Skeletal System: Structure, Function, and Bone Physiology Study Notes

Skeletal System Overview

Main Functions of the Skeletal System

The skeletal system is essential for multiple physiological and structural roles in the human body.

• Support: Provides a rigid framework that supports the body and maintains its shape.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Production: Houses bone marrow, which produces red and white blood cells (hematopoiesis).

Classification of Bones

Bones are classified based on their shapes and functions. There are six main types:

• Long Bones: Longer than they are wide; e.g., femur, humerus.

• Short Bones: Approximately equal in length and width; e.g., carpals, tarsals.

• Flat Bones: Thin and often curved; e.g., sternum, scapula.

• Irregular Bones: Complex shapes; e.g., vertebrae, pelvis.

• Sesamoid Bones: Embedded within tendons; e.g., patella.

• Sutural (Wormian) Bones: Small bones found within cranial sutures.

Bone Structure and Types

Main Parts of a Long Bone

Long bones have distinct anatomical regions:

• Diaphysis: The shaft or central part of the bone.

• Epiphysis: The ends of the bone, usually wider than the shaft.

• Metaphysis: The region between diaphysis and epiphysis, containing the growth plate.

• Medullary Cavity: The hollow region within the diaphysis, containing bone marrow.

• Periosteum: The outer fibrous membrane covering the bone.

• Endosteum: The inner membrane lining the medullary cavity.

Spongy Bone vs. Compact Bone

Bones contain two main types of tissue:

• Spongy Bone (Cancellous Bone): Found primarily in the epiphyses of long bones and inside flat bones. Characterized by a porous, trabecular structure that houses bone marrow.

• Compact Bone: Dense and solid, forming the outer layer of all bones and the bulk of the diaphysis in long bones.

Distinguishing Features:

• Spongy bone has a lattice-like network of trabeculae (Trabeculae are the thin, bony struts or plates that form the supportive framework inside spongy (cancellous) bone), while compact bone is organized into osteons (Haversian systems).

Bone Cells and Matrix

Bone tissue contains several specialized cell types:

• Osteocytes: Mature bone cells residing in spaces called lacunae.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells that break down bone matrix.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Bone Matrix: The basic unit is called the osteon (in compact bone), consisting of concentric lamellae around a central canal.

Bone Membranes

• Periosteum: The outer membrane covering the bone, involved in growth and repair.

• Endosteum: The lining of the medullary cavity, containing bone-forming cells.

Bone Growth and Development

Bone Growth in Thickness and Length

Bones grow by two main processes:

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

• Longitudinal Growth: Increase in length at the epiphyseal (growth) plate through endochondral ossification.

Types of Bone Formation (Ossification)

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; typical in flat bones (e.g., skull).

• Endochondral Ossification: Bone forms by replacing a cartilage model; typical in long bones.

Primary Steps in Endochondral Ossification

1. Development of cartilage model

2. Growth of cartilage model

3. Development of primary ossification center

4. Development of medullary cavity

5. Development of secondary ossification centers

6. Formation of articular cartilage and epiphyseal plate

Hormonal Regulation and Bone Disorders

Hormones Affecting Bone and Calcium Levels

Three primary hormones regulate blood calcium:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts; secreted in response to low calcium levels.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts; secreted by the thyroid gland.

• Calcitriol (active Vitamin D): Increases calcium absorption from the gut.

These hormones act mainly on the bones, kidneys, and intestines.

Growth Hormone (GH) Abnormalities

• Hypersecretion in Childhood: Leads to gigantism.

• Hypersecretion in Adulthood: Leads to acromegaly.

Bone Fractures and Disorders

Types of Bone Fractures

Fractures are classified based on their characteristics:

• Open (Compound) Fracture: Bone breaks through the skin.

• Closed (Simple) Fracture: Bone does not penetrate the skin.

• Displaced Fracture: Bone fragments are out of alignment.

• Non-displaced Fracture: Bone fragments remain in alignment.

• Complete Fracture: Bone is broken all the way through.

• Incomplete Fracture: Bone is only partially broken.

Fracture Type	Description
Open (Compound)	Bone pierces the skin
Closed (Simple)	Bone does not pierce the skin
Displaced	Fragments out of alignment
Non-displaced	Fragments in alignment
Complete	Bone broken through
Incomplete	Bone partially broken

Bone Disorders

• Osteomalacia: Softening of bones due to vitamin D deficiency in adults.

• Osteoporosis: Decreased bone mass and density, increasing fracture risk.

• Paget's Disease: Abnormal bone remodeling, leading to enlarged and misshapen bones.

Example: Osteoporosis is common in postmenopausal women due to decreased estrogen levels, which normally help maintain bone density.

Additional info: Some details, such as the steps of endochondral ossification and the hormonal regulation of calcium, were expanded for academic completeness.