1. Integumentary system: the epidermis and its layers

2. Integumentary system
Includes your entire outer layer of skin (16% of your total body weight), accessory organs like hair, nails, and glands.
Consists of two major parts:
(1) Cutaneous membrane
(2) Accessory structures

3. Cutaneous membrane (skin)
Made up of two layers:
(1) Epidermis (superficial epithelium), and Dermis (underlying connective tissue)
(2) Accessory structures – Including hair, nail , exocrine glands. Located mostly in the dermis and protrude into the epidermal layer
(3) Subcutaneous layer deep to the dermis is known as the hypodermis; often not considered part of the integumentary system, but because its fibers extend into the dermis, we will include it here

4. Structure of the skin

5. Function of the skin
(1) Protection of underlying tissues and organs against shocks, abrasion, and chemicals
(2) Excretion of salts, water, and organic wastes
(3) Maintenance of normal body temperature via insulation or evaporative cooling (sweating)
(4) Synthesis of vitamin D3
(5) Storage of nutrients – including lipids/adipose
(6) Detection of touch, pressure pain, and temperature stimuli and relaying information to the control center (brain)

6. epidermis
Recall epidermis composed of stratified squamous cells for the purpose of protection
Most abundant epithelial cells are called keratinocytes

7. epidermis
Keratinocytes make up five layers of thick skin, and four layers in thin skin
Thick skin is found in the palms of your hands and soles of your feet
Thin skin is found everywhere else in your body
Thick and thin refer to the thickness of the epidermal layer

8. Epidermal layers of thick skin
Top layer of skin
Bottom layer of skin

9. Stratum germinativum/stratum basale
Stratum germinativum, also known as stratum basale, is the bottom most layer of skin
Epidermal ridges are found here, and they extend into the dermis.
Projections called dermal papillae creates an interlocking arrangement that increases the strength of the bond between the dermal and epidermal layers

10. Stratum germinativum/stratum basale

11. Stratum germinativum/stratum basale
The contours of the skin surface follow the ridge patterns
Ridges on the palms and soles of feet increase the skin’s surface area and friction, helping to maintain grip
Ridge shapes are genetically determined, unique to you, and do not change throughout your life
Ridge prints located on your fingers are called fingerprints

12. Stratum germinativum/stratum basale
Basal cells dominate this layer. Basal cells are stem cells whose divisions replace skin cells lost to shedding at the skin’s surface
Merkel cells are also found in this layer. Merkel cells are sensitive to touch.
Basal Cell

13. Stratum germinativum/stratum basale
Melanocytes are pigment cells that give skin its color; they are also found in this layer

14. Stratum spinosum Means “spiny layer”
Contains Langerhans cells, which are part of the immune system
Langerhans cells are responsible for stimulating a defense against:
(1) microorganisms that penetrate the epidermis
(2) superficial cancer cells

15. Stratum granulosum Known as the “grainy layer”
By the time the cells reach this layer, they have stopped dividing and now begin to make lots of the protein keratin
Keratin makes up our hair and nails

16. Stratum lucidum Present only in thick skin
“Clear layer” filled with keratin

17. Stratum corneum
Top layer of epidermis; consists of layers of cells
Keratinization or cornification, is the formation of protective, superficial layers of cells made of keratin
Dead layers of cells so tightly connected that when they slough off they separate as sheets of cells

18. Epithelial Cell life cycle
Takes days for epithelial cells to move from stratum basale/germinativum to the stratum corneum
They stay in the stratum corneum for about two weeks before being washed/shed

19. Stratum corneum A dry layer, so few microorganisms can survive here
Layer is water resistant, but not waterproof. Water from inside the body slowly penetrates to the surface where it is evaporated. We lose ~500mL (1 pint) of water/day
This process is called insensible perspiration
If this layer is damaged or lost (as in burns), the rate of water loss dramatically increases and you are at risk of dehydration

20. Osmotic pressure
Recall from biology that freshwater will move into cells, causing them to SWELL
This is why when you take a bath your hands swell and look like raisins
Likewise, water will leave cells if placed in highly concentrated solutions, like oceanwater
Although the process occurs slowly, it will accelerate the dehydration process in shipwreck survivors
After 51 hours
of exposure to
seawater and
wearing gloves