SKIN + APPENDAGES Flashcards
Epidermis
- stratified squamous keratinized epithelium
- composed of keratinocytes x keratocytes cells
- lacks microvasculature; nutrients + O2 through diffusion by the dermis
- distinction ofrm the thic and thin skin depending on the thickness of the epidermal layer
(thick - 400-1400um and thin - 75-150 um).
- stratum basale
-single layer of cuboidal/columnar cells (basophilic)
- sit on the basement membrane of the dermis-epidermal junction
Structures
1.Hemidesmosomes - anchor the epith cells to the basal lamina (of the basement membrane)
2.Desmosomes - bind the cells laterally and upper surface (apical)
Cells
1.Keratinocytes
- basal stem cells
- cytoskeletal keratins (intermediate filaments 10 nm diamater)
2. merkel cells
3. melanocytes
2.stratum spinosum (thickest layer)
-polyhedral cells with a central nuclei + nucleoli
- cytoplasm that synthesizes keratins > Prickle cells
Structure
- keratins filaments are organized into the tonofibrils (in bundles) –> terminate at the desmosomes to hold the cell layers together
-> Some cells form the basal layer may still dvide and forms the stratum germinatiuvum (basal + spinous layer)
Cells
- desmosomes
- langerhans
- stratum granulosum
- 3-5 layers of the flattened polyhedral cells
> undergoing terminal keratinization - cytoplasm is filled with basophilic keratohyaline granules (dense , non-membrane-bound masses of Filaggrin.
- lamellar granules; small ovoid with lipids and glycolipids.
*produces a lipid rich impermeable layer (ectodermal barrier) which prevents water loss
desmosomes - tight junctions
- stratum lucidum
(only found in thick skin)
- flattened non-nucleated eosinophilic keratinocytes
- held by desmosomes
- the nuclei and the organelles are lost
> consists mainly of packed keratin filaments in an electron dense matrix
5.Stratum corneum
15-20 layers of squamous, non-nucleated keratinized cells filled with dead squamous of keratins filaments
>the fully keratinized cells shed at the epidermis
Melanocytes
neuroectodermal (neural crest cells derivatives)
- round pale-staining cells that attach to the basal lamina via the hemidesmosomes
- produce eumelanins ; black or brown pigment
- are found in the stratum basale
- between stratum basale and spinosum extend long irregular cytoplasmic processes to the keratinocytes (5-10), abundaunt mitochondria, golgi apparatus and cisternae of RER
- for red hair = pheomelanin pigment
Keratinocytes content of melanin and carotene.
Melanin synthesis
- tyrosine into DOPA (3,4-dihydroxyphenylalanin) by tyrosinase (enzyme) = melanin
- accumulates in vesicles = melanosomes- mature elliptical 1um long granules
- melanosomes are transported by kinesin to the cytoplasmic extensions - phagocytosis by the keratinocytes - dynein to the nuclei
- Melanosomes. degranulates and melanin accumulate in the keratinocytes
- absorbs and scatters UV light to protect the nuclear DNA
- UV light triggers MSH + paracrine factors secreted by the keratinocytes which accelerate melanin synthesis
- corneum and lucidum layers have NO NUCLEI = no damage to DNA
epidermal-melanin unit
one melanocyte + keratinoc0ytes that transfers melanosomes (5-6)
Langerhans cells
- antigen-presenting cells
- bone marrow derived (monocytes)
- star shaped cells found in the stratum spinosum (2-8% of epidermis)
- dendritic cells that have cytoplasmic extension through all the layer of keratinocytes
Function;
- bind, process and present the antigens to the T-lymphocytes.
- microorganism can penetrate the epidermis without alerting these cells (immune response of the epidermis)
- contain birbeck cells at the periphery (tennis racket cells)
Merkel cells/ epithelial tactile cells
- neuroectodermal origin
- mechanoreceptors fo the light touch sensation.
- are joined to the basal layer by the desmosomes
- highly abundant in the sensitive skin eg. finger tips
- basolateral surfaces of the cell that contact termnal discs of un-myelinated sensory fibers penetrating the basal lamina.
Dermis
- dermatome origin (somite)
- binds the hypodermis to the epidermis
- thickness depends on the area and reaches a max of 4mm at the back
- basement membrane
- Between the stratum basale and the the dermis - used form the diffusion of the nutrients to the keratinocytes
>dermis vasculature to the epidermis.
- outermost papillary
- irregular surface with projections = dermal papillae composed of loose CT.
> they interdigitate with the epidermal pegs, usually in frequent pressure skin (dermal-epidermal junction)
the loose CT contains cells
- types 1 and 3 collagen fibers
- fibroblasts
- mast cells + leukocytes + macrophages
- type 7 anchoring fibrils insert into the basal lamina to connect to the epidermis.
- reticular layer (deeper)
- dense irregular CT (thicker)
- contains less cells and more fibrils
>abundant in collagen type 1
> network of elastic fibers, for skin elasticity. - between the the collagen and elastic fibers there are proteoglycans rich in dermatan sulfate.
Vascular supply of the skin
Dermal regions with netwroks of the blood and lymphatics:
1.subpapillary plexus; is between the two dermis layers and capillaries branch to the dermal papillae to the epidermis
2. deep plexus; dermis and the hypodermis
3. arteriovenous anastomoses for thermoregulation function between the major plexi.
>regulate the flow to the dermal papillae for heat loss.
4. lymphatics at the dermal papillae and form 2 plexi
Dermis innervation
Subcutaneous tissue (hypodermis)
- loose CT that binds the skin to the subjacent organs
- contain adipocyte tissues, hair follicles, sweat glands and fibrocollagenous bands
- extensive vascular supply for rapid uptake of drugs into the tissue.
Non-encapsulated receptors
1.Merkel cells; associated with expanded nerve endings; light touch and object texture
- Free nerve ending; papillary dermis to the lower epidermal layers; temp,pain,itch (tactile receptors)
- root hair plexuses; sensory fibers at the bases of hair follicle in the reticular dermis; hair movement
Encapsulated receptors
1.Meissner corpuscles; elliptical structures 30-75um by 50-150um
- sensory axons amongst schwnn cells in the dermal papillae perpendicular to the epidermis; light touch or pressure deform skin shape
- Lamellated (pacinian);
oval structures, 0.5mm by 1mm found in the reticular layer and hypodermis
- flattened Schwann cells and collagen around branched unmyelinated axon; firm touch, pressure, vibrations
- impulses by the distortion of the capsule or tissue
-can also be found in CT of organs - Krause end bulbs;
ovoid composed of thin collagenous capsule penetrated by the sensory fibers
- penis and clitoris low frequency vibration
4.Ruffini; collagenous, fusiform capsules anchored to CT with sensory fibers stimulated by stretch
Sebaceous glands
- found at the dermis except the thick skin
- branched acinar HOLOCRINE glands with a duct covered in acini (secretes at upper portion of the hair follicle)
(holocrine secretion) - hairless areas the duct secretes directly onto the epidermis
-acini have a basal layer with flattened undiff. eipthelial cells on the basal lamina ; become lipid producing sebocytes w small fat droplets.
(the cells autophage and sebum is released near the duct)
- a mixture of lipids; wax esters,squalene,
cholesterol and triglycerides
Sweat glands
long epidermal invaginations embedded in the dermis
1.eccrine glands
- all body and but most on foot sole
- simple tubular coiled glands
a.secretory portions;
- more pale-stained than the ducts
- stratified cuboidal epith with 3 cells;
1. clear cells - on the basal lamina and produce sweat.
- abundant microvilli
- interstitial fluid is transported directly into the gland lumen or the intercellular canaliculi
2.dark cells- filled with eosinophilic granules lining the lumen of the glands
- merocrine secretion for the rlease of glycoprotein mix for innate immune function
3.myoepithelial cells - on basal lamina that contract to move the secretion into the duct.
- innervated by cholinergic fiber
b. Ducts - two more layer of acidophilic cells filled with mitochondria
- at the epidermis it merges with the stratum basale and flow through the epidermis layers to a sweat pore.
- absorb the Na+ ion
- apocrine sweat gland - axillary and perineal regions
- development depends on the sex hormones
- larger lumens than eccrine glands w simple cuboidal, eosinophilic cells + secretory granules.
<actually merocrine secretion)
b. Ducts are the same as eccrine but open into the hair follicle and contain protein rich product.
- initial secretion of odorless but may acquire form bacterial activity
- produces pheromones
- innervated by the adrenergic nerve endings
Hair
- elongated keratinized structures
- form epidermal invaginations = hair follicles
- all skin has hair except the GLABROUS SKIN.
face - 600 hairs /cm2 and the body 60hairs/cm2
1.Hair root matrix
- hair bulb as the dilatation of the follicile
- dermal papilla (loose CT) at the base of the bulb with a capillary network
- keratinocytes of the basal eoidermis covering the dermal papilla
2.Hair shaft - hair extending beyond the skin surface
- melanocytes at the matrix of the hair bulb will transfer melanosomes into the epithelial cells that will become the hair
- keratin in hair is more compact
Most thick hairs have a medulla formed by the vacuolates and moderately keratinized cells of root.
The cortex surrounding it is composed of heavily keratinized, densely packed fusiform cells
outer most is the cuticle formed by the a layer of heavily keratinized, squamous cells covering the cortex
outermost cells of the hair bulb continue with the epithelial root sheath
a. internal sheath surrounds the initial hair root but degenerates above the level of the sebaceous glands
b.external sheath covering the internal, extending to the epidermis.
c. hyaline layer and basement membrane - glassy membrane separating the follicle form the dermis
d. dermis ends up forming CT sheath
arrector pili muscle
- bundle of smooth muscle form the midpoint of the fibrous sheath to the dermal papillary
- contraction for the erection of hair (trap the warm air).
Hair growth
-asynchronously, cyclic and at diff rates
cycle has 3 phases:
1. anagen - long period of miotic activity and growth
2.catagen - arrested growth and regression of the hair bulb
3. telogen - inactivity when the hair may be shed
At phase 1 again the epidermal stem cells at the external root sheath produce progenitor cells for the new hair bulb.
