Histology of Skin and Accessory Structures of the Skin (APR)

Derman papilla

Location:

* Interface between dermis and epidermis

Description:

* Projection of papillary (external) layer of dermis
* Forms core of epidermal ridge
* Numerous and often branched
* Contains capillary loops

Function:

* Provides nourishment to avascular epidermis
* Provides fingerprints for grasping objects

Dermis

Location:

* Skin (between epidermis and hypodermis)

Description:

* Two layers: papillary (superficial), composed of areolar connective tissue; and reticular (deep), composed of dense irregular connective tissue
* Contains appendages of skin: hair follicles and glands (sweat and sebaceous)
* Contains sensory nerve endings and dense network of blood and lymphatic vessels
* Includes part of hair follicle (which develops as invagination from epidermis)

Function:

* Supports epidermis
* Dense concentration of collagen and elastin gives skin strength
* Receives general sensory stimuli (pain, touch, and temperature) via nerve endings and specialized receptors
* Regulates body temperature

Comment:

* Muscles of facial expression insert on dermis

Duct of merocrine sweat gland

Location:

* Dermis and epidermis of skin

Description:

* Lined by stratified cuboidal epithelium
* Has spiral course through dermis and epidermis from secretory part of gland
* Cells of duct epithelium are smaller and duct has a smaller lumen than secretory part of gland

Comment:

* Sweat from merocrine gland is a nonviscous, hypotonic watery secretion

Epidermis

Location:

* Skin (superficial to dermis)

Description:

* Keratinized stratified squamous epithelium of variable thickness
* Avascular
* Major cell type is keratinocyte
* Thick skin has five layers (superficial to deep): stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale
* Thin skin has four layers (stratum lucidum is missing)
* Cells of stratum lucidum and stratum corneum lack nuclei

Function:

* Physical barrier
* Protection of underlying structures

Comment:

* Also contains stem cells, melanocytes, Merkel cells, and dendritic cells
* Depends on underlying dermis for nutrients (i.e., vascular supply)

Stratum basale

Location:

* Skin
* Deepest layer of epidermis

Description:

* Single layer of columnar or high cuboidal cells resting on basement membrane
* Keratinocyte is predominant cell
* Also contains melanocytes, Merkel cells (touch receptors), and stem cells

Function:

* Keratinocytes undergo mitosis, producing stem cells and cells that migrate into stratum spinosum

Also known as:

* Basal layer or stratum germinativum

Comment:

* Cells in stratum basale and stratum spinosum responsible for turnover of epidermal keratinocytes
* Melanocytes of this layer produce melanin, a natural sunscreen

Stratum corneum

Location:

* Skin
* Outermost layer of epidermis

Description:

* Up to 30 layers of cornified, dead cells
* Dead cells contain keratin

Function:

* Creates barrier to fluids

Also known as:

* Keratin layer

Comment:

* Cells sloughed (shed) from surface and continually replaced from deeper layers
* Sloughed cells form major component of household "dust"
* Latin: cornu = horn

Stratum granulosum

Location:

* Skin
* Epidermis, between stratum spinosum and stratum lucidum

Description:

* Three to five layers of flattened keratinocytes
* Keratinocytes contain dark staining keratohyalin granules (precursor of keratin)
* Nuclei of keratinocytes in various stages of degeneration

Function:

* Protection for deeper layers

Also known as:

* Granular layer

Comment:

* Not a distinct layer in thin skin

Stratum lucidum

Location:

* Skin
* Epidermis, between stratum granulosum and stratum corneum

Description:

* Thin translucent layer composed of three to five layers of keratinocytes without nuclei or organelles
* Keratinocytes contain eleidin (an intermediate in keratin production)

Comment:

* Present only in skin of palm (hand) and sole (foot)

Stratum spinosum

Location:

* Skin
* Epidermis, between stratum basale and stratum granulosum

Description:

* Many layers of keratinocytes
* Deepest cells can divide
* Cells firmly attached to each other by desmosomes (cell junctions)
* Also contains dendritic (Langerhans) cells

Function:

* Helps maintain physical strength of epidermis
* Helps maintain barrier to infectious agents and harmful substances

Also known as:

* Prickle cell layer

Comment:

* Cells in stratum basale and stratum spinosum responsible for turnover of epidermal keratinocytes

Dermis

Location:

* Skin (between epidermis and hypodermis)

Description:

* Two layers: papillary (superficial), composed of areolar connective tissue; and reticular (deep), composed of dense irregular connective tissue
* Contains appendages of skin: hair follicles and glands (sweat and sebaceous)
* Contains sensory nerve endings and dense network of blood and lymphatic vessels
* Includes part of hair follicle (which develops as invagination from epidermis)

Function:

* Supports epidermis
* Dense concentration of collagen and elastin gives skin strength
* Receives general sensory stimuli (pain, touch, and temperature) via nerve endings and specialized receptors
* Regulates body temperature

Comment:

* Muscles of facial expression insert on dermis

Epidermis

Location:

* Skin (superficial to dermis)

Description:

* Keratinized stratified squamous epithelium of variable thickness
* Avascular
* Major cell type is keratinocyte
* Thick skin has five layers (superficial to deep): stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale
* Thin skin has four layers (stratum lucidum is missing)
* Cells of stratum lucidum and stratum corneum lack nuclei

Function:

* Physical barrier
* Protection of underlying structures

Comment:

* Also contains stem cells, melanocytes, Merkel cells, and dendritic cells
* Depends on underlying dermis for nutrients (i.e., vascular supply)

Stratum basale

Location:

* Skin
* Deepest layer of epidermis

Description:

* Single layer of columnar or high cuboidal cells resting on basement membrane
* Keratinocyte is predominant cell
* Also contains melanocytes, Merkel cells (touch receptors), and stem cells

Function:

* Keratinocytes undergo mitosis, producing stem cells and cells that migrate into stratum spinosum

Also known as:

* Basal layer or stratum germinativum

Comment:

* Cells in stratum basale and stratum spinosum responsible for turnover of epidermal keratinocytes
* Melanocytes of this layer produce melanin, a natural sunscreen

Stratum corneum

Location:

* Skin
* Outermost layer of epidermis

Description:

* Up to 30 layers of cornified, dead cells
* Dead cells contain keratin

Function:

* Creates barrier to fluids

Also known as:

* Keratin layer

Comment:

* Cells sloughed (shed) from surface and continually replaced from deeper layers
* Sloughed cells form major component of household "dust"
* Latin: cornu = horn

Stratum granulosum

Location:

* Skin
* Epidermis, between stratum spinosum and stratum lucidum

Description:

* Three to five layers of flattened keratinocytes
* Keratinocytes contain dark staining keratohyalin granules (precursor of keratin)
* Nuclei of keratinocytes in various stages of degeneration

Function:

* Protection for deeper layers

Also known as:

* Granular layer

Comment:

* Not a distinct layer in thin skin

Stratum lucidum

Location:

* Skin
* Epidermis, between stratum granulosum and stratum corneum

Description:

* Thin translucent layer composed of three to five layers of keratinocytes without nuclei or organelles
* Keratinocytes contain eleidin (an intermediate in keratin production)

Comment:

* Present only in skin of palm (hand) and sole (foot)

Stratum spinosum

Location:

* Skin
* Epidermis, between stratum basale and stratum granulosum

Description:

* Many layers of keratinocytes
* Deepest cells can divide
* Cells firmly attached to each other by desmosomes (cell junctions)
* Also contains dendritic (Langerhans) cells

Function:

* Helps maintain physical strength of epidermis
* Helps maintain barrier to infectious agents and harmful substances

Also known as:

* Prickle cell layer

Comment:

* Cells in stratum basale and stratum spinosum responsible for turnover of epidermal keratinocytes

Dermis

Location:

* Skin (between epidermis and hypodermis)

Description:

* Two layers: papillary (superficial), composed of areolar connective tissue; and reticular (deep), composed of dense irregular connective tissue
* Contains appendages of skin: hair follicles and glands (sweat and sebaceous)
* Contains sensory nerve endings and dense network of blood and lymphatic vessels
* Includes part of hair follicle (which develops as invagination from epidermis)

Function:

* Supports epidermis
* Dense concentration of collagen and elastin gives skin strength
* Receives general sensory stimuli (pain, touch, and temperature) via nerve endings and specialized receptors
* Regulates body temperature

Comment:

* Muscles of facial expression insert on dermis

Duct of sebaceous gland

Location:

* Dermis and epidermis of thin skin

Description:

* Short passage between secretory part of gland and hair follicle
* Lined by stratified squamous epithelium that is continuous with external root sheath of hair
* Most frequently opens into upper portion of hair follicle
* May open directly on skin surface of glans penis, clitoris, and lips

Function:

* Release of sebum into hair follicle or onto skin surface

Comment:

* Sebum is oily substance produced by sebaceous glands

Epidermis

Location:

* Skin (superficial to dermis)

Description:

* Keratinized stratified squamous epithelium of variable thickness
* Avascular
* Major cell type is keratinocyte
* Thick skin has five layers (superficial to deep): stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale
* Thin skin has four layers (stratum lucidum is missing)
* Cells of stratum lucidum and stratum corneum lack nuclei

Function:

* Physical barrier
* Protection of underlying structures

Comment:

* Also contains stem cells, melanocytes, Merkel cells, and dendritic cells
* Depends on underlying dermis for nutrients (i.e., vascular supply)

Hair

Location:

* Thin skin

Description:

* Accessory organ of skin
* Fine, keratinized filament
* Grows from oblique tube in skin called hair follicle
* Each hair has three zones: bulb, root, and shaft
* Projects from epidermal surface (length varies by body region)
* Usually pigmented (hair color due to melanin pigment granules in hair cortex)

Function:

* Protection (e.g., from sunburn)
* Heat retention (esp. on scalp)
* Cutaneous sensation
* Dispersion of chemical signals
* Facial recognition/visual identification

Comment:

* Hair not found in thick skin of palms (hands) or soles (feet)
* Genetic and hormonal factors determine body hair distribution
* Single hair also known as pilus (plural, pili)

Hair follicle

Location:

* Dermis
* May extend into hypodermis

Description:

* Tube that surrounds hair root
* Courses obliquely through dermis
* Two coats: epidermic (inner) and dermic (outer)
* Characteristic parts include: dermal papilla bulb, hair shaft, cuticle layer, internal follicular sheath, and external follicular sheath
* Associated with hair receptors and arrector muscle of the hair

Function:

* Hair formation and growth

Comment:

* Hair follicle develops as invagination from epidermis
* Follicle associated with a sebaceous gland, an arrector muscle of the hair, and an apocrine gland (in axilla and anogenital regions)
* Apocrine glands release part of apical cytoplasm with secretion
* Arrector muscle of the hair also known as arrector pili (piloerector) muscle

Sebaceous gland

Location:

* Skin

Description:

* Simple, saccular holocrine gland
* Secretory part lies in dermis
* Duct opens into hair follicle or onto skin surface

Function:

* Secrete sebum (oily substance)
* Lubricate and waterproof hair shaft

Comment:

* Not found in skin of palms (hands) or soles (feet)
* Holocrine glands produce secretion through fatty degeneration of cells in secretory part of gland

Dermis

Epidermis

Stratum basale

Stratum corneum

Stratum granulosum

Stratum spinosum

Hair shaft

Location:

* Surface of epidermis

Description:

* Filamentous, pigmented, keratinized structure
* Projects from epidermal surface, i.e., extends beyond skin surface (length varies by body region)

Function:

* Protection (e.g., from sunburn)
* Heat retention (esp. on scalp)
* Cutaneous sensation
* Dispersion of chemical signals
* Facial recognition/visual identification

Keratinocyte

Location:

* Skin

Description:

* Predominant cell of epidermis
* Originates in basal layer of epidermis and differentiates as they migrate toward surface of skin
* Fully differentiated keratinocyte composed of 85% keratin

Function:

* Helps form epidermal barrier to water and dehydration

Comment:

* Epidermis arranged in five layers (deep to superficial): stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum
* Individual keratinocytes in epidermis for approximately 1 month: they move from stratum basale to stratum corneum in approximately 2 weeks and then remain on the surface for an additional 2 weeks
* Keratin is major structural protein of epidermis

External root sheath

Location:

* Hair follicle

Description:

* Downgrowth of epidermis that forms external layer of hair follicle
* At skin surface composed of all layers of epidermis
* Thinner at dermal papilla, where it is composed only of cells corresponding to stratum basale of epidermis

Function:

* Support for hair production and growth

Hair matrix

Location:

* Hair bulb

Description:

* Surrounds dermal papilla of hair bulb
* Scattered melanocytes present in matrix

Function:

* Zone of dividing cells that give rise to all layers of hair and hair follicle
* Division and proliferation of cells in matrix account for hair growth

Comment:

* Hair color determined by amount of melanin pigment synthesized by melanocytes in hair matrix

Internal root sheath

Location:

* Hair follicle

Description:

* Multicellular sheath that surrounds hair
* Extends around inferior part of hair
* Separates hair from external root sheath
* Comprised of soft keratin

Function:

* Support for hair production and growth

Dermal papilla of hair bulb

Location:

* Inferior part of hair follicle (base of hair bulb)

Description:

* Invagination of loose connective tissue
* Contains capillary network

Function:

* Blood supply for hair follicle

Comment:

* Damage to dermal papilla results in death of hair follicle

Dermis

External root sheath

Location:

* Hair follicle

Description:

* Downgrowth of epidermis that forms external layer of hair follicle
* At skin surface composed of all layers of epidermis
* Thinner at dermal papilla, where it is composed only of cells corresponding to stratum basale of epidermis

Function:

* Support for hair production and growth

Hair matrix

Internal root sheath

Duct of sebaceous gland

Epidermis

Hair follicle

Sebaceous gland

Duct of sebaceous gland

Hair follicle

Sebaceous gland

Dermis

Duct of merocrine sweat gland

Location:

* Dermis and epidermis of skin

Description:

* Lined by stratified cuboidal epithelium
* Has spiral course through dermis and epidermis from secretory part of gland
* Cells of duct epithelium are smaller and duct has a smaller lumen than secretory part of gland

Comment:

* Sweat from merocrine gland is a nonviscous, hypotonic watery secretion

Epidermis

Hypodermis

Location:

* Deep to skin

Description:

* Layer of loose areolar connective and adipose tissue
* Contains cutaneous nerves and blood vessels, and glands (sweat and sebaceous)
* Contains lamellated (Pacinian) corpuscle (pressure receptor)
* Contains portions of hair follicles associated with thin skin

Function:

* Fat storage
* Thermal regulation
* Permits movement of skin

Also known as:

* Subcutaneous tissue or superficial fascia

Merocrine of sweat gland

Location:

* Dermis and hypodermis of skin

Description:

* Simple, coiled, tubular gland
* Secretory part of gland composed of simple cuboidal epithelium
* Duct opens directly on surface of skin at sweat pore
* Innervated by cholinergic nerve fibers

Function:

* Production and release of non-viscous, hypotonic, watery secretion known as sweat
* Plays a major role in temperature regulation through cooling that results from evaporation of sweat from body surface

Comment:

* Most numerous and widely distributed sweat glands in the body
* Palms, soles, and forehead have highest number of glands per square centimeter
* Some merocrine glands also called eccrine glands

Dermis

Duct of sweat gland

Location:

* Dermis and epidermis

Description:

* Lined by stratified cuboidal epithelium
* Has spiral course through dermis and epidermis from secretory part of gland
* Cells are smaller and duct has a smaller lumen than secretory part of gland

Function:

* Passage for release of sweat onto body surface

Epidermis

Epithelium of apocrine sweat gland

Location:

* Secretory part of apocrine sweat gland

Description:

* Simple cuboidal cells form secretory part of apocrine sweat gland
* Myoepithelial cells underlie cuboidal cells and line basement membrane of gland

Function:

* Production of viscous secretion (sweat) released into hair follicles in axilla, around areola of nipple, and anal region

Comment:

* Secretory products are odorless when released but when metabolized by bacteria on skin creates distinctive odor

Lumen of apocrine sweat gland

Location:

* Apocrine sweat gland

Description:

* Central opening in secretory part of gland
* Surrounded by secretory epithelial cells

Function:

* Receives secretions from epithelial cells of apocrine sweat
* Storage of apocrine secretion prior to release

Comment:

* Sweat from apocrine glands contains protein, carbohydrate, ammonia, lipid, and other organic substances

Dermis

Epidermis

Eponychium

Free edge of nail

Lateral nail fold

Nail bed

Nail body

Nail matrix

Nail root