Skin Layers and Functions

• The skin's primary function is protection and acting as a physical barrier.
• Organisms like bacteria and viruses are defended against by the skin.

Skin Layers

• Epidermis, dermis, and hypodermis are the main layers of the skin.
• The outer layer of the skin is the epidermis.
• The epidermis contains the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.
• The acronym "BSGLC" is used to remember the order of the epidermis layers.
• The Stratum basale contains melanocytes.

Keratinocytes and Keratin

• Keratinocytes make up 90% of epidermis cells.
• Keratinocytes produce keratin.
• The main functions of keratinocytes and keratin are physical protection, immunity (through cytokine production), and wound healing and repair.
• As keratinocytes age, they move up through the layers.

Cell Types/Proteins

• Keratinocytes, melanocytes, Langerhans cells (dendritic cells), and Merkel cells are key cell types in the skin.
• Collagen, elastin, and keratin are key proteins.
• Melanocytes live in the stratum basale and produce melanin for pigmentation and UV protection.
• Merkel cells are essential for touch sensation.
• Collagen provides structure.
• Elastin allows the skin to stretch and prevents sagging.
• Keratin provides strength and prevents excess water loss.

Stratum Corneum

• The top layer of the epidermis is the stratum corneum.
• It is composed of 10-20 layers of dead, flattened cells called corneocytes.
• Consists of fats and oils (ceramides, cholesterol, and fatty acids).
• It consists of anucleated cells.
• The stratum corneum appears in a basket weave pattern.

Stratum Lucidum

• The "extra" layer specific to the epidermis is the stratum lucidum
• This layer is present in areas characterized by thick skin.
• This layer is rich in eleidin (protein rich in lipids).
• It appears as a thin, homogenous eosinophilic layer in histological sections.

Stratum Granulosum

• In the stratum granulosum, keratinocytes start to flatten and accumulate keratohyalin granules.
• Profilaggrin is also present.
• Lamellar (Odland) bodies are located here.

Stratum Spinosum

• The stratum spinosum is the largest component of the epidermis.
• It is histologically characterized by the presence of desmosomes, which create dependent adhesion of epithelial cells.
• Desmoglein is present.
• Desmosomes are essential protein complexes that regulate calcium.

Stratum Basale

• The bottom layer of the epidermis is the stratum basale.
• Keratinocyte proliferation occurs here.
• It contains melanocytes.
• Characterized by cuboidal or low columnar cells with basophilic cytoplasm.

Dermal Layers

• Papillary and reticular dermis are layers of the dermis.
• The middle layer of the skin is the dermal layer.

Fibroblasts/Proteins of the Dermal Layer

• Fibroblasts are the key cells responsible for collagen and elastin production.
• Collagen and elastin are the major proteins, providing strength and elasticity.
• Keratin is absent.

Reticular Dermis

• The bottom layer of the dermis is the reticular dermis
• It is characterized by collagenous and elastic fibres, providing the skin with strength and elasticity.
• Contains thermoreceptors responsive to heat.

Papillary Dermis

• The top layer of the dermis is the papillary dermis.
• Capillary networks here supply nutrients and oxygen to the epidermis.
• Thermoreceptors responsive to cold are located here.

Hypodermal Layer

• Primarily composed of adipose tissues (fat storage).
• Capillary networks become thicker in the hypodermis than in the dermal layer.
• Connects to the rest of the body.

Cells/Proteins of the Hypodermal Layer

• Adipose cells store fat.
• Fibroblasts make collagen and elastin.
• Macrophages are important for immunity (white blood cells).
• Collagen and elastin give the hypodermal layer strength and elasticity.

Skin Anatomy - Hair

• Hair is made of keratin and is anchored into the skin by a hair follicle.
• Each hair follicle has a sebaceous gland and an arrector pili muscle.
• Arrector pili muscles make hair "stand up" on the skin.
• Important for regulating functions such as thermoregulation.

Skin - Physical Barrier

• The outer layer of skin protects the body from damage and water loss.
• It also keeps microorganisms from getting inside.
• Corneocytes are strong and protect from scrapes.
• An acidic environment (pH 4.5-5.5) is present.
• Skin secretions secrete antibacterial peptides

Skin - Thermoregulation

• Thermoregulation happens through controlling blood flow, sweating and hair movement.

Thermoreceptors

• Thermoreceptors are located in the dermis.
• The types of thermoreceptors are: surface receptors (fingers and toes), deep laying receptors (legs), low-threshold, high-threshold, warm-sensing nerves, and cold-sensing nerves.
• Low-threshold receptors activate when temperatures are between 15 and 45 °C.
• High-threshold receptors respond mainly to temperatures higher than 45 °C and lower than 15 °C.
• Warm-sensing nerves' firing rate increases as the temperature increases.
• Cold-sensing nerves' firing rate increases if the temperature decreases.
• During warm conditions: more blood flows through dilated capillaries and heat is lost by radiation.
• During cold conditions: less blood flows in capillaries and heat is conserved.

Skin - UV Protection

• The primary mechanism by which the skin protects from UV damage is melanin production.
• Melanin protects from UV radiation damage to DNA through the absorption of UV light
• Melanin absorbs a broad spectrum of light and converts energy into heat, which can then be dissipated.

Skin - Sensation

• Touch receptors allow for the processing of information regarding any sort of contact.
• Touch receptors are slowly or rapidly adapting.
• Four main types of receptors are Merkel discs, Meissner's corpuscles, Pacinian corpuscles, and Ruffini endings.
• Ruffini endings and Merkel discs are slowly adapting.
• Pacinian corpuscles, and Meissner's corpuscles are rapidly adapting

Other Functions

• Water retention, epidermal layer and lipid matrix
• Immune function: Immune cells (Langerhans, Keratinocytes, mast cells and macrophages) are required for innate immune activation, and adaptive immune activation (T/B cells) .
• Hormone production and metabolism of vitamin D are endocrine functions.

Skin Conditions - Skin Infections

• Skin can still develop infections despite protecting itself from bacterial/fungal infections.
• Wounds, toxins, insect bites, and hair follicles can lead to skin infections.
• Cuts, scraps, burns, punctures and surgical incisions are types of wounds
• Exfoliative toxins attack desmosomes causing skin breaks.
• Insect bites can provide an entry point for bacteria.
• Infections can occur when bacteria goes through hair follicles.
• Staphylococcus aureus and streptococcal species are the most common pathogens.