding part of the body known as the 'recipient site'. The technique is primarily used to treat male pattern baldness. In this minimally invasive procedure, grafts containing hair follicles that are genetically resistant to balding are transplanted to the bald scalp.
Bimatoprost, sold under the brand name Lumigan among others, is a medication used to treat high pressure inside the eye including glaucoma. Specifically it is used for open angle glaucoma when other agents are not sufficient. It may also be used to increase the size of the eyelashes. It is used as an eye drop and effects generally occur within four hours.
An apocrine sweat gland is composed of a coiled secretory portion located at the junction of the dermis and subcutaneous fat, from which a straight portion inserts and secretes into the infundibular portion of the hair follicle. In humans, apocrine sweat glands are found only in certain locations of the body: the axillae (armpits), areola and nipples of the breast, ear canal, eyelids, wings of the nostril, perineal region, and some parts of the external genitalia. Modified apocrine glands include the ciliary glands in the eyelids; the ceruminous glands, which produce ear wax; and the mammary glands, which produce milk. They are distinct from eccrine sweat glands, which cover the whole body.
False eyelashes are a cosmetic enhancement attaching synthetic or natural fibers to the eyelids to give the natural eyelashes a fuller, more dramatic look. They are available in various lengths, thicknesses, and curvatures.
An eyelash curler is a hand-operated cosmetic tool that curls eyelashes. Usually only the upper eyelashes are curled.
Madarosis is a condition that results in the loss of eyelashes, and sometimes eyebrows. The term "madarosis" is derived from the ancient Greek "madaros", meaning "bald". It originally was a disease of only losing eyelashes but it currently is the loss of both eyelashes and eyebrows. Eyebrows and eyelashes are both important in the prevention of bacteria and other foreign objects from entering the eye. A majority of patients with madarosis have leprosy, and it was reported that 76% of patients with varying types of leprosy had madarosis.
Sebaceous carcinoma, also known as sebaceous gland carcinoma (SGc), sebaceous cell carcinoma, and meibomian gland carcinoma is an uncommon malignant cutaneous tumor. Most are typically about 1.4 cm at presentation. SGc originates from sebaceous glands in the skin and, therefore, may originate anywhere in the body where these glands are found. SGc can be divided into 2 types: periocular and extraocular. The periocular region is rich in sebaceous glands making it a common site of origin. The cause of these lesions in the vast majority of cases is unknown. Occasional cases may be associated with Muir-Torre syndrome. SGc accounts for approximately 0.7% of all skin cancers, and the incidence of SGc is highest in Caucasian, Asian, and Indian populations. Due to the rarity of this tumor and variability in clinical and histological presentation, SGc is often misdiagnosed as an inflammatory condition or a more common neoplasm. SGc is commonly treated with wide local excision or Mohs micrographic surgery, and the relative survival rates at 5 and 10 years are 92.72 and 86.98%, respectively.
Trichomegaly is a condition in which the eyelashes are abnormally long, objectively defined as 12mm or greater in the central area and 8mm in the peripheral. The term was first used by H. Gray in 1944 in a publication in the Stanford Medical Bulletin, though he was only the third person to characterize the disorder; the first two reports were published in German in 1926 and 1931 by Reiter and Bab, respectively. Gray suggested the use of the term "movie lashes" to describe this condition, for long lashes were at the time being portrayed in film as a desirable characteristic in women.
The growth of human hair occurs everywhere on the body except for the soles of the feet, the palms of the hands, the inside of the mouth, the lips, the backs of the ears, some external genital areas, the navel, and, apart from eyelashes, the eyelids. Hair is a stratified squamous keratinized epithelium made of multi-layered flat cells whose rope-like filaments provide structure and strength to the hair shaft. The protein called keratin makes up hair and stimulates hair growth. Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair.
The accessory visual structures are the protecting and supporting structures (adnexa) of the eye, including the eyebrow, eyelids, and lacrimal apparatus. The eyebrows, eyelids, eyelashes, lacrimal gland and drainage apparatus all play a crucial role with regards to globe protection, lubrication, and minimizing the risk of ocular infection. The adnexal structures also help to keep the cornea moist and clean.
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