{"id":4257,"date":"2017-03-28T18:34:03","date_gmt":"2017-03-28T18:34:03","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=4257"},"modified":"2024-04-26T02:41:44","modified_gmt":"2024-04-26T02:41:44","slug":"hair","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/hair\/","title":{"raw":"Hair","rendered":"Hair"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Describe the structure and function of hair<\/li>\r\n<\/ul>\r\n<\/div>\r\n\r\n[caption id=\"attachment_4268\" align=\"alignright\" width=\"400\"]<img class=\"wp-image-4268\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/28185157\/506_Hair.jpg\" alt=\"This diagram shows a cross section of the skin containing a hair follicle. The follicle is teardrop shaped. Its enlarged base, labeled the hair bulb, is embedded in the hypodermis. The outermost layer of the follicle is the epidermis, which invaginates from the skin surface to envelope the follicle. Within the epidermis is the outer root sheath, which is only present on the hair bulb. It does not extend up the shaft of the hair. Within the outer root sheath is the inner root sheath. The inner root sheath extends about half of the way up the hair shaft, ending midway through the dermis. The hair matrix is the innermost layer. The hair matrix surrounds the bottom of the hair shaft where it is embedded within the hair bulb. The hair shaft, in itself, contains three layers: the outermost cuticle, a middle layer called the cortex, and an innermost layer called the medulla.\" width=\"400\" height=\"414\" \/> Figure 1. Hair follicles originate in the epidermis and have many different parts.[\/caption]\r\n\r\n<strong>Hair<\/strong> is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the <strong>hair follicle<\/strong>. The <strong>hair shaft<\/strong> is the part of the hair not anchored to the follicle, and much of this is exposed at the skin\u2019s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the <strong>hair root<\/strong>. The hair root ends deep in the dermis at the <strong>hair bulb<\/strong>, and includes a layer of mitotically active basal cells called the <strong>hair matrix<\/strong>. The hair bulb surrounds the <strong>hair papilla<\/strong>, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis (Figure 1).\r\n\r\nJust as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The <strong>medulla<\/strong> forms the central core of the hair, which is surrounded by the <strong>cortex<\/strong>, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the <strong>cuticle<\/strong>. These layers are depicted in a longitudinal cross-section of the hair follicle (Figure 2), although not all hair has a medullary layer.\r\n\r\n[caption id=\"attachment_2850\" align=\"alignright\" width=\"400\"]<img class=\"wp-image-2850\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/08171036\/511_Hair_Follicle.jpg\" alt=\"This micrograph is of the base of a hair follicle. The protruding hair is largely transparent, with only its dark outline visible. The inner root sheath is visible surrounding the very bottom of the hair as a circle of cells with dark-staining nuclei. The inner sheath extends up the hair shaft. The outer root sheath is much thicker than the inner root sheath, consisting of a large oval of lighter staining cells. The oval surrounds the bottom of the hair and extends into the hypodermis.\" width=\"400\" height=\"278\" \/> Figure 2. The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM \u00d7 4. (credit: modification of work by \u201ckilbad\u201d\/Wikimedia Commons)[\/caption]\r\n\r\nHair texture (straight, curly) is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible. The external hair is completely dead and composed entirely of keratin. For this reason, our hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to destroy the hair bulb so hair cannot grow.\r\n\r\nThe wall of the hair follicle is made of three concentric layers of cells. The cells of the <strong>internal root sheath<\/strong> surround the root of the growing hair and extend just up to the hair shaft. They are derived from the basal cells of the hair matrix. The <strong>external root sheath<\/strong>, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of the hair root and tends to be more keratinous in the upper regions. The <strong>glassy membrane<\/strong> is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis.\r\n<div class=\"textbox\">\r\n\r\nThe hair follicle is made of multiple layers of cells that form from basal cells in the hair matrix and the hair root. Cells of the hair matrix divide and differentiate to form the layers of the hair. Watch this video to learn more about hair follicles.\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1613496\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/aLY4EzDXwSM?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n\r\n<\/div>\r\nHair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. Each hair root is connected to a smooth muscle called the <strong>arrector pili<\/strong> that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft \u201cstand up.\u201d The primary purpose for this is to trap a layer of air to add insulation. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats.\r\n<h2>Hair Growth<\/h2>\r\nHair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the <strong>anagen<\/strong> phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The <strong>catagen<\/strong> phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle\u2019s active growth. Finally, during the <strong>telogen<\/strong> phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix then produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones.\r\n<h2>Hair Color<\/h2>\r\nSimilar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and\/or white.\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/3bb606be-18b9-4cc4-906b-b19be95d8017\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Describe the structure and function of hair<\/li>\n<\/ul>\n<\/div>\n<div id=\"attachment_4268\" style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-4268\" class=\"wp-image-4268\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/28185157\/506_Hair.jpg\" alt=\"This diagram shows a cross section of the skin containing a hair follicle. The follicle is teardrop shaped. Its enlarged base, labeled the hair bulb, is embedded in the hypodermis. The outermost layer of the follicle is the epidermis, which invaginates from the skin surface to envelope the follicle. Within the epidermis is the outer root sheath, which is only present on the hair bulb. It does not extend up the shaft of the hair. Within the outer root sheath is the inner root sheath. The inner root sheath extends about half of the way up the hair shaft, ending midway through the dermis. The hair matrix is the innermost layer. The hair matrix surrounds the bottom of the hair shaft where it is embedded within the hair bulb. The hair shaft, in itself, contains three layers: the outermost cuticle, a middle layer called the cortex, and an innermost layer called the medulla.\" width=\"400\" height=\"414\" \/><\/p>\n<p id=\"caption-attachment-4268\" class=\"wp-caption-text\">Figure 1. Hair follicles originate in the epidermis and have many different parts.<\/p>\n<\/div>\n<p><strong>Hair<\/strong> is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the <strong>hair follicle<\/strong>. The <strong>hair shaft<\/strong> is the part of the hair not anchored to the follicle, and much of this is exposed at the skin\u2019s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the <strong>hair root<\/strong>. The hair root ends deep in the dermis at the <strong>hair bulb<\/strong>, and includes a layer of mitotically active basal cells called the <strong>hair matrix<\/strong>. The hair bulb surrounds the <strong>hair papilla<\/strong>, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis (Figure 1).<\/p>\n<p>Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The <strong>medulla<\/strong> forms the central core of the hair, which is surrounded by the <strong>cortex<\/strong>, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the <strong>cuticle<\/strong>. These layers are depicted in a longitudinal cross-section of the hair follicle (Figure 2), although not all hair has a medullary layer.<\/p>\n<div id=\"attachment_2850\" style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2850\" class=\"wp-image-2850\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/08171036\/511_Hair_Follicle.jpg\" alt=\"This micrograph is of the base of a hair follicle. The protruding hair is largely transparent, with only its dark outline visible. The inner root sheath is visible surrounding the very bottom of the hair as a circle of cells with dark-staining nuclei. The inner sheath extends up the hair shaft. The outer root sheath is much thicker than the inner root sheath, consisting of a large oval of lighter staining cells. The oval surrounds the bottom of the hair and extends into the hypodermis.\" width=\"400\" height=\"278\" \/><\/p>\n<p id=\"caption-attachment-2850\" class=\"wp-caption-text\">Figure 2. The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM \u00d7 4. (credit: modification of work by \u201ckilbad\u201d\/Wikimedia Commons)<\/p>\n<\/div>\n<p>Hair texture (straight, curly) is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible. The external hair is completely dead and composed entirely of keratin. For this reason, our hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to destroy the hair bulb so hair cannot grow.<\/p>\n<p>The wall of the hair follicle is made of three concentric layers of cells. The cells of the <strong>internal root sheath<\/strong> surround the root of the growing hair and extend just up to the hair shaft. They are derived from the basal cells of the hair matrix. The <strong>external root sheath<\/strong>, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of the hair root and tends to be more keratinous in the upper regions. The <strong>glassy membrane<\/strong> is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis.<\/p>\n<div class=\"textbox\">\n<p>The hair follicle is made of multiple layers of cells that form from basal cells in the hair matrix and the hair root. Cells of the hair matrix divide and differentiate to form the layers of the hair. Watch this video to learn more about hair follicles.<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1613496\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/aLY4EzDXwSM?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<p>Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. Each hair root is connected to a smooth muscle called the <strong>arrector pili<\/strong> that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft \u201cstand up.\u201d The primary purpose for this is to trap a layer of air to add insulation. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats.<\/p>\n<h2>Hair Growth<\/h2>\n<p>Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the <strong>anagen<\/strong> phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The <strong>catagen<\/strong> phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle\u2019s active growth. Finally, during the <strong>telogen<\/strong> phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix then produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones.<\/p>\n<h2>Hair Color<\/h2>\n<p>Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and\/or white.<\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_3bb606be-18b9-4cc4-906b-b19be95d8017\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/3bb606be-18b9-4cc4-906b-b19be95d8017?iframe_resize_id=assessment_practice_id_3bb606be-18b9-4cc4-906b-b19be95d8017\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-4257\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Anatomy &amp; Physiology. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.79\">http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.79<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.79<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":9,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Anatomy & Physiology\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.79\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.79\"}]","CANDELA_OUTCOMES_GUID":"939a0072-a991-4dcd-b47e-b607cc13a258, 421bd267-d711-4e1b-a032-45535ba4c26b","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-4257","chapter","type-chapter","status-publish","hentry"],"part":3797,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4257","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":11,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4257\/revisions"}],"predecessor-version":[{"id":8712,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4257\/revisions\/8712"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/3797"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/4257\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=4257"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=4257"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=4257"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=4257"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}