The Skeletal System: Structure, Function, and Anatomy

Chapter 5: The Skeletal System

Main Types of Bone Tissue

The skeletal system is composed of specialized bone tissues that provide structure, protection, and support for the body. Two main cell types are involved in bone formation and maintenance:

• Osteoblasts: "Bone builders." Immature bone cells that secrete the organic components of the bone matrix, initiating bone creation and mineralization.

• Osteocytes: "Bone maintainers." Mature bone cells that maintain the protein and mineral content of the matrix.

The Skeletal System Composition

The skeletal system consists of bones, cartilage, and ligaments, each serving distinct roles:

• Bones: Rigid connective tissue providing structural support.

• Cartilage: Embryonic precursor to bones and covers joints.

• Ligaments: Connect bone to bone.

• Tendons: Connect muscle to bone.

Bone Matrix Composition

Bones are a type of supporting connective tissue with specialized cells and a solid matrix:

• Organic Component: 1/3 of bone mass. Includes collagen fibers and large protein-carbohydrate complexes that provide tensile strength.

• Inorganic Component: 2/3 of bone mass. Composed of hydroxyapatite crystals (crystallized calcium phosphate salts, 85%), along with calcium carbonate and other minerals. These provide hardness and resistance to compression.

Bone cells comprise about 2% of bone mass.

Main Functions of the Skeletal System

• Support and Movement: Provides a framework for the body and attachment for muscles, allowing movement (leverage).

• Protection: Shields vital organs (brain, heart, lungs, spinal cord, pelvic organs).

• Blood Formation: Red bone marrow produces erythrocytes, leukocytes, and platelets.

• Mineral Storage: Stores essential minerals (calcium, phosphate), maintaining electrolyte and acid-base balance.

Bone Structure

Osteon (Haversian System)

The functional unit of compact bone, consisting of concentric layers (lamellae) around a central canal.

• Central (Haversian) Canal: Center of an osteon, containing blood vessels and nerves.

Three Types of Lamellae (Layers of Bone Matrix)

• Concentric Lamellae: Layers surrounding the central canal, circular in shape.

• Circumferential Lamellae: Layers around the circumference of the bone, just beneath the periosteum.

• Interstitial Lamellae: Layers that fill spaces between osteons, remnants of older osteons.

Two Types of Bone Tissue

• Compact Bone: Dense and solid tissue forming the outer layer of bone. Osteons are aligned parallel to the long axis, providing resistance to compression.

• Spongy Bone (Trabecular/Cancellous): Found mainly in the epiphyses and inside flat bones. Made of branching plates called trabeculae, providing lightweight strength and distributing stresses from multiple directions.

Bone Anatomy: Long Bone Regions

• Diaphysis (shaft): Primarily compact bone, surrounds the medullary (marrow) cavity.

• Epiphyses: Ends of the bone, mainly spongy bone containing red marrow, covered by articular cartilage.

• Metaphysis: Region between diaphysis and epiphysis, contains the epiphyseal plate (growth plate) during development.

• Periosteum: Fibrous membrane covering the external surface of the bone except at joint surfaces; serves as attachment for tendons and ligaments, and plays a role in bone growth and repair.

Types of Bone Formation (Ossification)

• Intramembranous Ossification: Bones form directly within connective tissue (e.g., flat bones like the skull, face, mandible, clavicle).

• Endochondral Ossification: Bones replace a hyaline cartilage model, involving calcium deposition (e.g., most bones of the limbs, vertebrae, pelvis).

Bone Maintenance and Repair

Bones are remodeled yearly through absorption of old bone and deposition of new bone. Adaptation to stress involves reshaping, strengthening, and forming features for ligament and tendon attachments.

• Stressed bones become thicker and stronger.

• Inactivity causes bone degeneration; bones can lose up to one-third of their mass without use.

Wolff's Law: Bones adapt to stress; use strengthens bones, inactivity weakens them. "Form follows function."

Factors Regulating Bone Growth

• Hormones:

  • Growth Hormone: Stimulates bone growth plates and bone deposition.

  • Parathyroid Hormone (PTH): Stimulates bone breakdown, increases blood calcium by breaking down bones.

  • Calcitonin: Stimulates bone building, lowers blood calcium by building bones.

  • Sex Steroids (Estrogen and Testosterone): Stimulate long bone growth during puberty.

• Essential Minerals: Calcium, phosphate, magnesium, citrate, carbonate, sodium.

• Vitamins: A, C, D (calcitriol) are vital for bone health.

Aging and Bones

With age, bone remodeling becomes imbalanced. Osteoblast activity decreases while osteoclast activity remains constant, leading to loss of density and mass. This increases the risk of fractures and conditions like Osteoporosis (bones become porous).

Bone Fractures

A fracture is a break in a bone. Healing typically takes 1-2 weeks.

Classification

• Breaking of skin

• Direction of fracture

• Separation of bone pieces

Types of Fractures

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

• Closed (Simple) Fracture: Bone breaks but does not pierce the skin.

• Transverse Fracture: Break is straight across the bone.

• Spiral Fracture: Bone is twisted apart.

• Displaced Fracture: Bone ends do not line up correctly.

• Nondisplaced Fracture: Bone cracks but maintains proper alignment.

• Greenstick Fracture: Bone bends and cracks on one side without breaking completely (common in children).

• Compression Fracture: Bones are jammed together.

Bone Classification

Bones are classified by shape and structure:

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

• Irregular Bones: Complex shapes (e.g., vertebrae, facial bones).

• Short Bones: Cube-like bones (e.g., carpals, tarsals).

• Pneumatized Bones: Bones with air-filled cavities (e.g., sphenoid, ethmoid, maxilla).

• Flat Bones: Thin, flattened bones (e.g., sternum, ribs, scapula, skull).

• Long Bones: Upper and lower limbs (e.g., femur, humerus).

• Sesamoid Bones: Small bones resembling a sesame seed (e.g., patella).

Bone Markings

Bone markings are general landmarks for muscle attachment, articulation, and passage of nerves and blood vessels.

Projections and Processes

• Tubercle/Tuberosity/Trochanter: Bump or swelling for muscle attachment.

• Facet: Flat articular surface.

• Crest: Prominent ridge.

• Head: Rounded, prominent end of a bone.

• Condyle: Bump that joins into a joint.

• Trochlea: Pulley-shaped articular process.

Depressions

• Fossa: Broad, shallow depression.

• Sulcus (groove): Narrow depression or groove.

• Notch: Indentation at the edge of a bone.

Openings

• Fissure: Narrow slit-like opening.

• Foramen: Round or oval hole for blood vessels and nerves.

• Meatus: Passageway/canal through the bone.

The Skeleton: Axial and Appendicular Divisions

Axial Skeleton

Forms the central axis of the body (down the center).

• Skull Bones: Cranium + face + associated bones (hyoid bone, auditory ossicles).

• Vertebral Column: Includes sacrum and coccyx.

• Thoracic Cage: Ribs + sternum.

Functions

• Protects special sense organs (sight, smell, hearing, balance, vision).

• Framework for support and protection of dorsal and ventral body cavities.

• Attachment sites for muscles (posture, head, neck, trunk movement, respiration).

Sutures (Immovable Joints of the Skull)

• Coronal Suture: Separates frontal bone from parietal bones.

• Sagittal Suture: Separates two parietal bones.

• Lambdoid Suture: Separates occipital bone from parietal bones.

• Squamous Suture: Separates temporal bone from parietal bone.

The Skull & Associated Bones

The Skull

Composed of the cranium (enclosing the brain), facial bones (from the face), and associated bones (hyoid bone, auditory ossicles).

Cranial Bones

• Frontal Bone: Forms the forehead and upper part of the eye sockets. Key features include:

  • Supra-orbital foramen/notch

  • Ethmoid notch

  • Superciliary margin

  • Glabella

  • Frontal sinuses

  • Squamous part

  • Orbital part

  • Zygomatic process

  • Nasal part

Table: Types of Bone Fractures

Type of Fracture	Description
Open (Compound)	Bone breaks and pierces the skin
Closed (Simple)	Bone breaks but does not pierce the skin
Transverse	Break is straight across the bone
Spiral	Bone is twisted apart
Displaced	Bone ends do not line up correctly
Nondisplaced	Bone cracks but maintains proper alignment
Greenstick	Bone bends and cracks on one side
Compression	Bones are jammed together

Key Equations

• Bone Mass Composition: $\text{Bone Mass} = \text{Organic Component} + \text{Inorganic Component}$

• Hydroxyapatite Formula: $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$

Additional info: Academic context and expanded explanations have been added for clarity and completeness.