Cartilage and Bone: Structure, Function, and Growth

Cartilage

Structure and Composition

Cartilage is a specialized connective tissue that provides flexible support in various parts of the body. It is avascular, meaning it lacks blood vessels, and does not contain nerves. Its matrix contains glycosaminoglycans (GAGs), which help cartilage retain water and resist compression. Collagen fibers are present, and cartilage is composed of approximately 80% water.

• Perichondrium: The connective tissue lining around the cartilage.

• Chondroblasts: Immature cartilage cells that actively form cartilage.

• Chondrocytes: Mature cartilage cells responsible for maintaining cartilage.

• Lacunae: Small cavities within cartilage that house chondrocytes.

Types of Cartilage

• Hyaline Cartilage: The most abundant type, very firm and flexible. Contains lots of collagen. Found at the ends of long bones, nose, trachea, and ribs. Supports and resists stress.

• Elastic Cartilage: Similar to hyaline but with more elastic fibers, making it more flexible. Found in the ears and epiglottis. Maintains the shape of structures while providing flexibility.

• Fibrocartilage: Contains rows of chondrocytes alternating with thick collagen fibers. Found in intervertebral discs and knee joint menisci. Functions to absorb shock.

Bones

General Structure and Function

Bones are vascularized connective tissues that provide structural support, protection, and facilitate movement. Bone density increases with applied force and decreases with inactivity. Bone remodeling involves resorption (breakdown by osteoclasts) and deposition (formation by osteoblasts).

• Support: Enables standing, walking, and holding posture.

• Protection: Shields vital organs such as the brain.

• Movement: Facilitates locomotion and sports activities.

• Blood Cell Formation: Occurs in bone marrow (hematopoiesis).

• Fat Storage: Yellow marrow stores fat in long bones.

• Hormone Production: Bones influence insulin release and other hormonal functions.

Classification of Bones

• Long Bones: Longer than wide, with two rounded ends (e.g., femur, humerus). Contain spongy bone near the ends.

• Short Bones: Cubed-shaped and short (e.g., wrist, ankle). Mostly spongy bone with a compact outer layer.

• Flat Bones: Thin, flat bones (e.g., skull, ribs, sternum). Spongy bone sandwiched between compact bone.

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis). Primarily spongy bone with a compact covering.

Types of Bone Tissue

• Compact Bone: Dense, organized structure. Provides strength and support.

• Spongy Bone: Composed of trabeculae and red marrow. Found in the interior of bones and provides lightweight support.

Bone Linings

• Periosteum: Outer thin layer covering the bone and inner osteogenic layer.

• Endosteum: Covers the spongy bone and lines the canal of compact bone.

• Calcium Salts: Provide hardness and strength; aid in blood and fat storage.

Bone Cells

• Osteoblasts: Immature cells that regenerate bone.

• Osteocytes: Mature cells that maintain bone tissue.

• Osteoclasts: Cells that break down bone for remodeling and calcium release.

Structure of Long Bones

• Diaphysis: Shaft of the long bone; contains the medullary cavity with yellow marrow (fat storage).

• Epiphyses: Ends of the bone; contain spongy bone and are covered by hyaline cartilage.

• Epiphyseal Line: Separates the diaphysis from the epiphysis; site of bone growth during development.

Structure of Other Bone Types

All bones, regardless of type, have a shaft (diaphysis), epiphyses, and marrow cavity. Compact bone is on the outside, covered by periosteum; spongy bone is on the inside, covered by endosteum.

Structure of Compact Bone

• Osteon: The structural unit of compact bone, consisting of concentric rings (lamellae) around a central canal.

• Central Canal: Contains blood vessels and nerves.

• Perforating Canal: Runs perpendicular to the osteons, connecting them.

• Canaliculi: Small channels connecting osteocytes and allowing nutrient flow.

• Interstitial Lamellae: Fill spaces between osteons.

• Circumferential Lamellae: Sheets of bone under the periosteum, encircling the diaphysis.

Structure of Spongy Bone

• Contains trabeculae, lamellae, osteocytes, and canaliculi.

• Trabeculae are arranged along lines of stress for resistance.

• Spongy bone does not have osteons.

• Nutrients diffuse through canaliculi from marrow spaces to osteocytes.

Bone Formation (Ossification)

Intramembranous Ossification

Bones develop from fibrous connective tissue membranes containing mesenchymal cells. This process forms flat bones such as the skull and clavicles, beginning around the 8th week of embryonic development.

Endochondral Ossification

Bones develop by replacing hyaline cartilage. This process forms most bones except the clavicles and some skull bones. Begins in the second month of embryonic development.

Additional info: Endochondral ossification is more complex than intramembranous ossification because it requires the repair of cartilage and then ossification, whereas intramembranous ossification proceeds directly from mesenchymal cells to bone formation.

Bone Growth and Repair

Mechanisms of Bone Growth

• During infancy and youth, long bones lengthen by interstitial growth at the epiphyseal plates and grow in thickness by appositional growth.

• Most bone growth occurs during adolescence and early adulthood.

• Epiphyseal plates remain the same size during childhood; cartilage turns into bone as growth continues.

• Growth in width is called appositional growth.

Bone Repair (Fracture Healing)

1. Hematoma Formation: Local cells die due to lack of oxygen, causing inflammation and pain.

2. Fibrocartilaginous Callus Formation: Blood vessels invade the area, bringing macrophages, fibroblasts, chondroblasts, and osteoblasts to lay down collagen fibers.

3. Cartilage Conversion: Cartilage is converted to trabecular bone (within ~2 months).

4. Bone Remodeling: Excess material is removed, and compact bone is formed to restore shape.

Calcium Homeostasis and Bone Health

Negative Feedback Loop

1. Stimulus: Calcium levels drop.

2. Parathyroid gland releases parathyroid hormone (PTH).

3. PTH stimulates osteoclasts to degrade bone and release Ca2+ into the blood.

4. Calcium levels return to normal.

Osteoporosis

Osteoporosis is a condition where bones become less dense and weaker due to increased osteoclast activity compared to osteoblasts. Risk factors include aging, insufficient exercise, poor nutrition, and hormonal changes.

Summary Table: Types of Cartilage

Type	Main Features	Location	Function
Hyaline	Firm, flexible, abundant collagen	Ends of long bones, nose, ribs, trachea	Support, resist stress
Elastic	Flexible, elastic fibers	Ear, epiglottis	Maintain shape, flexibility
Fibrocartilage	Thick collagen fibers, alternating chondrocytes	Intervertebral discs, knee menisci	Absorb shock

Summary Table: Types of Bone Tissue

Type	Main Features	Location
Compact Bone	Dense, organized, osteons	Outer layer of all bones
Spongy Bone	Trabeculae, red marrow, lightweight	Interior of bones, ends of long bones

Key Equations

• Bone Remodeling Rate: $\text{Rate} = k[\text{Osteoclasts}][\text{Osteoblasts}]$

• Calcium Homeostasis: $\text{Ca}^{2+}_{\text{blood}} \uparrow \text{(osteoclast activity)}$