A Closer Look at Bones

Ossification

Bone Repair

• Begins in the embryo and continues as the skeleton grows during childhood and adolescence.
• Even after the adult bones have formed, ossification continues.
• By the eighth through twelfth weeks of embryonic development, the skeleton begins forming from thickened cartilage model of bone.
• These models are replaced by hard bone.

Bone and Calcium Homeostasis

• Broken bone causes bleeding and a blood clot forms.
• Callus forms which is a fibrous network between 2 fragments.
• Cartilage model forms first then, osteoblasts enter the callus and form cancellous bone this continues for 4-6 weeks after injury.
• Cancellous bone is slowly remodeled to form compact and cancellous bone.

• Bone is a major storage site for calcium
• Movement of calcium in and out of bone helps determine blood levels of calcium
• Calcium moves into bone as osteoblasts build new bone
• Calcium move out of bone as osteoclasts break down bone
• Calcium homeostasis is maintained by parathyroid hormone (PTH) and calcitonin

• Ossification center: = where bone formation begins
• Primary ossification center: where bone 1st begins to appear - forms diaphyses
• Secondary ossification center: forms epiphyses

Steps in Ossification

Bone Formation

• Chondroblasts build a cartilage model, the chrondroblasts become chondrocytes.
• Cartilage model calcifies (hardens).
• Osteoblasts invade calcified cartilage and a primary ossification center forms diaphysis.
• Secondary ossification centers form epiphysis.
• Original cartilage model is almost completely ossified and remaining cartilage is articular cartilage.

• Ossification = process of bone formation
• occurs in utero
• Osteoblast’s role:
• Build bone
• After an osteoblast becomes surrounded by bone matrix it becomes an osteocyte

Microscopic Anatomy of Bone

Bone Cells

Bone Growth

• Osteon
• A unit of bone
• Central (Haversian) canal
• Opening in the center of an osteon
• Carries blood vessels and nerves

• Infancy and youth:
• Long bones lengthen at epiphyseal plate
• Long bones widen by adding more lamella
• End of bone growth (in length):
• Epiphyseal plate is replaced by an epiphyseal line

• Osteoprogenitor Cells = stem cells
• Osteoblasts: build bone
• Osteocytes: maintain bone matrix
• Come from Osteoblasts
• Osteoclasts: break down bone
• to release Calcium and Phosphate
• to respond to forces placed on bone

• Perforating (Volkman’s) canal
• Canal perpendicular to the central canal
• Carries blood vessels and nerves
• Lacunae
• Cavities containing bone cells (osteocytes)
• Arranged in concentric rings
• Lamellae
• Rings around the central canal
• Sites of lacunae
• Canaliculi
• Tiny canals
• Radiate from the central canal to lacunae
• Form a transport system

A Closer Look at Bones

Bone Remodeling

Hematopoietic Tissue

• Removal of existing bone by osteoclasts and deposition of new bone by osteoblasts
• Occurs in all bones - responsible for changes in bone shape, bone repair, adjustment of bone to stress, and calcium ion regulation

• Tissue that makes blood cells
• Red marrow: location of blood forming cells
• Yellow marrow: mostly fat
• Location of hematopoietic tissue in newborns:
• most bones (red marrow)
• Location of hematopoietic tissue in adults:
• red is replaced with yellow marrow - red marrow is mainly in epiphyses of femur and humerus, also in axial skeleton