The Skin and its Functions

The Skin and its Appendages

• The skin is the largest organ in the human body, acting as a barrier between internal and external environments.
• It has diverse functions, including sensation, excretion, secretion, absorption, protection from UV radiation, maintaining a microbiome, and protection from trauma.
• Skin sensation involves receptors detecting stimuli like touch, pressure, temperature, and pain.
• Skin excretes waste products via sweat (water, salts, metabolites). Sebaceous glands secrete sebum to lubricate and protect skin and hair.
• Skin can absorb substances like medications through transdermal patches, influenced by thickness, hydration, and hair follicles.
• Melanin, produced by melanocytes, protects from harmful UV radiation by absorbing and dissipating it, thus reducing DNA damage and skin cancer risk.
• The skin's microbiome plays a crucial part in maintaining skin health, protecting against pathogens, and regulating immune response.
• Skin acts as a physical barrier, protecting underlying tissues from mechanical injury due to collagen fibers in the dermis, and epidermis' resistance to abrasion.
• Skin synthesises vitamin D, crucial for calcium metabolism and bone health. UVB radiation converts a precursor molecule in the skin into vitamin D3, then activated by the liver and kidneys.

Structure of the Skin

• The skin consists of three layers: epidermis, dermis, and hypodermis/subcutaneous tissue.
• Epidermis (outermost layer):
  • Stratum Basale: Deepest layer, where keratinocytes are produced. Also contains melanocytes (pigment production) and Merkel cells (touch sensation).
  • Stratum Spinosum: Above basale, cells connected by desmosomes. Contains Langerhans cells (part of the immune system).
  • Stratum Granulosum: Keratinocytes begin keratinization, accumulating keratohyalin granules.
  • Stratum Lucidum: Thin, clear layer only in thick, hairless skin (e.g., palms).
  • Stratum Corneum: Outermost layer, consisting of dead, flattened keratinocytes. Barrier to external threats.
• Dermis (beneath epidermis):
  • Papillary Dermis: Upper portion with finger-like projections (dermal papillae) containing capillaries, nerve endings, and Meissner's corpuscles (touch receptors).
  • Reticular Dermis: Deeper and thicker portion with dense collagen and elastin fibers. Houses hair follicles, sweat glands, sebaceous glands, and sensory receptors.

Dermo-Epidermal Junction

• The dermo-epidermal junction is the interface between epidermis and dermis.
• The epidermal ridges interlock with dermal papillae, providing mechanical stability and nutrient/waste exchange.

Hypodermis/Subcutaneous Tissue

• Located beneath the dermis, it insulates the body, absorbs shock, and anchors the skin to underlying structures.

Cells of the Epidermis

• Keratinocytes: Most abundant, producing keratin for skin strength and waterproofing.
• Melanocytes: Produce melanin, giving skin colour and protecting against UV radiation.
• Langerhans cells: Immune cells in the stratum spinosum. They present antigens to T-cells for pathogen protection.
• Merkel cells: Sensory cells in the stratum basale, which detect light touch.

The Process of Maturation, Keratinization, and Desquamation

• Keratinocytes mature and move upward through the epidermal layers, synthesizing keratin and other proteins.
• The process leads to a tough, protective layer.
• Dead keratinocytes form a strong barrier.
• Desquamation involves shedding these outermost dead cells.
• This process is continuous and maintains the skin's barrier function.

Skin Color and Langer Lines

• Skin colour primarily determined by melanin amount/type. Carotene, blood oxygen level are other factors.
• Langer lines are natural lines in the skin, corresponding to collagen fiber orientation. Incisions parallel to these lines heal better with less scarring.

Skin Glands

• Sebaceous glands: Associated with hair follicles, secrete sebum (oil) to lubricate and waterproof skin.
• Merocrine (Eccrine) glands: Found throughout the body, secrete watery sweat for thermoregulation.
• Apocrine glands: Located in the armpits and groin, produce thicker sweat (with potential body odour).
• Ceruminous glands: Modified apocrine glands found in the ear canal, secreting earwax (cerumen).

Hair

• Consists of hair shaft, hair bulb, hair follicle, and arrector pili muscle.
• Hair follicles contain cells that actively produce hair (anagen phase), then experience a transition (catagen), and resting phase (telogen).

Sensory Receptors in the Skin

• Meissner's corpuscles: light touch/texture.
• Pacinian corpuscles: deep pressure/vibration.
• Ruffini endings: skin stretch/sustained pressure.
• Merkel discs : light touch/texture.
• Nociceptors: pain.

Nails

• Composed of nail plate, nail bed, proximal nail fold, matrix, lunula, and cuticle.
• Nails are formed in the matrix, where keratinocytes produce hard keratin.
• Nails develop from ectoderm during fetal development, fully formed by week 32.