The Skeletal System: Structure, Composition, and Functions

The Skeletal System

Composition and Functions

The skeletal system is a specialized organ system composed of connective tissues that provide support, protection, and enable movement. It is essential for maintaining the structure of the body and facilitating various physiological processes.

• Bones: Rigid organs composed mainly of nonliving extracellular crystals, primarily calcium minerals. Bones also contain nerves, blood vessels, and living cells such as osteoblasts (bone-forming cells), osteocytes (mature bone cells), and osteoclasts (bone-resorbing cells).

• Ligaments: Dense fibrous connective tissue that attaches bone to bone, providing stability to joints.

• Cartilage: Flexible connective tissue that supports and cushions joints. Types include fibrocartilage (intervertebral disks, knee joints), hyaline cartilage (embryonic bone structures), and elastic cartilage (outer ear, epiglottis).

Main Functions of the Skeletal System:

• Support: Provides structural framework for the body.

• Protection: Encases and protects delicate internal organs (e.g., brain, heart, lungs).

• Movement: Bones interact with muscles to produce movement.

• Formation of Blood Cells: Site of blood cell production in bone marrow (hematopoiesis).

• Mineral Storage: Stores minerals such as calcium and phosphate for metabolic needs.

Bone Structure

Types of Bone Tissue

• Compact Bone: Dense, solid structure with central canals for blood vessels and nerves. Contains osteons (structural units).

• Spongy Bone: Porous in appearance, found at the ends of long bones (epiphyses), contains spaces that house red bone marrow.

Bone Cells

• Osteoblasts: Cells that synthesize bone matrix and promote bone formation.

• Osteocytes: Mature bone cells embedded in the bone matrix, residing in spaces called lacunae. They remain in contact with each other via cytoplasmic extensions through canaliculi.

• Osteoclasts: Cells responsible for bone resorption, breaking down bone tissue.

Bone Anatomy

• Epiphysis: The ends of long bones, primarily spongy bone.

• Diaphysis: The shaft of long bones, mainly compact bone.

• Periosteum: A membrane covering the outer surface of bones.

• Central Cavity: Contains yellow bone marrow (fat storage).

Bone Development and Growth

Ossification

Bone development, or ossification, is the process by which bone tissue forms. It involves:

• Chondroblasts: Cells that produce cartilage matrix during early development.

• Osteoblasts: Replace cartilage with bone matrix.

Bone growth is regulated by hormones:

• Growth Hormone: Stimulates bone lengthening during adolescence.

• Sex Hormones (Testosterone, Estrogen): Influence bone growth during puberty.

Bone Remodeling and Repair

Mature bone undergoes continuous remodeling and repair, influenced by diet, exercise, and age. The process involves:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Decreases blood calcium by promoting bone formation.

• Repair Sequence: Hematoma formation, fibroblast and chondroblast activity, osteoclast and osteoblast involvement, and deposition of osteoid matrix.

Classification of Bones

Types Based on Shape

• Long Bones: e.g., femur, humerus

• Short Bones: e.g., carpals, tarsals

• Flat Bones: e.g., cranial bones, sternum

• Irregular Bones: e.g., vertebrae

There are a total of 206 bones in the adult human body.

Organization of the Human Skeleton

Axial Skeleton

• Skull: Composed of flat cranial bones; contains sinuses lined with mucus-secreting tissue.

• Vertebral Column: Main axis of the body, supports the head, protects the spinal cord, and provides attachment for limbs and muscles. Consists of 33 irregular bones (vertebrae) classified into five regions: cervical, thoracic, lumbar, sacral, coccygeal.

• Ribs and Sternum: Protect thoracic organs; 12 pairs of ribs (bottom two pairs are floating), sternum is the breastbone formed by fusion of three bones.

Appendicular Skeleton

• Limbs and Attachments: Includes arms, legs, and their attachment to the trunk. Example bones: humerus, ulna, radius, carpals, metacarpals, phalanges (arms/hands); femur, tibia, fibula, tarsals, metatarsals, phalanges (legs/feet).

Joints (Articulations)

Classification by Movement

• Fibrous Joints: Immovable (e.g., fontanels in the skull).

• Cartilaginous Joints: Slightly movable, connected by cartilage (e.g., intervertebral disks).

• Synovial Joints: Freely movable, most common type in the body.

Diseases and Disorders of the Skeletal System

Common Conditions

• Sprain: Damage due to stretched or torn ligaments.

• Tendinitis: Inflammation of tendons following injury.

• Arthritis: Inflammation of joints; can be due to wear, friction, or genetic predisposition.

• Osteoarthritis: Degenerative joint disease, common in older adults; cartilage covering bone ends deteriorates.

• Rheumatoid Arthritis: Autoimmune disease causing joint inflammation.

• Osteoporosis: Condition where bones lose mass and become porous due to imbalance in osteoclast and osteoblast activity.

Summary Table: Types of Connective Tissue in the Skeletal System

Type	Main Function	Location/Example
Bones	Support, protection, movement, mineral storage, blood cell formation	All regions of the skeleton
Ligaments	Connect bone to bone, stabilize joints	Knee, elbow, ankle, etc.
Cartilage	Support, cushion joints	Intervertebral disks, ear, nose, joints

Key Equations

• Calcium Homeostasis:

$\text{PTH (Parathyroid Hormone)} \uparrow \Rightarrow \text{Bone Resorption} \uparrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Bone Formation} \uparrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \downarrow$

Example

When blood calcium levels drop, the parathyroid glands release PTH, which stimulates osteoclasts to break down bone and release calcium into the bloodstream.

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.