13.2 Skin appendages (hair, nails, and glands)
5 min read•august 14, 2024
The skin's appendages—hair, nails, and glands—play crucial roles in protecting our bodies and regulating temperature. These structures, though often overlooked, are essential for maintaining healthy skin and overall well-being.
Hair and nails provide physical , while sebaceous and sweat glands contribute to skin barrier function and temperature control. Understanding how these appendages work together helps us appreciate the complexity of our largest organ—the skin.
Hair follicle structure and growth
Hair follicle anatomy and hair shaft composition
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- are tube-shaped structures in the dermis and epidermis that produce and anchor hair shafts
- Each follicle contains a hair at the base, which is fed by the dermal papilla
- The hair consists of three main layers: an outer cuticle, cortex, and inner medulla
- The cuticle is formed by overlapping keratinocytes, providing protection and shine
- The cortex contains and pigment, determining the hair's strength and color
- The medulla is a loose core of cells in the center of the hair shaft (not always present)
Hair color and growth cycle phases
- Hair color is determined by the amount and type of produced by melanocytes in the hair bulb
- Eumelanin produces dark hair colors (black and brown)
- Pheomelanin produces lighter hair colors (blonde and red)
- Hair growth occurs in three distinct phases: anagen (active growth), catagen (transitional phase), and telogen (resting phase)
- Anagen phase lasts 2-6 years for scalp hair, during which the hair shaft continuously grows (85-90% of hair is in this phase at any given time)
- Catagen phase lasts 2-3 weeks, during which the follicle shrinks and detaches from the dermal papilla (1-2% of hair is in this phase)
- Telogen phase lasts 2-4 months, during which the follicle is dormant and the hair may shed (10-15% of hair is in this phase)
- A new anagen phase begins after telogen, restarting the
Sebaceous and sweat glands: Composition and function
Sebaceous glands and sebum production
- are holocrine glands associated with hair follicles that secrete an oily substance called sebum
- Sebum lubricates and waterproofs the skin and hair, helping to maintain the skin's barrier function
- Sebum is composed of triglycerides, wax esters, squalene, and dead sebocytes (its composition varies based on diet, genetics, and hormones)
- Sebaceous glands are most abundant on the face and scalp
- Overproduction of sebum can contribute to the development of acne
- Sebum production is regulated by hormones, particularly androgens (testosterone)
Eccrine and apocrine sweat glands
- Sweat glands are coiled tubular glands that produce sweat to regulate body temperature and excrete waste
- Eccrine glands are abundant throughout the skin and produce a clear, odorless fluid
- Eccrine sweat helps cool the body through evaporation and is innervated by the sympathetic nervous system
- They are most dense on the palms, soles, and forehead
- Apocrine glands are larger and open into hair follicles in the axillary and genital regions
- They produce a thicker, protein-rich sweat that can develop odor when metabolized by bacteria
- Apocrine glands become active during puberty and respond to emotional stimuli (stress, anxiety)
Nail layers and growth
Nail anatomy and structure
- Nails are hardened plates of keratin that protect the tips of fingers and toes
- They consist of several parts: nail plate, nail bed, nail folds, cuticle, , and matrix
- The nail plate is the visible part of the nail composed of dead, keratinized cells (translucent, allowing the underlying blood vessels to be seen)
- The nail bed is the skin beneath the nail plate containing blood vessels and sensory nerves (adheres tightly to the nail plate)
- The nail folds are the skin that frames the nail on three sides (the proximal fold extends over the nail and contains the cuticle)
- The cuticle is a thin strip of tissue that seals the nail plate to the proximal fold (prevents infection)
- The lunula is the white, half-moon shaped area at the base of the nail plate (part of the matrix, appears white due to its thickness)
Nail growth and health indicators
- Nail growth occurs in the matrix, a layer of rapidly dividing cells at the proximal end of the nail bed
- As new cells are produced, older cells are pushed forward and become keratinized, forming the nail plate
- Fingernails grow an average of 3 mm per month, while toenails grow about 1 mm per month (growth rates are affected by age, nutrition, and health status)
- Nails are an indicator of overall health
- Changes in color, texture, or shape can signify underlying medical conditions (anemia, malnutrition, liver disease, etc.)
- Healthy nails are smooth, uniform in color, and free of ridges or discoloration
Skin appendages: Thermoregulation and protection
Physical protection provided by hair and nails
- Hair and nails provide physical protection to the skin and underlying tissues
- Hair shields the scalp from UV radiation and trauma (acts as a barrier)
- Nails protect the tips of digits from injury and enhance fine motor skills
- Hair also plays a role in by trapping air close to the skin, providing insulation
- attached to hair follicles contract in response to cold or emotional stimuli, causing goosebumps and increasing insulation
Sebum and eccrine sweat in skin barrier function and thermoregulation
- Sebaceous glands contribute to the skin's barrier function by producing sebum
- Sebum waterproofs and lubricates the skin and hair, helping prevent moisture loss and entry of pathogens
- It also maintains the skin's slightly acidic pH, which inhibits bacterial growth
- Eccrine sweat glands are crucial for thermoregulation through evaporative cooling
- When body temperature rises, sweat production increases (controlled by the hypothalamus in response to changes in core body temperature)
- As sweat evaporates from the skin surface, it removes heat and cools the body
- Sweating is less effective in high humidity because the air is already saturated with moisture, reducing evaporation
- Apocrine sweat glands respond to emotional and hormonal stimuli (stress, fear)
- While their function in humans is not fully understood, they may play a role in pheromone production and social communication in other mammals
Key Terms to Review (24)
Alopecia: Alopecia refers to the loss of hair from the scalp or other parts of the body, which can occur in various forms and severities. It can be associated with genetic predisposition, autoimmune conditions, or other health issues, impacting not just appearance but also psychological well-being. Understanding alopecia is crucial in studying skin appendages, as hair loss directly relates to hair follicle health and functioning.
Apocrine secretion: Apocrine secretion is a type of glandular secretion characterized by the release of membrane-bound vesicles that contain the secretory product. This mechanism is primarily seen in certain glands, such as apocrine sweat glands, which are associated with hair follicles and play a role in scent production and pheromone release. Apocrine secretions are typically thicker and often contain proteins and lipids, contributing to their distinct odor when metabolized by skin bacteria.
Arrector pili muscles: Arrector pili muscles are small, smooth muscles attached to hair follicles in the skin that contract in response to various stimuli, such as cold or emotional responses like fear. When these muscles contract, they cause the hair to stand upright, a phenomenon commonly known as 'goosebumps.' This reaction is an important part of the body's thermoregulation and defense mechanisms.
Bulb: In human physiology, a bulb refers to a rounded, bulbous structure that is often associated with the base of certain hair follicles or glands in the skin. This structure plays a crucial role in the growth and development of hair, as well as the function of certain glands. The hair bulb, located at the base of the hair follicle, contains living cells that divide and form hair strands, while sebaceous glands can be found near these bulbs, providing essential oils that keep hair healthy.
Eccrine secretion: Eccrine secretion refers to the process by which eccrine glands produce and release a watery, clear fluid known as sweat directly onto the skin surface. This type of secretion is vital for thermoregulation and is primarily found in areas like the palms, soles of the feet, and forehead. Eccrine glands play a crucial role in maintaining body temperature through evaporative cooling, and their activity can be influenced by factors such as physical activity, heat exposure, and emotional stress.
Excretion: Excretion is the biological process by which organisms remove waste products from their bodies to maintain homeostasis and overall health. This process is crucial for eliminating harmful substances that could disrupt normal physiological functions, and it involves various systems and structures, including skin appendages and epithelial tissues, which play significant roles in the excretory functions of the body.
Growth cycle: The growth cycle refers to the series of stages that hair, nails, and skin glands go through as they develop and regenerate. This cycle includes phases of growth, rest, and shedding or renewal, which are essential for maintaining healthy appendages. Understanding this cycle is crucial for recognizing how various factors can influence the health and appearance of hair, nails, and glands over time.
Hair follicles: Hair follicles are specialized structures in the skin that produce hair and play a key role in the growth cycle of hair. These dynamic organs are embedded in the dermis and consist of various cell types, including keratinocytes, which generate the hair shaft, and melanocytes, which provide pigmentation. Hair follicles are essential for hair growth, regulation, and associated functions like sensory perception.
Hyperhidrosis: Hyperhidrosis is a medical condition characterized by excessive sweating beyond what is necessary to regulate body temperature. This condition can occur in specific areas like the palms, feet, underarms, or face, or it can be generalized across the entire body. The increased activity of sweat glands, which are important skin appendages, leads to this discomforting situation that can impact daily life and social interactions.
Keratin: Keratin is a fibrous structural protein that is a key component in the formation of hair, nails, and the outer layer of skin. This tough and durable protein provides strength, protection, and resilience to these appendages, helping to safeguard underlying tissues from damage and environmental stressors. Keratin also plays a vital role in maintaining the integrity of epithelial cells, making it essential for overall skin health.
Lunula: The lunula is the visible, white, crescent-shaped area at the base of a nail. It plays an important role in nail growth and health, serving as an indicator of underlying physiological conditions. This structure is particularly prominent on the thumb and may vary in visibility across different fingers.
Melanin: Melanin is a natural pigment found in the skin, hair, and eyes of many organisms, responsible for determining coloration and providing protection against UV radiation. This pigment is produced by specialized cells called melanocytes and plays a crucial role in skin pigmentation and photoprotection by absorbing harmful UV rays, which helps to prevent DNA damage and lower the risk of skin cancer.
Nail matrix: The nail matrix is the tissue located beneath the base of the nail, responsible for the production of new nail cells. This area plays a crucial role in nail growth and health, as it is where the nail plate originates. The health of the nail matrix directly influences the appearance and strength of the nails.
Onychomycosis: Onychomycosis is a fungal infection that affects the nails, often leading to nail discoloration, thickening, and in severe cases, nail loss. This condition can involve any part of the nail, including the nail bed and plate, and is primarily caused by dermatophytes, yeasts, or non-dermatophyte molds. It is significant in understanding skin appendages as it directly impacts the health and appearance of nails, which are vital appendages of the integumentary system.
Protection: Protection refers to the mechanisms and structures that safeguard the body from external threats such as pathogens, physical injury, and harmful substances. This is essential for maintaining homeostasis and overall health, relying on specialized features like skin, epithelial tissues, and associated structures that form barriers against environmental hazards.
Root: In the context of skin appendages, the root refers to the part of a hair or nail that is embedded within the skin, anchoring it in place and serving as the point of growth. The root is crucial for the structure and function of hair and nails, as it contains living cells that divide and produce the visible portions above the skin's surface. Additionally, it is associated with various glands that support the hair and nail's health and maintenance.
Sebaceous glands: Sebaceous glands are small exocrine glands in the skin that secrete an oily substance called sebum, which helps to lubricate and protect the skin and hair. These glands are typically associated with hair follicles and play a key role in maintaining skin health and hydration by preventing excessive dryness and providing a barrier against harmful microorganisms.
Secretion: Secretion is the process by which substances are produced and released from cells or glands to serve various functions in the body. This process plays a crucial role in maintaining homeostasis, facilitating digestion, regulating temperature, and providing protection. The mechanisms and types of secretion can vary widely depending on the specific cells or organs involved.
Sensation: Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. It allows us to perceive changes in our surroundings through various sensory modalities, such as touch, temperature, and pain, which are crucial for our interaction with the world. The skin plays a vital role in this process as it houses a variety of receptors that detect different types of stimuli, while skin appendages like hair, nails, and glands contribute to how sensations are experienced and processed.
Shaft: The shaft refers to the elongated structure of a hair or nail that extends above the surface of the skin. It is primarily composed of keratin, a fibrous protein, and plays a crucial role in providing strength and protection. The shaft's growth occurs in the follicle or matrix below the skin, where new cells are produced and push older cells upward, forming the visible part of hair and nails.
Sudoriferous glands: Sudoriferous glands, also known as sweat glands, are specialized exocrine glands located in the skin that play a crucial role in thermoregulation and excretion. These glands produce sweat, a fluid composed primarily of water, salts, and other substances, helping to regulate body temperature through evaporation and also assisting in the elimination of certain metabolic wastes.
Terminal hair: Terminal hair refers to the thick, long, and pigmented hair that typically grows on various parts of the human body, including the scalp, eyebrows, and during puberty in areas like the armpits and groin. It is distinct from vellus hair, which is finer and less pigmented. Terminal hair is characterized by its greater length and density, playing important roles in protection, thermoregulation, and sensory perception.
Thermoregulation: Thermoregulation is the process by which an organism maintains its internal body temperature within a certain range, despite external environmental conditions. This is crucial for sustaining metabolic functions and overall health, as extreme temperatures can disrupt cellular processes. The body's ability to regulate temperature involves various mechanisms, including sweating, shivering, and changes in blood flow, all of which contribute to maintaining homeostasis.
Vellus hair: Vellus hair is the fine, soft, and usually unpigmented hair that covers much of the human body. Unlike terminal hair, which is thicker and darker, vellus hair serves several purposes, including providing thermal insulation and contributing to the sensory functions of the skin. It is most commonly found on areas like the face, arms, and legs, playing a role in regulating body temperature and enhancing the sense of touch.