Skin: Structure & Function Flashcards
What is the integumentary system?
Refers to the skin, hair and nails.
Largest and heaviest organ of the body – ~15% of adult weight
Medically significant
What are the main functions of the skin?
Barrier (protection), against: - Dehydration - Infection - Injury / abrasion - Solar radiation Thermoregulation Sensation Repair Vitamin D production
What is the Integumentary system composed of?
3 layers of the skin Epidermis, Dermis, Hypodermis Hairs Glands Sense organs
What is H&E stain?
Stained with H+E (Haematoxylin and eosin)
Describe the structure of hypodermis
Hypodermis can be the thickest layer - composed of fat
Describe the epidermis structure
Thinnest (in abdomen)
Thickest on the heel of your foot
What are the 4 layers of epidermis?
Basal layer - (Stratum basale)
Stratum spinosum - (spiny layer)
Stratum granulosum - (Granular layer)
Stratum corneum - (cornified layer)
Outline the features of the basal layer
(Stratum basale)
First single layer, containing stem cells, and attached to dermis.
Stem cells constantly proliferate
Why is the basal layer referred to as dynamic?
Daughter cells constantly move “up” (distally) through the epidermis, differentiating as they go, until they are shed from the outer surface
This takes ~20-50 days.
Describe the structure and function of the stratum spinosum
(spiny layer)
Cells have many desmosomes, (junctions) here visible as “spines” between the cells
Strong bonds holding the epidermis together
Describe the composition of the stratum granulosum layer
(Granular layer)
1-4 layers of cells containing prominent granules of “keratohyalin” (keratin precursor)
Also contain lamellar bodies containing lipids (seen by TEM)
Describe the function of the stratum granulosum cells
Cells are differentiating to form the outermost layer
What is the stratum corneum ?
(cornified layer)
The outer protective layer of the epidermis. Many cells thick.
Outline the composition of the stratum corneum
Squamous cells which have lost their nuclei,
Cornified – full of “horny” keratin (from keratohyalin granules) thus tough and resistant to injury
Nonpolar lipids (waterproof) are between the cells – from lamellar bodies
Name examples of other epidermal cell types and their roles
Melanocytes (pigment) Langerhans cells (defence) Merkel cells (sensation
What stain is used to view melanocytes?
Special (DOPA) stain for melanin shows their dendritic form
What is the role of melanocytes?
Melanocytes in the epidermis synthesise melanosomes (pigment granules)
and transfer them to basal keratinocytes through long dendrites
What is melanin?
Pigment mainly in basal epidermis
Where in the skin is melanin found?
Keratinocytes arrange melanin pigment in a cap distal to the nucleus (sunny side)
especially in basal layer (stem cells
Outline the role of melanin in UV protection
UV protection - black-skinned people have only about 10% as many skin cancers as white people with the same lifestyle
What are Merkel cells?
Oval shaped mechanoreceptors (pale) cells in the basal layer
Touch-sensors
Hard to tell difference with H&E alone in white people
What is the function of Langerhans cells?
Immune system. Antigen-presenting cells (like macrophages)
Also are dendritic cells, forming a network
Describe the appearance of Langerhans cells in H+E stain
Small, pale cells in non-basal layers of epidermis. Hard to see with H&E only
Where is Vitamin D produced?
Vitamin D3, made in the epidermis. Mostly basal cells, also stratum spinosum
Why is Vit. D deficiency more prevalent in darker skin tones?
Requires UV light.
Requires more UV light in dark skin (melanin barrier)
Where in the body is Vitamin D converted to its active form?
Converted to active form in liver and kidney:
1,25 dihydroxyvitamin D3.
Commonly deficient in UK
What is the dermis?
Dense, irregular connective tissue.
Dense = full of collagen fibres;
Irregular = fibres run in all directions (not parallel)
What are the functions of the dermis?
Collagen provides tensile strength, (strength when pulled), hence protection against abrasion and impact
Contains elastin: protein complex providing elasticity
Dermis also carries blood and nerve supply for epidermis
Rich in blood vessels
Describe the structure of the dermal-epidermal border
The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways). Especially on hands, = fingerprints, also feet
What are rete ridges?
Epithelial extensions projecting into the underlying connective tissue in skin and mucous membranes
Which layer of the skin contains rete ridges?
Epidermis can have rete ridges
Dermis can have finger-like dermal papillae (sing. = papilla)
What is the hypodermis?
Aka fascia, or subcutis (Cutis = epidermis + dermis)
Where you put a hypodermic syringe, for a subcutaneous injection
Describe the structure of the hypodermis
Composed of fat, containing glands, hair follicles, nerves, blood vessels.
Often the thickest layer of skin. Thickness varies with age, body site, nutrition etc
What is the function of the hypodermis?
provides insulation, cushioning and energy storage
Name the 3 types of glands present in the skin
Sebaceous glands
Apocrine sweat glands
Eccrine sweat glands
What are Sebaceous glands?
Secrete oily sebum (“lanolin”) into hair follicle. Conditioner for hair and skin, prevents dryness and flaking
What are the roles of the Eccrine sweat glands?
These are normal sweat glands. Watery secretion on to skin surface, cools the body by evaporation
What is the function of the apocrine sweat glands?
Secrete into hair follicles
Oily fluid in humans, function unclear (contains pheromones in some mammals), but source of body odour after bacterial action
Found in armpits and anogenital region - Only after puberty
Less in Asian people
What is the role of hair/follicles?
Hair: rudimentary in humans over much of body
Keeps the head warm but can be bald
What infection is associated with hair follicles?
> also site of acne
Acne is when a follicle becomes blocked, and can get infected
What is the nail matrix?
The equivalent to basal layer in rest of epidermis, where cells divide and forming nail
What is the nail plate formed from?
Nail plate is made of keratin
Phalanx is the bone
What is the role of thermoreceptors?
Heat / Cold sensors
What do Meissner’s Corpuscle detect?
Senses touch
Merkel cells in basal epidermis also detect light touch
What are nociceptor fibres?
Free nerve endings - sense pain
What is the role of pacinian corpuscles?
Are baroreceptors that sense pressure
Describe the structure of the skin at the fingertip
Paler structure in the middle of the fingertip is the neural structure in a dermal papilla
How does the skin protect us against dehydration?
Epidermis: Keratin holds water, lipids stop it evaporating.
What skin layer helps protect against infection?
Epidermis: Impervious barrier, also immune system
Ho does the skin protect against injury?
All layers
Epidermis strong, rapid healing, thick where friction occurs.
Dermis has collagen – tough, leathery. Hypodermis cushions.
What layer protects against solar radiation?
Epidermis – stratum corneum and melanin pigment
How does the skin enable thermoregulation?
Hypodermis (insulation); thermoreceptors; blood supply regulation, sweating
What allows sensation in the skin?
Nerve supply and various receptors
How is skin repair enabled?
Epidermis – normal proliferation, and dermis – fibroblasts fill gaps with new collagen that epidermis can attach to
What skin layer is responsible for Vit. D production?
Epidermis
What structures innervate the skin?
- Thermoreceptors
- Meissner’s Corpuscle
- Nocicpetors
- Pacinian Corpuscle
