Endochondral ossification =  b0ne replaces cartilage
1. within cartilage model, chondrocytes enlarge, die and leave cavity in cartilage
2. blood vessels invade cavity  and perichondrium converts to periosteum
3. osteoblasts carried into cavity and produce spongy bone = primary ossification center
4. bone growth continues in both width and length
5. secondary ossification centers in epiphyses form spongy bone
​6. ossification centers meet at epiphyseal plate where epiphyseal cartilage remains. Cartilage also remains on ends of bone and serve as articular cartilage. 

​     Bone growth continues via interstitial (length) and appositional (circumference) growth
Interstitial growth: 
chondrocytes at epiphyseal side of plate grow and divide
while chondrocytes at the diaphyseal side of the plate die.
Osteoblasts move into area of dead chondrocytes and replace
with bone. Epiphyseal "closure" occurs when cartilage
production slows and bone production accelerates.
The epiphyseal plate then becomes the epiphyseal line
when all of the cartilage is replaced ​ with bone (post puberty).   

Appositional growth:

the periosteum exhibits a cellular inner layer of osteoblasts
which add a series of concentric layers (rings) on
ompact bone = the lamellae. During bone remodeling,
osteoblasts add to the outside while osteoclasts remove
​bone on the inside of the bone at the medullary cavity.  


 fibrous joints

Thyroid Hormones/ T3 T4


     Intramembraneous ossification = bone replaces embryonic tissue called mesenchyme
1. dermal bones are created when mesenchymal cells differentiate into osteoblasts which produce bone.
2. osteoblasts become trapped and differentiate into osteocytes.
3. bone growth continues in projections called spicules.
4. blood vessels get trapped between spicules.
5. osteoblasts on both surfaces differentiate into the cellular component of periosteum, producing compact bone.
6. the inner area sandwiched between two areas of compact bone is spongy bone. 

Arthritis is an inflammation of the joints. There are more than 100 different types of arthritis
including osteoarthritis (OA) and rheumatoid arthritis (RA).​

 Osteoblasts synthesize and secrete a matrix of collagen and calcium salts. When the area around the osteoblast calcifies, the osteoblast becomes trapped and transforms into an osteocyte. 

 Just an FYI re actual complexity of process


vitamin C and K also stimulate osteoblast differentiation

 The human body has 206 bones   
            There are 26 bones in the foot
​                        The hand and wrist contains 54 bones
                                  The femur is the longest and strongest bone
                                           The stapes, in the middle ear, is the smallest and lightest bone
                                                     Only the hyoid bone isn't connected to another bone
                                                              Only 10% of the world's animals have an internal skeletal system

 cartilagenous joints

 Welcome to Dr. Kate Brilakis' Learning Portal

Rickets the softening  of bones in children. Rickets occurs often because of extreme
vitamin D deficiency.

Bursitis is an inflammation
of a joint's bursa sac. 

 Bursa are fluid-filled sacs that works as cushions to reduce friction. Bursae are found next to the tendons near the large joints including shoulders, elbows, hips, and knees.

Osteoprotegerin (OPG)
block RANKL from binding to RANK so decreases osteoclast formation  

 Joints are articulations between two bones that are classified by the tissue which connects the bones. The three main types of joints are:
​synovial, cartilaginous and fibrous.

 synovial joints

 The synthesis and degradation of bone is controlled by the RANKL system.
RANKL is released by bone-forming osteoblasts which stimulates RANK receptors on the surface of stem cells to form osteoclasts = ligand/receptor system...


          bone remodeling

 Calcitonin acts to reduce blood Ca+ levels, opposing the action of parathyroid hormone (PTH). Calcitonin inhibits the activity of osteoclasts and decreases the resorption of calcium in the kidneys. 
PTH acts to resorb calcium from the bone, increase calcium resorption by the kidneys.


    compact bone structure

Excessive production of GH in children (before the epiphyseal plates fuse) results in  gigantism and continued growth of long bones.

 Hormone Control:
Calcitonin and Parathyroid Hormone (PTH)

          The Skeletal System

 Bone marrow
 is the spongy tissue found   in the medullary cavities of long bones. Red bone marrow (myeloid tissue) and yellow bone marrow (fatty tissue). Red marrow houses blood stem cells which are able to differentiate into WBCs used to fight infections, RBCs which carry O2 and platelets which participate in clot formation. Red bone marrow exhibits a highly vascular fibrous tissue containing hematopoietic stem cells/blood-forming stem cells. Yellow bone marrow exhibits mesenchymal (marrow stromal) stem cells which 
give rise to fat, cartilage, and bone.