155. Structure and Functions of the Skin – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• The skin is the largest organ of the body and forms the main barrier between the internal environment and the external world. It protects against mechanical, chemical and microbial injury and contributes to sensation, immunity, temperature regulation and vitamin D synthesis.
• The skin has two main layers: epidermis and dermis. The epidermis is keratinized stratified squamous epithelium and is avascular. From superficial to deep, its five layers are stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum and stratum basale. The stratum lucidum is present only in thick skin such as palms and soles.
• The stratum corneum contains dead, keratin-filled cells and forms the main protective barrier. The stratum granulosum contributes to keratinization and the water barrier. The stratum spinosum contains numerous desmosomes and Langerhans cells. The stratum basale contains actively dividing cells responsible for continuous epidermal renewal.
• Melanocytes are present in the stratum basale. They synthesize melanin from tyrosine through tyrosinase and transfer it to keratinocytes. Melanin absorbs ultraviolet radiation and protects nuclear DNA from ultraviolet-induced injury.
• The dermis is connective tissue divided into papillary and reticular layers. It contains blood vessels, lymphatics, nerves, hair follicles, glands, immune cells and smooth muscle. The papillary layer provides nourishment to the avascular epidermis, while the reticular layer provides strength and elasticity through dense collagen and elastic fibres.
• Skin colour depends mainly on melanin but is also influenced by blood flow and other pigments. Pallor may result from reduced skin blood flow, cyanosis from increased reduced haemoglobin, jaundice from bilirubin deposition, and carotene accumulation produces a yellow-orange colour.
• The principal skin glands are eccrine (merocrine), apocrine and sebaceous glands. Eccrine glands are widely distributed, especially on the palms, soles and forehead, and produce most sweat for evaporative cooling. Apocrine glands are mainly found in the axilla, areola, mons pubis and perianal region and become active after puberty.
• Sebaceous glands usually open into hair follicles and secrete sebum, which lubricates the skin and hair. Increased androgen activity and blockage of sebaceous ducts during puberty can contribute to acne vulgaris.
• Cutaneous blood flow varies greatly according to physiological needs. It increases during heat exposure to promote heat loss and decreases in cold conditions to conserve body heat. The skin can also act as a temporary blood reservoir.
• Hair and nails are epidermal appendages composed mainly of keratin. Hair may be lanugo, vellus or terminal hair. A hair consists of a shaft, root and bulb, while its main layers are medulla, cortex and cuticle. Sympathetic contraction of the arrector pili muscle produces hair erection or goosebumps.
• The main functions of skin include protection, thermoregulation, sensation, vitamin D synthesis, immune defense, water and electrolyte regulation and blood storage. Skin receptors detect touch, pressure, vibration, pain, temperature and itch. Sweat contains water and electrolytes, so excessive sweating can cause dehydration and electrolyte imbalance.
• Ultraviolet B radiation converts 7-dehydrocholesterol into cholecalciferol (vitamin D3) in the skin. Vitamin D is important for calcium absorption and normal bone mineralization.
• The skin contributes to innate immunity through its acidic surface, antimicrobial substances in sweat and sebum, Langerhans cells and resident immune cells. Severe burns destroy this barrier and can cause major fluid loss, infection and septic shock.