Skeleton, Bones, Bone and more Bones

Alright, this is a little post about Bones, just a little info to make you day get more boring.  These are the first segment on the skeleton. I hope you enjoy it.

Anatomy and Physiology

Skeletal System: BONES

The bone is an organ, made up for lots of different tissues all working together:

  • Bone
  • Cartilage
  • Dense connective tissue
  • Epithelium
  • Blood forming tissue
  • Adipose tissue (yellow bone marrow) and
  • Nervous tissue (funny bone).

Bones are always changing through life, a bit like a bank account = the calcium deposits and releases.
The function of a bone:

  • Support the whole body together as the skeleton to which muscles are attached.
  • Protection.
  • Assistance with movement, muscles contract and bones are pulled.
  • Mineral homeostatis. The calcium imbalance is maintained by the bone, if body is lacking the bone will release calcium into the blood (bank account refernce)
  • Blood cell production and Triglyceride storage ( think : long bone).


A Long Bone is made up of:

  • Diaphysis (hollow in the middle) – grows in the shaft of the bone = a long cylinder- it maintains the proportion of the bone.
  • Medullary Cavity (bone is hollow, cavity is the medulla – lined with connective tissue)
  • Epiphysis ( grows over the proximal and distal of bone – the ends)
  • Metaphyses ( the neck of the bone where Diaphysis meets the epiphysis (including the epuphseal plate – determiins the length of the bone. The calcium turns to bone when growing has stopped and is then called the Epiphyseal lining))
  • Articular cartilage (white covering that protects the joint)
  • Endosteum (the connective tissue lining inside the hollow bone (the medulla cavity), is a single layer of bone forming cells as well as the connective tissue)
  • Periosteum (connective tissue that covers the whole bone except for the articulate cartilage that covers the joint. Has two layers, the outer fiber layer and the inner osteogenic layer. It allows growth in width but not length) is an attachment point for ligaments and tendons.

Types of cells in bones:

  • Osteogenic  cells (osteoprogenic cells – are undiffrenciated, the divide to replace themselves and become osteoblast, they are found in the periosteum and the endosteum)
  • Osteoblasts –immature cells they produce matrix and collagen (job to produce bone) they can not divide.
  • Osteocytes – mature cells, they don’t produce matrix or collagen. They just maintain bone.
  • Osteoclasts – are specialised cells from White Blood Cells (WBC eat bacteria) move along surface of bone and provide calcium from bone if it is required. (Eat bone via secreting enzymes which release the calcium)

Matrix of a bone:

  • Inorganic material – salt and mineral makes it hardy
  • Organic collagen fibers  make it felxibleso it doent stretch or tear
  • Bone is not solid, it has small spaces, in spongy bone, there are a lot and compact bone has little space.

Compact Bone:

  • Arranges in units called osteons (contain central blood vessles, lymphatic vessels, nerves, calcified matrix and osteocytes), they are aligned along the lines of stress.
  • Makes up the long shaft of long bones and the external layer covering all bones.
  • Helps resist stress caused by weight and movement.

Spongy Bone:

  • Also called Cancellous bone
  • Doesn’t have osteons, it instead has trabecules  (latticle network that forms along the lines of stress, where blood cells develop in the red bone marrow) surrounding the red marrow filled spaces.
  • Forms the structure of short, irregular and flat bones. As well as the epiphysis of long bones.
  • It protects the red bone marrow, it forms columns that form interlaced networks.

Bone Formation:

  • All embryonic connective tissue begins as mesenchymes (bone is formed from this tissue).
  • Bone formation is called ossification or osteogenisis (bone creation). The mesenchyme cells provide the template for ossification.
  • There are two types of Ossification:
    • Intramembranous Ossification: bones form from or within fiberous  connective tissue membranes : bone forms between two layers
    • Endochondral Ossification: in the formation of the bone from hyaline cartilage models (inside cartilage)

Most bones in the body (not the mandible or the skull though) are made via the endochondral bone formation: the process is:

v  Replacement of cartilage by bone. To make the cartilage model (a template): mesenchymal cells form a template of the bone,

v   the growth is due to the chondrocyte dividing and the matrix production, the cells in the middle  area of the bone then calcify (the cells in the middle of the bone burst which changes the pH and chondrocyde death occurs as does the calcification)

v  In the same area, osteoclasts and osteoblasts are brought : the osteoblasts deposit the bone matrix over the already calcifiedcartilage = spongy bone lattice (trabeculae), the osteoclasts on the other hand form the medullary cavity.

v  The blood vessels dissolve paths in the new matrix to form the cavity.


Growth in Length:

  • The growth plate (epiphyseal) is made up of :
    • Zone of resting cartilage – holds the growth plate to the bone
    • Zone of proliferated cartilage – rapid cell division
    • Zone of hypertrophic cartilage – cells enlarge but remain in column
    • Zone of calcified cartilage – cells are dead since the matrix calcified, the osteoclasts remove the matrix and osteoblasts and capillaries create bone over calcified cartilage.
    • The activity on the growth plate  is the only way Diaphysis can growth in length, when the growth has finished the plate closes and is replaced by bone = bone is done growing. It eventually will calcify.
    • The cartilage cells are produced by mitosis, they are then destroyed and replaced by bone = stop in growth. And the epiphyseal line is created
    • Between 18-22 growth stops as the epiphyseal plate closes


Growth In Thickness:

  • Only can grow wider due to appositional growth (adding layers of diameter to the bone)
  • STEPS:
    • Periosteal cells turn into osteoblasts and secrete collagen and other organic matter to create matrix.
    • Ridges fuse and the periosteum becomes the endosteum (EVENTUALLY)
    • New concentric llamellae are formed
    • Osteoblasts under periosteum form  new circumferential lamellae.


Bone Remodelling:

  • Old bone is constantly brokendown by osteoclasts  and new bone is formed by osteoblasts
  • Hormones and calcitriol control the bone remodelling and growth.
  • Osteoclasts form a tight seal on the cell and secrete enzymes bellow them = release of calcium into body and the osteoblasts then build new bone.


Repairing Bone

  • Fracture repair is repairing a break in the bone
  • It involves:
    • Bleeding, a clot must be formed (fracture hematoma) bleeding ceases
    • The fracture hematoma will be organised into granulation tissue (procallus, which will change into soft tissue) the clot will take around 6 hours to form, the bone cells will die and the inflammation brings in phagocytes to clean up the mess and new capillaries will grow in damaged area
    • Soft tissue(soft tissue is along the area of damage) will turn into spongy bone of hard callus (callus is protection until healed) which will join the two broken ends (takes around 3-4 months)
    • Then the callus will be turned back to original form
    • Bone is quicker to heal than cartilage
    • If bones are not aligned correctly they will need to be moved, by hand or by surgery (screws).
    • Any injury in the body where cells are damaged or die is cause for inflammation and the healing process will begin with phagocytes coming in to remove the debris. Inflammation is a good thing BUT it can get out of control.





  • The bone is a huge supply of blood.
  • The more exercises you do  as a kid the better your bones will be as an adult. And the process of calcification only happens when collagen fibers are present.
  • Smoking stops growth.
  • The distal Femur is remodled completely every 4 months = due to impact and weight bearing.
  • Cardiac arrest if there is too much calcium in the body and Respiratory arrest will occur if too little. The parathyroid hormone is secreted if the level of calcium is low.
  • Loss of calcium in older people usually results in osteoporosis, bones also become fragile due to the decreased rate of protein synthesis.


Bone Disorders:

  • Rickets
    • Affects children
    • Calcium salts aren’t stored properly
    • Bones are soft (bow legs)
    • Bone deformities as a result
    • Osteomalacia
      • New adult bone doesn’t calcify
      • Hip fractures are common

Make sense? keep tuned for some more skeleton related anatomy and physiology notes by simply following my blog 🙂


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