Bone Tissue and the Axial Skeleton: Structure, Growth, and Anatomy

Bone Tissue and the Axial Skeleton

Overview

This lesson covers the structure, function, and development of bone tissue, as well as the anatomy of the axial skeleton. Understanding these topics is essential for comprehending how the skeletal system supports, protects, and enables movement in the human body.

Bone Tissue

Functions of Bone

• Support: Bones provide a rigid framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs, such as the brain (skull), heart and lungs (rib cage), and spinal cord (vertebrae).

• Assistance in Movement: Bones act as levers for muscles, enabling movement.

• Mineral Homeostasis: Bones store minerals, especially calcium and phosphorus, and release them into the bloodstream as needed.

• Blood Cell Production: Hematopoiesis occurs in red bone marrow, producing red and white blood cells and platelets.

• Triglyceride Storage: Yellow bone marrow stores fat as an energy reserve.

Classification of Bones by Shape

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and often curved (e.g., sternum, ribs, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, some facial bones).

• Sesamoid Bones: Small, round bones embedded in tendons (e.g., patella).

Macroscopic Structure of a Long Bone

• Diaphysis: The shaft or central part of a long bone.

• Epiphysis: The ends of a long bone, usually wider than the diaphysis.

• Periosteum: Dense connective tissue membrane covering the outer surface of bone (except at joints).

• Articular Cartilage: Hyaline cartilage covering joint surfaces, reducing friction and absorbing shock.

• Bone Marrow: Soft tissue in medullary cavities; red marrow (hematopoietic) and yellow marrow (fat storage).

Microscopic Structure (Histology) of Bone

• Compact Bone: Dense, organized into osteons (Haversian systems) with concentric lamellae around a central canal.

• Spongy Bone: Lattice-like network of trabeculae; spaces filled with marrow.

• Osteocyte: Mature bone cell residing in lacunae, maintaining bone tissue.

• Osteoblast: Bone-forming cell; synthesizes bone matrix.

• Osteoclast: Bone-resorbing cell; breaks down bone matrix.

• Lamellae: Layers of bone matrix in compact bone.

• Haversian Canal: Central canal in osteons containing blood vessels and nerves.

• Volkmann’s Canals: Perforating canals connecting Haversian canals transversely.

• Canaliculi: Tiny channels connecting lacunae, allowing communication between osteocytes.

Bone Formation (Ossification)

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms most flat bones (e.g., cranial bones).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most bones of the body, especially long bones.

Example: The flat bones of the skull are formed by intramembranous ossification, while the femur is formed by endochondral ossification.

Bone Growth

• Length: Occurs at the epiphyseal (growth) plate through endochondral ossification.

• Width (Appositional Growth): New bone tissue is added to the surface by osteoblasts in the periosteum.

Bone Remodeling and Repair

• Remodeling: Continuous process where old bone is replaced by new bone tissue, adapting to stress and repairing microdamage.

• Fracture Repair: Involves four main steps:

  1. Hematoma formation (blood clot at fracture site)

  2. Fibrocartilaginous (soft) callus formation

  3. Bony (hard) callus formation

  4. Bone remodeling (restoration of original shape)

The Axial Skeleton

Divisions of the Skeleton

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum), plus the hyoid bone.

• Appendicular Skeleton: Includes the bones of the limbs and girdles (not covered in this lesson).

Bone Markings

Bones have various surface features (markings) that serve as attachment points for muscles, passageways for nerves and blood vessels, and articulation sites for joints. Examples include foramina (holes), processes (projections), and fossae (depressions).

Components of the Axial Skeleton

• Skull: Composed of cranial and facial bones.

  • Cranial Bones (8): Frontal, Parietal (2), Temporal (2), Occipital, Sphenoid, Ethmoid.

  • Facial Bones: Include maxilla, mandible, zygomatic, nasal, and others.

• Hyoid Bone: U-shaped bone in the neck; does not articulate with other bones; supports tongue and muscles of the neck and pharynx.

• Vertebral Column: Consists of 33 vertebrae in five regions:

  1. Cervical (7)

  2. Thoracic (12)

  3. Lumbar (5)

  4. Sacral (5, fused)

  5. Coccygeal (4, fused)

  Main Functions of Intervertebral Disks:

  • Act as shock absorbers

  • Allow flexibility and movement of the spine

  • Prevent vertebrae from rubbing together

• Thoracic Cage: Includes the ribs and sternum; protects the heart and lungs and supports the shoulder girdles and upper limbs.

Structure of a Typical Rib

• Each rib has a head (articulates with vertebrae), neck, tubercle, and shaft.

• Ribs are connected posteriorly to the thoracic vertebrae and anteriorly (directly or indirectly) to the sternum.

Key Terms and Definitions

Term	Definition
Axial skeleton	Bones of the skull, vertebral column, thoracic cage, and hyoid bone
Appendicular skeleton	Bones of the limbs and girdles
Compact bone	Dense bone tissue organized into osteons
Spongy bone	Porous bone tissue with trabeculae
Diaphysis	Shaft of a long bone
Epiphysis	End of a long bone
Periosteum	Membrane covering bone surface
Articular cartilage	Cartilage covering joint surfaces
Bone marrow	Soft tissue in bone cavities (red or yellow)
Lamellae	Concentric layers of bone matrix
Haversian canal	Central canal in osteons
Canaliculi	Microscopic channels for cell communication
Osteocyte	Mature bone cell
Osteoclast	Bone-resorbing cell
Osteoblast	Bone-forming cell
Volkmann’s canals	Transverse canals connecting Haversian canals
Endochondral ossification	Bone formation from cartilage
Intramembranous ossification	Bone formation from mesenchyme
Cranial bones	Bones forming the cranium
Facial bones	Bones forming the face
Vertebrae	Bones of the vertebral column
Thoracic bones	Ribs and sternum
Hyoid bone	U-shaped bone in the neck

Self-Test Questions (with Academic Context)

1. List 5 functions of bones: Support, protection, movement, mineral storage, blood cell production.

2. Distinguish between compact bone and spongy bone: Compact bone is dense and forms the outer layer; spongy bone is porous and found inside bones.

3. Describe the histological structure of bone: Composed of osteons (compact bone), trabeculae (spongy bone), osteocytes in lacunae, and a matrix of collagen and minerals.

4. Describe the anatomy of the long bone: Consists of diaphysis, epiphyses, periosteum, articular cartilage, and marrow cavity.

5. The bone is a living tissue. Explain: Contains living cells (osteocytes, osteoblasts, osteoclasts), blood vessels, and nerves; capable of growth, repair, and remodeling.

6. Describe the process of endochondral ossification: Bone forms by replacing hyaline cartilage; primary ossification center develops in diaphysis, secondary centers in epiphyses.

7. Describe the process of intramembranous ossification: Bone develops directly from mesenchymal tissue; osteoblasts secrete matrix, forming flat bones.

8. How do bones grow in length and width? Length: at epiphyseal plate by endochondral ossification; width: appositional growth by osteoblasts in periosteum.

9. The cranium is composed of 8 bones. List these 8 bones: Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid.

10. Name the five major regions of the vertebral column: Cervical, thoracic, lumbar, sacral, coccygeal.

11. List the main functions of the intervertebral disks: Shock absorption, flexibility, prevent vertebral friction.

Health-Related Question

• Describe how a fracture is repaired:

  1. Hematoma formation (blood clot at fracture site)

  2. Fibrocartilaginous callus formation (soft callus stabilizes fracture)

  3. Bony callus formation (spongy bone replaces soft callus)

  4. Bone remodeling (restores bone to original shape)

Additional info:

• Bone growth is regulated by hormones (e.g., growth hormone, sex hormones), but detailed mechanisms are not required for this lesson.

• For lab study, focus on the bones and anatomical details emphasized in your lab manual.