Integumentary System

Epidermis

• Composed of epithelial tissue 

• Completely replaced every 4-6 weeks  

Dermis

• Composed of connective tissue 

Hypodermis 

• AKA: Subcutaneous layer 

• Also connective tissue, but different composition than dermis 

  Connects skin to the underlying fascia 

  • Fascia: fibrous covering of muscles and organs 

• “Tied” to other layers by the interweaving of collagen fibers between hypodermis and fascia 

• Well-vascularized, loose areolar connective tissue and adipose tissue 

  • Functions include fat storage, insulation and cushioning

Order of stratum top →bottom

Come → Stratum corneum 

Lets → Stratum lucidum 

Get → Stratum granulosum 

Sun → Stratum spinosum 

Burned → Stratum basale 

Stratum corneum 

• The outermost layer of the skin 

  • Exposed to the environment 

• 15-30 layers of dead skin cells 

  • Prevents penetration of microbes 

  • Prevents dehydration of underlying tissues 

• Shed periodically, replaced every 4 weeks 

Stratum lucidum 

• Reduces friction between stratum corneum and stratum granulosum 

• Found in palms of hands and soles of feet 

  • Referred to as “thick” skin 

Stratum granulosum 

• As the keratinocytes are pushed from the stratum spinosum, the cells become flatter, cells membranes thicken, and they accumulate large amounts of keratin 

• Characterized by the presence of granules, which are intracellular protein-filled vesicles. 

  • Some are filled with the precursor to keratin, keratohyalin 

  • Melanosomes are other granules filled with melanin 

Stratum spinosum 

• Contains young keratinocyte cells 

  • Widest at center, tapered at ends 

• Cells are joined to each other through desmosomes  

• Contains a dendritic cell called the Langerhans cell 

  • Functions similarly to macrophages 

  • Engulf bacteria, foreign particles, and damaged cells 

Stratum basale 

• Deepest layer of the epidermis 

• Made up of a SINGLE layer of cuboidal cells, responsible for continuously regenerating skin 

  • Contains stem cells that produce keratinocytes 

  • Basal stem cells → keratinocytes

• Attaches the epidermis to the “basement membrane”

• Closest to the blood supply 

Keratinocytes

• Manufacture and store short filaments of the protein keratin 

• Keratinocytes begin the synthesis of keratin AND a water-repelling glycolipid that makes skin waterproof

  • Keratin: an intracellular fibrous protein that gives hair, nails, and skin their hardness and water resistant properties

Birth of Cells 

• Cells move superficially as they age 

• New cells are “born” through mitosis in the basal layer 

  • As they mature they push up to the apical layer as cells are lost to friction 

• Epidermis is epithelial so avascular (no blood vessels)

  • Moving away from nutrients as it ages 

  • Die in the corneum and fall

Basement Membrane

• Ties the epidermis to the dermis below 

• Made up of a dense, sheet-like form of extracellular matrix 

• Critical for stem cell formation 

• Vertiligo: a skin condition that causes your skin to lose its color or pigment 

  • Some research suggests pigment producing cells have difficulty attaching to basement membrane 

Merkel cell

• Sensory receptor, signals to brain 

• Concentrated on surfaces of hands and feet 

Melanocyte

• melanin

• Pigment producing cell

• Protects from UV radiation damage 

• Melanocytes are dendritic cells with cellular extensions (dendrites) that allow them to contact keratinocytes 

  • Up to 40 at a time 

• Melanin granules protect the DNA of epidermal cells from UV ray damage 

• Prevents the breakdown of folic acid 

• Environments with less sun exposure suffer from insufficient production of Vitamin D

  • Vitamin D: Involved in calcium absorption

Desmosomes

strong connections that hold cells together

The Journey of Keratinocytes 

1. Originate in basale layer, plenty of nutrients to grow 

2. Keratinocytes connect to each other using desmosomes and produce keratin 

3. More tightly connected with visible molecules in the cell’s cytoplasm 

4. Cells are too far away from nutrients, no longer alive 

5. Dead cells are filled with keratin pushed to the surface

The Dermis

• The underlying connective tissue layer 

  • Contains blood, lymph vessels, nerves, follicles and sweat glands 

• Composed of 2 layers of connective tissue proper, with interconnected elastin and collagenous fibers (from fibroblast) 

Collagen fibers

•  provide structure and tensile strength 

  • Binds water 

Elastin fibers

• provide elasticity to the skin 

• Dermis: Stretch Marks 

• Skin affected by rapid growth 

• Stretch marks result when the dermis is stretched beyond its limits of elasticity 

• “Marks” are due to collagen fibers breaking

Dermis: Papillary Layer

• Made up of loose areolar connective tissue 

• Forms the papillae that project into the stratum basale 

• Papillary layer contains 

  • Fibroblasts, blood vessels, nerve fibers, fine collagen fibers 

Dermis: Reticular Layer 

• Much more substantial than papillary layer 

  • Composed of dense irregular connective tissue 

  • Dense weave of thick collagen fibers

• Vascularized with sensory and sympathetic nerves

Hypodermis: What is Fat?

• Adipose tissue (fat) is found in the hypodermis 

• Thickness varies greatly from one person to another 

• Aids in thermoregulation

  • Adults have white fat 

  • Infants have brown fat 

• Excess calories are stored as subcutaneous fat (hypodermis) or surrounding our organs (visceral)

Hair shaft 

• Long shaft of keratin 

• Exposed at the skin’s surface 

Hair root 

• Below surface section 

Arrector pilli 

• Muscle that moves hair shaft 

Hair bulb 

• Epithelial cells, similar to stratum basale 

• Rather than grow, keratinocytes fill themselves with keratin until they die 

• Hair papilla: connective tissue with blood vessels and nerves

• Unique Hair Love 

• Hair grows from the bulb within follicle 

• The hair we see is just the outer layer of the cuticle 

• Pigments are produced in the bulb 

• Textures across ethnicities/regions, determined by medulla 

Hair Damage 

• Healthy cuticle is smooth, shiny, and closed with intact scales 

• Damaged cuticles are rough, with raised scales, allows water to enter 

• Frizz is caused by swelling in the hair due to excess moisture entering hair