Download Skin (Integument)

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 20 - 40 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