Bone Is Considered An Important Storage Reservoir For A, The Basics Of Bone In Health And Disease

Summary

The musculoskeletal system is comprised of bones and connective tissue structures, such as cartilage, ligaments, and tendons. These structures are brought into motion by skeletal muscles. To withstand resultant forces, bone tissue resists pressure and tension and is minimally elastic. Bone tissue mainly consists of bone cells (osteoblasts, osteocytes, and osteoclasts) and a mineralized extracellular matrix that is primarily made up of collagen fibrils and hydroxyapatite crystals. Ossification, or bone formation, begins with a framework that consists of either mesenchymal connective tissue (intramembranous ossification) or cartilage (endochondral ossification). Woven bone is formed, which is replaced by the more solid and layered lamellar bone. The outer cortical layers can be macroscopically differentiated from the branched center of the trabeculae.

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Overview

Function of bone

Supportive function Protective function Storage (calcium and phosphorus reservoir)

Types of bone

Long bones: include the femur, humerus, ulna, radius, fibula, metacarpals, and phalanges Epiphysis Contains the articular surface of bones Diaphysis Flat bones Sesamoid bones Bones that develop within tendons (e.g., the patella) Function to reduce friction of the tendon and to reduce excessive wear Irregular bones Have several functions including: Attachment for the tendon of muscles

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Bone composition

Overview

The exact composition or organization of individual bone components differs in the various bones types and maturation stages. All human bones are composed of the same basic elements:

Bone cells

Characteristics of bone cells Definition Function <3> Location Osteoprogenitor cells

Osteoblasts <4>

Osteocytes

Osteoclasts

Bone-building cells that differentiate from osteoprogenitor cells

Bone matrix

Composed of organic and inorganic material:

Organic Inorganic

Bone membranes

Periosteum: a membrane of connective tissue that covers the outer surface of the bone in two distinct layers Endosteum

The periosteum and the endosteum consist of the same type of bone cells.

Bone marrow

Development and maturation

Development of bone

Bone is derived from mesoderm.

Ossification

Overview of the ossification process Endochondral ossification Intramembranous ossification Definition

Process

Examples

Mesenchymal cells differentiate to chondroblasts → formation of a cartilaginous model Bone collar formation: Bone forms around the cartilaginous model. Bone formation within the cartilaginous model

The skull undergoes both processes: some bones (e.g., frontal, parietal bones) are derived from the neural crest and undergo membranous ossification, whereas other bones (e.g., sphenoid, occipital bones) are derived from the paraxial mesoderm and undergo endochondral ossification.

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Stages of bone maturity

Bones are arranged into woven bone (primary bone) during embryonic development or bone healing. The structure of woven bone is disorganized and transformed into organized tissue of lamellar bone (secondary bone) through continuous remodeling.

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Stages of bone maturity Woven bone Lamellar bone Definition

Histology

Characteristics

Remodeling

Less mineralized bone substance with a high water content Rich in cells The extracellular matrix is organized into two types of tissues:
Mechanically strong
Usually successively remodeled to more stable lamellar bones Load-dependent, continuous remodeling of bone according to acting forces

The direction of collagen fibers of the bone extracellular matrix is an important distinguishing characteristic between immature woven bone and mature lamellar bone.

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Trabecular bone (spongy or cancellous bone)

Characteristics Trabeculae are aligned along large compressive and tensile forces (trajectory direction).

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