Skin and it’s appendages

• Skin is the largest organ of the body
• Consists of two layers:

  • Epidermis: superficial layer, an epithelium
  • Dermis: deeper layer, a connective tissue
• Hypodermis lies beneath the dermis and is not part of the skin
• Epidermis is a stratified squamous keratinized epithelium
• Keratinocytes are the most abundant cell type in the epidermis
• There are 5 layers of keratinocytes in thick skin and 4 layers in thin skin:

  • Stratum basale (stratum germinativum)
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum (in thick skin only)
  • Stratum corneum
• Stratum basale consists of 1 layer of cuboidal cells containing stem cells that undergo mitosis to produce new keratinocytes
• Stratum spinosum consists of several layers of cells with keratin filaments in their cytoplasm
• Stratum granulosum contains granular cells with keratohyalin granules and lamellar granules that contain lipid
• Stratum granulosum cells undergo changes to become cells of the stratum corneum, including increased keratin filaments, breakdown of organelles and nucleus, and thickening of the plasma membrane
• Stratum lucidum is only present in thick skin and consists of a thin layer of clear, poorly stained, flattened keratinocytes without organelles and nuclei
• Stratum corneum is the most superficial layer of the epidermis, consisting of flat cells with keratin bundles, a thick plasma membrane, and no nucleus or cytoplasmic organelles
• Melanocytes produce melanin pigments and transfer them into adjacent keratinocytes to protect cells' nuclei from ultraviolet radiation
• Merkel cells are touch receptors located in the stratum basale, forming Merkel discs with nerve endings
• Langerhans cells are antigen-presenting cells located in the stratum spinosum that phagocytose antigens and alert the immune system
• Dermis is made of connective tissue and consists of two layers: papillary layer and reticular layer
• The papillary layer is the more superficial and thinner layer of the dermis, consisting of areolar connective tissue with dermal papillae that interdigitate with epidermal ridges
• The reticular layer is deeper than the papillary layer, consisting of thicker dense irregular connective tissue with collagen bundles
• Hypodermis, also known as subcutaneous tissue, lies beneath the dermis, is not part of the skin, and consists of areolar connective tissue containing adipose tissue
• There are 5 skin appendages:

  • Hair follicles
  • Sebaceous glands
  • Sweat glands (Eccrine and Apocrine)
  • Nails
• Hair follicles are present all over the body except on specific areas, are tubular invaginations of the epidermis, and produce hair
• Hair follicles consist of external/outer root sheath, internal/inner root sheath, and hair
• The hair bulb is the lowest part of the hair follicle, containing matrix cells that divide to produce new cells forming hair, and melanocytes
• Hair bulb contains:

  • Matrix cells: divide and produce new cells that differentiate & keratinize, forming hair
  • Melanocytes
• Dermal papilla:

  • Loose connective tissue that projects into the base of hair bulb
  • Contains lots of capillaries
• Connective Tissue Sheath:

  • Part of dermis that surrounds hair follicle forming a sheath
  • Arrector pili muscle attaches to it
• Arrector Pili Muscle:

  • Bundles of smooth muscle cells
  • One end attaches to connective tissue sheath
  • The other end connected to the papillary layer of dermis
  • Contraction of the muscle pulls on the hair to make it erect and the skin producing 'goosebumps'
• Sebaceous Gland:

  • Simple, branched acinar gland associated with hair follicles
  • Ducts open into hair follicles
  • Secretes sebum (oily secretions)
  • Mechanism of secretion is holocrine secretion
  • Cells produce lipid droplets until they burst, discharged as sebum into hair follicle
• Sweat Glands:

  • Eccrine Sweat Gland:
  • Location: entire body surface except for lips & external genitalia
  • Not associated with hair follicles
  • Ducts open at surface of skin
  • Mechanism of secretion is merocrine secretion
  • Function: secrete sweat onto body surface for thermoregulation
  • Apocrine Sweat Glands:
  • Location: present only at axilla, areola of mammary gland, anogenital region
  • Mechanism of secretion is merocrine secretion
  • Ducts open into hair follicle
  • Become functional at puberty
• Thick Skin:

  • Location: palms of the hands, soles of the feet
  • Features: thick epidermis, presence of stratum lucidum, absence of hair follicles & their associated structures, eccrine sweat glands present
• Thin Skin:

  • Found elsewhere on the body
  • Features: thin epidermis, absence of stratum lucidum, hair follicles present, arrector pili muscle present, sebaceous glands present, apocrine & eccrine sweat glands present
• Skin Circulation:

  • Large arteries & veins located in hypodermis
  • Branch and form smaller arteries, venules & capillaries in dermis
  • Blood vessels form 3 plexuses in dermis: Subpapillary plexus, Reticular dermal plexus, Deep dermal plexus
• Sensory Receptors / Nerve Endings:

  • Free nerve endings respond to fine touch, heat, cold & pain
  • Merkel disc/corpuscle responds to touch
  • Pacinian corpuscle responds to pressure & vibration
  • Meissner corpuscle responds to touch
  • Ruffini corpuscle responds to skin stretch
• The Functions of Skin:

  • Protects body against bacteria, chemicals, ultraviolet rays, and mechanical forces
  • Impermeable to water to prevent water loss
  • Involved in thermoregulation, conveying sensory information, excreting substances, and forming active vitamin D
• Burns:

  • Classified by depth: First-degree burns, Second-degree burns, Third-degree burns
  • First-degree burns: superficial part of epidermis destroyed
  • Second-degree burns: whole epidermis & superficial part of dermis damaged
  • Third-degree burns: destroy entire epidermis, dermis, and skin appendages, requiring a skin graft for healing