Bones and Skeletal Tissues: Structure, Function, and Composition

CHAPTER 6: Bones and Skeletal Tissues

Skeletal Cartilages

Skeletal cartilages are specialized connective tissues that provide support and flexibility to various parts of the skeleton. They are avascular and lack nerves, relying on surrounding tissues for nutrient delivery.

• No blood vessels or nerves: Cartilage is nourished by diffusion from the perichondrium, a dense connective tissue girdle.

• Perichondrium: Contains blood vessels for nutrient delivery to cartilage.

Types of Cartilage

• Hyaline cartilage: Most abundant; provides support, flexibility, and resilience. Found in articular surfaces, costal cartilages, respiratory tract, and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains more elastic fibers, allowing greater flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength and the ability to absorb compressive shock. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

Growth of Cartilage

• Appositional growth: Cells in the perichondrium secrete new matrix on the external surface of existing cartilage.

• Interstitial growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding cartilage from within.

• Calcification: Occurs during normal bone growth and aging, where cartilage becomes hardened by deposition of calcium salts.

Bones of the Skeleton

The human skeleton is divided into two main groups based on location:

• Axial skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of the limbs and girdles that attach them to the axial skeleton.

Classification of Bones by Shape

Bones are classified by their shapes, which relate to their functions.

• Long bones: Longer than they are wide (e.g., femur, humerus). All limb bones except wrist and ankle bones.

• Short bones: Cube-shaped (e.g., carpals, tarsals). Includes sesamoid bones (e.g., patella) that form within tendons.

• Flat bones: Thin, flat, and slightly curved (e.g., sternum, scapulae, ribs, most skull bones).

• Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Functions of Bones

Bones perform several vital functions in the body:

1. Support: Provide a framework for the body and support soft organs.

2. Protection: Enclose and protect vital organs (e.g., brain, spinal cord, heart).

3. Movement: Act as levers for muscles to produce movement.

4. Storage: Store minerals (mainly calcium and phosphorus) and growth factors.

5. Blood cell formation (hematopoiesis): Occurs in red marrow cavities.

6. Triglyceride storage: Fat stored in yellow marrow cavities.

7. Hormone production: Osteocalcin, a hormone produced by bone, helps regulate bone formation and energy metabolism.

Bone Markings

Bones display various markings that serve as attachment sites, joint surfaces, or conduits for blood vessels and nerves.

• Projections (attachment sites):

  • Tuberosity: Rounded projection

  • Crest: Narrow, prominent ridge

  • Trochanter: Large, blunt, irregular surface

  • Line: Narrow ridge of bone

  • Tubercle: Small rounded projection

  • Epicondyle: Raised area above a condyle

  • Spine: Sharp, slender projection

  • Process: Any bony prominence

• Projections (joint formation):

  • Head: Bony expansion on a narrow neck

  • Facet: Smooth, nearly flat articular surface

  • Condyle: Rounded articular projection

  • Ramus: Armlike bar of bone

• Depressions and openings (passageways):

  • Groove: Furrow

  • Fissure: Narrow, slitlike opening

  • Foramen: Round or oval opening through a bone

  • Notch: Indentation at the edge of a structure

  • Meatus: Canal-like passageway

  • Sinus: Cavity within a bone, filled with air and lined with mucosa

  • Fossa: Shallow, basinlike depression

Bone Textures

• Compact bone: Dense outer layer that appears smooth and solid.

• Spongy (trabecular) bone: Internal honeycomb of trabeculae, with open spaces filled with bone marrow.

Structure of a Long Bone

• Diaphysis (shaft): Compact bone collar surrounds the medullary (marrow) cavity, which contains yellow marrow (fat) in adults.

• Epiphyses (ends): Expanded ends with spongy bone interior; covered by articular (hyaline) cartilage. The epiphyseal line is a remnant of the growth plate.

Membranes of Bone

• Periosteum: Double-layered membrane covering the external surface of bone. The outer fibrous layer is dense connective tissue; the inner osteogenic layer contains stem cells. Perforating (Sharpey's) fibers secure it to bone. Contains nerve fibers, blood vessels, and lymphatics.

• Endosteum: Delicate membrane lining internal bone surfaces; also contains osteogenic cells.

Structure of Short, Irregular, and Flat Bones

• Thin plates of periosteum-covered compact bone on the outside, with endosteum-covered spongy bone (called diploë in flat bones) inside.

• Bone marrow is found between the trabeculae.

Red Marrow: Hematopoietic Tissue

• In adults: Found in trabecular cavities of the heads of the femur and humerus, and in the diploë of flat bones.

• In infants: Found in medullary cavities and all spaces in spongy bone.

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic (osteoprogenitor) cells: Stem cells in periosteum and endosteum; give rise to osteoblasts and bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete bone matrix; responsible for bone growth.

• Osteocytes: Mature bone cells that maintain the bone matrix.

• Osteoclasts: Large cells that resorb (break down) bone matrix.

• Bone-lining cells: Flat cells on bone surfaces where bone remodeling is not occurring; help maintain matrix.

Compact Bone (Microscopic Structure)

• Osteon (Haversian system): Structural unit of compact bone; consists of concentric lamellae (matrix tubes) around a central (Haversian) canal containing blood vessels and nerves.

• Perforating (Volkmann's) canals: Run at right angles to the central canal, connecting blood vessels and nerves of the periosteum and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae to each other and to the central canal, allowing communication and nutrient/waste exchange.

Spongy Bone (Microscopic Structure)

• Trabeculae: Lattice-like structures aligned along lines of stress; contain irregularly arranged lamellae, osteocytes, and canaliculi.

• No osteons; nutrients supplied by capillaries in the endosteum.

Chemical Composition of Bone

Organic Components

• Cells: Osteogenic cells, osteoblasts, osteocytes, osteoclasts.

• Osteoid: Organic part of the matrix secreted by osteoblasts (about 1/3 of matrix); includes ground substance (proteoglycans, glycoproteins) and collagen fibers.

• Collagen fibers provide tensile strength and flexibility; sacrificial bonds between collagen molecules help dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up about 65% of bone by mass; mainly calcium phosphate crystals.

• Responsible for bone's hardness and resistance to compression.

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; most abundant	Articular surfaces, costal cartilages, respiratory tract, nose
Elastic	Similar to hyaline, more elastic fibers	External ear, epiglottis
Fibrocartilage	Thick collagen fibers, tensile strength, absorbs shock	Intervertebral discs, pubic symphysis, menisci

Summary Table: Bone Cell Types

Cell Type	Function
Osteogenic cells	Stem cells; differentiate into osteoblasts
Osteoblasts	Bone-forming cells; secrete matrix
Osteocytes	Mature bone cells; maintain matrix
Osteoclasts	Resorb (break down) bone matrix
Bone-lining cells	Maintain bone matrix on surfaces not being remodeled

Key Formula: Hydroxyapatite in Bone

• The main inorganic component of bone is hydroxyapatite, with the chemical formula: $Ca_{10}(PO_4)_6(OH)_2$

Example: Bone Function in Calcium Homeostasis

• Bones act as a reservoir for calcium. When blood calcium levels drop, osteoclasts break down bone matrix, releasing calcium into the bloodstream. When calcium levels are high, osteoblasts use calcium to build new bone matrix.

Additional info: The notes above expand on the original points by providing definitions, examples, and context for each bone and cartilage type, as well as the structure and function of bone tissue at both macroscopic and microscopic levels.