Accessory Structures of the Integumentary System: Hair, Glands, and Nails

  • Hair

    • Prevalence and Distribution
      • The majority of hair on the human body is not on the head; estimates suggest $75\%$ to upwards of $80-90\%$ is elsewhere.
      • There are approximately $2,500,000$ hairs on the body, with most of these not located on the head.
      • Hair is present almost everywhere on the body, including the face, posterior arms, hands, and knuckles.
      • The only two areas without hair are thick skin, specifically the palms of the hands and soles of the feet, to avoid affecting grip.
    • Functions of Hair
      • Protection: Eyelashes protect the eyes, hair in ear canals and nasal passages filters debris.
      • Sensation: Hair follicles are associated with nerve endings, allowing us to feel touch or detect objects before they make direct contact with the skin (e.g., something brushing the arm, or near the eye).
      • Thermoregulation/Stopping Heat Loss: Primarily from the head, and only effective if hair is sufficiently long. For individuals with no or very short hair, this function is minor in humans.
      • Visual Identification: Hair, especially eyebrows, plays a role in recognizing individuals.
      • Distribution of Secretions: Hair helps to distribute secretions from certain sweat glands (apocrine) and oil glands (sebaceous) across the body's surface, including pheromones (our particular scent).
    • Structure and Growth
      • Color: Occurs in many shades due to melanocytes located in the growing region of the hair (follicle and papilla), producing different colored melanin.
      • Chemotherapy and Hair Loss: Chemotherapy targets rapidly growing cells characteristic of cancer. Hair cells are also fast-growing epithelial cells, and thus are often inadvertently damaged, leading to hair loss. (Similarly, epithelial cells lining the digestive tract, which also turn over rapidly, can be affected).
      • Gray Hair: Occurs when hair contains both pigmented and unpigmented strands, or when follicles produce hair without pigment. This can happen with aging, disease processes, or air bubbles in the hair shaft. While stress is often correlated, a scientific causal link between stress and lack of pigmentation production has not been definitively proven.
      • Growth Cycle:
        • Scalp hair grows for about $2$ to $6$ years before falling out and being replaced.
        • Other body hair (e.g., eyebrows, nose hair, arm hair) has a much shorter growing cycle, perhaps around $3$ months, which prevents it from growing to extreme lengths.
    • Origin: All hair is of epidermal origin, produced from keratinocytes (keratin-producing cells). Significant damage to the epidermis leading to scar tissue will result in permanent hair loss in that area.

  • Glands

    • Types of Glands
      • Endocrine Glands: Produce and release hormones directly into the bloodstream. They lack ducts. Examples include the thyroid gland, pituitary gland, adrenal glands, and the pancreas (which has both endocrine and exocrine functions).
      • Exocrine Glands: Release secretions into a duct, which then carries the secretion to a body surface or into a body tube. Examples include sweat glands, oil glands (sebaceous), tear glands (lacrimal glands), salivary glands, and the pancreas (for digestive enzymes).
      • All glands, whether endocrine or exocrine, are derived from epithelial tissue.
    • Sebaceous Glands (Oil Glands)
      • Classification: Exocrine glands of the integumentary system.
      • Secretion: Produce an oily substance called sebum.
      • Functions:
        • Keeps hair and skin soft and pliable (flexible).
        • Makes skin waterproof, preventing excessive water loss and drying out.
        • If sebum production is low, skin can become dry and cracked.
      • Location: Absent on thick skin (palms and soles), which tend to dry out more easily.
      • Origin: Epidermal origin. Damage to the epidermis can lead to the inability to produce oil in affected areas.
      • Clinical Consideration: Acne is an inflammation of sebaceous glands, often caused by bacteria that naturally live on the skin's surface. Treatments may include anti-inflammatories or antibacterials.
      • Sebaceous glands typically dump their oils into hair follicles.
    • Sweat Glands
      • Merocrine (Eccrine) Sweat Glands
        • Prevalence: The most abundant type of gland in the entire body, found almost everywhere, including palms and soles.
        • Functions:
          • Thermoregulation: Releases water, salt, and some wastes onto the body surface, where evaporation cools the body.
          • Excretion: Contributes to the excretion of salts and metabolic wastes.
          • Protection: The slightly acidic nature of sweat helps decrease skin pH, inhibiting the growth of harmful microorganisms. The water content helps to dilute chemicals on the skin.
        • Origin: Epidermal origin.
      • Apocrine Sweat Glands
        • Characteristics: These are often referred to as