Chapter 6: Bones and Bone Tissue Study Notes

Chapter 6: Bones and Bone Tissue

Module 6.1 Introduction to Bones as Organs

Learning Outcomes

Describe the functions of the skeletal system.
Describe how bones are classified by shape.
Describe the gross structure of long, short, flat, irregular, and sesamoid bones.
Explain the differences between red and yellow bone marrow.

Functions of the Skeletal System

Support: Provides structural support for the body, serving as a framework that holds the body upright.
Protection: Encases and protects vital organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).
Movement: Facilitates movement by providing attachment points for muscles; joints allow for mobility.
Mineral Storage: Reservoir for minerals, particularly calcium and phosphorus, which are essential for metabolic functions.
Blood Cell Production: Houses bone marrow which facilitates the production of red blood cells, white blood cells, and platelets.
Energy Storage: Contains yellow marrow, a form of stored energy.

Classification of Bones by Shape

Long Bones: Characterized by a greater length than width; examples include the femur and humerus.
Short Bones: Approximately equal in length and width; examples such as the carpals and tarsals.
Flat Bones: Thin and flat in shape, providing protection and surface area for muscle attachment; examples include the sternum and skull bones.
Irregular Bones: Have complex shapes that do not fit into other categories; examples include the vertebrae and pelvis.
Sesamoid Bones: Bones embedded within tendons; the patella (kneecap) is a primary example.

Gross Structure of Bones

Long Bones: Composed of a diaphysis (shaft), epiphyses (ends), and metaphysis (region between the two).
Short Bones: Generally composed of spongy bone covered with a thin layer of compact bone.
Flat Bones: Two layers of compact bone with a layer of spongy bone in between.
Irregular Bones: Vary in composition but typically include both compact and spongy materials.
Sesamoid Bones: Usually small and round, embedded in the tendons.

Differences between Red and Yellow Bone Marrow

Red Bone Marrow: Involved in the production of blood cells; found in the spongy bone of certain bones like the pelvis and vertebrae.
Yellow Bone Marrow: Primarily composed of adipose (fat) tissue; serves as an energy reserve and can convert to red marrow under certain conditions.

Module 6.2 Microscopic Structure of Bone Tissue

Learning Outcomes

Describe the inorganic and organic components of the extracellular matrix of bone tissue.
Explain the functions of the three main cell types in bone tissue.
Describe the microscopic structure of compact bone and the components of the osteon.
Describe the microscopic structure of spongy bone.

Extracellular Matrix of Bone Tissue

Inorganic Components: Primarily composed of hydroxyapatite crystals ( $Ca<em>{10}(PO</em>4)<em>6(OH)</em>2$ ), which provide hardness and strength.
Organic Components: Composed mainly of collagen fibers (type I), which provide flexibility and tensile strength, as well as other proteins and growth factors.

Three Main Cell Types in Bone Tissue

Osteoblasts: Responsible for bone formation; produce the organic components of the bone matrix and facilitate mineralization.
Osteocytes: Mature bone cells that maintain bone tissue; derived from osteoblasts and reside in lacunae within the bone matrix.
Osteoclasts: Large, multinucleated cells responsible for bone resorption; break down bone tissue by dissolving hydroxyapatite and degrading collagen fibers.

Microscopic Structure of Compact Bone

Osteon: The basic structural unit of compact bone, consisting of concentric lamellae of bone matrix arranged around a central canal (Haversian canal) containing blood vessels and nerves.
Volkmann's Canals: Perpendicular to Haversian canals; allow for blood supply and communication between osteons.

Microscopic Structure of Spongy Bone

Trabeculae: Lattice-like structures of thin plates of bone; provide strength while reducing weight.
Bone Marrow Spaces: The spaces between trabeculae contain red or yellow bone marrow.

Module 6.3 Bone Formation: Ossification

Learning Outcomes

Explain the differences between primary and secondary bone.
Describe the process of intramembranous ossification.
Describe the process of endochondral ossification.

Primary vs. Secondary Bone

Primary Bone: Also known as woven bone, is the first type of bone formed during ossification, characterized by a random collagen fiber arrangement; it is weaker.
Secondary Bone: Also called lamellar bone, is stronger and organized into layers; replaces primary bone as skeletal development occurs.

Intramembranous Ossification

The process by which bone develops directly from mesenchymal tissue without a prior cartilage model; occurs in flat bones such as the skull.
Main steps include:
1. Osteoblasts cluster and secrete bone matrix within the membrane.
2. Osteoid is formed, becomes calcified, and turns into bone.
3. Osteoblasts become osteocytes upon maturation.

Endochondral Ossification

The process where bone develops by replacing hyaline cartilage; predominant method for forming long bones.
Main steps include:
1. Formation of a cartilage model.
2. Growth of the cartilage model and development of a primary ossification center.
3. Replacement of cartilage with bone, leading to the formation of secondary ossification centers.

Module 6.4 Bone Growth in Length and Width

Learning Outcomes

Describe how long bones grow in length.
Compare longitudinal and appositional bone growth.
Describe the hormones that play a role in bone growth.

Growth in Length of Long Bones

Longitudinal Growth: Occurs at the epiphyseal plate through a process where chondrocytes proliferate and then become ossified as the bone grows.

Longitudinal vs. Appositional Growth

Longitudinal Growth: Involves growth in length through the addition of new cartilage at the epiphyseal plate, which is then replaced by bone.
Appositional Growth: Involves an increase in the diameter of the bone through the addition of bone tissue to the outer surface; osteoblasts beneath the periosteum add new bone.

Hormones in Bone Growth

Growth Hormone: Stimulates overall growth and promotes growth plate activity; excess can lead to gigantism.
Thyroid Hormones: Regulate bone growth and metabolism.
Sex Hormones (Estrogen and Testosterone): Promote growth spurts during puberty and the closing of epiphyseal plates.

Module 6.5 Bone Remodeling and Repair

Learning Outcomes

Describe the process of bone resorption and bone deposition.
Describe the physical, hormonal, and dietary factors that influence bone remodeling.
Explain the role of calcitonin, parathyroid hormone, and vitamin D in bone remodeling and calcium ion homeostasis.
Describe the general process of bone repair.

Bone Resorption and Deposition

Bone Resorption: The process where osteoclasts break down bone tissue, releasing minerals like calcium into the bloodstream.
Bone Deposition: The process where osteoblasts build new bone, incorporating minerals and organic matrix into the structure.

Factors Influencing Bone Remodeling

Physical Factors: Mechanical stress or loading on bones stimulates bone formation; absence leads to bone loss.
Hormonal Factors: Hormones such as calcitonin and parathyroid hormone regulate calcium levels, influencing bone density and strength.
Dietary Factors: Adequate intake of calcium and vitamin D is essential for maintaining bone health and proper remodeling processes.

Role of Hormones in Bone Remodeling

Calcitonin: Lowers blood calcium levels by inhibiting osteoclast activity and promoting osteoblast activity, leading to bone formation.
Parathyroid Hormone (PTH): Raises blood calcium levels by stimulating osteoclasts, promoting bone resorption.
Vitamin D: Enhances intestinal absorption of calcium and phosphate, playing a critical role in maintaining mineral homeostasis.

General Process of Bone Repair

Involves several stages: hematoma formation, callus formation, callus ossification, and bone remodeling.
Following a fracture, a blood clot forms, and a soft callus develops, which is then replaced by a hard callus of bone tissue before reorganization into normal bone structure.