Anatomy Chapter 5

Chapter 5
The skin is composed of two distinct regions, the epidermis and the dermis. These two areas are firmly attached to one another along a wavy borderline. The epidermis (epi = upon) composed of epithelial cells, and is the outermost protective shield of the body.
Epidermis - Structurally, the epidermis is a thick keratinued stratified squamous epithelium consisting of four distinct cell types and five distinct layers.
Cells of the Epidermis - Cells populating the epidermis include: keratinocytes, melanocytes, Merkel cells, and tactile cells.
Keratinocytes: The most numerous cells are the keratinocytes which produce keratin, a fibrous protein responsible for protective properties of the epidermis.
Melanocytes: located at the base of the epidermis. Specialized cells that synthesize the pigment melanin. Melanin protects the cell nucleus from the destructive effects of UV radiation
Langerhans cells: arise from the bone marrow and migrate to the epidermis and other areas of the body containing stratified squamous epithelial tissue.
Merkel cells: present in small numbers at the epidermal-dermal junction. Associated with a disc-like ending of a sensory nerve fiber, called a Merkel disc, which functions as a sensory receptor.
Layers of the Epidermis:
In thick skin (which covers palms, fingertips, soles of feet) the epidermis consists of 5 layers or strata: (from deep to superficial) stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum.
Thin skin - which covers the rest of the body has only 4 layers, with the stratum lucidum absent.
Dermis:The second major skin region, is a strong but flexible connective tissue layer. The cell types found in the dermis are fibroblasts, macrophages, and occasional mast cells and white blood cells. Its gel-like matrix is heavily embedded with collagen, elastin, and reticular fibers. The dermis is your "hide" and is richly supplied with nerve fibers, blood vessels, and lymphatic vessels. The major portions of hair follicles, as well as oil and sweat glands, reside in the dermis, but are derived from epidermal tissue. The dermis varies in thickness and it has two major layers: papillary and reticular.

Chapter 4 Endocrine Glands * Ductless glands
– Secretions not released into a duct * (by exocytosis)Secrete hormones that travel through lymph or blood to their specific target organs * Target organs respond in some characteristic way

Exocrine Glands * Secretions released onto body surfaces (skin) or into body cavities * More numerous than endocrine glands * Secrete products into ducts * Examples include mucous, sweat, oil, and salivary glands

Unicellular Exocrine Glands * The only important unicellular glands are mucous cells and goblet cells * Found in epithelial linings of intestinal and respiratory tracts * All produce mucin
–Dissolves in water to form mucus
•Slimy protective, lubricating coating

Multicellular Exocrine Glands
• Multicellular exocrine glands are composed of a duct and a secretory unit
• Usually surrounded by supportive connective tissue * Supplies blood and nerve fibers * Extends into and divides gland into lobes

Classification of Multicellular Glands
• By structure and type of secretion * Structure
• Simple glands (unbranced duct) or compound glands (branched duct)
• Cells tubular, alveolar, or tubuloalveolal
–Type of secretion * • Merocrine – most – secrete products by exocytosis as produced * • Holocrine – accumulate products within then rupture * • Apocrine – accumulates products within but only apex ruptures – controversy if exist in humans

Covering and Lining Membranes - Composed of at least two primary tissue types

* An epithelium bound to underlying connective tissue proper * Are simple organs
Three types 1. Cutaneous membranes 2. Mucous membranes 3. Serous membranes

Cutaneous Membranes - is your Skin

* Keratinized stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis) * Dry membrane

Mucous Membranes - line body cavities that open to the exterior
• Mucosa indicates location not cell composition
• All called mucosae * Line body cavities open to the exterior (e.g., Digestive, respiratory, urogenital tracts)
• Moist membranes bathed by secretions (or urine)
• Epithelial sheet lies over layer of connective tissue called lamina propria
• May secrete mucus

Serous Membranes 0r Serosae — are Moist membranes found in closed ventral body cavity
• Consist of Simple squamous epithelium (mesothelium) resting on thin areolar connective tissue
• Parietal serosae line internal body cavity walls
• Visceral serosae cover internal organs
• Serous fluid between layers
• Pleurae, Pericardium, Peritoneum

Tissue Repair - outlining the process of tissue repair involved in normal healing of a superficial wound.
• when tissue injury occurs, these barriers are penetrated
• Tissue repair requires that Cells must divide and migrate
•Occurs in two major ways
–Regeneration - is replacement of destroyed tissue with the same kind of tissue * Which Original function restored

–Fibrosis - Connective tissue replaces destroyed tissue
•Original function lost

Steps in Tissue Repair: Step 1
Inflammation sets stage * Release of inflammatory chemicals * Dilation of blood vessels * Increase in vessel permeability * Clotting occurs Inflammation sets the stage: * Severed blood vessels bleed. * Inflammatory chemicals are released. * Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area. * Clotting occurs; surface dries and forms a scab.
Steps in Tissue Repair: Step 2

Organization restores blood supply * During organization the the blood clot is replaced with granulation tissue * Epithelium begins to regenerate * Fibroblasts produce collagen fibers to bridge the gap * Debris is phagocytized

Organization restores the blood supply: * The clot is replaced by granulation tissue, which restores the vascular supply. * Fibroblasts produce collagen fibers that bridge the gap. * Macrophages phagocytize dead and dying cells and other debris. * Surface epithelial cells multiply and migrate over the granulation tissue.

Steps in Tissue Repair: Step 3
Regeneration and fibrosis
–The scab detaches
–Fibrous tissue matures; epithelium thickens and begins to resemble adjacent tissue
–Results in a fully regenerated epithelium with underlying scar tissue

Regeneration and fibrosis effect permanent repair: * The fibrosed area matures and contracts; the epithelium thickens. * A fully regenerated epithelium with an underlying area of scar tissue results.
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Regenerative Capacity in Different Tissues

• Regenerate extremely well * Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue
• Moderate regenerating capacity * Smooth muscle and dense regular connective tissue
• Virtually no functional regenerative capacity * Cardiac muscle and nervous tissue of brain and spinal cord * New research shows cell division does occur
• Efforts underway to coax them to regenerate better

Developmental Aspects

Primary germ layers – Formed early in embryonic development
–Superficial to deep: Ectoderm, Mesoderm, and Endoderm
–Specialize to form the four primary tissues – (nervous, epithelium, muscle, connective) * Nerve tissue arises from ectoderm * Muscle and connective tissues arise from mesoderm * Epithelial tissues arise from all three germ layers

Chapter 5
Skin (Integument) - Consists of two distinct regions

Epidermis - superficial region
• Epithelial tissue

Dermis - underlies epidermis
• Mostly fibrous connective tissue
Hypodermis or (superficial fascia) – Subcutaneous tissue just deep to the skin
– Not part of skin but shares some functions
– Consists mostly of adipose tissue = that absorbs shock & insulates
– Anchors skin to underlying structures – mostly muscles

Epidermis
• Keratinized stratified squamous epithelium
• 4 or 5 distinct layers
1.)Stratum basale - is the deepest epidermal layer and is the site of mitosis.
2.)Stratum spinosum - is several cell layers thick and contains keratinocytes, melanin granules, and the highest concentration of epidermal dendritic cells.
3.)Stratum granulosum - contains keratinocytes that are undergoing a great deal of physical changes, turning them into the tough outer cells of the epidermis.
Stratum lucidum (only in thick skin) - is found only in thick skin and is composed of dead keratinocytes.
–Stratum corneum
Four cell types 1) Keratinocytes 2) Melanocytes 3) (langerhans) Dendritic cells 4) (merkel) Tactile cells

Cells of the Epidermis
• Keratinocytes
–Produce fibrous protein keratin
–Most cells of epidermis
–Tightly connected by desmosomes
• Melanocytes
–10–25% of cells in deepest epidermis
–Produce pigment melanin – packaged into melanosomes
• Protect apical surface of keratinocyte nucleus from uv damage
• Dendritic cells (langerhans)
–Macrophages – key activators of immune system
•Tactile cells (merkel)
–Sensory touch receptors

Layers of the Epidermis: Stratum Basale (Basal Layer)
• Deepest epidermal layer
• Also called stratum germinativum
• Firmly attached to dermis
• Single row of stem cells
–Actively mitotic
–Produces two daughter cells
• One cell journeys from basal layer to surface
–Takes 25–45 days
–Dies as moves toward surface
• One cell remains in stratum basale as stem cell
• Melanocytes compose 10-25% of this layer

Layers of the Epidermis: Stratum Spinosum (Prickly Layer)
•Several layers thick
•Cells contain web-like system of intermediate prekeratin filaments attached to desmosomes
•Abundant melanosomes and dendritic cells

Layers of the Epidermis: Stratum Granulosum (Granular Layer)
•Thin - four to six cell layers
•Cell appearance changes * Cells flatten * Nuclei and organelles disintegrate * Keratinization begins

•Cells accumulate keratohyaline granules * Help form keratin in upper layers * Cell accumulate lamellar granules

•Their water-resistant glycolipid slows water loss
•Cells above this layer die * Too far from dermal capillaries

Layers of the Epidermis: Stratum Lucidum (Clear Layer)
• Only in thick skin
• Thin, translucent band superficial to the stratum granulosum
• A few rows of flat, dead keratinocytes

Layers of the Epidermis: Stratum Corneum (Horny Layer)
•20–30 rows of dead, flat, anucleate keratinized membranous sacs
•Three-quarters of epidermal thickness
•Though dead, its cells have functions * Protect deeper cells from environment and water loss * Protect from abrasion and penetration * Barrier against biological, chemical, and physical assaults

Cell Differentiation in Epidermis
• Cells change from stratum basale to stratum corneum
• Accomplished by specialized form of apoptosis * Controlled cellular suicide * Nucleus and organelles break down * Plasma membrane thickens * Allows cells to slough off as dandruff and dander * Shed ~ 50,000 cells every minute

Dermis
• Composed of Strong, flexible connective tissue
• The cell types found in the dermis * Fibroblasts, macrophages, and occasionally mast cells and white blood cells
• Fibers in matrix bind body together * The dermis is your "Hide" used to make leather
• “Hide” Contains nerve fibers; blood and lymphatic vessels
• Contains epidermal hair follicles; oil and sweat glands
Two layers 1. Papillary 2. Reticular

Layers of the Dermis: Papillary Layer
• Areolar connective tissue with collagen and elastic fibers and blood vessels
• Loose tissue * Phagocytes can patrol for microorganisms

Dermal papillae - Superficial peglike projections

* Most contain capillary loops * Some contain meissner's corpuscles (touch receptors) * Some contain free nerve endings (pain receptors) * In thick skin lie atop dermal ridges that cause epidermal ridges * –Collectively ridges called friction ridges * Enhance gripping ability * Contribute to sense of touch * Pattern is fingerprints a) Friction ridges of fingertip
Layers of the Dermis: Reticular Layer
•~80% of dermal thickness
• Dense fibrous connective tissue
• Elastic fibers provide stretch-recoil properties
Collagen fibers * Provide strength and resiliency * Bind water
Cleavage lines because most collagen fibers parallel to skin surface * Externally invisible * Important to surgeons * Incisions parallel to cleavage lines gap less and heal more readily

Skin Markings
Flexure lines * Dermal folds at or near joints * Dermis tightly secured to deeper structures * Skin cannot slide easily for joint movement causing deep creases * Visible on hands, wrists, fingers, soles, toes

Other Skin Markings
Striae
–Silvery-white scars
–"Stretch marks"
–Extreme stretching causes dermal tears
Blister
–From acute, short-term trauma
–Fluid-filled pocket that separates epidermal and dermal layers

Skin Color
Three pigments contribute to skin color 1. Melanin - Only pigment made in skin 2. Carotene 3. Hemoglobin

Melanin * Two forms
–Reddish-yellow to brownish-black * Color differences due to amount and form * Produced in melanocytes
–Same relative number in all people * Migrates to keratinocytes to form "pigment shields" for nuclei * Freckles and pigmented moles
–Local accumulations of melanin * Sun exposure stimulates melanin production * Sunspots (tinea versicolor) are fungal infection; not related to melanin

Carotene and Hemoglobin
Carotene
–Yellow to orange pigment * Most obvious in palms and soles
– Accumulates in stratum corneum and hypodermis
–Can be converted to vitamin a for vision and epidermal health * Yellowish-tinge of some asians – carotene and melanin variations
Hemoglobin
–Pinkish hue of fair skin

Skin Color in Diagnosis
Cyanosis
* Blue skin color - low oxygenation of hemoglobin
Erythema (redness) * Fever, hypertension, inflammation, allergy
Pallor (blanching) * Anemia, low blood pressure, fear, anger
Jaundice (yellow cast) * Liver disorder
Bronzing
* Inadequate steroid hormones in addison's disease
Bruises
* Clotted blood beneath skin