The Integumentary System (Skin) Largest organ of the body (15% of body weight) Skin thickness variable, normally mm Protection –chemical barrier (waterproof, blocks UV rays) –physical barrier (toughness due to Keratin) –immune system activator Sensation –sense touch, temperature and pain provides information outside of the body

Vitamin D synthesis Body temperature regulation –Cutaneous vasoconstriction/vasodilation –sweat glands –hair

Skin (Integument) Consists of three major regions Epidermis –superficial region –5 Layers: stratum corneum stratum lucidem (tough skin only) stratum granulosum stratum spinosum stratum basale Dermis –middle region –2 Layers: papillary and reticular Hypodermis –deepest region

The Integumentary System (Skin)

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 –the major cell type 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 The exfoliated cells are constantly replaced by cells in the deepest strata of the epidermis which are constantly dividing (mitosis) maintaining a constant epidermal thickness –the rate of mitosis equals the rate of exfoliation

Stratum Corneum 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

Stratum Lucidum Thin translucent zone seen only in thick skin found on the palms of the hand and souls of the feet (calluses)

Stratum Granulosum Three to five cell layers deep made of keratinocytes Keratinocytes accumulate vesicles (granules) filled with the protein keratin and glycolipids which 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 the cells of the lucidum and corneum to die

Stratum Spinosum Several cell layers deep made of: Keratinocytes –become somewhat flattened and “spiny-shaped” Langerhans (dendritic) cells –phagocytose foreign substances such as bacteria and viruses that attempt to enter the body through the skin –alert the immune system to the presence of foreign substances

Stratum Basale Deepest of the epidermal strata made of a single layer of cells (consisting of 3 cell types) Keratinocytes –undergo mitosis rapidly to replace the cells that are exfoliated from the surface –as these keratinocytes “move” up through the more superficial strata Melanocytes –produce the brown pigment melanin to protect against UV radiation (sunlight) damage to DNA of keratinocytes Merkel (tactile) cells –touch receptors that provide sensory perception

Melanocytes Occur only in the stratum basale Melanin is shed from the melanocytes onto neighboring keratinocytes of the strata spinosum and basale. –2 Forms of Melanin Eumelanin: brownish-black pigment. Pheomelanin: reddish-yellow, sulfur-containing pigment.

Dark skin: produces greater quantities of melanin, and breaks it down more slowly. Melanin will accumulate into several epidermal layers. Light skin: produces less melanin, and breaks it down quickly so that it doesn’t spread beyond the stratum basale. UV exposure increases the productivity of melanocytes.

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 deep to the epidermis –Reticular deep layer deep to 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 (touch 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 soft keratin 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 follicle

Hair

Hair Function and Distribution Hair is distributed over the entire skin surface except –palms, soles, and lips, nipples and portions of the external genitalia 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 (goose bumps) increasing the thickness of the insulation barrier at the surface of the skin –Provide sensory perception

Hair Growth The Hair Cycle: –3 Stages: Anagen: 90% of hair, hair cells multiply in the follicle and push the hair deeper into the dermis. Also forms the hair root sheath. Hair root sheath cells transform into hair cells which produce keratin and then die as they are pushed away from the papilla. Catagen: Mitosis in the sheath ceases. The base of the hair hardens and is no longer anchored in the papilla. Telogen: Rest period. *When anagen begins anew, the new hair will push out the old.

Nails hard derivatives of the stratum corneum. composed of very thin, dead, densely packed cells, filled with parallel fibers of hard keratin. –Nail plate: the hard part of the nail. –Free edge: end which overhangs the tip of the finger or toe. –Nail body: the visible attached portion of the nail. –Nail root: extends proximally under the overlying skin. –Nail bed: the skin underlying the nail plate. It’s epidermis is called the hyponychium.

 Nail matrix: A growth zone at the proximal end of the nail, composed of stratum basale.  Mitosis (1mm/week).  Lunule: white crescent at the proximal end of a nail.  Cuticle (eponychium): a narrow zone of dead skin.

Exocrine glands 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 (sebum) into a hair follicle softens the skin Ceruminous glands –secrete a waxy substance (cerumen) in the ear canal keeps eardrum flexible

Hypodermis Subcutaneous layer deep to the dermis Composed of adipose and areolar connective tissue Functions to cushion and thermally insulate the body

Burns