Chapter 4 - Integumentary System
 
Integument
Functions:
1. Protection
2. Thermoregulation
3. Excretion (Secretion)
4. Energy Storage (lipids)
5. Synthesis of vitamin D
6. Sensation
7. Immune Defense
Components:
1. Cutaneous Membrane
a. Epidermis
b. Dermis
2. Accessory Structures
a. Hair
b. Nails
c. Exocrine Glands
 
Epidermis
   The epidermis is a keratinized stratified squamous epithelium.
Consists of four types of cells:
1. Keratinocytes
   These cells are the most abundant and form distinct layers. Five layers can be discerned in the thick skin (palms and soles) and four layers in thin skin. (Please note: the thickness of the "skin" in this context refers to the thickness of the epidermis not the cutaneous membrane.)
2. Melanocytes
   Melanocytes produce and distribute the pigment melanin.
3. Merkel Cell
   Epithelial cells specialized to sense touch.
4. Langerhans Cells
   Cells that initiate an immune response against pathogens and cancer cells.
Layers of Epidermis
1. Stratum Germinativum (Basale)
   The deepest layer of the epithelium and attaches to the basal lamina.
   This layer contains stem cells (basal cells) that provide a continual supply of new keratinocytes to replace those that are lost.
   Melanocytes are found in this layer.
   Merkel cells are also present in skin that lacks hair (e.g. lips, palms and soles).
2. Stratum Spinosum
   This layer is several cells thick.
   These cells form a strong attachment to one another because they are connected to one another by anchoring junctions (desmosomes) and these intercellular attachment are braced by bundles of keratin filaments called tonofilaments. 
   Some cells in this layer continue to divide after separating from the basal lamina.
   Melanocytes and Langerhans cells are also present in this layer.
3. Stratum Granulosum
   As the keratinocytes are pushed toward the surface they produce the proteins keratohyalin and keratin which accumulates in keratohyalin granules.
   The proteins in the keratohyalin granules organize the keratin filaments into bundles that cause the cells to become flattened.
   Insoluble proteins accumulate on the cytoplasmic side of the cell membrane while lipids are deposited on the other side. This makes the interlocked cells more impermeable to water.
   As the cells enter the next stratum the organelles disintegrate and the cells die.
4. Stratum Lucidum
   This layer is only seen in the thick skin of the palm and the sole and is considered a subdivision of the stratum corneum by many histologists.
   In the light microscope, it often has a refractile appearance and may not pick up stain. Hence, it appears clear or lucid. 
5. Stratum Corneum
   This is the most superficial layer in thin and thick skin and consists of layers of dead, flattened and tightly interlocked cells.
   The keratinized cells of this layer forms a dry environment unsuitable for microorganisms.
   This layer also forms a water-resistant layer that allows some water to be lost by insensible perspiration.
 
Dermis
   The connective tissue component of the skin. It has two layers:
1. Papillary Layer (superficial layer)
   This layer consists of areolar connective tissue.
   The papillary layer contains capillaries that supply the epidermis and nerve fibers that supply sensory receptors in the epidermis (Merkel cells) and in papillary layer itself (corpuscle of touch).
2. Reticular Layer (deep)
   This layer consists of dense irregular connective tissue formed by an interwoven network of collagen fibers.
   The collagen fibers surround blood vessels, nerves, hair follicles and glands and extend into the papillary layer above and the subcutaneous layer below.
 
Subcutaneous Layer (a.k.a. Hypodermis or superficial fascia)
   This layer stabilizes the position of the skin in relation to the underlying organs while allowing for some movement.
   It contains loose connective tissue with abundant fat cells.
   It is a good place to administer drugs (subcutaneous injection).
 
Accessory Structures
   During development these structures develop from invaginations of epidermis.
Hair Follicle and Hair
   Hair is a non-living structure formed in hair follicles.
Hair Production
   Hair follicles extend deep into the dermis and often into the subcutaneous layer.
   The hair follicle forms a hair bulb at its base which consists of epithelial cells that surround a peg of connective tissue called the hair papilla.
   Hair growth occurs in the epithelium directly over the hair papilla. This epithelium is called the hair matrix.
Hair Structure
Medulla - The center of the hair consisting of soft keratin. (Not always present.)
Cortex - The outer region of the hair with hard keratin.
Cuticle - A single layer of overlapping cells on the surface of the hair.
Hair Follicle Structure
   The follicle ensheaths the root and lower shaft of the hair and forms the following concentric layers around it:
1. Internal Root Sheath
   This layer is produced by cells at the periphery of the hair matrix.
   Cells in this layer disintegrate quickly so that it does not extend the entire length of the follicle.
2. External Root Sheath
   This layer is continuous with the epidermis and the hair matrix.
   Where it joins the hair matrix all the cells resemble those of the stratum germinativum.
3. Glassy Membrane
   This is the thickened basal lamina of the epithelium that forms the external root sheath. It is wrapped in a dense connective tissue sheath. 
Functions of Hair
   Hairs provide protection, insulation and sensation, e.g.
   Root Hair Plexus - A sensory nerve whose fiber surrounds the base of the hair follicle and detects the movements of the hair.
   Arrector Pili - A thin ribbon of smooth muscle that extends from the papillary layer of the dermis to the connective tissue sheath of the hair follicle that makes the hair stand up (gives "goose bumps").
 
Glands in the Skin
1. Sebaceous Glands
   Discharges a waxy, oily secretion called sebum into hair follicles by holocrine secretion.
   Sebum lubricates, conditions and inhibits the growth of bacteria on the surface of hair and skin.
2. Sweat Glands
   There are two kinds of sweat glands:
a. Apocrine Sweat Glands
   These are not true apocrine glands because the secretion is produced by merocrine secretion.
   These glands are found in the armpits, around the nipples and in the groin.
   The secretion is viscous, cloudy and potentially smelly if acted upon by bacteria.
b. Merocrine Sweat Glands (a.k.a. eccrine sweat glands)
   These are more numerous and widely distributed than apocrine sweat glands.
   They produce sweat or sensible perspiration and their is a high concentration of the glands on the palm and soles.
   The functions of these glands include:
1. Thermoregulation
     Cools the skin by evaporative cooling.
2. Excretion
     In addition to water and electrolytes sweat contains some metabolites and waste products and may also secrete some drugs.
3. Protection
     Sweat dilutes harmful chemicals and has antibacterial properties.
3. Mammary Glands
   Mammary glands are true apocrine glands.
   These glands are related to apocrine sweat glands.
4. Ceruminous Glands
   These modified sweat glands are found in the external ear canal.
   The secretions contain pigment granules and lipid droplets that combine with the secretion of sebaceous glands to produce cerumen or "ear wax".
 
Nails
   Found on the dorsal surface of the fingers and toes.
   Nails provide protection for the finger tips and support them against mechanical distortion. 
    Components of the nail include:
Nail body that is produced at the nail root and which covers the nail bed.
The nail body is bounded by the nail grooves and the nail folds.
Eponychium, also known as the cuticle, is a fold of the stratum corneum near the nail root.
Hyponychium is the stratum corneum under the free edge of the nail.
Lunula is the pale crescent near the nail root.