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 –sweat glands –hair Sensation –sense touch, temperature and pain provides information outside of the body

Skin (Integument) Consists of three major regions Epidermis –superficial region Dermis –middle region 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

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

The Dermis Its superior surface contains fingerlike projections called dermal papillae which keep it firmly attached to the superficially located epidermis –contain sensory receptors such as tactile corpuscles (touch receptors) and free nerve endings (sense pain) Location of: –hair and associated follicles –pacinian corpuscles (pressure receptors) –blood vessels –exocrine 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

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