Bio 210: Epidermis, Dermis, Hypodermis

Integumentary System: Skin

- Body’s largest and heaviest organ

- Consists of two layers: Epidermis and Dermis

- Accessory Organs: Hair, Nails, and Glands

- Functions: protection (abrasion, dehydration, UV exposure, pathogens), sensation (pressure, touch, temperature, pain), thermoregulation, vitamin D production

Epidermis

• Superficial

• 4-5 layers of Stratified Squamous Epithelial tissue

• Avascular and waterproof (keratin)

• Resists abrasion, prevents dehydration

Stratum basale

“base layer”

• Primarily basal cells (mitotic cells)

• A single layer of cuboidal to columnar cells

• Melanocytes and Merkel cells

Stratum spinosum

“Spiny layer”

• Thicker (8-10 layers)

• Keratinocytes begin dying

• Desmosome connections: keratinocytes shrink during fixation

• Langerhans cells

Stratum granulosum

“Granular layer”

• Thinner (3-5 layers)

• Granules

• Keratinocyte death: start to lose their organelles; cell membranes thicken

Stratum lucidum

“clear layer”

• found only in thick skin

• translucency

  • lack cellular components (organelles)

Stratum corneum

“corny layer”

• corneocytes

• Many layers (25+)

• Brick-and-mortar architecture

  • bricks of keratin

  • lipids form the mortar (cement)

Thick Skin

• All 5 epithelial strata (contains stratum lucidum)

• Found in areas subject to pressure/friction (palms, fingertips, soles of feet)

• Structure: contains friction (papillary) ridges; lacks hair follicles, sebaceous glands, and arrector pili

Thin Skin

• Only 4 epithelial strata (no stratum lucidum)

• Found everywhere that thick skin is not present

• Structure: Contains hair follicles, sebaceous glands, arrector pili muscles

Keratinocytes

• main type of cell in epidermis

• produce keratin (fibrous protein that helps give the epidermis its protective properties)

Corneocytes

• make up most of the outer layer of the skin

• Anucleate, flattened, and dead keratinocytes

Melanocytes

• produce melanin for skin pigmentation

Merkel cells

• light touch receptors

• found in stratum basale layer

Dendritic (Langerhans) cells

• boosts immune functions

• macrophage-like function: kills microorganisms, removes dead cells, and stimulates other immune system cells

Keratinization

• Increased levels of keratinization coincide with cellular death

• As cells move superficially, they become more keratinized

• Function: resist abrasion, prevent dehydration

Desquamation

• Skin slough/”skin peeling”

• Shedding the outermost layer of tissue

Skin pigmentation

• Melanin (eumelanin: black/brown pigment)(Peomelanin: red pigment)

• Carotene: yellow/orange pigment

• Hemoglobin: red pigment (with oxygen) and Blue pigment (without oxygen)

Dermis

- “True Skin:” core layer of cutaneous membrane

- Made up of Dense Irregular Connective Tissue

- Function: gives the skin strength and structural support

- Two layers: papillary and reticular

- Vascularized and derived from mesoderm

Papillary layer

• Loose areolar connective tissue = mesh network

• Dermal papillae: form friction ridges with epidermis

• Extensive capillary network

• Meissner’s (tactile) corpuscles: lighter touch.

Reticular

• Dense irregular connective tissue allows for strength and elasticity

• Thicker: 80% of the dermis

• Anchor for glands and hair follicles

• Pacinian (lamellar) corpuscles: strong pressure/vibrations.

Hypodermis

“Subcutaneous Membrane”

• NOT part of the cutaneous membrane

• Serves to connect skin to underlying fascia

• Made up of loose connective tissue

  • High adipose content (shock absorption and insulation

  • Vascularized