Chapter 6: Bones and Bone Tissue

Introduction to Bones as Organs

The skeletal system consists of bones, joints, and their associated supporting tissues. Bones are organs composed of dense osseous tissue and collagenous connective tissue, as well as bone marrow. The skeletal system performs several vital functions necessary for human physiology.

• Protection: Bones protect internal organs from mechanical injury. For example, the skull protects the brain, and the rib cage shields the heart and lungs.

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals such as calcium and phosphorus, which can be released into the bloodstream to maintain mineral balance and acid-base homeostasis.

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

• Fat Storage: Yellow bone marrow stores adipose tissue, serving as an energy reserve.

• Movement: Bones act as levers for muscles, enabling movement of the body.

• Support: The skeleton provides a rigid framework that supports the body and maintains its shape.

Classification of Bones by Shape

Bones are classified into five categories based on their shape, which reflects their function and location in the body.

Structure of a Typical Long Bone

Long bones have a unique structure that supports their function in movement and weight-bearing.

• Diaphysis: The shaft of the bone, containing a medullary cavity filled with yellow or red bone marrow.

• Epiphyses: The ends of the bone, covered with articular cartilage and containing spongy bone.

• Epiphyseal Plate: Hyaline cartilage plate in children, site of bone growth.

• Periosteum: Dense connective tissue membrane covering the outer surface of bone, containing blood vessels and nerves.

• Endosteum: Thin membrane lining the medullary cavity.

Structure of Short, Flat, Irregular, and Sesamoid Bones

These bones do not have a diaphysis or epiphyses. They are covered by periosteum and consist of two layers of compact bone with a middle layer of spongy bone (diploë in flat bones).

Blood and Nerve Supply to Bone

• Red Bone Marrow: Found in spongy bone of adults (e.g., pelvis, femur, sternum) and in all bones of children. Responsible for hematopoiesis.

• Yellow Bone Marrow: Composed of adipose tissue, found in the medullary cavity of long bones in adults.

Microscopic Structure of Bone Tissue

Bone tissue, or osseous tissue, is composed of an extracellular matrix (ECM) and a small population of cells scattered throughout.

• Extracellular Matrix:

  • Inorganic matrix: Makes up about 65% of bone's total weight, consisting mainly of calcium and phosphorus salts as hydroxyapatite crystals ().

  • Organic matrix: Makes up about 35% of bone's weight, consisting of collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins.

• Function of Matrix Components:

  • Inorganic matrix provides strength and resistance to compression.

  • Organic matrix provides flexibility and resistance to twisting and stretching forces.

Bone Cells and Their Functions

Bones are dynamic tissues, constantly remodeled by specialized cells.

• Osteoblasts: Bone-building cells found in the periosteum and endosteum. They synthesize bone matrix and initiate ossification.

• Osteocytes: Mature bone cells derived from osteoblasts, residing in lacunae. They maintain bone tissue and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption, breaking down bone matrix.

Histology of Bone Tissue

Bone tissue is organized into two types: compact bone and spongy bone, each with distinct histological features.

• Compact Bone:

  • Composed of osteons (Haversian systems), which are concentric rings (lamellae) surrounding a central canal containing blood vessels and nerves.

  • Osteocytes reside in lacunae between lamellae, connected by canaliculi.

  • Perforating (Volkmann's) canals connect osteons and allow blood vessels to penetrate deeper into bone.

• Spongy Bone:

  • Composed of a network of trabeculae, providing structural support and housing bone marrow.

  • Trabeculae are oriented along lines of stress to resist forces from multiple directions.

Bone Formation: Ossification

Ossification, or osteogenesis, is the process of bone formation, beginning in the embryonic period and continuing through childhood.

• Intramembranous Ossification: Forms many flat bones, including those of the skull and clavicles. Occurs within a mesenchymal membrane.

• Endochondral Ossification: Forms most bones below the head, including long bones. Occurs within a hyaline cartilage model.

Steps of Intramembranous Ossification

1. Mesenchymal cells differentiate into osteoblasts at the primary ossification center.

2. Osteoblasts secrete organic matrix, which calcifies.

3. Early spongy bone is formed as osteoblasts continue to lay down bone.

4. Mesenchymal cells form the periosteum; compact bone is formed beneath the periosteum.

Steps of Endochondral Ossification

1. Hyaline cartilage model is formed.

2. Chondroblasts in the perichondrium differentiate into osteoblasts.

3. Osteoblasts build bone on the external surface; the primary ossification center forms.

4. Cartilage in the center calcifies and is replaced by bone.

5. Secondary ossification centers form in the epiphyses.

Bone Growth and Remodeling

Bones grow in length at the epiphyseal plate and in width by appositional growth. Remodeling is a continuous process involving bone resorption by osteoclasts and bone deposition by osteoblasts, allowing bones to adapt to stress and repair damage.

Summary Table: Bone Cell Functions

Key Terms and Definitions

• Ossification: The process of bone formation.

• Hematopoiesis: The formation of blood cells in the bone marrow.

• Periosteum: Dense connective tissue covering the outer surface of bone.

• Epiphyseal Plate: Growth plate of hyaline cartilage in children.

• Trabeculae: Lattice-like network of spongy bone.

• Osteon: Structural unit of compact bone.

Example: The femur is a long bone that supports body weight, enables movement, and contains both red and yellow bone marrow for hematopoiesis and fat storage.

Additional info: Some details, such as specific locations of red bone marrow in adults and the chemical formula for hydroxyapatite, were inferred from standard anatomy and physiology knowledge.