skin Flashcards
3 layers:
epidermis- stratified squamous epithelial layer. made up of keratinocytes.
dermis- supportive connective tissue matrix, made up of fibroblasts and immune cells. hypodermis- (adipose or fat layer) made up of adipocytes.
skin functions:
-prevents mechanical abrasion of underlying tissues and coordinated wound healing.
-prevents desiccation or water absorption. -prevents injury by chemicals/radiation.
-barrier to pathogens.
-mechanism of sensation.
-some metabolic functions.
-mechanism of thermoregulation.
the hypodermis:
loose connective tissue and fat.
gives stability to structures about.
reduces heat loss from body (insulation).
absorbs blows to the body.
energy reserve.
the epidermis:
made up of keratinocytes, melanocytes and merkel cells. rete ridges highly developed in areas exposed to stress- hands/feet. increase contact with dermis to prevent epidermis falling off (blister formation)
why is the epidermis stratified:
stratification has advantages over monolayer- stem cell population protection deep inside. upper cells provide protection. monolayer is more difficult to replace if abraded (barrier function would be lost), dividing population and protective population are same.
stem cells in the epidermis:
stem cells at tips of rete ridges- unlimited division possible. transit amplifying cells (not stem cells) make up rest of basal layer- 1,2 divisions.
basal layer:
columnar cells anchored to basement membrane- hemidesmosomes.
proliferative- divide every 200/400 hours. undifferentiated.
spinous and granular layer:
spinous layer: cells larger and flattened- 8-10 cells thick. differentiation starts- biochemical change. desmosomes. langerhans cells (travel from the bloodstream to the epidermis).
granular layer: keratohyalin granules containing profilaggrin. lipid filled lamellar granules.
transition zone:
between granular (living) and cornified (dead) layers (recycled granular layer). enzymatic activity and cellular restructuring. profilaggrin cleaved to filaggrin- causes keratin filaments to be restructured. organelles are destroyed by proteases, DNase, RNase, acid hydrolases, and plasminogen activator. lamellar granules fuse with the plasma membrane and release lipid into extracellular space- lipids form sheets for waterproofing. cornified envelope forms.
cornified layer:
terminally differentiated. cell loses 70% dry weight. forms corneocyte- flattened cell, forms protective surface, desmosomes and interlocking ridges/grooves. 80% is keratin encased in cornified envelope- covalently linked protein (involucrin) structure formed beneath the plasma membrane during terminal differentiation. remnants of organelles, melanin pigment. corneocytes sloughed from skin surface. interstitial fluid (tissue fluid) lost by insensible perspiration (not the same as sweat from sweat glands).
reasons for keratinocyte maturation:
normal epidermal turnover (homeostasis). wound healing. prevention of skin cancer or premature ageing.
how is damage from radiation prevented:
melanocytes synthesise melanin. melanin- absorbs UV radiation, scavenges free radicals. its production is increased by sun exposure. 2 types- eumelanin is brown/black, phaemelanin is yellow/red.
melanocyte function:
melanosomes are vesicles containing melanin. move along arm like structures called dendrites and passed to keratinocytes. form cap over keratinocyte nucleus for protection.
the dermis:
directly below epidermis. thickness from 0.6mm on eyelids to 3mm on palms and soles.
contains- mast cells (immune cells involved in inflammation allergy and wound healing), dermal appendages (hair follicles, sweat glands, blood vessels), fibroblasts.
functions of fibroblasts:
produce chemicals/stimulants which diffuse to epidermis to control proliferation.
produce collagen for tensile strength for resistance to longitudinal stress.
produce elastin for stretch and recoil for flexibility and movement.
produce elastin for stretch and recoil for flexibility and movement.
produce structural proteoglycans/glycosaminoglycans for hydration and turgor.
make extracellular matrix (cells embedded without being in direct contact).
2 dermis layers:
superficial papillary layer- containing sensory nerves and loose connective tissue.
underlying reticular layer- contains collagen fibres. anchors papillary layer to subcutaneous layer.
mechanism of sensation:
nerves give perception of sensation- most common where sensation important.
sensors in epidermis and dermis- merkel cells in basal layer of epidermis, touch (meissners corpuscles- in dermal papillae of hands/feet). pressure/vibration- paccinian corpuscles encapsulated nerve endings deep in dermin/hypodermis of hands and feet.
pain/temperature/itchiness- myelinated and non myelinated free sensory nerve endings.
hair:
5 million hairs on the body- 98% on the skin surface.
no hair on parts of genitals or palms/soles.
dense on scalp.
emerge from follicles from deep dermis/subcutis to surface. 2 parts called the root and shaft.
hair structure:
dermal papilla formed from connective tissue- holds capillaries and nerves.
hair bulb surrounds papilla with epithelial cells- proliferative cells.
3 layers of hair: inner medulla contains flexible keratin.
outer cortex contains hard keratin.
hair fibre coated in keratin called cuticle.
2 layers of epithelial wall- internal root sheath surrounds the root, external root sheath covers entire length of follicle.
hair colour and hair types:
melanocytes at base of hair produce melanin pigment. over time, less pigment is made- grey colour.
lanugo hairs- form at 20 weeks gestation, usually shed before birth, fine and long.
vellus hairs- commonest hair type over body surface, short fine and light colour.
terminal hairs- thick and long, 10,000 on scalp, also on eyebrows, eyebrows eyelashes beard and pubic areas.
hair production:
growth is cyclical at rate of 0.4mm/day. hair root firmly attached to matrix.
growth stops- follicle inactive, shrinks so attachment broken down.
growth restarts- old hair pushed upwards and is shed. some drugs can halt hair growth- cytotoxins, heparin, warfarin.
nail:
Dense plate of cells
Packed with hardened keratin (0.3-0.5 mm thick).
Protects finger tip and help grasping.
Fingernails grow at 0.1 mm/day.
– Toenails more slowly
nail structure:
Nail plate from nail matrix (root) overlies nail bed– Pink due to dermal capillaries
* Nail bed produces some keratin
* Growth in root – Keratinocytes divide and
mature (keratinize) into nail plate
* Epithelial cuticle protects nail
