Bone Development: Ossification Processes

Intramembranous Ossification

• Intramembranous ossification is the process by which certain flat bones form from mesenchymal model
• Mesenchymal cells in the primary ossification center differentiate into osteoblasts
• Osteoblasts secrete organic matrix, which calcifies, trapping osteoblasts that transform into osteocytes
• Trabeculae of early spongy bone are laid down by osteoblasts and surrounding mesenchyme differentiates into periosteum
• Osteoblasts in the periosteum lay down early compact bone

Endochondral Ossification

• Endochondral ossification begins in the fetal period for most bones, starting as hyaline cartilage
• Bones of the wrist and ankle ossify much later
• Chondroblasts in the perichondrium differentiate into osteoblasts
• Osteoblasts build a bone collar on the bone's external surface as ossification begins from the outside
• Internal cartilage calcifies, leading to chondrocyte death
• In the primary ossification center, osteoblasts replace calcified cartilage with early spongy bone, forming the secondary ossification centers and medullary cavity
• The medullary cavity enlarges, replacing remaining cartilage with bone, and the epiphyses finish ossifying

Epiphyseal Plate Structure and Bone Growth

• The epiphyseal plate is composed of hyaline cartilage responsible for bone growth in length
• It contains five zones of cells:
  • Zone of reserve cartilage: Cells not directly involved in bone growth, but can be recruited for division
  • Zone of proliferation: Actively dividing chondrocytes in lacunae
  • Zone of hypertrophy and maturation: Mature chondrocytes
  • Zone of calcification: Dead chondrocytes with calcification
  • Zone of ossification: Calcified chondrocytes and osteoblasts

Epiphyseal Plate Injuries

• Injuries to the epiphyseal plate can be problematic, potentially leading to shorter bones in adulthood
• Damage to the cartilage can accelerate plate closure, hindering lengthwise bone growth

Appositional Growth

• Appositional growth involves osteoblasts depositing new bone between the periosteum and bone surface
• Circumferential lamellae are formed first, and deeper layers are incorporated into osteons
• Appositional growth increases bone width

Resorption and Deposition

• Resorption: Breakdown of bone tissue by osteoclasts
  • Osteoclasts secrete hydrogen ions, making the pH more acidic, which breaks down hydroxyapatite crystals in the inorganic matrix
  • Enzymes break down proteoglycans, glycosaminoglycans, and glycoproteins
• Deposition: Formation of new bone by osteoblasts
  • Osteoblasts secrete proteoglycans and glycoproteins that bind to calcium ions, and vesicles containing calcium ions, ATP, and enzymes
  • This initiates the process of calcification

Blood Calcium Homeostasis Feedback Loop

• Parathyroid hormone (PTH) is involved in maintaining blood calcium homeostasis
• Stimulus: Blood calcium level drops below normal range
• Receptor: Parathyroid gland cells detect low blood calcium levels
• Control center: Parathyroid gland cells release PTH into the blood
• Effector/response: PTH stimulates effects that increase blood calcium levels
• Homeostasis and negative feedback: Calcium levels return to normal, and negative feedback decreases PTH secretion

Osteoporosis

• Osteoporosis is a bone disease characterized by inadequate inorganic matrix in the extracellular matrix
• It increases susceptibility to bone fractures

Functions of the Skeletal System

• Protection of vital organs, such as the brain
• Mineral storage, including calcium and phosphate, crucial for electrolyte and acid-base balance
• Blood cell formation (hematopoiesis) in red bone marrow
• Fat storage in yellow bone marrow
• Aids in movement through muscle attachment and leverage
• Provides support for body weight

Connective Tissues in the Skeletal System

• Compact bone
• Spongy bone
• Hyaline cartilage
• Dense regular collagenous connective tissue (tendons & ligaments)
• Dense irregular connective tissue (periosteum)

Compact vs. Spongy Bone

• Compact bone: Hard and dense bone tissue on the exterior of a bone, composed of osteons, providing resistance to stress
• Spongy bone: Inner honeycomb-like bone composed of trabeculae, resisting forces from multiple directions and housing bone marrow

Bone Classification by Shape

• Long bones: Longer than wide, e.g., humerus
• Short bones: About as long as wide, e.g., trapezium carpal bone
• Flat bones: Broad, flat, and thin, e.g., sternum
• Irregular bones: Shape doesn't fit into other categories, e.g., vertebra
• Sesamoid bones: Round, flat bones within tendons, e.g., patella

Bone Structures and Functions

• Diaphysis: Shaft forming the long axis of the bone
• Epiphysis: Expanded end of the bone, covered by articular cartilage
• Medullary cavity: Contains red or yellow bone marrow
• Red bone marrow: Hematopoietic tissue producing blood cells
• Yellow bone marrow: Stores triglycerides and consists of blood vessels and adipocytes
• Periosteum: Dense irregular collagenous connective tissue covering the bone, rich in blood vessels and nerves
• Endosteum: Membrane lining the inner surfaces of bone, containing bone cells
• Perforating (Sharpey's) fibers: Secure the periosteum to the bone matrix
• Nutrient foramen: Hole in the diaphysis allowing entry of nutrient arteries

Bone Extracellular Matrix (ECM)

• Organic components: Osteoid – provides flexibility and tensile strength, containing collagen, proteoglycans, GAGs, and glycoproteins
• Inorganic components: Hydroxyapatite crystals (calcium and phosphorus salts) along with other salts and ions, providing compressional strength

Bone Cell Types

• Osteoblasts: Build bone by secreting organic and inorganic matrix components
• Osteocytes: Maintain ECM and recruit osteoblasts for bone building
• Osteoclasts: Break down bone ECM through resorption
• Osteogenic cells: Stem cells that differentiate into bone-building cells

Intramembranous vs. Endochondral Ossification

• Intramembranous ossification: Forms flat bones directly from mesenchymal tissue
• Endochondral ossification: Forms most bones, replacing hyaline cartilage with bone
• Both processes involve osteoblast activity and matrix deposition, but differ in starting material.