Overview
This lecture covers the structure, function, and cellular components of bone tissue, focusing on bone formation, remodeling, and maintenance, as well as the differences between compact and spongy bone.
Bone Marrow & Blood Cell Production
- Red bone marrow produces blood cells, especially red blood cells (RBCs), which lack nuclei and carry oxygen.
- Hematopoiesis (blood cell formation) mainly occurs in the axial skeleton (not appendages).
Types of Bone Cells
- Osteoprogenitor cells are stem cells committed to becoming bone cells, derived from mesenchyme.
- Osteoblasts secrete osteoid (primarily collagen), which later calcifies during bone formation.
- Osteocytes are mature bone cells residing in lacunae, maintaining bone matrix.
- Osteoclasts, derived from white blood cell lineage, resorb bone by dissolving matrix and releasing calcium into the blood.
Bone Matrix Components
- The organic component (osteoid) consists mainly of collagen, giving bone flexibility and tensile strength.
- The inorganic component is primarily hydroxyapatite (calcium and phosphate crystals), providing hardness.
- Proteoglycans and glycoproteins fill spaces in the matrix.
Hormonal Regulation of Bone
- Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclast activity.
- Calcitonin decreases blood calcium by promoting calcium deposition in bone.
- Vitamin D enables calcium absorption in the gut; vitamin C is essential for collagen synthesis.
Bone Remodeling & Clinical Conditions
- Bone remodeling involves both osteoblasts and osteoclasts and maintains bone health and shape.
- Osteopenia refers to decreased bone density; osteoporosis is advanced loss leading to fragile bones, often due to hormonal changes or imbalances.
- Osteitis deformans (Paget’s disease) involves excessive bone resorption and formation, leading to deformities.
Compact vs. Spongy Bone
- Compact bone’s structural unit is the osteon (Haversian system), with a central canal for blood vessels.
- Osteocytes in compact bone occupy lacunae and are connected by canaliculi.
- Spongy bone consists of trabeculae (latticework), filled with red bone marrow; lacks organized osteons.
Cartilage & Bone Growth
- Hyaline cartilage is involved in bone formation; chondroblasts lay down the cartilage matrix, and chondrocytes maintain it.
- Growth in bone length occurs by interstitial growth at epiphyseal plates; growth in width is appositional, beneath the periosteum.
- Intramembranous and endochondral ossification are the two main processes of bone formation during development.
Key Terms & Definitions
- Osteoprogenitor cell — bone stem cell committed to bone lineage.
- Osteoblast — cell that forms bone matrix.
- Osteocyte — mature bone cell in lacunae, maintains bone tissue.
- Osteoclast — bone-resorbing cell, releases calcium to blood.
- Osteoid — organic bone matrix, mainly collagen.
- Hydroxyapatite — inorganic, crystalline component of bone.
- Lacuna — small space housing an osteocyte or chondrocyte.
- Canaliculi — small canals connecting lacunae to central canal.
- Osteon (Haversian system) — structural unit of compact bone.
- Trabeculae — lattice-like structures in spongy bone.
- Interstitial growth — bone growth in length at growth plates.
- Appositional growth — bone growth in width.
- PTH (Parathyroid hormone) — raises blood calcium via bone resorption.
- Calcitonin — lowers blood calcium via bone deposition.
- Osteopenia/Osteoporosis — decreased bone density/condition with fragile bones.
Action Items / Next Steps
- Review textbook and e-text sections on bone tissue, osteon structure, and bone growth.
- Study histology slides of compact and spongy bone in lab and virtual environments.
- Memorize the roles of bone cell types and key terms.
- Prepare for questions relating to bone structure, function, and hormone regulation for the next exam.