Skeletal System: Structure, Function, and Physiology Study Guide

Skeletal System: Structure, Function, and Physiology

General Structure of Bones

The human skeleton is composed of various types of bones, each with unique structures and functions. Understanding the anatomy of a generalized long bone and the microscopic structure of bone tissue is fundamental in Anatomy & Physiology.

• Long Bone Anatomy: A typical long bone consists of the diaphysis (shaft), epiphyses (ends), metaphyses (regions between diaphysis and epiphyses), articular cartilage, periosteum (outer covering), medullary cavity (marrow cavity), and endosteum (lining of the medullary cavity).

• Osteon (Haversian System): The structural unit of compact bone, consisting of concentric lamellae (layers) surrounding a central canal containing blood vessels and nerves.

• Spongy Bone (Cancellous Bone): Composed of trabeculae (lattice-like structures) filled with red bone marrow; found mainly in the epiphyses of long bones and inside flat bones.

• Compact Bone: Dense bone tissue forming the outer layer of bones; provides strength and protection.

Bone Cells and Their Functions

Bone tissue contains several specialized cell types, each with distinct roles in bone formation, maintenance, and remodeling.

• Osteoclast: Large, multinucleated cells that break down bone matrix (bone resorption).

• Osteoblast: Bone-forming cells that secrete bone matrix and initiate calcification.

• Osteocyte: Mature bone cells derived from osteoblasts; maintain bone tissue and communicate with other bone cells.

• Chondroblast: Cells that produce cartilage matrix during bone development and growth.

• Chondrocyte: Mature cartilage cells that maintain the cartilage matrix.

Bone Formation (Ossification)

Ossification is the process by which bone tissue is formed. There are two main types, each responsible for forming different types of bones.

• Intramembranous Ossification: Forms flat bones (e.g., skull, clavicle) directly from mesenchymal tissue.

• Endochondral Ossification: Forms most bones of the body, especially long bones, by replacing hyaline cartilage with bone.

Steps of Endochondral Ossification

1. Development of a cartilage model

2. Growth of the cartilage model

3. Development of the primary ossification center (diaphysis)

4. Development of the medullary cavity

5. Development of secondary ossification centers (epiphyses)

6. Formation of articular cartilage and epiphyseal plate

Bone Growth

Bones grow in length and thickness through two main processes:

• Longitudinal Growth: Occurs at the epiphyseal plate (growth plate) through interstitial growth of cartilage, followed by ossification.

• Appositional Growth: Increase in bone thickness by addition of new bone tissue at the surface (periosteum).

Epiphyseal Plate Layers and Growth

The epiphyseal plate consists of several zones, each with specific cellular activities:

• Zone of Resting Cartilage: Anchors the plate to the epiphysis.

• Zone of Proliferating Cartilage: Rapid cell division (chondrocytes multiply).

• Zone of Hypertrophic Cartilage: Chondrocytes enlarge and mature.

• Zone of Calcified Cartilage: Matrix becomes calcified; chondrocytes die.

• Zone of Ossification: New bone is formed by osteoblasts.

This process causes longitudinal bone growth.

Hormonal Regulation of Bone Metabolism

Bone metabolism is regulated by hormones that control calcium levels in the blood and bone.

Hormone	Action	Increase/Decrease Calcium in Bones	Released in Response to
Thyroid (Calcitonin)	Inhibits osteoclasts, stimulates calcium deposition in bones	Increase	High blood calcium
Parathyroid Hormone (PTH)	Stimulates osteoclasts, increases blood calcium	Decrease	Low blood calcium
Calcitonin	Inhibits bone resorption	Increase	High blood calcium

Bone Fractures and Repair

Bone fractures occur when the physical force exerted on the bone is stronger than the bone itself. The body repairs fractures through a series of steps:

1. Hematoma formation (blood clot at the fracture site)

2. Fibrocartilaginous callus formation (soft callus)

3. Bony callus formation (hard callus)

4. Bone remodeling (restoration of original bone structure)

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption and formation, allowing bones to adapt to stress and repair microdamage.

• Osteoclasts: Resorb (break down) bone tissue.

• Osteoblasts: Form new bone tissue.

• Osteocytes: Regulate the activity of osteoblasts and osteoclasts.

Ongoing remodeling is essential for maintaining bone strength and mineral homeostasis.

Applied Anatomy: Bone Landmarks and Joints

Understanding bone landmarks and joint structures is crucial for identifying skeletal features and their functions.

• Scapula: The large bony ridge running obliquely from the lateral border to the acromion is the spine of the scapula.

• Humerus: The rounded projection on the medial proximal end is the head of the humerus.

• Shoulder Joint: Formed by the articulation of the glenoid cavity of the scapula and the head of the humerus.

• Carpal Bones: The hamate has a hook and is located in the distal row.

• Shoulder Ligament: The coracohumeral ligament stabilizes the shoulder posteriorly.

• Ilium: The small projection on the posterior aspect of the iliac crest is the posterior superior iliac spine.

• Femur: The small ridge connecting the lesser trochanter with the linea aspera is the pectineal line.

• Nasal Septum: Formed by the vomer and the perpendicular plate of the ethmoid bone.

• Skull (Inferior View): The two rounded areas lateral and anterior to the foramen magnum that articulate with the first cervical vertebra are the occipital condyles.

Additional info:

• Bone remodeling is also important for adapting to mechanical stress and for the regulation of calcium and phosphate levels in the body.

• Hormonal regulation ensures that blood calcium levels remain within a narrow range, which is vital for muscle contraction, nerve function, and blood clotting.