Bone & Cartilage Development - Anatomy & Physiology

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Introduction

Osteogenesis is the formation of bone. Bone forms from one of three lineages; the skull forms from neural cest; the limb skeleton forms from lateral plate mesoderm; and the axial skeleton forms from paraxial mesoderm (sclerotome). There are two methods of osteogenesis. Both involve the transformation of pre - existing mesenchymal tissue into bone.

1. Intramembranous Ossification - direct conversion of mesenchymal tissue into bone. Occurs in production of the skull.
2. Endochondrial Ossification - mesechymal tissue first differentiates into cartilage, then into bone. This occurs in the production of limb and axial skeleton.
   • Limb cartilage is produced in a proximal to distal direction as a continuous structure. It becomes punctuated with joints later, and then converted to bone.

Endochondrial Ossification

• Bone formation via cartilage.

1. Proliferating limb mesenchyme condenses.
2. The increased cell density triggers mesenchymal cells to differentiate into chondrocytes. Proliferation continues.
3. The bone shape is formed, with a diaphysis (shaft) and epiphysis (bulbous ends).
4. Chondrocytes in the middle of the diaphysis stop proliferating and undergo hypertrophy.
   • The shaft is surrounded by the perichondrium, a restrictive membrane. This means that the bone can only grow in length, not width. The epiphysis are pushed away from each other.
5. After chondrocytes have undergone hypertrophy, they die.
   • Osteoblasts on the inner side of the perichondrium invade the cartilage matrix.
   • They then secrete the extra - cellular matrix characteristic of bone.
   • Ossification occurs from the outside - in. This is called primary ossification, or perichondrial ossification.
6. Blood vessels invade the central region of the diaphysis to transport bone marrow stem cells and osteoclasts.
   • The osteoclasts break down the cartilage matrix to allow room for the bone marrow.