Eyelash

The eyelashes of the human embryo develop from the ectoderm[5] between the 22nd and 26th week of pregnancy.[6] 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[7] and is therefore not affected by puberty. Lash follicles do not have an arrector pili muscle associated with them, making the lashes static.[8]

Human lashes do not vary in length by sex[9] or ethnicity,[10] with the upper lashes of humans typically 7 to 8 mm in length,[9] and generally do not exceed 10 mm in length.[11] Lower lashes average between 5 and 6 mm in length.[9] Eyelashes are considered to be long when they are 10 mm or greater in length.[12] Human eyelash length, thickness, and darkness decrease significantly with age.[13]

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.[4] The typical width of the human eye is 24.2 mm,[14] 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.[11]

The length of eyelashes in humans vary greatly by individual, but not by sex or ethnicity. Short eyelashes are shown in this subject

Lashes grow thick at the roots, and have tapered ends. People of Asian ethnicities have significantly thicker lashes than people of Caucasian ethnicities.[10] However, the number of lashes on the eyelids is fewer in Asians than in Caucasians.[10]

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.[10] 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.[15]

Human lashes grow in up to six distinct layers on each upper and lower eyelids, in some cases resulting in sets of over 200 lashes surrounding each eye

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.[8]

The follicles of eyelashes are associated with a number of glands known as the glands of Zeis and the glands of Moll.

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".

Green mascara

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.[29]

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.[30] 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.[31]

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.[32] Some cosmetic brands have begun using peptides in their formulation rather than prostaglandins because of regulatory rules in places like Canada and California.[33]