Skin Structure and Layers

Main Layers of the Skin

The skin is composed of two primary layers: the epidermis and the dermis. These layers differ in structure, function, and location.

• Epidermis: The outermost layer, made of stratified squamous epithelial cells. It provides a protective barrier against environmental damage.

• Dermis: The deeper layer beneath the epidermis, composed mainly of connective tissue. It contains blood vessels, nerves, hair follicles, and glands.

• Hypodermis (Subcutaneous Layer): Not considered a true layer of skin, but lies beneath the dermis. It consists of adipose tissue and provides insulation and cushioning. Additional info: The hypodermis anchors skin to underlying tissues.

Strata of the Epidermis

The epidermis is organized into distinct layers (strata), which differ between thick and thin skin. The layers, from deepest to most superficial, are:

• Stratum basale (germinativum): Deepest layer; contains mitotically active cells responsible for generating new keratinocytes.

• Stratum spinosum: Several layers of keratinocytes; provides strength and flexibility.

• Stratum granulosum: Cells begin to die and accumulate keratin granules.

• Stratum lucidum: Present only in thick skin (palms, soles); thin, clear layer of dead cells.

• Stratum corneum: Outermost layer; consists of dead, flattened keratinocytes that are continuously shed and replaced.

Thick skin (palms, soles) contains all five strata, while thin skin (most of the body) lacks the stratum lucidum.

Stratum with Mitotic Cells

The stratum basale contains mitotic cells (keratinocyte stem cells) and is responsible for the replacement of cells lost from the skin surface.

Diffusion Between Epidermis and Dermis

Since the epidermis is avascular, nutrients and oxygen must diffuse from the underlying dermis. This diffusion is essential for cell survival and function in the epidermis.

• Nutrients and oxygen diffuse from dermal capillaries into the lower layers of the epidermis.

• Waste products diffuse back into the dermis for removal.

Cell Types and Their Functions in the Epidermis

Major Cell Types

• Melanocytes: Produce the pigment melanin, which protects against UV radiation.

• Keratinocytes: Most abundant cell type; produce keratin, a protein that provides strength and waterproofing.

• Dendritic (Langerhans) cells: Immune cells that detect and respond to pathogens.

• Merkel cells: Sensory cells associated with nerve endings; involved in touch sensation.

Keratin is a tough, fibrous protein that helps protect the skin and underlying tissues.

Dermis Structure and Layers

Layers of the Dermis

The dermis consists of two main layers:

• Papillary layer: Superficial layer; composed of loose connective tissue. Contains dermal papillae that interlock with the epidermis, increasing surface area for exchange of nutrients.

• Reticular layer: Deeper layer; composed of dense irregular connective tissue. Provides strength and elasticity to the skin.

Hair Structure and Function

Parts of Hair

Hair is a filamentous structure that grows from follicles in the dermis. Its main parts include:

• Hair root: The portion of hair embedded in the skin.

• Hair shaft: The visible part of hair above the skin surface.

• Hair follicle: The sheath surrounding the hair root; anchors the hair into the skin.

• Hair papilla: A small, capillary-rich structure at the base of the follicle; supplies nutrients to growing hair.

• Hair matrix: Region of actively dividing cells that produce the hair.

Function: Hair provides protection, sensory input, and helps regulate body temperature.

Arrector Pili Muscle

The arrector pili is a small, smooth muscle attached to hair follicles. Its contraction causes hair to stand upright ("goosebumps") and helps with thermoregulation.

Glands of the Skin

Sebaceous Glands

Sebaceous glands secrete sebum, an oily substance that lubricates and waterproofs the skin and hair.

• Function: Prevents drying and protects against some bacteria.

Sudoriferous (Sweat) Glands

Sudoriferous glands produce sweat and are classified into two types:

• Eccrine glands: Widely distributed; produce watery sweat for thermoregulation.

• Apocrine glands: Located in axillary and genital areas; secrete a thicker, milky sweat that can develop odor due to bacterial action.

Function: Sweat glands help regulate body temperature and excrete waste products.

Nail Structure and Function

Parts of the Nail

Nails are protective coverings on the dorsal surface of fingers and toes. Key parts include:

• Free edge: The part of the nail that extends beyond the finger or toe.

• Nail bed: The skin beneath the nail plate.

• Eponychium (cuticle): The fold of skin at the base of the nail.

• Lunula: The crescent-shaped, whitish area at the base of the nail.

• Nail root: The portion of the nail under the skin where growth occurs.

• Nail body: The visible part of the nail.

Function: Nails protect the distal phalanges and aid in grasping objects.

Pigments Affecting Skin Color

Main Skin Pigments

Skin color is determined by several pigments:

• Melanin: Produced by melanocytes; provides brown to black coloration and protects against UV radiation.

• Carotene: A yellow-orange pigment obtained from diet; accumulates in the stratum corneum and fatty tissue.

• Hemoglobin: The red pigment in blood; gives skin a pinkish hue, especially in areas with rich blood supply.

Summary Table: Layers and Cell Types of the Skin

Key Equations

Diffusion of nutrients and oxygen from dermis to epidermis can be described by Fick's Law:

Where: J = diffusion flux D = diffusion coefficient \frac{dC}{dx} = concentration gradient