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 20 - 40 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
-also contains some antimicrobial chemicals
Hypodermis
-Subcutaneous layer below to the dermis
-Composed of adipose and areolar connective tissue
-Functions to cushion and thermally insulate the body