Chapter 7B Skeletal system mod

7.4 Bone Formation

Overview of Ossification

Ossification (ostogenesis)

• The formation and development of bone connective tissue (CT). It begins early in embryonic development and continues through childhood and adolescence, with significant changes occurring at various life stages.

Key Milestones:

• By the 8th-12th week of embryonic development, the skeleton begins to form through:

  • Intramembranous ossification: A process where bone develops directly from sheets of mesenchymal connective tissue.

  • Endochondral ossification: Involves the replacement of hyaline cartilage with bone, forming the majority of bones in the body.

Intramembranous Ossification

• Definition: Bone growth that occurs within a membrane; primarily responsible for the formation of flat bones.

• Key Contributions:

  • Produces the flat bones of the skull, which protect the brain and support facial structures.

  • Contributes to forming certain facial bones, such as the nasal and zygomatic bones.

  • Forms the mandible, the lower jawbone crucial for jaw movement and functions such as chewing.

  • Involves the central part of the clavicle, which connects the arm to the body.

Endochondral Ossification

• Definition: A process where bone develops by replacing a cartilage model.

• Key Contributions:

  • Produces most bones of the skeleton, including:

    • Bones of the upper and lower limbs, essential for mobility and manipulation.

    • Pelvis, providing support for the body and housing organs in the reproductive and digestive systems.

    • Vertebrae, which protect the spinal cord and support the body's structure.

    • Ends of the clavicle, completing the shoulder girdle.

• Notable Process:

  • Long bone development, which includes the growth of the diaphysis and epiphysis from a cartilage template.

7.5a Bone Growth

Interstitial Growth

• This growth occurs at the epiphyseal plate and is crucial for increasing bone length.

Five Zones of Epiphyseal Plate:

1. Zone of resting cartilage: Small, inactive cartilage cells awaiting signals to grow.

2. Zone of proliferating cartilage: Rapidly dividing cartilage cells that contribute to bone lengthening.

3. Zone of hypertrophic cartilage: Larger cells that signal the growth of bone.

4. Zone of calcified cartilage: Cartilage cells begin to die and are replaced by bone.

5. Zone of ossification: The area where new bone is formed, solidifying the structure.

• Growth in Length: This process specifically occurs in:

  • Zone 2 (proliferating cartilage) and Zone 3 (hypertrophic cartilage), which together push the resting cartilage toward the epiphysis, effectively elongating the bone.

7.5b Bone Remodeling

Overview of Bone Remodeling

• Bone remodeling is a continuous process throughout adulthood and occurs at the periosteal (outer surface) and endosteal (inner surface) layers of bones.

Dependent on Coordinated Activities of:

• Osteoblasts: Cells responsible for forming new bone by synthesizing bone matrix and promoting mineralization.

• Osteocytes: Mature bone cells that maintain the bone matrix and communicate signals for remodeling.

• Osteoclasts: Cells that resorb bone tissue during the remodeling process to regulate bone mass and mineral homeostasis.

• Influence: Bone remodeling is heavily influenced by hormones and mechanical stressors from activity.

Wolff’s Law

• This principle states that mechanical stress induces bone remodeling that adapts its structure to withstand the forces it encounters.

• The process involves:

  • Detection of stress by osteocytes, leading them to send signals to osteoblasts to enhance bone formation.

Consequences:

• Increased bone mass can result from weight-bearing activities.

• Conversely, decreased bone mass happens when mechanical stress is removed, such as in prolonged bed rest.

Hormonal Influence on Bone Growth

Key Hormones

• Growth hormone (somatotropin):

  • Secreted by the anterior pituitary gland, stimulating the liver to produce insulin-like growth factor (IGF), crucial for bone growth.

• Thyroid hormone:

  • Regulates metabolic rate and essential for normal development at epiphyseal plates.

• Sex hormones (Estrogen and Testosterone):

  • Secreted during puberty, they accelerate the growth of bones and promote the closure of epiphyseal plates.

• Glucocorticoids:

  • High doses can impair growth of the epiphyseal plates leading to decreased bone density and growth.

• Serotonin:

  • Influences normal bone remodeling; excessive levels can contribute to adverse effects on bone density.

  • Importance of calcium in the bones

    • nerve unokukse control

Effects of Aging on Bone

Aging Changes

• Decreased tensile strength:

  • Results from a reduced rate of protein synthesis by osteoblasts, leading to weaker bone structure.

• Increased relative amount of inorganic material:

  • Causes bones to become more brittle and fragile.

• Bone Mass Changes:

  • A significant increase in bone loss of calcium and other minerals, leading to osteopenia and a higher risk of fractures. Women tend to lose skeletal mass more significantly than men as they age.

Osteoporosis

• A condition characterized by a significant reduction in bone mass, which compromises normal bone function and increases frailty.

• Contributing Factors:

  • Reduced levels of hormones like Vitamin D and estrogen, which critically support bone mass.

Fracture Classification and Healing

Types of Fractures

Classifications:

• By position of bone ends:

  • Nondisplaced: Ends retain normal position.

  • Displaced: Ends are out of normal alignment, potentially requiring realignment.

• By completeness of break:

  • Complete: The fracture goes all the way through the bone.

  • Incomplete: The fracture does not entirely break the bone.

  • Open (compound): Skin is penetrated by the broken bone, increasing the risk of infection.

  • Closed (simple): The skin remains intact despite the fracture.

Stages of Bone Repair

1. Hematoma Formation: Blood vessels torn by the fracture cause swelling and inflammation, leading to a hematoma.

2. Fibrocartilaginous Callus Formation: Capillaries grow into the hematoma; fibroblasts secrete collagen to physically connect the broken ends.

3. Bony Callus Formation: Osteoblasts invade the fibrocartilaginous callus, and trabeculae form, creating a bone template.

4. Bone Remodeling: Excess material is removed, and compact bone is reconstructed, aiming to restore the original structure and strength.