Histology of Skin Flashcards
What is the skin and what are its main functions?
- Largest organ in the body
- Forms a boundary between the body and the external environment
- Protection
- Thermoregulation
- Sensation
What is skin made up of?
An epithelium of keratinised stratified squamous cells on a base of connective tissue called the epidermis
What determines skin colour?
Amount of melanin pigment secreted by melanocytes of the epidermis
How does the degree of keritanisation in skin change?
Skin thickness changes depending on the level of mechanical stress (such as the skin on the sole of the foot)
What improves the ability of skin to grip? What are these also important for?
- Dermal ridges
- Important for texture recognition and touch
How does skin change as you age?
- Elasticity is reduced
- Skin is lined, sags and recovers poorly after deformation
- Epidermal appendages, epidermis-derived downgrowths into the dermis become visible
What is the structure under nails?
- Cover the dorsal aspects of the distal phalanges of the limbs
- Provide a firm base for the finger or toe pulp
- Nail bed appears pink due to the underlying capillaries supplying the dermis
What differs between regions of skin?
- Hair density and coarseness
- Palms of hands, soles of feet and eyelids are hairless
What are the types of sweat glands?
- Eccrine
- Apocrine
- Sebaceous
What are eccrine sweat glands?
- Cover the vast majority of the body
- Simple tubular glands
- Important in temperature control through evaporation
- Activity is under the control of the sympathetic nervous system
What are apocrine sweat glands?
- Large, and in humans are limited to the axilla, the anogenital region and the mammary areola
- Their duct opens into a hair follicle
- Saline-based product is more viscous than the product of eccrine sweat glands
What are sebaceous sweat glands?
- Present everywhere but the palm of the hand or sole of the foot
- The sebum they produce provides a protective covering for the skin
How does the skin change with temperature?
- Hot weather the skin is richly perfused
- Cold conditions cutaneous blood supply is reduced through the activity of arteriovenous anastomoses
What are arteriovenous anastomoses?
- Networks of vessels which allow the reduction of perfusion through the most superficial tissues
- Conserves heat and protects the central portions of the body housing the vital organs
What is the function of the top most layer of the skin?
- Physical protection
- Protects against UV light and mechanical and chemical stress
- Waterproof
- Thermoregulation achieved through arteriovenous shunts
- Sensations
- Metabolic functions
What is the topmost layer of the skin formed of?
- Keratin formed through keratinisation
Why does skin sometimes go red during exercise or when flushed?
- Blood supply shifts towards the periphery of skin for heat loss
What is the main metabolic function of adipose tissue below the skin?
- Subcutaneous fat or adipose tissue involved in production of vitamin D
- Triglyceride storage
What are the histological layers of the skin?
- Epidermis (most superficial and epithelial tissue)
- Dermis
- Hypodermis or subcutis (deepest layer)
How is the epidermis viewed on a micrograph?
- Purple H&E stain
- Reflects the number of cells present
What is the dermis?
- Dense connective tissue (mainly dense irregular)
- Some loose connective tissue underlying epidermis
- Many collagen fibres present
How is the dermis viewed on a micrograph?
Pale pink
What is the hypodermis mainly formed of?
- Adipose tissue
- Connects underlying tissues
What other structures does the hypodermis have?
- Sweat glands
- Ducts that secrete their sweat onto the surface
- Hair follicles with some glands attached
What is the full histological classification for epithelial tissue?
Keratin stratified squamous epithelium
What is the main cell type in the epidermis? What is distinct about this?
- Keratinocyte
- About 4/5 different discrete layers
What cells are located at the basal layer of the epidermis? What is this layer called?
- Cuboidal cells - layer which adheres to the basement membrane
- Stratum basale
What cells are the most superficial in the epidermis?
- Flattened squamous cells
Describe the basal layer
- One cell thick
- Attaches to the basement membrane via hemidesmosomes
- Only layer where mitosis occurs - to replenish the above layers
- Melanocytes only present in this layer
Describe the layer above the basal layer?
- Called the stratum spinosum
- Around 8-10 layers thick
- In a micrograph can sometimes see tiny spines connecting each cell together known as desmosomes
- Involved in the production and maturation of keratin
How is keratin formed?
- Cytoskeleton protein called cytokeratin
- In the stratum spinosum these cytokeratins aggregate as tonofibrils (collections of cytokeratin that are found attached to desmosomes)
- Move upwards from the spinosum to the stratum granulosum as keratohyalin
- Moves to stratum corneum as mature keratin (combination of keratohyalin and cytokeratin
- Keratin so overproduced in the epidermis that the outermost layer of cells begin to die
Describe the stratum granulosum
- Characterised by basophilic (acidic) granules
- These stain strongly with basic dye
- With haematoxylin these stain very dark purple
- Usually 3-5 cell layers thick
- Granules packed full of keratohyalin
Describe the stratum corneum
- Mature keratin is overproduced here
- Anuclear squames
What happens to cells at the epidermis from overkeratinisation?
- Cells begin to die
- Kick out all the organelles
- No nuclei present
- Layers become known as squames which can flake off from your skin (dust)
How often is the epidermis replenished?
- Every 25-30 days
- New skin cells come up from mitosis that occurs in the stratum basale
- Mature as they move up the cell layers and become part of the stratum corneum where there are dead keratinocytes
Describe the stratum lucidum
- Found between the stratum granulosum and stratum corneum
- Only in thick skin
- Palms of hands, finger and soles of feet
- Very thick layer of keratin
- No hairs on this skin
- Clearer on a micrograph
What are melanocytes?
- Only found in the basal layer
- Long cellular processes that extend into the stratum spinosum
- Involved in UV protection
How can melanocytes be viewed on a micrograph?
- Immunohistochemistry used to stain melanin
- Can’t differentiate between keratinocytes and melanocytes using normal H&E stain
- Can see individual cells when staining melanin
- See cytoplasmic processes involved in transferring melanin to above layer of the stratum spinosum
How are keratinocytes viewed on a micrograph?
- Brown layer in melanin staining
- Some melanin in the keratinocytes
What is the function of melanin in keratinocytes/
Creates a barrier around the nucleus to protect from UV and involved in giving skin tones
What are Merkel cells and their function?
- Found in the stratum basale
- Associate with free nerve involved in light touch
What are Langerhans cells?
- Immune cells
- Act as an antigen presenting cell similar to macrophages or monocytes
- Only found in the skin
- Found in all layers of the epidermis and dermis
What are the two sections of the dermis/
- Papillary
- Reticular
Where is the papillary dermis found? How is it charcaterised?
- Directly underneath the epidermis
- Ridges and extension of the dermis into the epidermis called dermal papillae (elevations seen in micrographs)
- Smaller layer than reticular and loose connective tissue
What is the reticular dermis?
- Dense irregular connective tissue
- Larger layer than the papillary dermis
How is the reticular dermis viewed on a micrograph with elastic van Gieson staining?
- Black fibres of elastic tissue
- Red is collagen
- Densely arranged
