Bone Structure and Cells Lecture Review

Bone Markings

• Holes (Openings): Allow passage of structures.

  • Foramen: Small hole for blood vessels and nerves.

  • Canal: Larger passage (e.g., vertebral canal).

  • Meatus: Entrance to a canal (e.g., auditory meatus).

  • Sinus: Large empty space in bone.

• Depressions: Serve as attachment points for muscles or other tissues.

Osseous Tissue Composition

• Cells embedded in a dense collagen fiber matrix.

• Collagen: Main structural component.

• Hydroxyapatite: Major deposited mineral, a composite of calcium phosphate $(Ca<em>5(PO</em>4)_3(OH))$ and calcium carbonate.

Bone Cells

• Osteogenic Cells: Bone stem cells; differentiate into osteoblasts.

• Osteoblasts: Bone-building cells; deposit minerals onto collagen to form bone; become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells, found in lacunae (small holes); maintain bone matrix; do not build new bone.

• Osteoclasts: Bone-resorbing cells; derived from macrophages; break down old/damaged bone (resorption) to recycle nutrients or release minerals (e.g., calcium) into the body.

Bone Remodeling

• Continuous balance between bone deposition by osteoblasts and bone resorption by osteoclasts.

• Imbalance (e.g., increased osteoclast activity) can lead to conditions like Osteoporosis, causing fragile, hollow bones.

Compact Bone

• Dense and sturdy, provides primary support (e.g., diaphysis of long bones).

• Composed of osteons (concentric rings).

• Central Canal: Runs through the center of osteons; contains blood vessels and nerves.

• Lamella: Bony matrix organized in rings around the central canal.

• Canaliculi: Tiny canals connecting osteocytes within the lamella, facilitating nutrient exchange.

Spongy Bone (Cancellous Bone)

• Less dense, with an open lattice-like network.

• Trabeculae: Functional units; branching plates or bars of bone forming the lattice.

• Contains osteocytes, osteoblasts, and osteoclasts within its structure.

• Blood vessels navigate through the open spaces of the lattice.

Ossification (Osteogenesis)

• Process of bone formation.

• Embryonic Development: Begins with a cartilaginous prototype of the skeleton ($7$ weeks), later replaced by bone.

• Mineralization of the skeleton is not complete until around $25$ years of age.

• Intramembranous Ossification:

  • Direct bone formation from undifferentiated mesenchymal tissue, without a cartilage template.

  • Mesenchymal cells differentiate into osteoblasts, forming ossification centers.

  • Osteoblasts secrete osteoid (unmineralized bone matrix).

  • Spongy bone forms first, followed by compact bone around it, with blood vessels integrating.