Skin basics .

Integumentary system
Integument = skin
Only one organ in this system --but it's the largest organ of the body
Therefore, the skin is an organ AND a system

Functions of skin
Protection - mechanical, UV radiation, immune "first line" and "second line," water conservation
Excretion - sweat glands excrete "waste"
Chemical synthesis - vitamin D
Thermoregulation - can regulate heat loss or conservation
Sensation - various sense of touch, temperature, vibration, pain

General structure
Epidermis - stratified squamous epithelial outer layer
Dermis - dense fibrous connective inner layer (usually thicker than the epidermis)
Hypodermis - loose fibrous tissue under skin (hypodermis is NOT part of skin, but where else can we discuss it?)
Thickness varies
Thick skin {numerous sweat glands ,but lacks hair follicles,sebaceous glands & smooth muscle fibre}- Thickest skin found on palms, fingertips, sole
Thin skin - Thinnest skin found on scalp, near lips.

Epidermis:4 types of cells present

• keratinocytes{dominant type}
• melanocytes
• langerhan cells
• merkel cells.

epidermal layers:5

• stratum basale{stratum germinatum}
• stratum spinosum
• stratum granulosum
• stratum lucidum
• stratum corneum

stratum basale:

• deepest layer & single layer
• serve as stem cells for epidermis{divide & move up}
• contain intermediate keratin filaments that increase in number as cells move up.

stratum spinosum:

• 4 to 6 rows of cells
• why spiny?-in routine histologic preparations,cells in this layer shrink.as a result,the developed intercellular spaces b/w cells appear to form numerous cytoplasmic extensions or spines.spines represent where desmosomes are anchored to bundles of keratin filaments or tonofilaments & to neighboring cells
• tonofilaments-resistance to abrasion.

stratum granulosum:from here on to top, cells dead & no nucleus.

• 3 to 5 layers of cells
• keratohyalin granules present
• As cells from stratum germinativum die, they enter stratum granulosum
• All cells here are dead (and look grainy when stained/ no nuclei) --but biochemical changes are happening inside them
  Keratohyalin (a precursor to keratin) forms in granules here
• in addition cytoplasm of these cells contain lamellar granules formed by lipid bilayers-these are discharged into intercellular spaces as layers of lipid & seal skin.

stratum lucidum:

• Keratohyalin is transformed into eleidin, which is almost transparent, making this layer look almost "clear"

stratum corneum:

• Keratin is fully formed
• Flattened cells filled with keratin form "keratinized layer" and make this "keratinized stratified squamous epithelium"

other skin cells:

melanocytes:

• in basal layer of epidermis
• synthesize melanin{protect from ultraviolet radiation}

langerhans cells:[antigen presenting cells]

• in stratum spinosum
• part of immune response{recognize,phagocytose & process foreign antigens & present to T lymphocytes}

merkel cells:

• in basal layer
• most abundant in fingertips
• function as mechanoreceptors to detect pressure

Dermis:
Epidermis joins dermis at the glue-like dermal-epidermal junction
Dermis is irregular dense fibrous connective tissue
Two layers of the dermis:

Papillary region

• Outer (superficial) region of dermis has bumps called dermal papillae
  
• Increase surface area for glue to "hold" more tightly
• Arranged in rows to form "prints" of hands/fingers and feet/toes to improve grip (and to identify the perpetrators of crimes against professors)

Reticular region

• Deeper area of the dermis has irregular swirls of collagen fibers plus nerves and nerve endings, blood vessels, sweat glands, and more
• Reticular = "network".

this is first skin posting,next is skin lesions.