The Integumentary System (Skin) Largest organ of the body (15% of body weight) Skin thickness variable, normally 1-2 mm Protection –chemical barrier (waterproof) –physical barrier (tough) –immune system activator Body temperature regulation –blood flow through the skin –sweat glands –hairs Sensation –sense touch, temperature and pain provides information outside of the body

Skin (Integument) Consists of three major regions Epidermis –outermost superficial region Dermis –middle region Hypodermis –deepest region (subcutaneous)

The Skin (Integumentary system)

Epidermis -Outer portion of the skin is exposed to the external environment and functions to protect the organs within the body Composed of stratified squamous epithelium which can be cell layers thick depending on location on the surface of the body -the major cell type of this region is the keratinocyte The cells of the epidermis are arranged into 5 structurally and functionally distinct layers called strata -each strata is made of one or more types of cells each providing a specific function for the epidermis

The Epidermis

Layers of the Epidermis The most superficial layer of the epidermis are the cells that are subjected to the physical forces that come from the outside of the body –friction, abrasion, puncture… As a result, these cells continuously fall off from the surface of the skin –exfoliation

Layers of the Epidermis As the most superficial layer of cells are exfoliated, the cells just deep to them now become the most superficial cells If this process were allowed to continue without the replacement of exfoliated cells, the epidermis would disappear, as it becomes progressively thinner However, this is not the case because the exfoliated cells are constantly replaced by cells in the deepest strata of the epidermis which are constantly dividing (mitosis) –under normal circumstances the rate of mitosis equals the rate of exfoliation which maintains the skin at constant thickness

5 Strata of the Epidermis

Stratum Basale (Basal Layer) Deepest of the epidermal strata made of a single layer of cells (consisting of 3 cell types) that is firmly attached to the deeper dermis Melanocytes –produce the brown pigment melanin to protect against UV radiation (sunlight) damage to DNA Merkel cells –touch receptors that provide sensory perception Keratinocytes –undergo mitosis rapidly to replace the cells that are exfoliated from the surface –as these keratinocytes “move” up through the 3 next strata, the cellular anatomy and physiology change

Stratum Spinosum (Prickly Layer) Several cell layers thick made of: Keratinocytes –become somewhat flattened and “spiny-shaped” –begin to synthesize the protein keratin Langerhans (dendritic) –phagocytose foreign substances –help activate the immune system

Stratum Granulosum (Granular Layer) -Three to five cell layers thick made of keratinocytes -Keratinocytes accumulate vesicles (granules) filled with keratin (a thick tough protein) and glycolipids that are exocytosed -the exocytosed keratin wraps around the cell membrane of the keratinocyte creating a thick coat which provides protection from abrasion and puncture -the exocytosed glycolipids fills the extracellular spaces between the keratinocytes and provides a waterproofing property to skin forms a barrier between surface cells and deeper layers of the epidermis cuts off nutrient supply to more superficial strata

Stratum Lucidum Thin translucent zone seen only in thick skin Keratinocytes have no nucleus or organelles –dead cells –does not stain well Stratum Corneum (Horny Layer) Outermost strata made of keratinized cells Made of up to 30 layers of dead keratinocytes –accounts for up to 75% of the epidermal thickness Most superficial layer of keratinocytes exfoliate from the surface of the body

Dermis Second major skin region containing strong, flexible connective tissue, sensory cells, hair, muscles, blood vessels and glands Composed of two layers: –Papillary superficial layer just below the epidermis –Reticular deep layer beneath the papillary layer

Dermis Papillary layer Reticular layer

Layers of the Dermis: Papillary Layer Papillary layer –20% of the thickness of the dermis –Its superior surface contains fingerlike projections called dermal papillae which keep it firmly attached to the superficially located epidermis –Dermal papillae contain sensory receptors such as tactile corpuscles (pressure receptors) and free nerve endings (sense pain)

Layers of the Dermis: Reticular Layer Reticular layer –80% of the thickness of the dermis -Elastic fibers provide stretch-recoil properties –Location of: hair and associated follicles pacinian corpuscles (pressure receptors) blood vessels glands

Hair -Filamentous strands of dead keratinized cells produced by hair follicles -Contains hard keratin which is tougher and more durable than the keratin (soft) of the epidermis The long thin hair shaft is visible above epidermis The hair root is in the dermis within a follicle Pigmented by melanocytes at the base of the hair

Hair

Hair Function and Distribution Functions of hair include: –Retain body heat The follicle of the hair is connected to piloerector muscles that contract when cold –raises hair on skin surface increasing the thickness of the insulation barrier –goose bumps –Provide sensory perception –Guarding the scalp against physical trauma, heat loss, and sunlight Hair is distributed over the entire skin surface except –palms, soles, and lips, nipples and portions of the external genitalia

Glands of the skin 3 different types that secrete their products onto the surface of the skin Sweat glands –Merocrine sweat glands secrete water to cool of the body –Apocrine sweat glands scent glands that secrete a milky substance rich in fatty acids in response to stress or sexual stimulation Sebaceous glands –secrete an oily substance called sebum into a hair follicle leading to the epidermis softens the skin (and hair)

Glands

Glands of the skin Ceruminous glands –found in the external ear canal and secretes cerumen (wax) keeps eardrum flexible Hypodermis -Subcutaneous layer below to the dermis -Composed of adipose and areolar connective tissue -Functions to cushion and thermally insulate the body -also contains some antimicrobial chemicals