Overview
This lecture introduces the skeletal system, focusing on bone tissue, bone structure, types of bone cells, bone formation, bone repair, calcium homeostasis, and the general functions of the skeletal system.
Introduction to the Skeletal System
- The skeletal system is divided into bones, cartilage (padding at bone ends), and ligaments (connect bones together).
- These structures are primarily made of connective tissue.
Types of Bone Cells
- Osteogenic cells are stem cells that generate other bone cells.
- Osteoblasts build bone by secreting the bone matrix.
- Osteocytes maintain bone tissue once surrounded by matrix.
- Osteoclasts break down bone tissue; they are multinucleated.
Bone Matrix and Structure
- The bone matrix consists of hard minerals (mainly calcium) and strong collagen fibers.
- Bone cells occupy spaces called lacunae within the matrix.
- Two bone tissue types: compact bone (dense outer layer) and spongy bone (porous inner layer with trabeculae).
Compact and Spongy Bone Details
- Compact bone is organized in osteons (Haversian systems) with central canals for blood vessels and nerves.
- Lamellae are rings of matrix around osteon canals; canaliculi are tiny channels for nutrient passage.
- Spongy bone has trabeculae with spaces filled by red bone marrow, where blood cells are produced by hemocytoblasts.
Bone Markings and Classifications
- Bone markings include openings (foramen), depressions, and projections (for muscle attachment).
- Bones are classified by shape: flat, long, short, round (sesamoid), and irregular.
Anatomy of a Long Bone
- Major parts: epiphysis (ends), diaphysis (shaft), compact bone (surface), spongy bone (interior), medullary cavity (central space), periosteum (outer covering), endosteum (inner lining).
- Red marrow fills spongy bone; yellow marrow (fat) fills the medullary cavity and can convert to red marrow if needed.
- Articular cartilage covers bone ends; nutrient foramina allow vessels/nerves entry.
Bone Formation (Osteogenesis)
- Endochondral ossification: most bones form from hyaline cartilage models, with primary and secondary ossification centers.
- Growth hormone stimulates lengthening via the epiphyseal (growth) plate.
- Intramembranous ossification forms mainly flat bones from fibrous membranes, without a cartilage stage.
Bone Repair and Remodeling
- Fracture healing involves forming a hematoma (blood clot), osteoblast invasion (new bone formation), and osteoclast activity (smoothing the repair).
- Bone remodeling maintains shape and function throughout life.
Calcium Homeostasis
- Hypercalcemia (high blood calcium) triggers calcitonin from the thyroid, stimulating osteoblasts to store calcium in bones.
- Hypocalcemia (low blood calcium) triggers parathyroid hormone, activating osteoclasts to release calcium from bones.
Functions of the Skeletal System
- Provides body support and shape.
- Stores minerals (especially calcium) and lipids (yellow marrow).
- Produces blood cells in red marrow (hematopoiesis).
- Protects organs (e.g., skull, ribcage).
- Enables movement via joints.
- Allows growth in height via cartilage in growing bones.
Key Terms & Definitions
- Osteogenic cell â bone stem cell that produces all other bone cells.
- Osteoblast â bone-building cell that secretes matrix.
- Osteocyte â mature bone cell maintaining tissue.
- Osteoclast â bone-resorbing (breaking down) cell.
- Lacunae â small cavities in bone matrix housing bone cells.
- Osteon (Haversian system) â structural unit of compact bone.
- Lamellae â rings of bone matrix in osteon.
- Trabeculae â bars forming spongy bone structure.
- Epiphyseal plate â cartilage growth plate in developing bones.
- Hematopoiesis â blood cell production in red bone marrow.
- Calcitonin â hormone that lowers blood calcium by stimulating bone formation.
- Parathyroid hormone (PTH) â hormone raising blood calcium by stimulating bone breakdown.
Action Items / Next Steps
- Review textbook chapters on bone tissue, bone structure, and bone growth.
- Learn types of bone markings and practice identifying bone shapes.
- Read about different types of bone fractures as supplemental material.