Skin Structure and Function

• The purpose of the skin is to protect the body via a physical barrier
• Skin protects the body from pathogens such as bacteria and viruses

Epidermal Layer

• It is the outermost layer of Skin
• The layers are: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale
• The Stratum basale contains melanocytes

Keratinocytes

• They make up 90% of the Cells in the Epidermis
• They produce keratin
• Key functions of keratinocytes include physical protection, immunity, and wound healing and repair
• Keratinocytes move up as they age

Other Key Cell types and Proteins

• Melanocytes live in the stratum basale and produce melanin for pigmentation and protection against UV radiation
• Langerhans cells which are dendritic cells, are associated with immune responses
• Merkel cells are important for touch sensation
• Collagen is a protein
• Elastin's elasticity allows the skin to stretch and prevents sagging
• Keratin provides strength and prevents excess water loss

Stratum Corneum: Top Layer

• it is composed of 10-20 layers of dead, flattened cells called corneocytes
• It has very strong cells
• Also consists of fats and oils (ceramides, cholesterol and fatty acids)
• cells lack a nucleus (anucleated) and appear in a basket weave pattern

Stratum Lucidum

• It is an extra layer of the epidermis
• Only present in areas characterized by thick skin
• It is rich in Eleidin which is protein rich in lipids
• Thin homogenous eosinophilic layer on histological sections

Stratum Granulosum

• Keratinocytes begin to flatten and accumulate keratohyalin granules.
• Contains Profilaggrin and lamellar (Odland) bodies/filaggrin

Stratum Spinosum

• It is the largest component of the epidermis
• Histologically characterized by the presence of desmosomes
• Desmosomes: important protein complexes which regulate calcium-dependent adhesion of epithelial cells
• Desmoglein is a very interesting protein

Stratum Basale

• This is the bottom layer
• Keratinocyte proliferation occurs here
• Contains melanocytes responsible for pigmentation
• Characterized by cuboidal or low columnar cells with basophilic cytoplasm and melanocytes

Dermal Layer

• It lies in the middle of the Skin
• The layers of the dermis are; the Papillary and Reticular

Cells and Proteins

• Fibroblasts are a primary cell type responsible for the production of collagen and elastin
• Collagen and Elastin are major proteins found within the Dermal layer
• No keratin in the dermal layer
• Provides Strength and elasticity

Reticular Dermis

• It is the bottom layer of the Dermis
• It is Characterized by collagen and elastic fibers, giving the skin strength and elasticity
• Sensation occurs here via Thermoreceptors which respond to heat

Papillary Dermis

• It is the top layer of the Dermis
• Capillary networks here supply the epidermis with nutrients and oxygen
• Sensation occurs via Thermoreceptors which respond to the cold

Hypodermal Layer

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

Cells and Proteins

• Adipose cells are a primary cell type responsible for fat storage
• Fibroblasts responsible for production of collagen and elastin
• Macrophages are major white blood cells responsible for immunity
• Collagen and Elastin: are also the MAJOR proteins found within the Hypodermal layer
• Provides Strength and elasticity.

Skin Anatomy - Hair

• Hair is made of keratin and is anchored into the skin by the hair follicle
• Each hair follicle has a sebaceous gland and an arrector pili muscle
• The arrector pili muscle makes hair stand up on the skin
• hair is important for regulating functions such as thermoregulation

Skin Anatomy - Physical Barrier

• The skin acts as a physical barrier to protect from damage, water loss, and prevents infiltration of microorganisms.
• Corneocytes are strong and provide physical protection from abrasions.
• Acidic environment with a pH of 4.5-5.5
• Skin secretes antibacterial peptides such as RNase 7, dermcidin, and DCD-1/DCD-1L

Skin Anatomy - Thermoregulation

• It helps body thermoregulation through blood flow, sweating, and hair movement

Thermoreceptors

• These are found in the Dermis
• Their location is dependent on where in the body
• Surface receptors: ie, fingers and toes
• Deep Laying receptors: ie, Legs
• Low-threshold receptors: Activated by temperatures between 15 and 45 °C
• High-threshold receptors: Respond mainly to temperatures higher than 45 °C and lower than 15 °C
• Warm-sensing nerves: Their firing rate increases as the temperature increases
• Cold-sensing nerves: Their firing rate increases if the temperature is reduced

Skin Anatomy - UV Protection

• The skin produces melanin to protect from UV damage
• There is UV exposure from UVA and UVB
• Then DNA Damage occurs
• There is the cAMP/CREB Signaling Pathway
• Melanosome Formation occurs
• There is UV protection
• Melanin protects from UV radiation damage of DNA via absorption of UV light
• Melanin absorbs a broad spectrum of light and can convert the energy into heat, which can then be dissipated

Skin Anatomy - Sensation

• Touch receptors allow for processing information related to contact
• Touch receptors are either slowly-adapting or rapidly-adapting
• There are four main types of receptors: Merkel discs, Meissner's corpuscle, Pacinian corpuscle, and Ruffini's endings

Skin Anatomy - Other Functions

• Functions include water retention via the epidermal layer/lipid matrix
• Immune cells (Langerhans, Keratinocytes, mast cells, and macrophages) required for innate immune activation
• An adaptive immune activation (T/B cells)
• Hormone production and metabolism (Vitamin D)

Skin Conditions

• There are skin infections
• Despite the skin protecting from bacterial/fungal infections, opportunistic microorganisms can lead to development of skin infections
• This includes wounds from cuts, scrapes, burns, punctures, and surgical incisions
• Includes toxins, Exfoliative toxins attack desmosomes causing skin breaks
• Insect bites: insect bites can provide an entry point for bacteria
• Hair Follicles: infections can occur when bacteria enter through hair follicles
• Most common pathogens are Staphylococcus aureus and Streptococcal spp. (Pyogenes/hygiene and Pneumoniae/upper respiratory tract infections)