Biology: Structure of Bone

Study Notes

Bone is a specialized type of dense supporting connective tissue composed of calcified intercellular substance, bone matrix, and bone cells.
Bone matrix is composed of inorganic (75% of dry weight) and organic materials (25% of dry weight).
Inorganic materials mainly consist of calcium in the form of hydroxyapatite.
Organic materials are mostly type I collagen fibers (about 90%).

Periosteum is the connective tissue that covers the bone, consisting of an outer vascularized dense fibrous layer and an inner osteogenic layer.
Endosteum is a thin layer of flattened osteogenic cells and osteoblasts that lines the bone marrow cavity and spaces.

Osteogenic cells:
- Found on or near the free surfaces of bones in the endosteum and the inner layers of periosteum.
- Able to proliferate and differentiate into osteoblasts.
- Active during bone growth and repair.
Osteoblasts (bone-forming cells):
- Large oval cells with highly basophilic cytoplasm.
- Secrete type I collagen and bone matrix proteins.
- Responsible for calcification of bone matrix (activity of alkaline phosphatase).
- Have well-developed RER, Golgi, and mitochondria with secretory vesicles containing procollagen molecules and alkaline phosphatase.

Osteoclasts:
- Large multinucleated cells (6-12 nuclei or up to 50 nuclei) derived from the fusion of monocytes.
- Have acidophilic cytoplasm and occupy pits in the bone surface (Howship's lacunae).
- Function: bone resorption and remodeling.

Oval branched cells present inside lacunae.
Extend cytoplasmic processes through canaliculi filled with extracellular fluid and originating from lacunae.
Maintain the calcified matrix, and death of osteocytes is followed by resorption of the matrix.