Lecture 1: Epidermis, Dermis, Hypodermis

The skin makes up _% of total body weight.

15-20

The skin is home to _ species of bacteria.

> 1000

What is the function of the skin?

Protect and preserve the rest of our body

Where is the thickest layer of skin?

1. Palms of hands

2. Soles of feet

*Withstand friction, provide better barrier for moisture

Where is the thinnest layer of skin?

Eyelids

What is healthy skin involved in?

1. Fluid, protein, electrolyte loss

2. Vit. D production

3. Protection from injury

4. Defense against infection

5. Sensation

6. UV protection

7. Temperature regulation

8. Wound healing

9. Appearance

10. Conduit for secretion

What are the layers of the skin?

3 layers:

1. Epidermis

2. Dermis

3. Hypodermis (subcutaneous)

What is the anatomy of the epidermis?

1. Dead cells flaking off at the skin surface

2. Stratum corneum

3. Stratum lucidum (white layer)

4. Stratum granulosum

5. Stratum spinosum

6. Stratum basale

7. Dermis

What are the primary cells composing the epidermal layer?

1. Keratinocytes

2. Melanocytes

3. Langerhans cells

4. Merkel cells

What are characteristics of the epidermis?

Avascular and keratinized

How many layers does the epidermis have with thick skin?

All 5 layers

How many layers does the epidermis have with thin skin?

4 layers

There is a new epidermis every _ days.

25-45

What percentage of the epidermis is made up of keratinocytes?

90%

What do keratinocytes produce?

Keratin

What are characteristics of keratinocytes?

- Tightly connected by desmosomes

- Callus: accelerated keratin formation

What percentage of the epidermis is made up of melanocytes?

8%

What do melanocytes produce?

Melanin

What are characteristics of melanocytes?

- Spider shapes

- Melanin taken in by keratinocytes

- Shields nucleus from UV

What are characteristics of Langerhans cells?

Star shaped phagocytes

What is the purpose of Merkel cells?

Sense skin curvature

What is the purpose of the stratum basale?

Constantly reproduce keratinocytes

In the stratum basale, keratinocytes are attached by _ to the _.

hemidesmosomes; basal lamina or basal membrane of dermis

In other epidermal layers (besides stratum basale), keratinocytes are attached by _.

desmosomes (cells to cell adherent discs composed of transmembrane glycoproteins: Cadherins)

What are melanocytes in the stratum basale?

Dendrites extend from their cells bodies and contain organelles known as melanosomes

What do melanosomes contain?

Melanin that is produced by the amino acid Tyrosine

What is melanin responsible for?

For everyone's unique skin color

- Photoprotective properties allowing it to absorb harmful ultraviolet light and protect cells against DNA damage

In the stratum spinosum, what do keratinocytes do?

Keratinocytes become active in keratin and protein synthesis

What are tonofibrils and where are they found?

Keratin filament bundles found in the stratum spinosum

What is the purpose of tonofibrils?

They converge with hemidesmosomes and desmosomes to give the skin strength

- Help skin withstand friction and sheer forces

What is the purpose of Langerhans cells in the stratum spinosum?

Immune cells phagocytic T lymphocytes and antigen-presenting cells

What is the stratum spinosum known as?

The thickest layer of the epidermis

What makes up the stratum granulosum?

Flattened keratinocytes 3-5 layers of cells

- Filaggrin binds to tonofibrils which forms an insoluble keratin matrix

What organelles are contained in the stratum granulosum?

Odland bodies: aide with the barrier function of epidermis

What is the purpose of Odland bodies?

They contain lipids and enzymes

- Form a lipophilic layer between stratum granulosum and stratum corneum

- Creates a barrier to water loss

What is an important lipid in the stratum granulosum?

Ceramides

What do ceramides do in the stratum granulosum?

Aide in helping keep the normal brick and mortar configuration

What is the purpose of the stratum granulosum?

Keeps extrinsic foreign material out

Why is the stratum lucidum a unique layer?

Not found on thinner skin areas of the body

- Found on hairless skin of palms of hands and soles of feet ONLY

What is the stratum lucidum composed of?

- Flattened eosinophilic cells

- Dead clear keratinocytes

What is the purpose of the stratum lucidum?

Provides thickness and strength to withstand friction to soles and palms

- Also serves a greater moisture barrier

What is the stratum corneum?

15-20 layers of dead flat and stacked keratinocyte cells embedded with lipid layers to form the main protective shield of the skin

What is the stratum corneum held together by?

Corneodesmosomes

- As they migrate to the surface, they are termed corneocytes

What is desquamation?

Process where corneocytes shed or slough off from stratum corneum

What happens when desquamation is completed?

Over a period of 28-30 days the entire process of migration and desquamation is completed

- New skin cells of the epidermis are renewed

What are examples of skin conditions with quicker migration?

1. Psoriasis: 14 days migration time causing reduced differentiation in keratinocytes causing silver plaque

2. Eczema (atopic dermatitis)

What is atopic dermatitis caused by?

A mutation in the gene responsible for making Filaggrin

- Causes issues with protective barrier of the skin formed in the Stratum Granulosum layer

- Moisture can escape, bacteria and viruses can enter

- Dry infection prone skin

What are the layers of the dermis?

1. Papillary

2. Reticular

What is the papillary layer of the dermis made up of?

Upper most layer of dermis and immediately below epidermis

- Connective tissue

- Capillary loops

- Type VII collagen

What is the purpose of the papillary layer of the dermis?

Allows nutrients from dermal vascular supply to pass through avascular epidermis

What is the purpose of the type VII collagen in the papillary layer of the dermis?

Links dermal papillae and basal lamina

What is a unique characteristic of the papillary layer of the dermis?

Rete ridges = dermal-epidermal layer junction

What is the reticular layer of the dermis?

Elastic fibers are intertwined with other collagen fibers to give flexibility and elasticity to the skin

What is the reticular layer of the dermis made up of?

Extracellular matrix composed of Type I collagen, elastic fibers, glycosaminoglycans (GAGs), proteoglycans, fibroblasts, mast cells, macrophages

What is a distinct substance in the reticular layer of the dermis?

Hyaluronan (hyaluronic acid): anionic nonsulfated GAG in the ECM and contributes to cellular proliferation and migration

- Integral piece of wound healing process

What is the hypodermis also known as?

Subcutaneous layer

What is the hypodermis?

Structure that binds the skin to underlying structure

- Anatomically NOT part of the skin

What is the hypodermis composed of?

1. Connective tissue

2. Vascular supply

3. Adipose cells

What is the purpose of the hypodermis?

1. Allow skin to move freely over underlying structures facilitating fluid muscle and joint movement

2. Provide insulation and pressure redistribution