Integumentary System and Skin Structure

• The integumentary system includes the skin, hair, nails, glands muscles, and nerves.
• The skin's functions include protection, temperature regulation and sensation.
• Dermatology is the study and treatment of skin disorders.

Structure of the Skin

• The skin consists of two main layers, the epidermis and the dermis.
• Below the dermis lies the hypodermis, made of areolar and adipose tissue, anchoring the skin.

Epidermis

• The epidermis is the outermost layer, made of stratified squamous keratinized epithelial cells.
• Epidermal thickness varies, being thickest on palms and soles and thinner elsewhere.
• The epidermis does not contain blood vessels itself, and it gets nutrition from capillaries in the dermis.
• The epidermis is composed of five layers, the layers of the epidermis include:
  • Stratum corneum (horny layer).
  • Stratum lucidum (clear layer).
  • Stratum granulosum (granular layer).
  • Stratum spinosum (spiny layer).
  • Stratum germinativum or stratum basale (regenerative layer).
• Cells in the stratum basale divide by mitosis, pushing older cells upward to the surface.
• The epidermis is maintained by shedding keratinized cells, cell movement toward the surface undergoing keratinization, and continuous cell division in the stratum germinativum.
• The entire process of cell turnover takes about 4 weeks.
• Excessive shedding of keratinized cells from the scalp is called dandruff.

Stratum Corneum

• The stratum corneum is the outermost layer.
• This layer is composed of 25-30 layers of dead, nonnucleated, keratinized cells.
• Keratin provides structural strength, and lipids make the layer abrasion-resistant and a barrier.
• Cells in the the stratum corneum are constantly shed and replaced by cells from the stratum germinativum.
• Calluses form due to constant skin exposure to friction, resulting in abnormal thickening of the stratum corneum.

Stratum Lucidum

• The stratum lucidum is directly beneath the stratum corneum, made of 3-5 layers of dead keratinized cells.
• The cells of the stratum lucidum are flattened, transparent, dead keratinized cells.

Stratum Granulosum

• The stratum granulosum is a thin layer of 2-5 layers of flattened keratinocytes.
• Keratohyalin granules convert tonofilaments into keratin.
• Lamellar granules release a lipid-rich, water-repellent secretion.
• Cells undergo apoptosis in this layer, marking a transition between dead and alive cells.

Stratum Spinosum

• The stratum spinosum lies above the stratum germinativum.
• The cells of the stratum spinosum is made of 8-10 layers of spiny keratinocytes connected by spine like projections.
• The stratum spinosum provides strength and flexibility.

Stratum Germinativum/Basale

• The stratum germinativum is the deepest, the most important layer responsible for forming upper epidermal layers.
• The stratum germinativum consists of a single row of cuboidal or columnar keratinocytes and stem cells that can divide by mitosis forming new keratinocytes.
• Tonofilaments attach to desmosomes binding cells of this layer to each other and the dermis.
• Melanocytes in this layer produce melanin for skin color.

Dermis

• The dermis lies directly beneath the epidermis.
• The dermis is made of dense connective tissues with collagen and elastin fibers.
• The dermis contains blood vessels, nerves, lymph vessels, smooth muscles, glands, and hair follicles.
• The dermis consists of the papillary and reticular layers.

Papillary Layer

• The papillary layer is adjacent to the epidermis and consists of fine elastic fibers.
• Cells called papillae project towards the epidermis, forming epidermal ridges unique to each individual.
• Epidermal ridges form fingerprints that serve as the basis for identification.

Reticular Layer

• The reticular layer lies between the papillary layer and the hypodermis.
• This layer consists of bundles of collagen fibers, fibroblasts, and some elastic fibers.
• Collagen and elastic fibers in the reticular layer provide extensibility, elasticity, and strength.
• Overstretching of the skin can cause rupture of elastic fibers, leading to stretch marks.

Skin Color

• Skin color depends on pigmentation and hemoglobin (Hb) in the blood.
• Melanocytes in the stratum germinativum produce melanin.
• Melanin production is from tyrosine, influenced by tyrosinase.
• The amount of melanin produced determines skin color differences among races.
• Exposure to sunlight increases melanin production, protecting against UV radiation.
• Hemoglobin in cutaneous blood vessels also contributes to skin coloration, where light-skinned individuals exhibiting pink to red skin.
• Decreased Hb leads to paleness (anemia) while reduced Hb due to CO₂ leads to cyanosis.

Accessory Structures of the Skin

• Accessory structures include hair, nails, and various glands.

Hair

• Hair or pili is a main characteristic of mammals.
• Hair covers body surface except for the palms, soles, and parts of external genitalia.
• Distribution is heaviest on the head, eyebrows and axillae.
• Hair thickness and distribution depend on genetic and hormonal factors.
• Scalp hair provides protection from sun and injury; eyebrows protect the eyes.

Anatomy of Hair

• Each hair consists of dead keratinocytes bonded by proteins.
• The shaft is the visible portion above the skin surface.
• The root penetrates the dermis surrounded by the hair follicle.
• The bulb at the base of the follicle is where cell division occurs for hair growth.
• The shaft and root have three layers: cuticle, cortex, and medulla.
• The arrector pili muscle causes "goose flesh" when it contracts due to fear or cold.

Hair Growth

• Hair follicles grow in cycles with growth and resting stages.
• During growth, bulb cells divide by mitosis and push hair upwards.
• New cells become keratinized, and the resting stage begins.
• Hair grows for 2-6 years and rests for about 3 months.
• Normal hair loss is 70-100 hairs per day replaced by new growth.
• Testosterone affects hair follicles, causing baldness in some men. Radiation therapy, chemotherapy, illness, and severe stress also affect hair growth.
• Hair is straight, curled, or tightly curled due to genetic factors affecting keratin cross-linking.
• Hair color is determined by genetic factors controlling melanin.
• A decline in melanin causes grey, while lack of melanin and air accumulation causes white hair.

Nails

• Nails are tightly packed, hard, dead keratinized epidermal cells protecting fingertips and toes.
• A nail consists of a nail body, free edge, and nail root.
• The nail body is visible and pinkish due to blood flow.
• The free edge forms the lanula.
• The nail root is attached to the nail bed, where cells divide for growth.
• The nail root is covered by the cuticle or eponychium.
• Finger nails grow at 1 mm/week on average, influenced by temperature, season, and health conditions.

Skin Glands

• The skin contains sebaceous and sweat glands.

Sebaceous Glands

• Sebaceous glands or oil glands develop along hair follicles.
• Sebaceous glands secrete sebum (oil).
• Oil is secreted into the hair, and is all over the skin except in the soles of feet and palms of hands.
• Sebum lubricates the skin and hair, acts as a bactericidal and fungicidal agent, and prevents drying.

Sweat Glands

• Sweat glands or sudoriferous glands are simple tubular glands distributed throughout the skin.
• Sweat glands are numerous in palms, soles, axillae, and groins at ~300 glands per square inch on palsm.
• Sweat is released by exocytosis.
• Sweat glands either release sweat on the skin, or empty the secretions in hair follicles, and do not become active until puberty
• Sweat glands are important for body temperature regulation, helps to lower body temperature.

Functions of the Integumentary System

• The skin's functions include protection, temperature regulation, and sensation.

Protective Function

• The skin protects organs from microbial invasion, toxic chemicals, UV rays, and dehydration.
• Keratinocytes resist microbial invasion.
• Langerhans cells and macrophages phagocytize bacteria and viruses.
• The skin acts as a chemical barrier.
• Melanin protects tissue against UV radiation.
• Lipids prevent excessive water loss.

Regulation of Body Temperature

• Skin regulates body temperature through sweat evaporation and blood flow.
• High temperature increases sweat reducing body temperature.
• Low temperature decreases sweat production conserving heat.
• High temperature increases blood flow, increasing heat loss.
• Low temperature causes vasoconstriction and conserves heat.

Other Functions

• The skin excretes some salts, ammonia, and urea.
• The skin absorbs fat-soluble vitamins (A, D, E, K), oxygen, carbon dioxide, and some toxic chemicals.
• The skin contains sensory nerve endings forming cutaneous receptors that convey touch, pain, pressure, and temperature to the brain.
• The skin synthesizes vitamin D from 7-dehydrocholesterol when exposed to UV rays, which is necessary for calcium and phosphorus intake.

Disorders of the Integumentary System

Dermatitis

• Dermatitis is an inflammatory condition with redness, swelling, exudation, and itching.
• Dermatitis is caused by allergens or irritants.

Pressure Ulcers

• Pressure ulcers are also known as decubitus ulcers or bedsores.
• Pressure ulcers arise from lack of blood flow when skin is compressed.
• Blistering may indicate surface damage, and reddish blue color means deep tissue damage.

Skin Cancer

• Skin cancer develops due to sun exposure of UV light.
• Basal cell carcinoma is common, producing an open ulcer, and is easily treated.
• Squamous cell carcinoma produces nodular tumors and can spread.
• Malignant melanoma is dangerous, associated with dark moles or lesions, and can be fatal.

Other Disorders

• Moles are grouping of melanocytes that develop during the first years of life.
• Darkening and enlarging moles may indicate skin cancer around 30 years.

Wound Healing

• Damage to the skin starts a sequence of events that repairs the skin.
• Epidermal wound healing occurs when only the epidermis is wounded.
• Deep wound healing occurs when the dermis is penetrated.
• Epidermal wounds heal as basal cells enlarge and migrate across.

Deep Wounds Healing

• The following are the four phases in deep wounds healing:
  • Inflammation: cut surfaces become inflamed, and blood clot and debris fill the gap.
  • Migration: phagocytes and fibroblasts migrate to the blood clot secreting collagen fibers.
  • Proliferation: epithelial cells proliferate across the wound, and the clot becomes the scab.
  • Maturation: collagen fibers rearrange and blood vessels restore.

Burns

• Burns are tissue damage from heat, cold, electricity, radiation, or corrosive chemicals.
• The burn classifications are:
  • First-degree: Involves epidermis, redness, pain, and slight swelling, heals in 7 days.
  • Second-degree: Involves epidermis and dermis, redness, pain, swelling, and blisters, potentially takes months to heal.
  • Third-degree: Full-thickness, destroys epidermis and dermis, painless, and requires grafts.
• First and second degree burns are called partial thickness burns, where third degree burns are called full thickness burns.

Complications of Burns

• Complications include dehydration, hypothermia, infection, and kidney failure.

Ageing

• Ageing causes dry skin, wrinkles, sagging, grey hair, and brittle nails.
• Collagen decreases in the dermis, elastic fibers lose elasticity, creating sagging skin and wrinkles.
• Reduction in melanocytes can cause grey hair.
• Skin glands reduce, which leads to dry broken skin.
• All of these factors leads to higher risk to skin infections, skin cancer, itching, and pressure sores.

Embryonic Development

• During embryonic development the ectoderm forms epidermal layers, mesoderm differentiates into the dermis.
• Nails develop at 10 weeks, hair follicles at 12 weeks, sebaceous glands at 16 weeks, and sweat glands at 20 weeks.

Healthy Skin Tips

• Healthy skin tips include diet containing omega-3 fatty acids, vitamin A, zinc, and fresh fruits and vegetables.
• Healthy skin tips include sleep, drinking water, green tea/buttermilk, and reducing stress.