Anatomy & Physiology[Type text]Ms. Buchalski

The Breakdown of Ossification J

*Before you begin to learn the process you must know the following definitions…*

OSTEO=BONE… CHONDRO=CARTILAGE

·  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

  1. Occurs in the bones of the skull
  2. Bone tissue replaces sheets of connective tissue
  3. Steps:
  4. Mesenchymal cells (stem cells) found in the tissue in the head will differentiate into osteoblasts
  5. Osteoblasts secrete the osteoid (bone matrix) which calcifies (when calcium and phosphorus are present) and hardens to form the flat bones of the skull
  6. Spongy bone is created first
  7. Outer layer that is still membraneous tissue becomes the periosteum
  8. Osteoblasts inside the periosteum form a layer of compact bone surrounding the spongy bone.
  9. Areas in the skull called fontanels remain uncalcified until after birth when they will fuse and form a suture.

2.  Endochondral Ossification

  1. Occurs in most bones of the body before and after birth
  2. Bone replaces cartilaginous models (made up of hyaline cartilage)
  3. Mesenchymal cells differentiate into chondroblasts which lay down the framework for the new bone.
  4. The cartilaginous model grows appositionally (length-wise) and interstitially (width-wise) until they are a decent size.
  5. 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)
  6. The bone collar deprives the cartilage of nutrients, killing the cells, and creating space for the bone matrix.
  7. Blood vessels now invade the open space and form spongy bone (by delivering osteoblasts secreting osteoid)
  8. 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)
  9. The epyphysis contains secondary ossification centers which form spongy bone in the ends of the bone.
  10. 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.