Overview of the Skeletal System

Skeletal System Overview

• The skeletal system is composed of bones, cartilage, ligaments, and tendons, working together to provide support, protection, movement, mineral storage, and blood cell production.
• Each bone is an organ, made of bone tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue. Bone remodeling is a continuous process involving the building of new bone and the breakdown of old bone.
• Osteology is the study of bone structure and bone disorders.

Functions of the Skeletal System

• Support: Provides a framework for soft tissues and attachment points for muscles.
• Protection: Shields internal organs like the brain (cranium) and heart/lungs (rib cage).
• Movement: Skeletal muscles attach to bones, generating movement when they contract.
• Mineral Homeostasis: Stores and releases minerals like calcium and phosphorus, crucial for maintaining homeostasis and distributing these minerals throughout the body. Bone tissue stores ~99% of the body's calcium.
• Blood Cell Production: Red bone marrow within certain bones creates red blood cells, white blood cells, and platelets (hemopoiesis).
• Triglyceride Storage: Yellow bone marrow stores triglycerides as a potential energy source.

Macroscopic Bone Structure

• Diaphysis: The long, cylindrical shaft of a long bone.
• Epiphyses: The proximal and distal ends of a long bone.
• Metaphyses: The regions between the diaphysis and epiphyses (often containing growth plates).
• Articular Cartilage: A thin layer of hyaline cartilage covering the epiphyses reducing friction and absorbing shocks.
• Periosteum: A tough connective tissue sheath covering the bone surface, assisting in fracture repair, nourishing bone tissue, and providing attachment points for ligaments and tendons.
• Medullary Cavity: A hollow cylindrical space within the diaphysis containing fatty yellow bone marrow and blood vessels minimizing bone weight.
• Endosteum: A thin membrane lining the medullary cavity.

Bone Tissue Histology

• Bone tissue is ~15% water, ~30% collagen fibers, and ~55% crystallized mineral salts.
• The main mineral salt is calcium phosphate (Ca3(PO4)2), which combines with calcium hydroxide (Ca(OH)2) to form hydroxyapatite crystals (Ca10(PO4)6(OH)2).
• Calcification occurs as mineral salts are deposited in the collagen framework, hardening the tissue, and initiated by osteoblasts.

Bone Cells

• Osteoprogenitor Cells: Unspecialized bone stem cells derived from mesenchyme, the only bone cell that divides.
• Osteoblasts: Bone-building cells that synthesize and secrete collagen and other organic components to form bone tissue.
• Osteocytes: Mature bone cells, the main cells in bone tissue, maintaining daily metabolism.
• Osteoclasts: Large cells derived from monocytes that break down bone tissue (bone resorption).

Bone Tissue Types

• Compact Bone: Strongest bone type, with few spaces. Forms the outermost layer of all bones, and much of the diaphysis of long bones.
• Spongy Bone (Trabecular/Cancellous): Contains many open spaces and is found in the interior of bones, filled with bone marrow.

Bone Formation

• Intramembranous Ossification: Formation of bones directly from mesenchymal tissue.
• Endochondral Ossification: Formation of most bones by replacing hyaline cartilage models.

Factors Affecting Bone Growth & Remodeling

• Various hormones (Insulin-like growth factors, thyroid hormones, sex hormones) and minerals (Mg, P, Mn) influence bone growth and remodeling.

Clinical Connections

• Flatfoot: Reduced height of the medial longitudinal arch due to weakened supporting tissues.
• Bone fractures and structural abnormalities.

Appendicular Skeleton

• The appendicular skeleton includes the pectoral (shoulder) and pelvic girdles, as well as upper and lower limbs.

Axial Skeleton

• The axial skeleton includes the skull (cranial and facial bones), vertebral column, and hyoid bone.