Chapter 7B Skeletal system

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Chapter Overview

Chapter 7 part B Lecture Outline Anatomy & Physiology AN INTEGRATIVE APPROACH Fourth EditionAuthors: Michael P. McKinley, Valerie Dean O’Loughlin, Theresa Stouter Bidle

Formation and Growth of Cartilage

Interstitial Growth
- Definition: A process where cartilage grows from within, increasing its size and volume.
- Key Figure: Figure 7.9a illustrates this growth process, showing how the matrix expands from within the cartilage tissue.
Appositional Growth
- Definition: The process wherein new cartilage material is added to the surface of existing cartilage, increasing its thickness.
- Key Figure: Figure 7.9b depicts how new layers of cartilage are deposited onto the outer surface.

Bone Formation

Ossification (Osteogenesis)
- Definition: The process of formation and development of bone connective tissue (CT) crucial for structuring the skeleton.
- Timeline: This process begins early in the embryo and continues through childhood and adolescence, reflecting the dynamic nature of skeletal development.
- Key Milestone: By the 8th to 12th weeks of embryonic development, the skeletal framework begins to take shape, forming the basis for future growth.
- Two Main Processes:
  - Intramembranous Ossification: Formation of bone directly within fibrous connective tissue.
  - Endochondral Ossification: Formation of bone by replacing hyaline cartilage, essential for the development of most skeletal components.
Intramembranous Ossification
- Definition: Bone growth that occurs directly within a membrane of mesenchymal tissue.
- Bones Produced: This process leads to the formation of the flat bones of the skull, some facial bones, the mandible, and the central part of the clavicle.
- Key Figure: Figure 7.10 provides a visual representation of this type of ossification process.
Endochondral Ossification
- Definition: This begins with a hyaline cartilage model that gradually ossifies to form bone.
- Bones Produced: Most bones of the skeleton, including the upper and lower limbs, pelvis, vertebrae, and the ends of the clavicle, are formed through this process.
- Example: Long bone development showcases this process in detail.
- Key Figures: Figures 7.11 (depicting a 10-week fetus and a 16-week fetus) illustrate significant stages of this development.

Bone Growth

Interstitial Growth
- Dependence: This growth is largely dependent upon the growth of cartilage in the epiphyseal plate, where new cartilage cells are formed.
- Five Zones of the Epiphyseal Plate:
  - Zone of Resting Cartilage: Composed of small, inactive cartilage cells.
  - Zone of Proliferating Cartilage: Contains actively dividing cells that contribute to lengthening bones.
  - Zone of Hypertrophic Cartilage: Where older cartilage cells enlarge and then die, paving the way for ossification.
  - Zone of Calcified Cartilage: Minerals are deposited, making the tissue hard and preparing it for the final stage.
  - Zone of Ossification: Where bone tissue replaces the cartilage, leading to increased length of the bone.
Appositional Bone Growth
- Mechanism: Occurs within the periosteum where bone matrix is systematically deposited in layers that are parallel to the surface, contributing to the growth in thickness of the bone.
- Osteoclast Activity: Osteoclasts are responsible for resorbing bone matrix along the medullary cavity, allowing the bone to maintain its proportions and strength.
Bone Remodeling
- Definition: The process of continual replacement of old bone with new bone, occurring throughout adulthood.
- Location: This process occurs at the periosteal (outer) and endosteal (inner) surfaces of bone.
- Mechanism: It is regulated by the coordinated activities of osteoblasts (bone-building cells), osteocytes (mature bone cells), and osteoclasts (bone-resorbing cells), influenced by hormonal signals and mechanical stress experienced by the bone.
Mechanical Stress
- Ull’s Law: This principle states that bones remodel themselves in response to the physical stress exerted upon them, leading to structural adaptations.
- Impact of Weight-Bearing Movements:
  - Increased mechanical stress from activities such as walking or lifting increases bone mass and strengthens the bone structure.
  - Conversely, a lack of mechanical stress can lead to a decrease in bone mass, increasing the risk of osteoporosis and fractures.

Hormones Influencing Bone Growth and Remodeling

Growth Hormone (Somatotropin)
- Production: It is produced by the anterior pituitary gland and plays a critical role in growth regulation.
- Mechanism: Stimulates the liver to produce insulin-like growth factor (IGF), which promotes cartilage growth at the epiphyseal plate and overall bone growth.
Thyroid Hormone
- Source: Secreted by the thyroid gland.
- Function: Influences metabolism, regulates the growth and activity of the epiphyseal plate, and ensures proper bone formation.
Sex Hormones
- Types: Estrogen and testosterone.
- Impact: Secreted in large amounts during puberty, these hormones accelerate the growth of bones, leading to the dramatic changes associated with adolescence.
Glucocorticoids
- Category: A group of steroid hormones produced by the adrenal cortex.
- Effects: High levels can lead to increased bone loss by inhibiting osteoblast function and promoting osteoclast activity.
Serotonin
- Function: Acts as both a neurotransmitter and hormone, playing a role in regulating bone remodeling rates.
- Effects of High Levels: Excessive serotonin can inhibit osteoblast differentiation, contributing to disorders related to low bone density.

Blood Calcium Regulation

Calcitriol Production
- Role: Essential for raising blood calcium levels by increasing intestinal absorption of calcium and phosphate.
Parathyroid Hormone (PTH) Effects
- Function: Increases blood calcium levels through multiple mechanisms, including increasing calcium resorption from bones and enhancing calcium absorption in the intestines.
Calcitonin
- Response Mechanism: Released by the thyroid gland in response to elevated blood calcium levels.
- Impact: Inhibits osteoclast activity and stimulates the renal excretion of calcium, helping to lower blood calcium levels.

Effects of Aging on Bone

Decreased Tensile Strength
- Phenomenon: Aging is associated with a reduction in bone tensile strength due to decreased protein synthesis by osteoblasts, resulting in increased brittleness and susceptibility to fractures.
Osteopenia
- Definition: A condition characterized by gradual bone loss that begins around age 35-40 years.
- Sex Differences: Women typically experience a greater loss of bone mass compared to men due to hormonal changes post-menopause.
Osteoporosis
- Consequences: Significant bone mass reduction that compromises normal function and integrity of the skeletal system.
- Influencing Factors: Key factors include hormonal decreases with aging, lifestyle choices, and genetic predisposition.

Bone Fractures

Classification
- Types of Fractures: Includes comminuted (shattered), compression (crushed), and transverse (straight across) fractures, each with unique healing considerations.
- Key Figure: Figure 7.16 provides a visual guide to these fracture types.
Fracture Repair
- Process: Involves a detailed sequence of events, including inflammation, organization of fibrous tissue, and the formation of new bone to replace the damaged area.
- Key Figure: Figure 7.17 illustrates the stages of fracture healing and the complexity involved.

Learning Objectives

Intramembranous and Endochondral Ossification
- Comparison: Recognize how intramembranous ossification occurs earlier and creates flat bones, while endochondral ossification consists of the majority of bones and continues into late adolescence and adulthood.
Zones of the Epiphyseal Plate
- Understanding Growth: Compare and contrast the various growth zones of the epiphyseal plate and their specific roles in lengthening bones.
Hormonal Influence
- Impact of Hormones: List key hormones affecting bone growth, outlining their function and implications for overall health and disease prevention.