Integumentary System: Skin Structure and Function

• The integumentary system is the only system seen every day and makes up about 16% of total body weight, roughly 10-11 lbs.

Functions of the Integumentary System

• Protection
• Excretion of salt, water, and wastes
• Maintenance of body temperature
• Synthesis of vitamin D for calcium metabolism
• Storage of nutrients such as lipids
• Detection of touch, pressure, pain, and temperature

Cutaneous Membrane: Epidermis

• About 40 pounds of skin flakes are shed in a lifetime
• A new epidermis replaces the old one every 25-45 days
• It does not have a blood supply
• Keratinocytes are the most abundant cells
• The epidermis is thick with 5 layers, such as on the palms and soles at .5 mm, similar to paper towel thickness
• It is thin with 4 layers elsewhere at .08 mm

Stratum Corneum

• Outermost layer that consists of 15-30 rows of dead cells filled with keratin, an insoluble protein that waterproofs and protects skin
• Keratinization is the process
• Cells are joined by desmosomes, shedding in sheets
• Exposed cells last about 2 weeks before shedding

Stratum Lucidum

• Found only on the thick skin of the palms and soles
• It has 3-5 rows of clear, flat, dead cells filled with eleidin, which turns into keratin

Stratum Granulosum

• Characterized by 3-5 rows of flat cells that develop darkly stained granules that will turn into keratin, or keratinocytes
• This layer has no more cell division
• Keratin fibers and keratohyalin link fibers and dehydrate the skin
• Cells in this layer die because of a lack of nutrients

Stratum Spinosum

• There are 8-10 rows of cells fitting closely together
• They contain pre-keratin filaments which allow cells to join tightly together via desmosomes
• Langerhan's cells, macrophages that activate the immune system, protect against microorganisms and superficial skin cancer

Stratum Basale (Stratum Germinativum)

• Innermost layer consisting of a single layer of cuboidal to columnar cells, including basal cells, melanocytes, and Merkel cells for touch sensation where there is no hair
• Mitosis occurs here
• Melanocytes produce melanin, the skin pigment, in a 1:4 or 1:20 ratio
• Melanin moves toward the skin surface once inside the cell
• Epidermal ridges form, increasing surface area of contact with the dermis, creating fingerprints

Dermis

• Composed mostly of connective tissue with collagen and elastin fibers, nerve fibers, blood vessels, and lymph vessels
• Thicker than the epidermis, it serves to hide and protect

Papillary Layer

• Consists of areolar connective tissue
• Outer 20% of the dermis forms the epidermal ridges on thick skin
• It has an extensive capillary blood supply, which supplies the epidermis with nutrients
• Contains dermal papillae that fit to epidermal ridges

Reticular Layer

• Lower 80% of the dermis
• Composed of dense irregular connective tissue with collagen and elastin fibers
• Contains arterioles and venules, few capillaries, nerves, hair follicles, and glands
• This layer forms fissure lines in the palms and stretch marks
• Retin A increases blood flow to the dermis and stimulates repair
• A blister is a separation of epidermal and dermal layers by a fluid-filled pocket

Subcutaneous Layer (Hypodermis or Superficial Fascia)

• It is not part of the integument
• Consists of loose connective tissue, which is heavily adipose
• Accommodates large volumes of fluid, facilitating subcutaneous injections, such as hypodermic needles
• Contains large arteries and veins

Epidermal Pigmentation and Color Shifts

• Carotene, orange/yellow, is converted into vitamin A. Too many carrots can cause an orange tint
• Melanin, brown/black from melanocytes, is concentrated in cheeks, forehead, nipples, scrotum, and labia majora
• All races have the same melanocytes but different abilities to produce melanin, more melanin the more protection from the sun
• Dermal circulation with red color means increased blood flow resulting in a blush, erythem: while a lack of blood flow results in pale, pallor
• Low oxygen in blood can cause cyanosis, a blue color most apparent in lips and nails and disease coloration such as jaundice (yellow) and vitiligo

Hair: Accessory Structures and Epidermal Derivatives

• Functions include insulation, sense, visual, and protection
• Hair grows about 1mm every three days, varying by person
• Head hair lasts about 2-5 years, with about 2.5 million hairs, 75% on the body
• Normal hair loss in an adult is 70-100 hairs per day
• Hair color is due to melanin; blonde and red hair have more sulfur and iron
• Color is genetic but can be affected by the environment
• White hair lacks pigments or contains air bubbles in the medulla

Parts of Hair

• Cuticle: The outermost layer made of keratinized scale-like cells
• Shaft: The visible portion of the hair
• Root: The part of the hair below the skin surface
• Follicle: Surrounds the root and has a sebaceous gland, root hair plexuses sensitive to touch, and external and internal root sheath
• Bulb: The enlarged area at the base of the root houses the papilla which consists of areolar CT with blood vessels to nourish the hair
• Matrix (germinal layer): Responsible for growth of existing hairs and production of new hairs by cell division
• Arrector pili muscle: Associated with each follicle, causing involuntary contraction under stresses, resulting in "goose-bumps"

Types of Hair

• Vellus: peach fuzz
• Terminal: eyelashes, eyebrows, head hair
• Puberty causes hormones to turn vellus hair to terminal hair in the pubic area, arm pits, and limbs
• Male pattern baldness occurs when terminal hair switches to vellus type due to hormonal changes

Nails

• Plates of tightly packed, hard, keratinized cells of the epidermis
• The lunula is a whitish, semilunar area with a thick stratum germinativum where vascular tissue does not show through
• The nail root is buried in a fold of skin and epithelium under the nail root, which functions to provide nail growth
• The nail body covers the nail bed
• Nails grow roughly 1 mm a week

Glands

• Sebaceous glands use holocrine secretion, the sebum is released by squeezing action of arrector pili muscles
• These glands are active in the last months of fetal development, almost stopping after birth until puberty
• Sebum consists of a mix of fats, cholesterol, protein, and inorganic salts, which keeps hair from becoming brittle, prevents evaporation, keeps skin soft, and inhibits bacterial growth
• They are absent in palms and soles
• Usually associated with hair, but can occur without hair follicle on the face, back, external genitalia, and nipples

Sweat Glands

• There are 3-4 million sweat glands including

  • Apocrine glands that are located in arm pits, groin, and nipples and secrete products onto hair follicles
    • Act merocrine, controlled by the nervous system and circulating hormones
  • Merocrine (eccrine) glands are more numerous than apocrine glands, are have the highest number on palms and soles of feet
    • Discharge directly to skin surface
      • Cooling surface of skin
      • Excreting water and electrolytes (salt)
      • Protection
      • Sweat is a mix of water, salt, urea, uric acid, amino acids, ammonia, sugar, lactic acid, and ascorbic acid
  • Ceruminous glands, modified sweat glands in the ear canal, produce cerumen which has hairs that provide a barrier against stuff

• Sebaceous glands and Apocrine sweat glands are controlled by the autonomic nervous system where as Merocrine sweat glands are under regional control

Sense Endings

• Meissner’s Corpuscles: These nerve endings detect touch and are found in the papillary layer of the dermis
• Pacinian Corpuscles: These nerves endings detect pressure and are found in the reticular and subcutaneous layer
• Free Nerve Endings: These nerve endings detect pain
• When stimuli for other sensations (touch, cold) reach an intensity it causes pain
• Somatic pain arises from stimulation of receptors in the skin
• Visceral pain stimulates receptors in the viscera
• Referred pain originates in an internal organ but pain is detected as coming from a body surface
• Phantom pain is detected as coming from a part that is no longer there

Injury and Repair

1. Bleeding
2. Scab forms with fibrin, fibers of blood and RBC’s, to remove debris, phagocytes
3. Collagen fibers dissolve the clot
4. Scar tissue begins to form

Burns using the Rule of Nines

1. If the anterior and posterior surfaces of the head and neck are affected, the burns cover 9% of the body surface
2. The anterior and posterior surfaces of each shoulder, arm, forearm, and hand constitute 9% of the body surface
3. The anterior and posterior surfaces of the trunk, including buttocks, constitute 36%
4. The anterior and posterior surfaces of each foot, leg, and thigh total 18%
5. The perineum represents 1%

Burn Depth

• First-Degree: Involves the surface epidermis only such as sunburns. Skin functions remain intact
• Second-Degree: Involves the entire epidermis and part of the dermis. Some skin function is lost and will heal 1-2 weeks
• Third-Degree: Destroys the epidermis, dermis, and epidermal derivatives Skin functions are lost
• Less painful because nerves are destroyed
• Requires skin grafts, and sepsis is the leading cause of death
• Forth-Degree: Due to electrical injury
  • Extent of damage is influenced by voltage, type of current, and length of contact
  • True electrical injury produces an entrance wound and a least one exit wound
    • The most extensive damage commonly occurring at the exit point
    • Electrical current follows the path of least resistance with blood vessels, nerves, tendons, and bone
      • Skin is highly resistant

Skin Disorders

Aging

• Integument thins, blood flow decreases, cellular activity decreases, and repair slows down

Skin Cancer

• Basal Cell Carcinoma
  • 75% of all skin cancer
  • Arises in the epidermis, and stratum germinativum
  • Rarely metastasizes and has a genetic predisposition
• Squamous Cell Carcinoma
  • Arises from the epidermis
  • Rarely metastasizes
  • Arises from sun-exposed skin or a preexisting lesion
• Malignant Melanoma
  • Arises from melanocytes
  • Metastasizes rapidly and can kill in months
  • Has a 10-20 year delay between sun damage and cancer
• Acne: Inflammation of a sebaceous gland where bacteria live on the sebum and starts at puberty
• Dermatitis: Inflammation of the papillary layer
• Decubitus Ulcer, or bedsores, are caused by a constant deficiency of blood to tissues overlying a bony projection subjected to prolonged pressure
  • Blood deficiency results in an ulceration, which is a crater-like opening in a membrane