Human Anatomy - Bones and Skeletal Tissues chap[ter 6.1

Found throughout the adult body, including:
- External ear
- Nose
- Articular cartilages (covering bone ends at joints)
- Costal cartilage (connecting ribs to sternum)
- Cartilages in the larynx and trachea
- Intervertebral discs (between vertebrae)
- Pubic symphysis (joint uniting hip bones anteriorly)
- Articular discs (e.g., menisci in the knee joint)
Structures Illustrated in Figure 6.1 (Cartilages in the Body):
- Cartilage in external ear
- Cartilages in nose
- Epiglottis (elastic cartilage of larynx)
- Thyroid cartilage and Cricoid cartilage (hyaline cartilages of larynx)
- Larynx and Trachea (respiratory tube cartilages)
- Costal cartilage
- Articular cartilage of a joint
- Meniscus (padlike cartilage in knee joint)
- Pubic symphysis
- Cartilage in intervertebral discs
Categorization by Type within Figure 6.1:
- Hyaline cartilages
- Elastic cartilages
- Fibrocartilages

Definition: A layer of dense irregular connective tissue.
Function: Surrounds cartilages, providing nutrients and resisting outward pressure.
Role in Growth: Functions in the growth and repair of cartilage.
Composition: Primarily consists of water.
Characteristic: Is a resilient tissue that springs back to its original shape after compression.

Shared Similarities Among All Cartilages:
- Cell Type: Chondrocyte (mature cartilage cell).
- Cell Location: Chondrocytes are located within $lacunae$ (small cavities within the matrix).
- Matrix Composition: Contains fibers and a jellylike ground substance.
1. Hyaline Cartilage
- Abundance: Most abundant type of cartilage in the body.
- Chondrocyte Appearance: Chondrocytes appear spherical.
- Fiber Type: Collagen unit fibril is the only type of fiber found in the matrix.
- Ground Substance: Holds a large amount of water, contributing to its resilience.
- Function: Provides support through flexibility and resilience.
2. Elastic Cartilage
- Fiber Type: Contains many elastic fibers.
- Characteristic: Able to tolerate repeated bending.
- Locations: Epiglottis (flap protecting the airway during swallowing) and cartilage of the external ear.
3. Fibrocartilage
- Resistance: Resists strong compression and strong tension forces.
- Nature: Considered an intermediate type between hyaline and elastic cartilage.
- Locations: Pubic symphysis, menisci of the knee joint, and the anulus fibrosus (outer ring of intervertebral disc).

1. Appositional Growth
- Mechanism: Chondroblasts (cartilage-forming cells) in the surrounding perichondrium produce new cartilage matrix on the external surface of existing cartilage.
2. Interstitial Growth
- Mechanism: Chondrocytes within the cartilage divide and secrete new matrix from within, expanding the cartilage from the inside.
Cessation of Growth: Cartilage stops growing when the skeleton stops growing, typically post-puberty.

Dominant Tissue: Primarily composed of bone connective tissue.
Other Tissues Present:
- Nervous tissue (e.g., nerves innervating bone).
- Blood connective tissue (within bone marrow and blood vessels).
- Cartilage (specifically articular cartilages at joint surfaces).
- Epithelial tissue (lining blood vessels within bone).

1. Support: Provide a hard framework for the body, supporting soft organs.
2. Movement: Skeletal muscles use bones as levers to facilitate movement.
3. Protection: Shield underlying organs from injury (e.g., skull protects the brain, rib cage protects heart/lungs).
4. Mineral Storage: Serve as a reservoir for important minerals, primarily calcium and phosphate, releasing them into the bloodstream as needed.
5. Blood Cell Formation: Bone contains red marrow, which is the primary site of hematopoiesis (blood cell production).
6. Energy Metabolism: Osteoblasts (bone-forming cells) secrete the hormone osteocalcin, which helps regulate bone formation and protects against obesity, glucose intolerance, and diabetes mellitus.

A. Organic Components ( $35%$ of bone mass)
- Living Cells: Osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts.
- Fibers: Collagen fibers (provide tensile strength).
- Ground Substance: Proteoglycans and glycoproteins.
- Contribution: Contribute to the flexibility and tensile strength of bone, preventing it from being brittle.
B. Inorganic Components ( $65%$ of bone mass)
- Mineral Salts: Primarily calcium phosphates (hydroxyapatite crystals).
- Function: Invade the bony matrix.
- Contribution: Provide exceptional hardness and resistance to compression, making bone rigid.

1. Osteoprogenitor Cells
- Nature: Stem cells found in the periosteum and endosteum.
- Function: Can differentiate into osteoblasts.
2. Osteoblasts
- Location: Bone surfaces.
- Function: Actively produce and secrete new bone matrix (osteoid).
- Osteoid: Consists of collagen fibers and ground substance; later mineralized.
3. Osteocytes
- Nature: Mature bone cells (osteoblasts that have become trapped in the matrix they secreted).
- Location: Reside in lacunae within the bone matrix.
- Function: Keep the bone matrix healthy, playing a role in maintaining bone tissue and responding to mechanical stress.
4. Osteoclasts
- Location: Found within bone tissue, typically in resorption bays on bone surfaces.
- Function: Responsible for the resorption (breakdown) of bone matrix.
- Origin: Derived from a line of hematopoietic stem cells that also give rise to macrophages (a type of white blood cell).
- Secretions: Secrete hydrochloric acid and lysosomal enzymes to dissolve the mineral and organic components of bone matrix.

1. Long Bones
- Characteristics: Longer than they are wide; consist of a shaft (diaphysis) and two ends (epiphyses).
- Example: Humerus (arm bone).
2. Short Bones
- Characteristics: Roughly cube-shaped.
- Example: Talus (ankle bone), carpals (wrist bones), tarsals (ankle bones).
3. Flat Bones
- Characteristics: Thin, flattened, and usually curved.
- Example: Sternum (breastbone), skull bones, scapulae (shoulder blades), ribs.
4. Irregular Bones
- Characteristics: Have various complex shapes that do not fit into the other categories.
- Example: Vertebra (spinal bone), hip bones.

1. Compact Bone
- Location: Dense outer layer of bone.
- Structure: Appears smooth and solid.
2. Spongy (Cancellous) Bone
- Location: Internal network of bone.
- Structure: Consists of trabeculae (little "beams" of bone, a network of bony spicules).
- Spaces: Open spaces between trabeculae are filled with bone marrow (red or yellow).