Describe the major functions of the skin.

- Protection from mechanical trauma
- Barrier to water loss
- Protection from UV damage
- Protection from microbial agents & Immunologic response
- Thermoregulation

- Photoactivation of Vitamin D

- Sensory Reception (touch, pressure, heat, cold, pain)

Layers of skin: EBM-DPRS

Epidermis
Basement Membrane
Dermis
1. Papillary dermis
2. Reticular dermis
Subcutis (Hypodermis)

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Stratum corneum: distinguishes thin from thick skin; very flattened apoptotic keratinized cells; held together by desmosomes, full of cross-linked cytokeratin filament, and covered by the phospholipid products of lamellar bodies between cells -> water-proof; undergoes desquamation by enzymatic degradation

Stratum lucidem: subset of corneum; thin pale layer found only in thin skin; cells lost nuclei

Stratum granulosum: sets up the formation of stratum corneum; keratinocytes produce kertohyaline granules (when activated as filaggrin is involved in cross-linking the cell's cytokeratin w/ tough kertains of the stratum corneum) and lamellar bodies; cells are undergoing apoptosis, but not being phagocytized or fragmented

Stratum spinosum: keratinocytes mature and enlarge, filling cytoplasm with keratin intermediate filaments; variable thickness; have prickle cell appearance; langerhans cells also are present, as well as melanocytes cell processes

Statum basale: single layer of cells on the basement membrane; keratinocytes; cuboidal; mitotic figures are common; have desmosomes and hemidesmosomes, which attach basale to basement membrane

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Define: keratinocytes

keratinocytes: predominant cell type in epidermis, stratified squamous

Produce: involucrin (cross-linking), lamellar bodies ( secrete impermeable, lipid-containing sheet -> prevent evaporation),

keratohyaline granules -> keratinization

Immune: cytokine production

UV protection: phagocytize melanocyte tips
and melanosome move to cover
the keratinocyte nucleus.

Define: hemidesmosomes

hemidesmosome: cell-ECM connection mediated by integrins; often connect basement membrane to other cells, e.g. stratum basale

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Define: tonofilaments/tonofibrils/keratin intermediate filaments

keratin intermediate filaments: with the cell membrane, are cross-linked, producing a thickened cornified layer

Define: keratohyaline granules (distinguish from lammellar bodies)

keratohyaline granules: produce filaggrin which is involved with crosslinking process

produced by keratinocytes

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Define: lamellar bodies / membrane coating granules

Contrast the papillary dermis, the reticular dermis, and the subcutaneous tissue (hypodermis), listing the characteristic structures found in each, including blood supply.

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Papillary: "dermal papillae"; thinner; fine collagen fibers; capillary loops that deliver nutrients to epidermis and some sensory nerve endings

Reticular: thicker; coarse bundles of collagen fibers, with elastic fibers; "dense irregular connective tissue"; has some of the skin appendages (e.g. eccrine glands & small hair follicles)

Subcutis (hypodermis): subcutaneous CT; adipose tissue; large blood vessels; large terminal hair follicles; Pacinian corpuscles; sometimes skeletal muscle

Describe the various nerve endings in skin. Identify characteristic features in photographs and diagrams.

Efferents: Postganglionic fibers from ganglia in the paravertebral chain supply blood vessels, sweat and sebaceous glands.

Afferents:

Naked nerve endings: (pain)

Networks around hair follicles:

Meissner corpuscle(tactile): dermal papillae[image]

Pacinian (pressure):subcutaneous

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Merkel endings

Describe the origin, structure, and function of melanocytes. Describe the process of melanin production and factors influencing skin pigmentation. Describe how melanin protects against damage by UV radiation. Describe additional UV protective mechanisms.

Melanocytes: terminally differentiated cells

Location: stratum basale->stratum spinosum; (also outer root sheath of the hair follicle, iris of the eye, the retinal pigment epithelium and other sites)

Function: produce melanin for UV protection, servicing the keratinocytes melanin; keratinocytes phagocytize the melanin, providing helmet above it's nucleus.

Production: Tyrosine → DOPA -> melanin family (via tyrosinase)

Pigmentation: not function of melanocyte numbers, but in type of melanin, size of melanosomes, and rate of degradation by kertinocytes; ratio of types of melanin lead to differences in skin color, etc.

Additional UV protection:

-Antioxidants from keratinocytes (catalases, peroxidases, etc.)
-Macrophages
-Langerhans cells

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Describe the differences between eccrine and apocrine sweat glands. Describe the role of the integument in thermoregulation.

Eccrine: simple coiled tubular glands; most numerous on soles, least on back; heat stablization via evaporation; innervated by unmyelinated sympathetic C fibers (ACh, as opp. to norepinephrine); neural control via hypothalamus

Secretion: merocrine

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Apocrine sweat glands:

Location: subcutaneous of axilla, anus, and areola-> hair follices; viscous secretion; myoepitehlial cells present; innervation by sympathetic adrenergic

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Describe the origin, structure, and function of Langerhans' cells and Merkel cells. Describe the contribution of the skin to both innate (nonspecific) and acquire (speicifc) immunity.

Langerhans: antigen-presenting member of the MPS (mononuclear phagocytic system) in the epidermis. It is related to the bone marrow-derived monocyte.

innate (nonspecific) immunity: internalize & process antigen in endosomes, binding processed antigens to antigen-presenting molecules on cell surface; migrate to reional lymph nodes and present antigen to naive lymphocytes-> T lymphocytes activation and proliferation -> memory cells

acquired (specific) immunity: upon 2nd presentation of antigen, Langerhans process and present antigen to sensitzed lymphocytes in nodes, or present in skin-> activation and proliferation

Merkel cells: An epidermal cell located in the stratum basale associated with sensory perception. It has cytoplasmic dense core granules, similar to cells of the dispersed neuroendocrine system

Describe the structure of sebaceous glands.

Sebaceous glands:

part of thepilosebaceous apparatus of the hair follicle (except the tarsal glands of eyelid).

Location: thin skin; and are an outgrowth of external root sheath of hair shaft.

Secretion: holocrine -> The short duct opens into the hair follicle.
Arrector pili muscles help expel the secretion.

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Describe the structure of the adult female inactive breast. Discuss how inactive and senile mammary glands differ from the active stage. Identify each stage in photographs and diagrams.

Prepuberty: Composed entirely of the duct system; no
secretory alveolar units (terminal duct-lobular units).

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Inactive (non pregnant) Post-pubertal female
Enlargement of the breast is due primarily to the accumulation of adipose tissue.
Rising estrogen levels stimulate the growth and
branching of the duct system.
No secretory alveoli are present, but may have mild
secretory activity of ducts during mid to late phases of
the menstrual cycle.

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Early to mid pregnancy
Prominent inc. in duct branching induced by estrogen;
Development of alveoli as extensions from terminal ducts is induced by progesterone.

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Late pregnancy

Significant breast enlargement due to hypertrophy of alveolar cells

Lumens of ducts and alveoli widen as secretory products
accumulate

Describe the structure of the nipple and areola.

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-Latiferous ducts dilate slightly into lactiferous sinuses, and then open onto the skin

-Skin of nipple and surrounding areola is typical thin skin with the following variations:

--increased sensory nerve endings

--increased smooth muscle -> nipple erection

--increase sebaceous & apocrine glands

--increased pigmentation (esp. during pregnancy)

Describe the structure of nails. Idenitfy the major components in photographs and diagrams.

Describe how the different layers of the integement contribute to protection against mechanical trauma.

Germinative (basal) layer:
-Adhesion of basal cells to basal lamina
- Hemidesmosomes connect to intracellular keratin filaments, providing additional strength
- The ability of basal cells to proliferate provides for
regeneration after physical trauma
- Basal keratinocytes are resistant to apoptosis via expression of growth factors receptors, which disappear as cell progresses towards the cornified layer

Spinous layer - Desmosomes & their interaction with the keratin network

Dermo-epidermal junction
- Increased surface area for interface improves strength
of interface & increases potential number of proliferating basal cells
- More ridges in areas of high abrasion

Reticular dermis – cushion

Subcutis – allows skin to move freely relative to underlying structures

Describe how the different layers of the integement contribute to preventing water & electrolyte loss.

Keratinization

Granular layer

- Set up the formation of stratum corneum:
- Keratinocytes produce involucrin, lamellar bodies, keratohyaline granules
- Keratin intermediate filaments and the cell membrane are cross-linked, producing a thickened cornified layer
- Lamellar bodies (membrane-coating granules) released via exocytosis
- Apoptotic cells are not phagocytized or fragmented, but are desquamate (shedded)

Stratum corneum
- Barrier to penetration by external substances and to
transepidermal water loss

- Dead corneocytes remain intact because of crosslinking of cytoskeleton and plasma membrane (filaggrin from keratohyaline granules participates in crosslinking process)
- Lamellar bodies (membrane-coating granules) released via exocytosis

- Phospholipid lamellae sandwiched b/w layers of dead corneocytes prevent evaporation

CC: Loss of large areas of epidermis can result in oozing of interstitial fluid with loss of water, protein & electrolytes, especially potassium (e.g. scalded baby syndrome)

Describe the contribution of the skin to production of Vitamin D3.

Cells of skin synthesize vitamin D3, needed in calcium metbolism and proper bone formation, through the local action of UV light on the vitamin's precursor, 7-Dehydrocholesterol, primarily found in the stratum basale and stratum spinosum.

Describe the major types of sensation experienced by the skin and major types of neural sensory transducers.

Types & specificity of sensation vary with location:  
vibration, pressure, pricked, stroked, hairs bent (and direction) or pulled, etc.

Sensory receptors are present in both epidermis & dermis
The area of sensation transmitted to a single dorsal root
& its ganglion is a ‘dermatome’; dermatomes overlap

Describe the function and control of sweat production and the signifance of sweat components.

Contents of sweat: NaCl - secretory coil; K+ (excessive amounts has CC)

controlled by heat and emotional stress;

Distinguish between glabrous and non-glabrous skin, between apical and non-apical skin. Describe the neural control of dermal vasculature.

Glabrous skin: w/out hair ("free from hair")

non-glabrous: w/ hair

corresponds to thick vs. thin

apical skin: found in the exposed but poorly insulated areas of the body, e.g. palms, soles, face, and ears; has AV anastamoses

non apical skin: everywher else, lack AV anastomoses

heat exposure dilates the arterioles; cold exposure constricts the arterioles via cutaneous nerves

Describe the structure of hair. Identify these strctures and their characteristic components in photographs and diagrams. Distinguish between 'lanugo', 'vellus', and 'terminal' hair. Describe the hormonal control of hair differentiation.

Lanugo hair – fetal hair, shed shortly before or after
birth
Vellus hair – located over most of the body; short, non-pigmented
Terminal hair follicles –located on ‘hairy areas’

Hair bulb
Contains matrix cells which produce the hair shaft
Melanocytes in the germinal layer make melanin, producing the color of the hair shaft.
Dermal papilla
Lies within the hair bulb

Contains capillaries and specialized fibroblasts
Controls number of matrix cells & thus hair size;

DP produces growth factors and sends it to bulge to proliferate secondary stem cells; have androgen receptors-> which increase follicle size; can cause male-pattern balding

Outer root sheath

Epithelium is continuous with surface epidermis
Contains melanocytes, Langerhans cells & Merkel cells,
all of which may repopulate the epidermis after damage

A bulge of the outer root sheath
located near the insertion of arrector pili muscle, has multipotent stem cells for hair, epidermal & sebaceous gland regeneration

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