Structure and Function of Epidermis and Dermis

Skin

function: barrier; temperature regulation; sensory organ; physical protection; immunity; psychosocial/aesthetic importance

layers: epidermis; dermis; subcutaneous fat

Keratin (major structural component of epidermis)

type of intermediate filament formed as obligate heteropolymer composed of two types of keratin monomers

5/14 Þ basal cells (genetic defects in 5 or 14 Þ epidermolysis bullosa simplex)

1/10 Þ spinous cells (become 6/16 in hyperproliferative epidermis i.e. psoriasis or atopic dermatitis)

85% of total protein in terminally differentiated keratinocytes

Epidermis

stratified squamous epithelium; continually renewed Þ need stem cell population Þ basal cells

functions: produce stratum corneum Þ barrier function (lost in burn victims); hold itself together (pemphigus - autoimmune disease where antibodies are directed against cell adhesion junctions Þ loss of function Þ blisters); contribute to production of epidermal/dermal basement memb

basal layer: single layer; columnar; keratin 5/14; only skin cells that can divide Þ stem cells Þ transient amplifying cells (50% of basal cells) Þ divide several times Þ differentiate

stratum spinosum: 3-10 cells thick (largest layer); keratin 1/10

migrate towards surface Þ lose water, form desmosomes, become larger and flatter

stratum granulosum: 2-3 cells thick; organelles begin degrading

keratinocytes with keratohyaline granules: contain proflaggrin Þ flaggrin (component of stratum corneum)

stratum corneum (cornified layer): 15 cells thick; flat, polyhedral cells (corneocytes); no organelles or nuclei

cytosol: mostly keratin filaments and flaggrin; encased in protein shell (involucrin, loricrin)
lipid enriched membranes make up the extracellular space

‘mortar’ between keratinocytes; provides water barrier

Epidermal/Dermal Junction

adhesion; semi-permeable barrier; Ý frictional resistance Þ stability (anchoring complexes)

composed of basement membrane - restricts molecular on basis of size/charge; permits migrating/invading cells

ubiquitous proteins (laminin I; collagen IV) and some unique (bullous pemphigoid Ag; laminin 5; collagen VII)

four zones via electron microscope

hemidesmosomes: attach basal cells to the junction via interaction with keratin filaments

lamina lucida: anchoring filaments (attach to hemidesmosomes); (a₆b₄ integrin, laminin 5, bullous pemphigoid Ag)

lamina densa: laminin I; collagen IV (found in all BM)

sublamina densa: anchoring fibrils (collagen VII)

diseases:

epidermolysis bullosa: genetic disease Þ defects in various adhesion molecules Þ separation Þ blisters

bullous pemphigoid: autoimmune disease Þ antibody to bullous pemphigoid antigen in epi/derm junction Þ blisters

Dermis

fibroelastic connective tissue

divided into papillary (superficial) and reticular (deep) layers

major components via fibroblast: collagen; elastin; interfibrillar gel (proteoglycans, salt, water) Þ protect against trauma

collagen: 80% of fat-free weight of skin; major type is I; minor type is III

glycine every 3^rd amino acid; also high in proline and hydroxyproline
triple helix: highly stable; provides tensile strength; holds connetive tissue matrix together

elastin: ‘perfect oil’ of 1/3 randomly spaced glycine and mostly other hydrophobic aa (hydrophobic aa’s can slide easily)

elastic fibers: core of elastin held together by fibrillin and microfibrillar-associated proteins 1,2,3 and 4

Structures in the Skin

Vasculature

function: nutrients, wound healing, control of hemostasis, modulation of inflammation

vascular plexus in deep reticular dermis Þ three superficial plexi Þ smaller arterioles Þ superficial capillaries in upper

papillary dermis (a single papillary loop of capillaries extends into one dermal papilla)

epidermis is avascular thus gets all nutrients by diffusion from vessels in dermis

Glands

eccrine: sweat glands found throughout the skin - facilitates evaporative cooling

apocrine: scent glands mainly in axilla and anogenital region; not involved in cooling

Hair Follicles (formed from epidermal-dermal interaction)

all are formed at birth; no new follicles during life (character of hair may change: size, cyclic behavior, color)

vellus hairs: fine, short hairs

terminal hairs: thicker pigmented i.e. on head

cyclic growth phase: anagen phase of growth, for years; telogen phase of rest, for months

50-100 hairs shed daily

layers of follicle: medulla, cortex, inner root sheath, outer root sheath

base is in subcutaneous fat; arrector pili muscle attached to side; sebaceous gland above arrector pili attached to side

hair grows at base of follicle Þ telogen (lower third of follicle reabsorbed) Þ papillae moves upward to pili Þ anagen

Subcutaneous Fat

shock absorption; insulation (maintains body heat); energy storage

microlobule (basic unit; numerous adipocytes; 1mm) Þ secondary lobules (separated by connective tissue septa)