Skeletal System - I (Bones)

Skeletal System - I (Bones)

• The skeletal system is comprised of osseous (bone) tissue, dense connective tissue, cartilage tissue, adipose tissue, and nervous tissue.
• Bones are dynamic tissues that continuously grow, remodel, and repair themselves.
• Bone functions include support, protection, assisting in movements, mineral homeostasis, hematopoiesis (blood cell formation), and triglyceride storage.

Expected Learning Outcomes

• Describe bone structure (gross and microscopic).
• List bone functions.
• Outline bone classifications.
• Describe bone formation and remodeling.

Introduction

• Bone is a complex tissue composed of different tissues.
• Bone is a dynamic tissue that continually grows, remodels, and repairs itself.

Functions of Bones

• Support
• Protection
• Assist in movements
• Mineral homeostasis
• Hematopoiesis (blood cell formation)
• Triglyceride storage

Structure of Bones (Macroscopic)

• Long bones (e.g., humerus, femur): Have a shaft (diaphysis) and two ends (epiphyses).
• Diaphysis: Shaft or body of a long bone.
• Epiphysis: The ends of a long bone, covered by articular cartilage.
• Metaphysis: Region between the diaphysis and epiphysis.
• Articular cartilage: Hyaline cartilage covering the epiphysis.
• Periosteum: Dense connective tissue covering the bone, except at articular cartilage.
• Medullary cavity: Cylindrical space within the diaphysis.
• Endosteum: Thin connective tissue lining the medullary cavity.

Structure of Bones (Microscopic)

• Bone (osseous) tissue: Cells and extracellular matrix.
• Cells: Osteoprogenitors (stem cells), osteoblasts (bone-building cells), osteocytes (main cell type maintaining bone metabolism), osteoclasts (bone-resorbing cells).
• Matrix: Collagen fibers and other substances.

Classification of Bones

• Long bones: Longer than wide (e.g., femur, humerus, metacarpals, metatarsals).
• Short bones: Cube-shaped (e.g., carpals, tarsals).
• Flat bones: Thin, flattened, plate-like surfaces (e.g., skull bones, ribs, sternum, scapula, hipbone).
• Irregular bones: Varied shapes, do not fit other categories (e.g., vertebrae, facial bones).

Bone Formation (Ossification)

• Intramembranous ossification: Mesenchymal stem cells within the mesenchymal become osteoprogenitors cells then become osteoblasts forming ossification centers in skull and facial bones and clavicle bones.
• Endochondral ossification: A hyaline cartilage model is developed from mesenchymal and cartilage cells, and then replaced by bone formation.

Postnatal Growth of Endochondral Bones

• During childhood and adolescence, bones lengthen due to growth of the epiphyseal plates.
• Cartilage is replaced by bone tissue as quickly as it grows.
• Epiphyseal plate maintains constant thickness.
• Whole bone lengthens.
• As adolescence ends, chondroblasts divide less often, epiphyseal plates become thinner, cartilage stops growing, is replaced by bone tissue, and long bones stop lengthening when diaphysis and epiphysis fuse.

Hormonal Regulation of Bone Growth

• Growth hormone (pituitary gland): Stimulates epiphyseal plates.
• Thyroid & parathyroid hormones: Maintain proper proportions.
• Sex hormones: Promote bone growth and induce epiphyseal plate closure.

Bone Remodeling

• The physiological process of ongoing replacement of old bone with new bone.
• Involves bone resorption (removal) by osteoclasts and bone deposition (addition) by osteoblasts.

MCQ Question

• Which bone is ossified by membranous ossification? -Answer: Clavicle (A)

OSPE Question

• In an X-ray of the shoulder and rib cage, what does the red arrow indicate? -Answer: Epiphysis (A)

Reference

• Totora's Principles of Anatomy and Physiology