Overview
This lecture explains the structure of a long bone, including its main parts, types of bone tissue, cellular composition, coverings, and microscopic organization.
Types and Parts of Bones
- Bones are classified by shape as long, short, flat, or irregular.
- Long bones have three main regions: diaphysis (shaft), epiphysis (ends), and metaphysis (flared regions between shaft and ends).
- In growing bones, the epiphyseal growth plate (cartilage) joins diaphysis and epiphysis before fusing after ossification.
Macroscopic Bone Structure
- Diaphysis contains a medullary (marrow) cavity surrounded by thick compact bone and thin cancellous (spongy) bone.
- Epiphysis has abundant cancellous bone surrounded by a thin compact bone layer, with no medullary cavity.
- Bone marrow in long bones may be red (hematopoietic, blood cell synthesis) or yellow (fatty, adipocytes); adult diaphysis has mainly yellow marrow.
Bone Tissues and Matrix
- Two main bone tissues: compact (dense/cortical, outside) and cancellous (spongy/trabecular, inside, with trabeculae).
- Bone tissue is a specialized connective tissue with cells in a calcified extracellular matrix.
- Organic matrix (type I collagen, ground substance) provides tensile strength; inorganic matrix (mainly hydroxyapatite) provides compressional strength.
Bone Cells and Their Roles
- Osteoprogenitor cells: stem cells differentiating into osteoblasts for bone formation.
- Osteoblasts: produce organic bone matrix (osteoid), type I collagen, ground substance, and bone-specific proteins (osteocalcin, osteonectin).
- Osteocytes: mature bone cells trapped in matrix, maintain bone tissue.
- Osteoclasts: multinucleated cells from monocytes, responsible for bone resorption.
- Bone lining cells: inactive cells on bone surfaces.
Bone Coverings: Periosteum and Endosteum
- Periosteum: dense connective tissue sheath covering outer bone, with an outer fibrous layer (type I collagen, nerves, blood vessels) and an inner cellular layer (bone-forming cells).
- Perforating fibers anchor periosteum to bone, especially at tendon attachment sites.
- Endosteum: cellular lining of marrow cavities and trabeculae, contains osteoprogenitor cells, osteoblasts, osteoclasts, and bone lining cells.
Microscopic Structure: Lamellar and Woven Bone
- Woven bone is immature, with irregular collagen; remodels into mature lamellar bone (strong, regularly layered).
- Lamellar bone layers (lamellae) are arranged around Haversian (central) canals, forming osteons.
- Osteocytes reside in lacunae between lamellae, connected by canaliculi for nutrient exchange.
- Volkmann's canals (transverse canals) connect Haversian canals across osteons.
- Interstitial lamellae are remnants of old osteons; circumferential lamellae line the outer and inner bone surfaces.
Cancellous Bone and Marrow
- Cancellous bone consists of trabeculae (rods/plates) containing osteocytes, providing support with less weight.
- Spaces between trabeculae are filled with bone marrow and lined by endosteum.
Key Terms & Definitions
- Diaphysis â Shaft of a long bone.
- Epiphysis â End part of a long bone.
- Metaphysis â Region between diaphysis and epiphysis.
- Epiphyseal growth plate â Cartilage plate for bone growth.
- Compact (cortical) bone â Dense outer bone tissue.
- Cancellous (spongy/trabecular) bone â Lattice-like inner bone tissue.
- Periosteum â Dense connective tissue covering outer bone.
- Endosteum â Cellular lining on inside of bone cavities.
- Osteon (Haversian system) â Structural unit of compact bone.
- Osteoblast â Cell that makes bone matrix.
- Osteocyte â Mature bone cell maintaining bone.
- Osteoclast â Multinucleated cell that resorbs bone.
- Trabeculae â Rods/plates in cancellous bone.
- Medullary cavity â Central cavity containing bone marrow.
Action Items / Next Steps
- Review diagrams of long bone structure.
- Memorize key cell types and their functions in bone.
- Understand the differences between compact and cancellous bone.