• epidermis: ectoderm, keratinized stratified squamous epithelium
• dermis: derived from mesoderm; dense connective tissue
• hypodermis-subcutaneous tissue/fascia deep to dermis; provides anchorage
• epidermal appendages-hair, hair follicles, sweat glands, sebaceous glands, nails

Thickness of skin= due to thickness of epidermis (drv ectoderm, keratinized stratified squamous edpithelium)

ex: hairless skin of palm and foot.

layers of epidermis

CLGSB

from bottom to top:

• stratum basale: (stratum germinativum) deepest layer, separate from dermis by basement membrane (basal lamina) and attached by hemidesmosomes. cells are cuboidal to columnar and are mitotically active stem cell.
• stratum spinosum: "prickle cell layer" irregular, polyhedral cells with processes that extend outward and contact neighboring cells by demosomes. tonofibrils found in processes.
• stratum granulosum: diamond shaped cells which contain keratohyalin granules; granules have cystein rich and histidine rich protein precursors of filaggrin that aggregates keratin filaments present in cornified cells.
• stratum lucidum-thin clear layer consist of eleidin (see in thick skin only)
• stratum corneum- outermost layer-made up of keratin and horny scales which were once living cells. dead cells known as squames (anucleated)--calluses.

• Keratinocytes
• Melanocytes
• Langerhan's cell
• Merkel's cell

• cell divide in stratum germinativum (basale). one cell remains, other cell is pushed up toward the surface.
• cells are pushed into the next layer, stratum spinosum where tonofibrils are present.
• cells pushed toward surface and become flattened and diamond-shaped=stratum granulosum. cells are keratohyalin granules or tonofibril.
• cells continue to be pushed toward surface where they flatten, lose organelles and nuclei. keratohyalin granules turn into keratin.
• finally cell reach surfae and are shed, so keratinocytes produce keratin layer.

• epidermis cells, orginiated in basal layer
• produce keratin-->water barrier
• In stratum spinosum: cells produce keratohyalin granules,intermediate associated proteins, filaggrin and trichohyalin (keratiniation) and lamellar bodies.
• In stratum granulosum, karatinocyte discharges lamellar bodies. remainder of cytoplasm has many keratohyalin granules associated with tonofilaments.

Epidermal water barrier: determined by 2 factor in terminally differentiating keratnocytes:

1. deposition of insoluble proteins on the inner surface of the plasma membrane (cell envelope)-crosslinked of proline-rich proteins and larger proteins.
2. a lipid layer attached to outer surface of plasma membrane (lipid envelope)- formed by lemellar bodies exocytose into intercellular spaces between the stratum granulosum and corneum.

• derived from neural crest cells, found in stratum basale, protective agains UV. 
• melanin granules from melanocytes are transferred via the long processes to cytoplasm of basal keratinocyte.
• melanin Production: oxidation of tyrosin to 3,4 DOPA, DOPA transformed into melanin. occur in premelanosomes.
• melanin transferred to neighboring keratinocytes by pigment donation involve phagocytosis of tips of melanocyte processes.

• dendritic, antigen-presenting, need special stains to visualize in stratum spinosum
• mesenchymal origin, derived from bone marrow-part of mononuclear phagocytic system
• birbeck granules: tennis rackete shaped.
• uptake Ag in skin and take to lymph node.

• acute sensory perception
• mechanoreceptor
• modified epidermal cells in stratum basale
• found in finger tips
• bound to adjoining keratinocyte by desmosomes, contain neurosecrtory granules similar to adrenal medulla.

1. free nerve endings: most numerous, terminate in stratum granulosum, lack connective tissue and schwann cells, fine touch, temperature and mechanoreceptors. surround hair follicle -"whisker"
2. Paccinian COrpuscle: deep pressure receptor for mechanical and vibratory pressure. dermis, hypodermis,myelinated, onion shaped.
3. meissner's corpuscles: touch receptors within dermal papillae, response to low-frequency stimuli in papilarry layer of hairless skin, look like Q-tip.
4. Ruffini's corpuscles:dermis, mechanical displacement of collagen fiber.

2 layers of connective tissue that merged:

1. papillary layer: thinner, composed of loose connective tissue and contacts epidermis
2. reticular layer: dense irregular bundles of connective tissue with coarse elastic fiber and collagen fiber.

regular lines of tension in the skin formed by collagen and elastic fibers in reticular dermis.

Less scarring occur if cut along these line.

• thermoregulation (temperature)
• Nutrition

• invagination of peidermis, deepest down growth becomes geminal matrix.
• three segments: 1. infundibulum (surface to opening of sabaceous gland) 2. isthmus (infundibulum to insertion of arrector pili muscle) 3. inferior segment (growing follicle portions, at base expands to bulb)

• germinal matrix sits over connective tissue, papilla brings capillaries into dividing cells
• 3 layers:

1. cuticle: squamous cells
2. Huxley's layer: flattened cells
3. Henley's layer: cuboidal cells in direct contact with outermost part of hair follicle, the external root sheath.

• keratogenous zone
• hair emerges from follicle it is entirely keratinized as hard heratin

• Medulla-middle part, soft keratin, large vacuolated cells, only present in thick hair.
• cortex-peripheral to medulla, cells here undergo differentiation into keratin-filled cells, contains pigment
• cuticle of hair shaft-outermost layer of hair composed of very thin scale-like cells.

secrete sebum: fatty material oil and lubricate hair. possess bactericidal and fugicidal properties.

Sebacoeus glands are holocrine glands (cell dies in excretory process): sebum come out cell suicide.

• eccrine: distribute all over body but lips and external genitalia; no loss of cytoplasm (merocrine excretion)
• aprocine: limited to axilla, areola, nipple, skin around anus and external genetilia, thought to be aprocine (membrane budding) but actually mecrocrine.

• simple tubular gland with secretory part and excretory part.
• excretory part go to surface of the skin.
• thermoregulation-cholinergic-start at scalp and forehead
• emotional-stress adrenergic: start in palm, sole, axilla.

• clear cells: glycogen, pale cytoplasm, secrete water, nacl, urea.
• dark cells: secretory granules, secrete glycoprotein
• myoepithelial cells: help expell sweat.  only secretory segment have this not excretory segment.

• aprocine produce odor (adrenergic)
• eccrine does not (cholinergic)

• nail bed has stratum basale, spinosum
• no granulosum
• nail itself is stratum corneum
• hig sulfur content make nail hard
• crescent-shaped white aread near nail root- lunula
• eponychium (cuticle)-edge if skin fold covering nail root
• hyponychium-epidermis underlying distal tip of nail.