Accessory Structures of the Skin Summary Questions

Accessory Structures of the Skin

Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands.

• These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis.

Hair

Hair 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 hair follicle.
• The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s 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 hair root.
• The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix.
• The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis.

 

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 medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the cuticle.

 

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.

 

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 arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft “stand up.”

• 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.

 

Hair Growth

Hair grows and is eventually shed and replaced by new hair.

• This occurs in three phases.

1. The first is the anagen 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.
2. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s active growth.
3. Finally, during the telogen 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.

 

Hair Color

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.

 

Nails

The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes.

• The nail body is formed on the nail bed, and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress.
• In addition, the nail body forms a back-support for picking up small objects with the fingers.
• The nail body is composed of densely packed dead keratinocytes.
• The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously.

 

The lateral nail fold overlaps the nail on the sides, helping to anchor the nail body.

• The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium.

 

The nail bed is rich in blood vessels, making it appear pink, except at

the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little moon”).

• The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a thickened layer of stratum corneum.

Sweat Glands

When the body becomes warm, sudoriferous glands produce sweat to cool the body.

• Sweat glands develop from epidermal projections into the dermis.
• There are two types of sweat glands, each secreting slightly different products.

 

An eccrine sweat gland is type of gland that produces a hypotonic sweat for thermoregulation.

• These glands are found all over the skin’s surface, but are especially abundant on the palms of the hand, the soles of the feet, and the forehead.
• They are coiled glands lying deep in the dermis, with the duct rising up to a pore on the skin surface, where the sweat is released.
• This type of sweat, released by exocytosis, is hypotonic and composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide.
• Eccrine glands are a primary component of thermoregulation in humans and thus help to maintain homeostasis.

An apocrine sweat gland is usually associated with hair follicles in densely hairy areas, such as armpits and genital regions.

• Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis, with the duct normally emptying into the hair follicle.
• In addition to water and salts, apocrine sweat includes organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent smell.
• The release of this sweat is under both nervous and hormonal control, and plays a role in the poorly understood human pheromone response.

 

Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop sweat.

• When the antiperspirant enters the sweat gland duct, the aluminum-based compounds precipitate due to a change in pH and form a physical block in the duct, which prevents sweat from coming out of the pore.

 

Sebaceous Glands

A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair.

• Most sebaceous glands are associated with hair follicles.
• They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping it pliable.
• The fatty acids of sebum also have antibacterial properties, and prevent water loss from the skin in low-humidity environments.

 

The secretion of sebum is stimulated by hormones, many of which do not become active until puberty.

• Overactive sebaceous glands can cause acne.
• Thus, sebaceous glands are relatively inactive during childhood.

 

5.3 | Functions of the Integumentary System

The skin and accessory structures perform a variety of essential functions, such as protecting the body from invasion by microorganisms, chemicals, and other environmental factors; preventing dehydration; acting as a sensory organ; modulating body temperature and electrolyte balance; and synthesizing vitamin D.

• The underlying hypodermis has important roles in storing fats, forming a “cushion” over underlying structures, and providing insulation from cold temperatures.

 

Protection

The skin protects the rest of the body from the basic elements of nature such as wind, water, and UV sunlight.

• It acts as a protective barrier against water loss, due to the presence of layers of keratin and glycolipids in the stratum corneum.
• It also is the first line of defense against abrasive activity due to contact with grit, microbes, or harmful chemicals.
• Sweat excreted from sweat glands deters microbes from over-colonizing the skin surface by generating dermicidin, which has antibiotic properties.

 

Tattoos and Piercings

The skin, in its own way, functions as a form of armor—body armor.

• It provides a barrier between your vital, life-sustaining organs and the influence of outside elements that could potentially damage them.

 

For any form of armor, a breach in the protective barrier poses a danger.

• The skin can be breached when a child skins a knee or an adult has blood drawn—one is accidental and the other medically necessary.
• However, you also breach this barrier when you choose to “accessorize” your skin with a tattoo or body piercing.
  • Because the needles involved in producing body art and piercings must penetrate the skin, there are dangers associated with the practice.
  • These include allergic reactions; skin infections; blood-borne diseases, such as tetanus, hepatitis C, and hepatitis D; and the growth of scar tissue.
  • Despite the risk, the practice of piercing the skin for decorative purposes has become increasingly popular.

 

According to the American Academy of Dermatology, 24 percent of people from ages 18 to 50 have a tattoo.

• Tattooing has a long history, dating back thousands of years ago.
• The dyes used in tattooing typically derive from metals.
• A person with tattoos should be cautious when having a magnetic resonance imaging (MRI) scan because an MRI machine uses powerful magnets to create images of the soft tissues of the body, which could react with the metals contained in the tattoo dyes.

 

What these videos on tattoos:

• How does a tattoo work?

 

• How do you remove tattoos?

 

Sensory Function

The fact that you can feel an ant crawling on your skin, allowing you to flick it off before it bites, is because the skin, and especially the hairs projecting from hair follicles in the skin, can sense changes in the environment.

• The hair root plexus surrounding the base of the hair follicle senses a disturbance, and then transmits the information to the central nervous system (brain and spinal cord), which can then respond by activating the skeletal muscles of your eyes to see the ant and the skeletal muscles of the body to act against the ant.

 

The skin acts as a sense organ because the epidermis, dermis, and the hypodermis contain specialized sensory nerve structures that detect touch, surface temperature, and pain.

• These receptors are more concentrated on the tips of the fingers, which are most sensitive to touch, especially the Meissner corpuscle (tactile corpuscle), which responds to light touch, and the Pacinian corpuscle (lamellated corpuscle), which responds to vibration.
• Merkel cells, seen scattered in the stratum basale, are also touch receptors.

 

In addition to these specialized receptors, there are sensory nerves connected to each hair follicle, pain and temperature receptors scattered throughout the skin, and motor nerves innervate the arrector pili muscles and glands.

• This rich innervation helps us sense our environment and react accordingly.

 

Thermoregulation

The integumentary system helps regulate body temperature through its tight association with the sympathetic nervous system, the division of the nervous system involved in our fight-or-flight responses.

• The sympathetic nervous system is continuously monitoring body temperature and initiating appropriate motor responses.
  • Recall that sweat glands, accessory structures to the skin, secrete water, salt, and other substances to cool the body when it becomes warm.
  • Even when the body does not appear to be noticeably sweating, approximately 500 mL of sweat (insensible perspiration) are secreted a day.
• If the body becomes excessively warm due to high temperatures, vigorous activity, or a combination of the two, sweat glands will be stimulated by the sympathetic nervous system to produce large amounts of sweat, as much as 0.7 to 1.5 L per hour for an active person.
  • When the sweat evaporates from the skin surface, the body is cooled as body heat is dissipated.

In addition to sweating, arterioles in the dermis dilate so that excess heat carried by the blood can dissipate through the skin and into the surrounding environment.

• This accounts for the skin redness that many people experience when exercising.

 

When body temperatures drop, the arterioles constrict to minimize heat loss, particularly in the ends of the digits and tip of the nose.

• This reduced circulation can result in the skin taking on a whitish hue.
• Although the temperature of the skin drops as a result, passive heat loss is prevented, and internal organs and structures remain warm.
• If the temperature of the skin drops too much (such as environmental temperatures below freezing), the conservation of body core heat can result in the skin actually freezing, a condition called frostbite.

 

Integumentary System

All systems in the body accumulate subtle and some not-so-subtle changes as a person ages.

• Among these changes are reductions in cell division, metabolic activity, blood circulation, hormonal levels, and muscle strength.
• In the skin, these changes are reflected in decreased mitosis in the stratum basale, leading to a thinner epidermis.
• The dermis, which is responsible for the elasticity and resilience of the skin, exhibits a reduced ability to regenerate, which leads to slower wound healing.
• The hypodermis, with its fat stores, loses structure due to the reduction and redistribution of fat, which in turn contributes to the thinning and sagging of skin.

Generally, skin, especially on the face and hands, starts to display the first noticeable signs of aging, as it loses its elasticity over time.

• The accessory structures also have lowered activity, generating thinner hair and nails, and reduced amounts of sebum and sweat.
  • A reduced sweating ability can cause some elderly to be intolerant to extreme heat.
• Other cells in the skin, such as melanocytes and dendritic cells, also become less active, leading to a paler skin tone and lowered immunity.
• Wrinkling of the skin occurs due to breakdown of its structure, which results from decreased collagen and elastin production in the dermis, weakening of muscles lying under the skin, and the inability of the skin to retain adequate moisture.

 

Many anti-aging products can be found in stores today.

• In general, these products try to rehydrate the skin and thereby fill out the wrinkles, and some stimulate skin growth using hormones and growth factors.
• Additionally, invasive techniques include collagen injections to plump the tissue and injections of BOTOX that paralyze the muscles that crease the skin and cause wrinkling.

 

Vitamin D Synthesis

The epidermal layer of human skin synthesizes vitamin D when exposed to UV radiation.

• In the presence of sunlight, a form of vitamin D3 called cholecalciferol is synthesized from a derivative of the steroid cholesterol in the skin.
• The liver converts cholecalciferol to calcidiol, which is then converted to calcitriol (the active chemical form of the vitamin) in the kidneys.

 

Vitamin D is essential for normal absorption of calcium and phosphorous, which are required for healthy bones.

• The absence of sun exposure can lead to a lack of vitamin D in the body, leading to a condition called rickets, a painful condition in children where the bones are misshapen due to a lack of calcium, causing bowleggedness.
• Elderly individuals who suffer from vitamin D deficiency can develop a condition called osteomalacia, a softening of the bones.

 

In present day society, vitamin D is added as a supplement to many foods, including milk and orange juice, compensating for the need for sun exposure.

• In addition to its essential role in bone health, vitamin D is essential for general immunity against bacterial, viral, and fungal infections.
• Recent studies are also finding a link between insufficient vitamin D and cancer.