Wednesday, June 9, 2010

In Depth on... Bone Tissue!

First off, the study of bone structure and the treatment of disorders relating to bones is osteology. 
There are 206 bones in the human body!! I'll get into where they are a bit later...

Bones are good for support, protection, movement, mineral homeostasis (we mainly discussed Ca2+) and red blood cell production. 

Bones are classified as (and examples of each...):
  1. long 
    1. humerus
    2. radius
    3. ulnar
    4. metacarpals
    5. femur
    6. tibia
    7. fibula
    8. metatarsals
    9. clavicle
  2. short
    1. carpals
    2. tarsals
    3. patella*
    4. calcaneus
  3. irregular
    1. vertebra
    2. illium
    3. ischium
    4. pubis
    5. bones of pelvic girdle
  4. flat
    1. skull
    2. sternum
    3. scapula
    4. ribs
  5. sesamoid*
    1. patella
Next we talked about the structure of long bones.  Long bones have a compact layer of bone that surrounds some spongy bone.  
The ends of a long bone are called the epiphyses.  On the superficial layer of the epiphyses,   Next come the metaphyses.  The metaphyses includes the epiphyseal plates.  Between the two ends of epiphyses and metaphyses is the diaphysis.  
The membranes of the long bone include the endostuem and the periosteum.  The endostuem (endo-in) is the lining of the marrow cavities.  The periosteum (perio-outer) is a tough membrane covering only the bone, not any cartilage and is composed of two layers.  The two layers of the periosteum are the outer fibrous layer and the inner osteogenic layer.  The outer fibrous layer is composed of dense irregular tissue.  The inner osteogenic layer is bone cells and blood vessels that nourish or help with repairs. 

The matrix of the bone is abundant with mineral salts.  The primary one is calcium phosphate (plus some other stuff, aka hydroxyapitite).  The salts are deposited in the membrane, then they crystalize and harden.  This process is known as crystalization or mineralization.  Mature bones contain much more matrix than immature bone.  

Osteogenic cells develop into osteoblasts which form the matrix that is made of osteocytes.  'blasts get stuck in the matrix and become 'cytes'  Osteoclasts are the cells that reabsorb the old bones/break down old bone matrix.  

There are two types of bone: compact and spongy.
Compact bone is really, really organized and is composed of tubular osteons (more on those in a second).  Osteocytes lie in the lacunae (tiny chambers arranged in concentric rings around a central [Hasverian] canal).  Compact bone lies over spongy bone and composes most of the bone tissue of the diaphysis.  It protects, supports and resists stress.   An osteon is a concentric ring (lamellae) of calcified matrix surrounding a vertically oriented blood vessels.  Osteocytes are found in spaces called lacunae.  Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next.  Interstitial lamellae are the older osteons that have been partially removed by osteoclasts during tissue remodeling (a little more detail a bit later..).  

Spongy bone is not made of osteons.  It is not hard and solid like compact bone.  Instead, spongy bone consists of trabeculae that surrounds many red marrow filled spaces.  It forms most of the structure of short, flat and irregular bones as well as the  epiphyses of long bones.  Spongy bone is very light and supports and protects the red bone marrow.  

Blood and Nerve supply of bones:
Periosteal (perio-outer) arteries supply the periosteum.  Nutrient arteries supply the diaphyis of the long bone  and red marrow.  Metaphyseal and epiphyseal arteries supply the red marrow and spongy bone of the epiphysis.  Nerves follow blood supply into the bone tissue and periosteum where they sense damage and transmit pain messages.  

To grow in length: cartilage grows on the epiphyseal side of the epiphyseal plate.  Then osteoclasts break down the cartilage on the diaphyseal side and eventually the plate closes and is completely bone. 
To grow in width, periosteal cells differentiate into osteoblasts and secrete collagen and matrix to form bony ridges and then a tunnel around periosteal blood vessel.  Concentric lamellae fill in the tunnel to form an osteon. 

Factors that affect bone growth:
  1. the parathyroid horomone and calcitonin regulate calcium salt storage
  2. growth hormone (from the pituitary gland) stimulates the liver to releast growth factors that stimulate the growth plate.  
  3. thyroxin makes sure growth hormone stimulates growth in proper proportions (deficiencies delay bone growth)
  4. steroids (testosterone and estrogen) stimulate growth spurts in puberty by stimulating growth plates (cartilage cells)
  5. Vitamins A, C, D deficiencies make bones weak and develop slower.  
 Bone Remodeling: 
remodeling is the ongoing replacement of old osteons/interstitial lamellae.  Old bone is constantly destroyed by osteoclasts and new bone is built by osteoblasts.  Orthodontics uses this practice by forcing the deep teeth cells to be broken down and moved and rebuilt in the right spot.  

Bone fractures:
A fracture is any break in the bone.  
Open fractures break the bone and go through the skin.
Simple fractures break the bone but not the skin.
Comminuted fracture is when the bone splinters at site of impact
Greenstick fractures only occur in children.  One side of the bone breaks and the other just bends.  
Impacted fracture occurs when one end of the break is pushed up into the other.  
Stress fractures are many microfissures in the bone.  

The bone healing process occurs in steps.  The first step is bleeding.  Then a clot forms.  Fibroblasts then secrete cartilage and collagen fibers that form a callus.  Osteoblasts transform cartilage into new bone.  

Calcium homeostasis is very important for the body because calcium is involved in a lot of systems.  Bone is the major reservoir of Calcium.  Bone will take in calcium when blood levels are too high.  
when we wake up our blood levels of calcium are low, so we release parathyroid hormone to get some more calcium out of our bones.  then we eat which should bring calcium levels above the norm, so the PTH slows down it's releasing.  Because levels are now too high, calcitonin is released from the thyroid and sends calcium back into the bone.   Calcium ions help nerve and muscle cells function, they aid in blood clotting and in enzyme functions for biochemical reactions.  Small changes in calcium levels can be deadly.  

Exercise and Bone Tissue:
Bones get stronger in response to mechanical stress by increasing deposition of mineral salts and production of collagen fibers.  If you don't have any mechanical stressor, then bones weaken through demineralization and collagen reduction.  This often occurs while being in a cast, to astronauts in outer space and people who are bedridden.  Weight-bearing activities like walking or moderate weight-lifting can help build and retain bone mass.  

Osteoporosis is more common in women.  The rate of bone formation/deposition increases until age 20-30.  Then there is an equal rate of deposition and resorbtion until age 40-50.  After that, resorbtion rates exceed deposition rates.  This causes the total amount of bone mass to decrease.  Men lose approximately 25% while women lose 35% Estrogen therapy can help women slow down the onset of osteoporosis

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