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Anatomy of Hair Follicle

Location of Hair Follicles

  • Hair follicles are predominantly located in the dermis, which is situated beneath the epidermis.
  • Accessory structures within the dermis include hair and nails, both of which originate from embryonic epidermis.

Structure of Hair Follicle

  • Each hair follicle consists of two main components:
    • Tough connective tissue shaft.
    • Hair shaft.
  • Each hair follicle is associated with a sebaceous (oil) gland.

Function of Sebaceous Gland

  • The sebaceous gland secretes oil (sebum), which travels through a duct to the hair shaft and is pulled up to the surface of the skin.

Nerve Endings

  • The base of the hair follicle contains naked nerve endings that detect hair movement and send signals to the brain.

Arrector Pili Muscle

  • The arrector pili muscle is attached to the hair follicle.
  • When this muscle contracts, it causes the hair to stand on end, resulting in goosebumps.
  • Functions of Goosebumps:
    • Historically served as a protective mechanism to make creatures appear larger to predators.
    • Contemporary functions are attributed to various stimuli including:
    • Fear
    • Sudden temperature changes
    • Emotionally moving music.
  • Vestigial Nature of Arrector Pili Muscle:
    • There is ongoing debate regarding whether this muscle is a vestigial organ.
    • Some researchers believe it may still serve functional purposes related to hair movement and sensory detection.

Hair Root Anatomy

Hair Root Structure

  • The hair root broadens into a bulb-like structure at its base.
  • This structure is associated with a blood supply.
  • Contains a papilla that is rich in capillaries and adult stem cells.
    • Adult Stem Cells:
    • They are mitotically active, pushing new daughter cells upward, which leads to hair growth.

Process of Hair Growth

  • Living cells proliferate and move upward, die, and keratinize as they become part of the hair shaft.
  • The lengthening of the hair shaft is attributed to the continual division and upward movement of these cells.

Nutrition of Hair

  • Hair cannot be nourished externally; nutrients reach the living cells in the hair root solely through diffusion from the cardiovascular system.

Structure Layers of the Hair Shaft

  • Cuticle:
    • Outer layer made of dead keratinized cells.
    • This layer does not allow for the absorption of external substances.
  • Cortex:
    • Located beneath the cuticle.
    • Composed of hard keratin, which provides structure to the hair.
  • Medulla:
    • Central portion of the hair shaft.
    • Contains soft keratin that grants flexibility to the hair.

Color of Hair

  • Hair color is determined by the amount of melanin synthesized by melanocytes located in the hair follicle.
  • The pigmentation mechanisms responsible for red hair remain an area of ongoing inquiry.

Exocrine Glands in the Dermis

Overview of Gland Types

  • Endocrine Glands:
    • Ductless glands that secrete substances directly into the bloodstream, playing hormonal roles.
  • Exocrine Glands:
    • These glands secrete substances through ducts to specific sites, including:
    • Sebaceous glands
    • Sweat glands.

Sebaceous Glands (Holocrine Secretion)

  • The mode of secretion involves the entire cell rupturing, releasing sebum and cell debris into the hair follicle.
  • Functions of Sebum:
    • Protects and moisturizes skin and hair.
    • Possesses antibacterial properties.

Sweat Glands

  • Eccrine Glands:
    • Responsible for producing sensible perspiration (visible sweat).
    • Key role in thermal regulation.
    • Sweat Composition:
    • 99% water
    • Contains sodium chloride and antibodies.
    • Functions via an evaporative cooling mechanism.
  • Apocrine Glands:
    • Concentrated in areas like the axilla (armpits), around the nipples, and the groin.
    • Produce thick sweat that is high in lipids and proteins; this sweat is broken down by bacteria to generate body odor.
    • Not active until puberty, possibly linked to pheromone production.

Sensory Receptors in the Skin

Types of Sensory Receptors

  • Meissner's Corpuscles:
    • Located in the papillary dermis beneath the epidermis.
    • Sensitive to light pressure and discriminative touch.
  • Pacinian Corpuscles:
    • Located deeper within the dermis.
    • Responsive to deep pressure and vibration.
    • Function Summary:
    • Meissner's corpuscles are sensitive to texture.
    • Pacinian corpuscles are sensitive to deep stimuli.

Test Preparation Note

  • Expected exam questions will involve:
    • Identification of the functions and anatomical positions of these sensory receptors in the skin.
    • Comparisons between Meissner's and Pacinian corpuscles.

Summary Notes for Exam

  • Discuss the functions and anatomy of hair follicles.
  • Differentiate between eccrine and apocrine glands.
  • Understand the mechanisms of secretion for both sebaceous and sweat glands.
  • Review the processes of hair growth, structure, and nutrition, as well as the complexities associated with skin sensory receptors.