Histology of the Integument

What is the largest organ of the body?

- Skin (Integumentary System)

Integumentary System: Functions

- Barrier for protection

- Sensation/Excretion

- Vitamin D3 Synthesis

Epidermal Derivatives

- Hair

- Nails

- Exocrine Glands (Sebaceous + Sweat)

Sebaceous vs Sweat Glands

- Sebaceous: Secretes oil lubricating skin and hair

- Sweat: Secretes sweat from dermis and epidermis

Components of the Skin

- Epidermis ↓

- Dermis ↓

- Subcutaneous (Hypodermis)

Epidermis and Dermis are derived from which germ layers?

- Epidermis: Ectoderm

- Dermis: Mesoderm

Epidermis vs Dermis: Composition

- Epidermis: Keratinized stratified squamous epithelia

- Dermis: Dense irregular connective tissue + a thin layer of loose (areolar) CT just deep to the epidermis

What are the Layers of the Epidermis?

C'mon, Let's Get Sun Block

- Corneum

- Lucidum

- Granulosum

- Spinosum

- Basale

Thick vs Thin Skin

- Thick: 5 Layers (+ Lucidum) // Found on Fingers, Palms, Soles

- Thin: 4 Layers (- Lucidum) // Widespread w/ hair follicles + sebaceous glands

What is the predominant cell type of the Epidermis? What links the cells together?

- Keratinocytes linked by desmosomes

What occurs to Keratinocytes as they move from deep to superficial?

- Keratinization: The process by which cells form fibrils of keratin and harden → then desquamation (falling off)

New cells continuously grow upward from basal layer to replace lost cells

What are Melanocytes derived from?

- Neural Crest Cells

Melanocytes: Location and Function

- Location: Stratum Basale

- Function: Produces melanin to transfer to Keratinocytes for protection against UV (gives their nuclei little melanin hats to protect against DNA damage)

What are Eumelanin and Pheomelanin?

- Eumelanin: Brownish-black pigment

- Pheomelanin: Reddish-yellow

The amount of each determine hair/skin color

Melanoma

- Cancer of melanocytes w/in the skin, serious

Epidermal-Melanin Unit

- One melanocyte and the keratinocytes that get melanin from it. In average is 1 melanocyte per 10 keratinocytes.

How is Melanin Produced?

1. Enzyme Tyrosinase converts L-tyrosine → L-DOPA

2. L-DOPA oxidized → DOPAquinone (melanin precursor)

3. DOPAquinone → Eumelanin/Pheomelanin

4. Melanin accumulates w/in membrane-bound premelanosomes, which mature into melanosomes

How is Melanin Transferred to Keratinocytes for UV protection?

1. Melanocytes extend dendritic processes containing melanosomes toward keratinocytes

2. Keratinocytes phagocytose melanocyte processes → receive melanosomes (pigment donation)

3. Melanosomes accumulate above the nuclei of Keratinocytes, forming protective umbrellas that shield DNA from UV

Do the number of melanocytes differ between individuals with darker or lighter skin?

- No, the difference in skin color is based on the RATE of melanin production and extent of epidermal distribution

What are Langerhans cells? Describe their origin, location, and function.

- Origin: Bone marrow-derived immune cells (myeloid)

- Location: Stratum spinosum (epidermis)

- Function: APCs → phagocytose antigens and present on MHC II → presents processed antigen to T-cells → Immune response

What is a characteristic feature of Langerhans Cells? Their function?

- Birbeck Granules: Tennis racket shaped → involved in antigen uptake and processing

What are the major categories of sensory receptors found in the skin?

- Non-encapsulated receptors and encapsulated receptors

Non-encapsulated vs Encapsulated Receptors

- Non-encapsulated: Nerve endings w/ no capsule (free nerves/Merkel discs)

- Encapsulated: Nerve endings surrounded by capsule (Meissner/Pacinian Corpuscles)

What are Free Nerve Endings? Where are they located?

- Detect pain/temp/itching

- Location: Papillary dermis + deeper epidermal layers

Non-encapsulated

What are Tactile (Merkel) Cells? Where are they abundant?

- Non-encapsulated receptors used to sense light touch

- Abundant in highly sensitive skin (fingertips, hair follicles)

What is Merkel Cell Carcinoma?

- Rare, fast growing, Spreads easily

- Way less common than malignant melanoma, but 2X mortality

What are Tactile (Meissner) Corpuscles? Where are they abundant?

- Encapsulated receptors for light touch + low frequency stimuli

- Located in dermal papillae of lips/fingertips/palms/soles

What are the differences between Merkel vs Meissner Receptors?

- Merkel: Non-Encapsulated, in Epidermis, Light touch + object texture (slow)

- Meissner: Encapsulated, In Dermis, light touch + low-frequency stimuli (fast)

Pacinian (lamellated) corpuscles: Function and Location

- Encapsulated receptors for deep pressure and vibration, located in reticular dermis + subcutaneous layer

Which receptor is located in the connective tissue of visceral organs?

- Pacinian (Lamellated) Corpuscles → found in rectum + bladder and produce pressure sensations when distorted

Stratum Basale

- Deepest layer of the epidermis, contains stem cells for skin renewal

(Basically new cells form here and are pushed superficially to replace old/lost keratinocytes)

Stratum Basale: What binds the cells together in this layer? To the basement membrane?

- Single layer of keratinocytes + stem cells + melanocytes bound via desmosomes

- Rests on the basement membrane → bound by hemidesmosomes

What assembles in the Stratum Spinosum?

- Keratin filaments → bundles called tonofibrils

Stratum Spinosum: Function and Composition

- A layer of the epidermis that provides strength and flexibility to the skin

- Desmosomes between cells form "spines" that holds onto the upper layers of the epidermis above it

Stratum Granulosum: Histology

- A layer of the epidermis that marks the transition between the deeper, metabolically active strata and the dead cells of the more superficial strata

(Dark band on histo)

Stratum Granulosum: Composition

- 3-5 layers of flattened cells w/ keratohyaline granules that stain basophilic; also has lamellar granules that help against water loss

Keratohyaline vs Lamellar Granules: Compositon

- Keratohyaline: Tonofibrils + fillagrin (bundling protein)

- Lamellar: Lipids + Glycolipids

Keratinization and a lipid-rich layer results in what?

- An almost impenetrable skin barrier

Stratum Lucidum: Composition

- 4-6 rows of CLEAR, flat dead keratinocytes with large amounts of keratin (nuclei and organelles have been lost)

Only found in thick skin

Stratum Corneum: Composition

- Most superficial layer, 15-20 layers of squamous keratinized cells; cells are anucleate and coated in lipids

Squames

- Fully keratinized cells of the stratum corneum that are normally shed from the epidermis as new skin cells emerge and desmosomes break down

What initiates epidermal cell replacement?

- Division of stem cells in the stratum basale

What occurs after cells divide in the stratum basale?

- The move upward and differentiate into keratinized cells → eventually lost by exfoliation on the skin surface

Epidermal-Dermal Junction

- The top of the papillary layer (dermis) where it joins the epidermis

Layers of the Dermis

- Papillary layer (Superficial)

- Reticular layer (Deep)

Papillary Layer: Composition

- Loose CT immediately beneath epidermis, contains vasculature and nerve endings

How do cells of the epidermis receive nutrients if it is avascular?

- Vasculature of the papillary layer immediately beneath supplies blood to all of skin → diffusion feeds up into epidermal component

Reticular Layer: Composition

- Deep to papillary layer, contains dense irregular CT w/ elastic fibers that forms lines of tension (Langer's Lines)

Subpapillary Plexus

- A rich, nutritive capillary network located bt/ papillary and reticular layers

Deep Plexus

- Larger blood and lymphatic vessels lies near the interface of the dermis and the subcutaneous layer.

Subcutaneous (Hypodermis): Composition

- Loose/areolar CT that sits right above musculature and binds skin closely to the subjacent organs

- Contains adipocytes and extensive vasculature

The extensive vasculature of the hypodermis promotes what?

- Rapid uptake of insulin or drugs injected into this tissue

Sebaceous vs Eccrine vs Apocrine: Products

- Sebaceous: Sebum

- Eccrine: Sweat

- Apocrine: Sweat w/ carbs + lipids + proteins

Hair: Composition and Location

- Consists of elongated keratinized structures that form within epidermal invaginations (hair follicles)

Hair: Growth pattern

- Grows discontinuously, alternating between periods of growth and rest

Sebaceous Glands

- Secrete sebum (oil) into the hair follicles where the hair shafts pass through the dermis

Sebum

- Oily, waxy substance produced by sebocytes → lubricates, softens, and waterproofs skin

Sebaceous Glands use what type of secretion?

- Holocrine Secretion: A type of exocrine gland release where the entire secretory cell disintegrates and dies, turning the cell itself into the product

Eccrine Sweat Glands: Composition and Location

- Composition: Simple coiled glands, duct is stratified cuboidal

- Location: Throughout skin except for lips, nail beds, a part of external genitalia

What aids in Eccrine Gland Secretion?

1. Myoepithelial cells: Smooth-muscle like cells in the basal lamina that squeeze on the cells producing sweat so gland can release

2. Sweat continues down spiraling channel through epidermis to sweat pore on skin surfaces

Eccrine Glands aid in what?

- Temperature regulation via cooling d/t evaporation of H2O from body surface

Apocrine Sweat Glands: Composition and Location

- Composition: Large-lumen (wide open), tubular glands; duct is stratified cuboidal

- Location: Axilla, areola, anal skin, external genitalia

Apocrine Sweat Glands: What aids in secretion? What facilitates body odor?

- Myoepithelial cells facilitate expulsion of secretions

- Secretions are protein/carb/ammonia rich → mixed with bacteria → generates odor

When do Apocrine Sweat Glands become functional?

- At puberty