Overview
This lecture introduces the skeletal system, focusing on osseous (bone) tissue, main bone cell types, and the differences between compact and spongy bone.
Functions of the Skeletal System
- Supports body structure and protects internal organs.
- Provides leverage for movement via muscles pulling on bones.
- Stores minerals, mainly calcium (98% stored in bones).
- Maintains calcium balance by releasing or absorbing calcium as needed.
- Stores fat in yellow bone marrow.
- Produces blood cells in red bone marrow.
- Bones are dynamic, undergoing constant growth, repair, and remodeling.
Structure and Composition of Osseous Tissue
- Osseous tissue is a connective tissue with specialized cells and an extracellular matrix.
- The matrix includes collagen fibers (flexibility and tensile strength) and hydroxyapatite (calcium salts for hardness).
- Bones are strong yet flexible and self-repairing, unlike static plastic models.
Types of Bone Cells
- Osteogenic (osteoprogenitor) cells: Stem cells that generate new bone cells during growth or repair.
- Osteoblasts: Build new bone (osteogenesis); become osteocytes once surrounded by bone.
- Osteocytes: Mature bone cells maintaining the bone matrix; found in lacunae and connected by canaliculi.
- Osteoclasts: Large cells with many nuclei that break down bone (osteolysis), releasing minerals into blood.
- Osteoblasts and osteoclasts have opposite roles in bone remodeling and calcium homeostasis.
Compact vs. Spongy Bone
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Compact bone: Dense, forms the outer shell (cortex) of bones, made of cylindrical osteons.
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Osteons have central canals with blood vessels and nerves.
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Lamellae are rings of bone matrix; types include concentric, circumferential, and interstitial.
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Lacunae house osteocytes; canaliculi connect them for nutrient exchange.
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Perforating (Volkmann's) canals run perpendicular to central canals for blood supply.
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Spongy (cancellous) bone: Porous, contains bone marrow, found inside bones and between layers in flat bones.
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Composed of trabeculae—branching plates that lighten bone weight and provide support.
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No osteons, but has osteocytes and lamellae in an irregular arrangement.
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Osteoporosis thins trabeculae, reducing bone density.
Key Terms & Definitions
- Osseous tissue — bone tissue, a type of connective tissue.
- Osteocyte — mature bone cell maintaining bone matrix.
- Osteoblast — bone-forming cell.
- Osteoclast — bone-resorbing (breaking down) cell.
- Osteon — structural unit of compact bone.
- Lamella — ring of bone matrix.
- Lacuna — cavity containing an osteocyte.
- Canaliculus — tiny canal connecting lacunae.
- Trabecula — branchlike plate in spongy bone.
- Hydroxyapatite — mineral (calcium salt) giving bone hardness.
Action Items / Next Steps
- Review diagrams/models of compact and spongy bone structures.
- Study the functions and life cycle of different bone cells.
- Read more about bone remodeling and calcium homeostasis.