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 fundamental for students of anatomy and physiology, as bones provide support, protection, and play a key role in movement and homeostasis.

Bone Tissue

Functions of Bone

• Support: Bones form the framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

• Assistance in Movement: Skeletal muscles attach to bones, and their contraction moves the skeleton.

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

• Blood Cell Production: Hematopoiesis occurs in the 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, cranial 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: A dense layer of vascular connective tissue enveloping the bone except at the surfaces of the joints.

• Articular Cartilage: Hyaline cartilage covering the surfaces of bones at joints, reducing friction and absorbing shock.

• Bone Marrow: Soft tissue in bone cavities; red marrow is involved in blood cell production, yellow marrow stores fat.

Histology of Bone Tissue

Bone tissue is a connective tissue with a matrix hardened by calcium phosphate and other minerals. It is highly vascularized and innervated.

• Compact Bone: Dense outer layer; provides strength and protection.

• Spongy Bone (Cancellous Bone): Lighter, less dense, found at the ends of long bones and inside flat bones; contains trabeculae.

Microscopic Structure

• Osteon (Haversian System): The structural unit of compact bone, consisting of concentric lamellae around a central (Haversian) canal.

• Lamellae: Layers of bone matrix in which collagen fibers run in different directions to resist twisting.

• Haversian Canal: Central channel containing blood vessels and nerves.

• Canaliculi: Tiny canals connecting osteocytes, allowing for nutrient and waste exchange.

• Osteocyte: Mature bone cell that maintains bone tissue.

• Osteoblast: Bone-forming cell.

• Osteoclast: Bone-resorbing cell.

• Volkmann’s Canals: Perforating canals that connect blood vessels and nerves of the periosteum to those in the Haversian canals.

Bone Formation (Ossification)

Bone formation occurs through two main processes:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue. This process forms most flat bones, such as cranial bones and the clavicle.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. This process forms most bones of the body, including long bones.

Steps of Endochondral Ossification

1. Development of cartilage model

2. Growth of cartilage model

3. Development of primary ossification center

4. Development of medullary cavity

5. Development of secondary ossification centers

6. Formation of articular cartilage and epiphyseal plate

Steps of Intramembranous Ossification

1. Development of ossification center

2. Calcification

3. Formation of trabeculae

4. Development of periosteum

Bone Growth

• Growth in Length: Occurs at the epiphyseal plate through interstitial growth of cartilage, which is then replaced by bone.

• Growth in Width (Appositional Growth): Osteoblasts in the periosteum add new bone tissue to the surface.

Bone Remodeling and Repair

• Remodeling: Ongoing replacement of old bone tissue by new bone tissue, involving osteoclasts (resorption) and osteoblasts (deposition).

• Fracture Repair: Involves four main steps:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

The Axial Skeleton

Overview and Components

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage. It supports and protects the brain, spinal cord, and thoracic organs.

• Axial Skeleton: Skull, vertebral column, thoracic cage (ribs and sternum), and hyoid bone.

• Appendicular Skeleton: Limbs and girdles (not covered in this lesson).

Bone Markings

Bones have various surface markings that serve as attachment sites for muscles, passageways for nerves and blood vessels, and articulation points for joints.

Type of Marking	Description	Example
Foramen	Round or oval opening through a bone	Foramen magnum (skull)
Process	Any bony prominence	Mastoid process (temporal bone)
Fossa	Shallow depression	Mandibular fossa
Condyle	Rounded articular projection	Occipital condyle
Crest	Narrow, prominent ridge	Iliac crest
Spine	Sharp, slender projection	Ischial spine
Tubercle/Tuberosity	Small/large rounded projection	Greater tubercle (humerus)

Skull

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

• Facial Bones: Form the structure of the face (not all listed here).

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

Vertebral Column

• Functions: Protects the spinal cord, supports the head, and serves as an attachment point for ribs and muscles.

• Regions:

  1. Cervical (7 vertebrae)

  2. Thoracic (12 vertebrae)

  3. Lumbar (5 vertebrae)

  4. Sacral (5 fused vertebrae)

  5. Coccygeal (4 fused vertebrae)

• Intervertebral Disks: Pads of fibrocartilage between vertebrae that absorb shock, allow movement, and prevent friction.

Thoracic Cage

• Components: Sternum, ribs, and thoracic vertebrae.

• Functions: Protects the heart and lungs, supports the shoulder girdles and upper limbs, and provides attachment points for muscles.

• Ribs: 12 pairs; true ribs (1-7), false ribs (8-12), floating ribs (11-12).

• Structure of a Typical Rib: Head, neck, tubercle, shaft, and costal groove.

Key Terms and Definitions

Term	Definition
Axial skeleton	Bones along the central axis of the body
Appendicular skeleton	Bones of the limbs and girdles
Compact bone	Dense, solid bone tissue forming the outer layer
Spongy bone	Porous bone tissue inside bones
Diaphysis	Shaft of a long bone
Epiphysis	End of a long bone
Periosteum	Membrane covering bone surface
Articular cartilage	Cartilage covering bone ends at joints
Bone marrow	Soft tissue in bone cavities
Lamellae	Concentric rings of bone matrix
Haversian canal	Central canal in osteon
Canaliculi	Small channels connecting osteocytes
Osteocyte	Mature bone cell
Osteoclast	Bone-resorbing cell
Osteoblast	Bone-forming cell
Volkmann’s canals	Perforating canals connecting osteons
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	Bones of the thoracic cage
Hyoid bone	U-shaped bone in the neck

Sample Self-Test Questions

• List 5 functions of bones.

• Distinguish between compact bone and spongy bone.

• Describe the histological structure of bone.

• Describe the anatomy of the long bone.

• Explain why bone is considered a living tissue.

• Describe the process of endochondral ossification.

• Describe the process of intramembranous ossification.

• Explain how bones grow in length and width.

• List the 8 cranial bones.

• Name the five major regions of the vertebral column.

• List the main functions of the intervertebral disks.

• Describe how a fracture is repaired.

Additional info:

• Bone is a living tissue because it contains cells, is vascularized, and undergoes constant remodeling and repair.

• Bone growth in length occurs at the epiphyseal plate, while growth in width is due to appositional growth at the periosteum.

• Fracture repair involves hematoma formation, callus formation, and remodeling to restore bone structure.