Anatomy & Physiology
Skeletal System
The Breakdown of Ossification 
*Before you begin to learn the process you must know the following definitions…*
OSTEO=BONE… CHONDRO=CARTILAGE
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Osteocytes- bone cells
Osteoblasts- secrete new bone matrix (osteoid) **think blast=build**
Osteoclasts- break down bone **think clast=crumble**
Osteoid- bone matrix
Chondrocytes- cartilage cells
Perichondrium- membrane that covers cartilage
Periosteum- membrane that covers bone
Two Types of Bone Growth (Ossification/Osteogensis=Growth)
1. Intramembranous Ossification
a. Occurs in the bones of the skull
b. Bone tissue replaces sheets of connective tissue
c. Steps:
i. Mesenchymal cells (stem cells) found in the tissue in the head will differentiate into osteoblasts
ii. Osteoblasts secrete the osteoid (bone matrix) which calcifies (when calcium and phosphorus are
present) and hardens to form the flat bones of the skull
1. Spongy bone is created first
2. Outer layer that is still membraneous tissue becomes the periosteum
3. Osteoblasts inside the periosteum form a layer of compact bone surrounding the spongy
bone.
iii. Areas in the skull called fontanels remain uncalcified until after birth when they will fuse and
form a suture.
2. Endochondral Ossification
a. Occurs in most bones of the body before and after birth
b. Bone replaces cartilaginous models (made up of hyaline cartilage)
i. Mesenchymal cells differentiate into chondroblasts which lay down the framework for the new
bone.
ii. The cartilaginous model grows appositionally (length-wise) and interstitially (width-wise) until
they are a decent size.
iii. Blood vessels penetrate the perichondrium and deliver osteoblasts inside this membrane which
form the bone collar. (The membrane around the bone collar is now called the periosteum)
iv. The bone collar deprives the cartilage of nutrients, killing the cells, and creating space for the
bone matrix.
v. Blood vessels now invade the open space and form spongy bone (by delivering osteoblasts
secreting osteoid)
vi. Bone matrix continues to be laid down forming the diaphysis. (the diaphysis is hollowed out by
osteoclasts which form the medullary cavity, containing the bone marrow)
vii. The epyphysis contains secondary ossification centers which form spongy bone in the ends of
the bone.
viii. The epiphyseal plate remains cartilaginous and the cells will continue to divide until the bone is
fully grown. Eventually the cells of the epiphyseal plate die and are replaced with bone, and the
bone can no longer lengthen.