Key Points

• Integumentary system serves to both protect the body and assist in the maintenance of homeostasis.
• Comprises the skin, which divides into:
  • The Epidermis
  • The Dermis
  • The Skin Accessory Organs (derivatives): hair, glands, and nails

EPIDERMIS

Superfical to Deep:

Stratum corneum

• Outermost layer; it comprises numerous rows (25-30) of flat, dead cells.
  • Contain keratin.
  • Continually sloughed.

Stratum lucidum

• Comprises clear-appearing cells.
  • Lack nuclei and organelles.
  • Contain keratin and eleidin.

Stratum granulosum

• Comprises 3-5 rows of cells that contain keratohyaline granules.
  • Help form the keratin cells of the epidermis.

Stratum spinosum

• Comprises, most notably, several rows of keratinocytes.
  • They are called prickle cells due to their spiny appearance.
  • Within the stratum spinosum layer, desomosomes exist, which form intercellular bridges.

Stratum basale

• Forms the deepest layer of the epidermis.
  • It comprises a single layer of cuboidal keratinocytes that are active in cellular reproduction (mitosis).
  • Responsible for hair growth
  • Occurs when stratum basale generates new hair cells that push the old, keratinized hair cells through the follicle to form hair fibers.
• Note that melanocytes and Merkel cells also exist within this layer.
  • Melanocytes are responsible for the formation of melanin, which derives from tyrosine.

“Come Let’s Get a Sun Burn”

Use the “Come Let’s Get a Sun Burn” to remember the layers

• Come – Corneum
• Let’s – Lucidum
• Get a – Granulosum
• Sun – Spinosum
• Burn – Basale

TYPES OF SKIN

Thin skin

• Most ubiquitous form of skin
• Thin skin lacks a stratum granulosum and stratum lucidum and has a relatively small amount of stratum corneum.
• 75 to 150 μm thick.

Thick skin

• Restricted to discrete areas that are commonly traumatized.
  • Key regions: palms and soles.
• Thick skin contains both stratum granulosum and stratum lucidum.
  • Has a prominent stratum corneum.
• 400 to 600 μm thick.

DERMIS AND SKIN ACCESSORY ORGANS

Papillary layer, with dermal papillae

• Forms a thin layer of loose connective tissue with finger-like projections, called dermal ridges (aka dermal papillae).
  • They interdigitate with the epidermal down-growths (epidermal papillae) — they form fingerprints.

Reticular layer

• Forms the bulk of the dermis.
  • It comprises a dense layer of collagen bundles and elastic fibers
  • (as opposed to the papillary layer, which is loosely constructed)

Hypodermis

• Superficial fascia layer (and not a layer of the skin).

ACCESSORY ORGANS

Derivatives/appendages of the skin.

Hair

• Shaft, root, and dermal papilla.
• Hair follicle invaginates the epidermis.
• The outer root sheath of the hair follicle is continuous with the epidermis.
• Hair follicle comprises:
  • Hair shaft, which is the slender filament at the center of the follicle that extends above the epidermis.
  • Hair root, which is the expanded region of the hair follicle, deep within the dermis at the root of the hair follicle.
  • Dermal papilla, which contains the capillary network that nourishes the hair follicle; not to be confused with dermal papilla that interdigiate with epidermis.

Arrector pili

• Smooth muscle attached to the dermal sheath surrounding the hair follicle.

Glands

• Sebaceous glands
  • Commonly associate with hair follicles;
  • Sebaceous glands are branched acinar, holocrine glands, which have a lobulated appearance.
  • They secrete sebum (an admixture of an oily/lipid substance and degenerating epithelial cells) into hair follicles.
  • Keeps hair supple, skin soft, protects against microorganisms, and maintains the hydration status of the epidermis.
• Sweat Glands: Eccrine and Apocrine
  • Eccrine glands secrete sweat and are most prominent on the palms, soles of the feet, and forehead.
  • Sweat is formed through the filtration the blood into a hypotonic solution that is primarily water with small amounts of such products as salt, antibodies, metabolic waste, vitamin C, and dermcidin.
  • Apocrine glands are specialized, and (unlike the nearly ubiquitous eccrine glands) are confined to specific regions, namely: the axilla, areolar nipple, and perianal region.
  • Other apocrine, non-sweat producing glands are ceruminous glands, which lie in ear canals and secrete cerumin, and mammary glands, which produce and secrete milk.

Nails

• Comprise a nail bed, nail plate, cuticle, lunula, hyponchium, and nail root.

Other stuctures:

• Blood vessels
  • Vascularize the dermal papilla

• Sensory nerve receptors

Within papillary layer

• Meissner copuscles
  • Mechanosensitive (light touch detection)
• Unmyelinated nerve fibers
  • Transmit pain and temperature sensation.

Within reticular layer

• Pacinian corpuscles
• Krause end-bulbs
  • Mucocutaneous (eg, oral cavity) receptors.
• Clinical Correlations
  • Neuropathy
• See Sensory Receptors

CLINICAL CORRELATIONS: COMMON SKIN LESIONS

Flat, Discolorations

• Macules
• Patches

Raised Lesions

• Papules
• Nodules
• Plaques
• Blisters

Vascular Lesions

• Petechiae
• Purpura
• Ecchymosis

Infectious Lesions

• Pustules
• Vesicles

Venous-backflow Lesions

• Varicose Veins
• Stasis Dermatitis

PHASES OF HAIR GROWTH

• Human hair grows at 2mm per day.
• Human head has ~ 150,000 hairs.

Anagen phase

• Growing phase

Catagen phase

• Growth respite phase.

Telogen phase

• Terminal resting phase (when hair falls out).

HAIR COLOR

• Hair derives its color from the melanocytes that lie within the hair follicle basement membrane on the surface of the dermal papilla.
• Clinical Correlation