PSB225 Chapter 7 The Skeletal System
Learning Objectives
Describe the role and locations of various components of the skeletal system
Define the functions of different types of bone cells
Discuss the process of bone remodeling
Explain the functions of various hormones affecting bone
7.1 Introduction to the Skeletal System
Components of the Skeletal System
Bones: Primary structures, provide support and protection.
Cartilage: Flexible connective tissue found at joints and growth plates.
Ligaments: Connect bone to bone, providing joint stability.
Tendons: Connect muscle to bone, facilitating movement.
Joints: Areas of contact between bones and/or cartilage.
Cartilage
Characteristics: Flexible, semi-rigid connective tissue.
Functions:
Covers joint surfaces and growth plates of long bones.
Reduces friction in joints and absorbs shock during movement.
Growth: Initially covers growth plates but is replaced by bone as development occurs.
Ligaments
Function: Connect bones to each other at joints.
Examples:
Posterior Cruciate Ligament (PCL)
Medial Collateral Ligament (MCL)
Anterior Cruciate Ligament (ACL)
Tendons
Function: Connect muscles to bones, instrumental for movement.
Example: Achilles tendon.
Joints
Definition: Contact areas between bones and/or cartilage.
Mobility vs. Stability: Greater mobility typically means less stability in joint function.
Synovial Joints
Characteristics: Feature a synovial cavity which allows free movement.
Bones
Characteristics: Light yet strong, supporting body structure.
Functions:
Support and protection of organs.
Act as levers for movement.
Facilitate hematopoiesis (blood cell production) in red bone marrow.
Store minerals like calcium and phosphate.
7.2 Microscopic Anatomy: Bone Connective Tissue
Cells of Bone
Osteoprogenitor cells: Stem cells that can differentiate into osteoblasts and osteocytes.
Osteoblasts: Make and secrete bone matrix (osteoid) and become osteocytes.
Osteocytes: Mature cells that maintain bone matrix.
Osteoclasts: Large cells that resorb bone, derived from bone marrow cells.
Bone Formation
Process: Begins with osteoblasts secreting osteoid, which calcifies and forms bone.
Inorganic Components: Calcium phosphate, calcium carbonate, etc., contribute to bone hardness and rigidity.
Bone Remodeling Cycle
Continuous process of bone resorption (osteoclast activity) and formation (osteoblast activity) throughout life.
Rate: About 20% of skeleton is replaced annually, influenced by hormonal activity.
Hormonal Regulation of Bone Growth and Remodeling
Hormones
Functions: Molecules that influence osteoblast and osteoclast activity, affecting bone composition.
Growth Hormone: Stimulates growth at epiphyseal plate.
Thyroid Hormone: Affects metabolic rate of osteoblasts.
Sex Hormones: Estrogen and testosterone promote bone growth at puberty.
Glucocorticoids: Can negatively impact bone density if levels are high.
Regulating Blood Calcium Levels
Importance: Calcium is crucial for muscle function, neuronal signaling, and blood clotting.
Primary Hormones:
Calcitriol: Active form of Vitamin D that promotes calcium absorption.
Parathyroid Hormone (PTH): Released in response to low blood calcium, enhances osteoclast activity, increasing blood calcium levels.
Activation of Vitamin D to Calcitriol
Process: UV light and dietary intake convert 7-dehydrocholesterol into Vitamin D3, which is metabolized to calcidiol and then calcitriol in the kidney, increasing calcium absorption.
Effects of PTH and Calcitriol on Blood Calcium Levels
Both hormones work together to increase calcium levels by stimulating osteoclasts, reducing calcium excretion in the kidneys, and enhancing intestinal absorption.
Calcitonin
Function: Hormone released from the thyroid that lowers blood calcium by inhibiting osteoclast activity and promoting renal calcium loss.
Important Bones to Know
Cranium
Mandible
Clavicle
Humerus
Radius
Ulna
Sternum
Ribs
Vertebrae
Femur
Tibia
Fibula