Overview
This lecture introduces bone histology, the functions of the skeletal system, bone types, the structure of long bones, and the microscopic anatomy of bone tissue.
Skeletal System Overview
- The skeletal system includes bones, joints, ligaments, and tendons.
- Bones provide structure; joints connect bones; ligaments connect bone to bone; tendons connect muscle to bone.
- Adults have 206 bones in the body.
Functions of the Skeletal System
- Protects vital organs (e.g., skull protects brain, ribs protect heart/lungs).
- Supports body weight and maintains posture.
- Produces blood cells in red bone marrow (hematopoiesis).
- Stores calcium and phosphate minerals.
- Stores fat in yellow bone marrow.
- Provides attachment and leverage for movement.
Bone Classification by Shape
- Long bones: longer than wide (e.g., femur, humerus).
- Short bones: about equal in length and width (e.g., carpals, tarsals).
- Flat bones: thin, broad, and flat (e.g., ribs, sternum, skull bones).
- Sesamoid bones: form inside tendons (e.g., patella).
- Irregular bones: complex shapes (e.g., vertebrae).
Structure of Long Bones
- Periosteum: dense connective tissue covering outer bone.
- Diaphysis: shaft of the bone.
- Epiphyses: ends of the bone (proximal and distal).
- Articular cartilage covers joint surfaces.
- Medullary cavity: central cavity containing bone marrow.
- Endosteum: connective tissue lining the medullary cavity.
- Compact bone: dense outer layer, provides strength.
- Spongy bone: inner layer with spaces, houses bone marrow.
- Epiphyseal line/plate: growth region in children (cartilage), becomes bone in adults.
- Nutrient foramen: hole for blood vessel entry.
Structure of Other Bones
- Short, flat, sesamoid, and irregular bones also have periosteum, compact bone, and spongy bone with marrow.
Bone Marrow Differences
- Red bone marrow: produces blood cells, abundant in children.
- Yellow bone marrow: stores fat, increases with age as red converts to yellow.
Microscopic Structure of Bone Tissue
- Bone tissue has an extracellular matrix (ECM) with organic (collagen) and inorganic (mineral) components.
- Inorganic matrix (calcium/phosphate) gives bones hardness; organic matrix (collagen) gives flexibility.
- Three bone cells: osteoblasts (build bone), osteocytes (maintain bone), osteoclasts (break down bone).
- Compact bone is organized into osteons with concentric lamellae ("rings").
- Osteons contain a central canal with nerves and blood vessels.
- Osteocytes live in lacunae and communicate via canaliculi.
- Spongy bone consists of trabeculae (struts), with spaces for marrow.
- Compact bone bears weight; spongy bone provides a scaffold for marrow—not for weight-bearing.
Key Terms & Definitions
- Articulate — to connect or join together.
- Ligament — connective tissue joining bone to bone.
- Tendon — connective tissue joining muscle to bone.
- Periosteum — connective tissue covering outer bone surface.
- Diaphysis — shaft of a long bone.
- Epiphysis — ends of a long bone.
- Medullary cavity — hollow space inside bone containing marrow.
- Endosteum — connective tissue lining medullary cavity.
- Compact bone — dense bone tissue; strength and support.
- Spongy bone — porous bone tissue; houses marrow.
- Osteon — structural unit of compact bone.
- Lamella/lamellae — concentric rings of bone tissue in osteons.
- Osteoblast — cell that builds new bone tissue.
- Osteocyte — mature bone cell maintaining bone tissue.
- Osteoclast — cell that breaks down bone tissue.
- Lacuna — small cavity housing an osteocyte.
- Canaliculi — small channels for cell communication in bone.
- Epiphyseal line/plate — growth plate region in long bones.
- Trabeculae — struts of spongy bone.
Action Items / Next Steps
- Review diagrams showing bone structure and osteon organization.
- Memorize bone classifications and key cell types.
- Read textbook sections on microscopic bone anatomy and bone cell function.