SKIN: Structure and Function of Skin Flashcards
What does integument refer to?
Integument refers to the skin and its appendages which are hair, nails and glands
The skin is the largest and heaviest organ of the body (making up about 15% of an adult’s weight).
Structures of the Integument
Skin (3 layers)
- epidermis (outer epithelial layer of the skin)
- dermis (Nerves & blood vessels)
- hypodermis ( fat & connective tissue)
Hairs
Glands
Nails
Sense Organs
What are some functions of the skin?
- acts as a barrier against physical (injury), chemical and biological threats (infections)
- homeostatic functions in temperature and water balance regulation
- sensory functions
- secretory/’nutritional’ function producing Vitamin D
- insulation (really, a property of the hypodermis/subcutis)
- repair
- cosmetic (tattoos, piercings, etc.)
Epidermal Layers (deep to superficial)
EPIDERMIS: (has no blood supply)
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum basale
DERMIS: (everything below the stratum basale and above the adipose tissue)
- dermal papillae
- papillary layer
- reticular layer
(don’t need to know this in detail, but could help)
Stratum Corneum (cornified)
- outer protective layer of the epidermis
- many cells thick
- squamous keratinocyte cells which have lost their nuclei and are cornified (full of horny keratin from keratohyalin granules), thus tough and resistant to injury
- non-polar lipids from lamellar bodies are now between cells for waterproof purposes
- cells flaking off and constantly being replaced
Stratum Basale (basal layer)
- keratinocyte stem cells attached to the dermis and constantly proliferate
- dynamic daughter cells move “up” distally through the epidermis, differentiating as they go until they are shed from the outer surface (takes 20-50 days)
Stratum Spinosum (spiny layer)
- keratinocyte cells held together by many desmosomes (junctions), which are visible as “spines”
- strong bonds hold the epidermis together
Stratum Granulosum (granular layer)
1-4 cell layers, but cells are flattened, so the layer is thin
Keratinocyte cells contain prominent keratohyalin granules, a precursor of the protein keratin
Cells also contain lamellar bodies (granules) containing lipids and they help with waterproofing the skin
Cells are differentiating to form the outermost layer
Lack of nuclei in these cells as they don’t need to divide
Epidermal cells other than keratinocytes
Melanocytes (pigment) Langerhans cells (defence) Merkel cells (sensation)
What would you see if you stained a section of skin?
The cells in the epidermis are very close together, so they would give a very intense strain, whereas tissue in the dermis is much looser, with cells further apart, so it stains less intensely.
Fat (the rounded bits in the subcutis) doesn’t stain at all.
Melanocytes
synthesize melanosomes (in which melanin pigment is synthesized) and transfer them to basal keratinocytes through long dendrites
How do keratinocytes protect the skin from UV radiation?
they arrange melanin pigment on top of the nucleus (in a cap distal), protecting the nucleus from UV radiation from the sun
Why are melanocyte pale cells?
Why are melanocyte pale cells?
because their melanin pigment tends to be transferred to other basal cells, meaning not much pigment is in the melanocyte itself
What are the two types of melanin?
Two types of melanin account for variations in skin and hair colour. The result depends on how they are mixed.
1) Eumelanin (brown/black)
2) Pheomelanin (red/yellow)
How would you be able to identify melanocytes by H&E?
They would be pale cells, in or protruding from the basal layer.
Some pale cells in the basal layer are Merkel cells: touch-sensors. It’s hard to tell the difference with H&E alone.
Merkel cells
pale cells in the basal epidermis (like melanocytes)
touch-sensors/receptors
Langerhans cells
what are they?
function?
where are they?
dendritic cells which form a network and function as antigen-presenting cells (like macrophages) to protect against infection
small pale cells in the upper layers of the epidermis (non-basal layers)
They’re hard to see with H&E only though.
Describe the dermis.
It is dense (full of collagen fibres), irregular (fibres run in all directions, not parallel) connective tissue.
Collagen provides tensile strength (made from fibroblasts), hence it acts as protection against abrasion and impact.
It also contains elastin, a protein complex that provides elasticity.
The dermal-epidermal border is often wavy, to resist shear forces (rubbing sideways).
What are the two layers of the dermis
Thin papillary layer
Deeper reticular layer
Describe the elastic fibres in the dermis.
Elastin van Gieson stain shows the collagen fibres in red, and the elastic fibres in black.
These are stretchy and give the skin its elasticity.
One effect of UV light on the skin is damage to elastic fibres, resulting in the loss of elasticity (ie. wrinkles).
Dermal-epidermal layer
Wavy to resist shear forces (rubbing sideways)
-especially on hands (fingerprints) and feet
Rete ridges - epidermal ridges that appear at the dermal-epidermal border and are responsible for cementing together the two layers
Dermal ridges - Dermal projections into the epidermis (finger-like papillae)
More apparent in the thick skin of hands =
fingerprints, as well as feet
Describe the hypodermis.
the subcutaneous tissue just deep to the dermis made up of mostly adipose (fat) tissue containing:
- glands
- hair follicles
- nerves and blood vessels
richly supplied with blood so the body can take up nutrition/energy from fat if it needs or it or it can lay down new fat
Appendages are things that are attached to something larger.
What are the three main appendages of the skin?
- hair follicles: each has an erector pili muscle which controls the angle of the hair
sweat glands:
- Eccrine Sweat Glands: normal sweat glands which respond primarily to elevated body temperature by secreting water onto skin surface, cooling the body by evaporation
- Apocrine Sweat Glands: secrete their products into hair follicles in the armpits and anogenital region. oily fluid in humans with an unclear function, but is the source of odour after bacterial action. function increases only after puberty
- sebaceous glands: these are holocrine glands which release sebum (the oily/waxy substance that waterproofs the skin and hair, to prevent dryness and flaking) [when these sebaceous glands become overactive, we get acne]
Site of acne
HAIR FOLLICLES become blocked and get infected
if follicle becomes blocked, secretions of the sebaceous gland (oily sebum) surrounding the follicle can trap dirt, forming a black plug known as a blackhead
a whitehead is where a membrane accidentally forms over the oily sebum and can’t get out
a papule is where the epidermis itself is sealed over the top of the blocked follicle
a spot is when the entire structure is infected and bacteria proliferate
components of the nail
Nail matrix - responsible for nail growth
Nail plate - hard part of the nail made of keratin
Hyponychium - The slightly thickened layer of skin that lies beneath the free edge of the nail plate..Site of infection sometimes
List the different sensory innervation in the skin.
1) free nerve endings (multimodal: pain, heat, mechanosensitive)
2) hair follicle receptor (abundant mechanoreceptors)
3) Ruffini endings/corpuscles (slowly-adapting mechanoreceptors)
4) Meissner corpuscles (sense stretching of incorporated collagen fibres, sense light touch, present in hairless skin, such as palms and lips)
5) Pacinian corpuscles (rapidly-adapting mechanoreceptors, sense pressure, lots in fingertips)
6) Merkel cell disks (slowly adapting mechanoreceptors)
7) Thermoreceptors (in dermis): to sense heat or cold
8) Nociceptors (in dermis): sense pain (free nerve endings)
9) Pacinian corpuscle (in hypodermis): senses pressure
Each of these sensory structures responds to slightly different modalities (pressure, vibration, temperature, etc.). They also differ in how they adapt (rapidly versus slowly). Finally, they differ in their abundance in the skin from different parts of the body.
