The Integumentary System

The Integumentary System

1. Structure of the Skin

   • also called Skin / Cutaneous Membrane / Integument
   • covers entire surface of human body
   • largest organ in body 1.8 m^2
   • all 4 types of tissues: epithelial, connective, nervous, muscular
   • 2 regions
     • Dermis
       • Dermal papillae – dermal ridges
       • Collagenous fibers prevent skin from being torn
       • Elastic fibers stretch to allow movement of muscles and joints
       • Vascularization of dermis supplies oxygen and nutrients to cells of dermis and epidermis and causes temporary changes to skin color
       • Numerous sensory nerve fibers
     • Epidermis
       • It is the outer and thinner region of skin
       • Made of stratified squamous epithelium divided into 5 layers
       • Five layers – deep to superficial
         • Stratum corneum
           • Over much of the body, keratinization is minimal. But, in areas contianingan underlying stratum lucidum, a particularly thick layer of dead, keratinized cells affords extra protection
           • Hardening is caused by keratinization of the entire cell, which causes the uppermost cell layers of the epidermis to die
           • Keratin (waterproof) prevents water loss and water gain
           • Serves as a mechanical barrier against microbe invasion
         • Stratum lucidum
           • Deep to stratum corneum
           • Found only in thick skin: the palms of the hands, soles of the feet, elbows etc
           • Calluses provide protection from constant friction
         • Stratum granulosum
           • flattened cells that get their name from the dark-staining protein granules found in their cytoplasm
           • Cells are tightly bound and form a barrier
           • Contain more keratin than stratum spinosum cells
         • Stratum spinosum
           • Cells can reproduce by mitosis like stratum basale
           • Name is derived form their spiny appearance, which is created by keratin fibers
           • Protects against foreign materials
           • Produces and retains lipid molecules that prevent moisture loss from the skin
         • Stratum basale
           • Constantly dividing; new cells pushed to the surface, then die and are sloughed off
           • Cells
             • Keratinocytes – most numerous; produce keratin, a water-proofing protein
             • Langerhans cells – macrophages
             • Melanocytes – produce melanin; skin color and protection from UV radiation
           • Sensory nerves
             • Free nerve endings – supply pain and temperature sensations to the brain
             • Tactile cells (Merkel cells) – signal the brain that an object has touched the skin (touch sensations)
   • basement membrane
     • Hypodermis
       • Subcutaneous layer located below the dermis
       • Composed of loose connective tissue, including adipose (fat) tissue
         • Adipose tissue also helps insulate the body
         • Provides protective padding against external assaults
         • Excessive development of adipose tissue in the hypodermis layer results in obesity
       • Adipose tissue provides energy storage, insulation, and protective padding
       • Subcutaneous injections performed with a hypodermic needle

2. Functions of Skin

   • Sensation
     • Felt by somatic sensory receptors
       • Mechanoreceptors – feel pressure, vibration, and texture
       • Thermoreceptors – feel temperature (divided into hot and cold)
       • Nociceptors – feel pain; noci means hurt/injured/pain in latin
       • Proprioceptors – sense the position of body parts in relation to each other (proprius means “ones own”)
   • Endocrine
     • Synthesis of vitamin D
     • Vitamin D3 is made when the organic chemical (7-dehydrocholesterol) reacts with UVB light that falls on the skin
   • Immunity
     • Langerhans cells constantly monitor the skin for unsafe situations
     • Then sends out immune cells (T cells and B cells) to bring back the information
     • The body decides whether to send out inflammatory cells/hormones to cause an allergic reaction or to form a scab/scar, etc
   • Excretion
     • Excretion of water, urea, ammonia and uric acid
     • Waste products are released and regulated with sweat
   • Protection
     • UV radiation, Dehydration Microorganisms, Mechanical trauma/physical injury
     • It is able to do this because of the strength of tissues that form the skin, keratin (waterproof a protein), and melanocytes (pigment).
   • Temperature Regulation
     • Releases sweat from sweat glands to promote cooling
     • Regulates blood flow to different areas of the body to maintain warmth

3. Accessory Structures of the Skin

   • Hair
     • Found on all body parts except the palms, soles, lips, nipples, and portions of the external reproductive organs
     • Most hair is fine and downy
     • After puberty there is noticeable hair in the axillary and pelvic regions (ex: in male, beards develop)
     • Hair in different parts of the body has different growing and resting time periods
     • Sebaceous (oil) glands empty into follicle
     • Arrector pili – a smooth muscle attached to hair follicle. Contraction of the muscle causes the hair to stand on end = “goosebumps”
     • Hair follicles -> complex structures that hair projects from
       • Formed from epidermal cells
       • Located in dermis
       • Cells become keratinized as they are pushed out
       • Hair matrix -> where hair follicles are found
       • Hair root – portion of hair within follicle
       • Hair shaft – portion of hair that continues beyond the skin
     • Hirsutism
       • when women produce more male sex hormone than usual; it’s a condition characterized by excessive body and facial hair
         • hormonal injections and procedures are possible treatments to kill hair
     • Alopecia – hair loss
       • Alopecia areata – sudden onset of patchy hair loss
       • Androgenic alopecia – male pattern baldness
   • Nails
     • Formed from specialized epithelial cells
     • Lunula (moon) – area of rapidly dividing cells ; the whitish half moon of nail
     • Pink color due to vascular dermal tissue under the nail body (nail itself has no color)
     • Epithelial cells become keratinized as they move away from the root
     • Nail root – base of the nail
     • Cuticles – fold of skin that hides the nail root
     • Nail body – visible portion of the nail
   • Skin Glands
     • Glands – groups of specialized cells that produce and secrete substances into ducts
       • Sweat (sudoriferous) glands
         • Present in all regions of the skin; some become more active under stress
         • Are tubular – its coiled at its origin within the dermis
         • Apocrine glands
           • Open into hair follicles in anal region, groin, and armpits
           • Begin to secrete at puberty
           • Mammary glands are modified apocrine glands that produce milk only after childbirth
         • Eccrine sweat glands
           • Open onto surface of skin
           • Active when body heats up; helps lower body temperature as sweat evaporates
           • Sweat (perspiration) is mostly water, but also contains salts and some urea (wastes)
         • Ceruminous glands
           • modified sweat glands that produce cerumen (earwax)
           • Line the external ear canal
       • Sebaceous glands
         • Most are associated with a hair follicle
         • Secrete an oily substance called sebum
           • Weakens or kills bacteria on skin surface
           • Lubricates and waterproofs hair and skin
         • If sebum collects, whiteheads or blackheads form; if bacteria present, a pimple will develop
         • Acne vulgaris – inflammation of the sebaceous glands
           • Most common form of acne during adolescence

4. Effects of Aging

   • Rate of cell mitosis decreases
   • Dermis becomes thinner and the dermal papillae flatten
   • Adipose tissue in the hypodermis decreases
   • Collagen decreases
   • Wrinkles form because of
     • Loose epidermis
     • Fewer fibers
     • Less padding in hypodermis
     • UV damage
   • Elastic fibers in upper layer of dermis are lost and those in the lower layer become thicker, less elastic, and disorganized
   • Number of hair follicles decreases
   • The baldness is on the X chromosome (from mom)
   • Number of melanocytes decrease
   • Limited homeostatic adjustment to heat because of
     • Fewer sweat glands
     • Less vasculature (fewer blood vessels)
   • Reduced number of sebaceous glands