Supporting Connective Tissue: Cartilage and Bone

Supporting Connective Tissue: Cartilage and Bone

  • Types of Connective Tissue

    • Connective Tissue (CT) is divided into three main types:

    • CT Proper

    • Supporting CT

    • Fluid CT

    • There are two types of Supporting CT:

    • Cartilage

    • Bone

    • Supporting CT forms the skeleton that supports the body.

Components of Supporting Connective Tissue

  • Supporting CT has three basic components, similar to CT Proper:

    • Cells

    • Protein Fibers

    • Ground Substance

  • Supporting CT contains very few cells, which are widely separated by the extracellular matrix (ECM). This ECM contains fibers and ground substance.

    • In Cartilage: Chondrocytes

    • In Bone: Osteocytes

Extracellular Matrices of Supporting CT

  • Comparison of ECM in Different CT Types:

    • Areolar CT Proper: ECM forms a gel.

    • Cartilage: ECM forms a flexible solid.

    • Bone: ECM forms a rigid solid.

  • Flexibility Ranking:

    • Areolar CT Proper > Cartilage > Bone

Cartilage Cells

  • Cartilage contains Chondrocytes.

    • Terminology: "chondro" means cartilage, "cytes" means cells.

  • Lacunae: Chondrocytes are found in fluid-filled spaces within the ECM of cartilage, known as lacunae.

  • Function: Chondrocytes produce the protein fibers and ground substance that form the ECM of cartilage.

Cartilage Extracellular Matrix: Fibers and Ground Substance

  • Cartilage contains:

    • Collagen Fibers

    • Elastic Fibers

  • The ground substance in cartilage comprises water and Proteoglycans.

Perichondrium

  • Definition: A sheath that surrounds most cartilage.

  • Structure: Contains an inner cellular layer and an outer fibrous layer.

    • The inner layer has stem cells that divide to form daughter chondrocytes involved in new cartilage formation.

    • The outer fibrous layer is dense irregular CT providing support, protection, and attachment to other structures.

Blood Supply in Cartilage

  • The perichondrium is rich in capillaries that supply chondrocytes with oxygen and nutrients.

  • Diffusion: O2 and nutrients diffuse through the ECM to reach chondrocytes.

  • Healing: The ECM of cartilage lacks blood vessels:

    • Slight injuries heal slowly.

    • Severe injuries cannot repair.

Types of Cartilage

  • Categories:

    • Hyaline Cartilage

    • Elastic Cartilage

    • Fibrous Cartilage

  • Characteristics: The composition of the ECM determines specific cartilage characteristics.

Hyaline Cartilage

  • Description: Most common cartilage type, characterized by a smooth, glassy appearance.

    • Greek Origin: "hyalinos" means glass.

  • Contains chondrocytes in lacunae.

  • ECM features thin, closely packed bundles of collagen fibers and ground substance.

  • Strength: It is the weakest type of cartilage.

Locations of Hyaline Cartilage

  • Found In:

    • Costal cartilages (connects ribs to sternum with flexibility)

    • Articular cartilage (covers ends of bones in synovial joints to prevent friction)

    • Cartilage rings (around airways to keep them open).

Elastic Cartilage

  • Contains chondrocytes in lacunae.

  • The ECM consists of many elastic fibers, a few collagen fibers, and ground substance.

  • Unique Property: Can be twisted and return to original shape.

  • Locations:

    • External ear

    • Auditory canal

    • Epiglottis (flap covering trachea during swallowing).

Fibrous Cartilage

  • Also known as Fibrocartilage.

  • Contains chondrocytes in lacunae and some fibroblasts.

  • ECM features dense bundles of collagen fibers, regularly arranged along stress lines in the tissue.

  • Contains minimal ground substance and no perichondrium, resulting in limited blood supply.

Locations of Fibrous Cartilage

  • Found In:

    • Intervertebral discs (between vertebrae)

    • Pubic symphysis (pelvis)

    • Menisci (knee joint)

  • Function: Prevents bone-to-bone contact, limits joint movement, resists compression, and absorbs shock.

  • Strength: It is the strongest type of cartilage.

Growth of Cartilage

  • Types of Growth:

    • Appositional Growth: Adds to the surface of cartilage.

    • Interstitial Growth: Adds from within the tissue.

Appositional Growth of Cartilage

  • Process:

    • Stem cells in the inner cellular layer of the perichondrium divide and differentiate into Chondroblasts.

    • Chondroblasts produce ECM surrounding them and eventually differentiate into chondrocytes in lacunae.

Interstitial Growth of Cartilage

  • Process:

    • Chondrocytes divide within lacunae, producing additional ECM, allowing cartilage to expand from within.

Bone Cells

  • Bone contains:

    • Osteoprogenitor cells (stem cells)

    • Osteoblasts

    • Osteocytes

    • Osteoclasts

  • Terminology: "osteo" means bone.

Osteoprogenitor Cells

  • Definition: Stem cells that divide to produce daughter cells differentiating into osteoblasts.

  • Function: The division rate increases after bone fractures.

Osteoblasts

  • Definition: Cells that secrete Osteoid, the organic component of bone ECM.

  • Composition of Osteoid:

    • Collagen fibers

    • Ground substance

    • Enzymes for calcium phosphate crystal formation.

  • Location: Found in a single layer on the inner and outer surfaces of a bone.

  • Function: Build new bone in response to mechanical and hormonal stimuli.

Osteocytes

  • Definition: Formed when osteoblasts become surrounded by ECM.

  • Location: Reside in lacunae.

  • Function: Maintain the ECM of bone.

  • Cellular Connections:

    • Osteocytes have processes passing through Canaliculi, connecting via Gap Junctions for nutrient and oxygen delivery.

Osteoclasts

  • Definition: Large cells with multiple nuclei originating from immune system stem cells.

  • Location: Layered on the inner and outer surfaces of bone alongside osteoblasts.

  • Function: Secrete enzymes and acids to dissolve collagen fibers and calcium phosphate crystals.

  • Property: Have a ruffled border that increases the surface area for enzyme and acid secretion.

Bone Remodeling

  • Process: Involves continual building and breakdown of bone.

  • Importance: Regulates calcium and phosphate levels in the blood. Also affected by Vitamin D, hormones, intestinal absorption, and kidney excretion.

    • Key Factors:

    • Low calcium in blood

    • High calcium in blood

    • Small intestine

    • Kidney

Role of Osteocytes in Bone Remodeling

  • Osteocytes monitor:

    • Calcium and phosphate levels in bone and blood

    • Mechanical stresses on bone

  • Function: Help control bone remodeling based on these factors.

Role of Osteoblasts and Osteoclasts in Bone Remodeling

  • Osteoblasts build bone and take up calcium and phosphate from the blood for storage.

  • Osteoclasts break down bone and release calcium and phosphate back into the blood, dinstinguishing two essential functions:

    • Calcium and Phosphate Regulation

    • Bone Maintenance

Response to External Stress on Bone

  • Bone remodels in response to stress, adapting to weight-bearing exercises, such as walking, running, and lifting weights.

  • Effect of Exercise: Promotes thicker, stronger bones due to increased ECM production at stress locations.

  • Effect of Inactivity: Leads to thinning and weakening of bones.

Bone Strength in Response to Injury

  • If a bone breaks, it is repaired and can become stronger than before recovery.

  • Orthodontic Changes: In cases such as braces, osteoclasts and osteoblasts adjust bone in response to positional changes of teeth.

Bone Extracellular Matrix (ECM)

  • Composition:

    • Collagen fibers

    • Small amount of ground substance

    • Calcium phosphate combined with calcium hydroxide forming Hydroxyapatite crystals.

  • Bone Structure Strength: Collagen fibers and hydroxyapatite crystals create mineralized collagen fibrils aiding in overall strength.

  • Strength Comparison: Bone's properties allow it to compete with steel-reinforced concrete.

Structure of Long Bones

  • Parts:

    • Epiphysis: End of the bone

    • Metaphysis: Area between end and shaft

    • Diaphysis: Shaft of the bone.

Articular Surface and Cartilage

  • Articular surface contacts another bone in a joint and is covered with articular cartilage.

  • Function of Articular Cartilage: Decreases friction in joints.

Dense and Spongy Bone

  • Types of Bone:

    • Compact Bone: Forms outer layer (cortex) of bones.

    • Spongy Bone: Fills centers of bones, present entirely within bones except for long bone shafts.

  • Compact Bone Characteristics: Dense and heavy, resists parallel forces.

  • Spongy Bone Characteristics: Lighter than compact bone and oriented for multidirectional stress.

Trabecular Configuration of Spongy Bone

  • Formed from Trabeculae.

  • Contains open spaces and oriented according to stress patterns on bone.

Osteons in Bone Structure

  • Osteons are cylindrical structures present in compact bone and larger trabeculae of spongy bone.

  • Each osteon consists of:

    • Osteocytes in lacunae

    • Concentric lamellae (circular ECM layers)

    • Central canal

    • Canaliculi (tiny canals for communication)

    • Perforating canals (not shown).

Lamellae and Canaliculi Structure

  • Osteons contain concentric rings of osteocytes in lacunae, supported by layers of ECM known as lamellae.

  • Types of Lamellae:

    • Concentric lamellae: Surround osteocytes in osteons

    • Interstitial lamellae: Fill spaces between osteons

    • Circumferential lamellae: Found on the outer surface of the bone.

Central Canals in Bone

  • Central canals are located at the center of osteons and contain arteries, veins, lymph vessels, and nerves, connecting throughout the bone.

  • Central canals run parallel to the surface of the bone, surrounded by concentric rings of osteocytes and lamellae.

Nutrient Diffusion in Bone

  • Interstitial Fluid: Filled in central canals, canaliculi, and lacunae with oxygen and nutrients diffusing from arteries into the fluid, then to osteocytes.

  • Cellular Extensions: Osteocytes extend processes into canaliculi, connecting through gap junctions.

Periosteum

  • The periosteum covers bone surfaces and comprises:

    • Inner osteogenic layer (contains cells)

    • Outer fibrous layer (dense irregular CT).

  • Function: The osteogenic layer contains osteoprogenitor cells capable of becoming osteoblasts that build bone and remodel after injury, while the fibrous layer supports the bone and connects with tendons, ligaments, and other tissues.

Fiber Interaction in Periosteum

  • Collagen fibers of the periosteum interweave with those in tendons, ligaments, and the bone's ECM.

  • Perforating Fibers: Enhance the bond strength, making it so a strong pull on a tendon or ligament can break bone instead of fibers.

Endosteum

  • Definition: A single, incomplete cell layer covering the inner surface of compact bone and trabeculae of spongy bone.

  • Contains osteoprogenitor cells, osteoblasts, and osteoclasts, similar to the inner cellular layer of periosteum.

Blood Supply in Bone

  • Blood is supplied via Periosteal Arteries which enter bones through perforating canals.

  • Perforating canals branch into central canal arteries within the osteons, connecting throughout the bone.

Patterns of Bone Blood Supply

  • Nutrient Arteries: Pass through nutrient foramen into the bone center, branching out to supply most bone, connecting with the periosteal arteries.

Blood Supply Distributions in Bone

  • Arteries and veins correlate with:

    • Epiphyseal artery and vein

    • Metaphyseal artery and vein

    • Articular cartilage

    • Periosteum

Compact vs. Spongy Bone

  • Similarities: Both contain osteocytes in lacunae, ECM arranged in lamellae, and canaliculi.

  • Differences:

    • Compact bone organizes osteocytes and lamellae around a central canal

    • Most spongy bone does not have this central canal organization.

Differences in Canaliculi Connections

  • Canaliculi in spongy bone open onto the trabeculae surface and connect to lacunae, allowing for nutrient diffusion from nutrient artery system into interstitial fluid.

Bone Nerve Supply

  • Bones have extensive innervation; sensory nerves are present in periosteum and endosteum.

  • Bone injuries are associated with significant pain.

Bone Marrow Types

  • Two types of bone marrow exist:

    • Red Bone Marrow: Contains stem cells producing red blood cells, white blood cells, and platelets.

    • Yellow Bone Marrow: Contains adipocytes storing fat.

Distribution of Red Bone Marrow

  • Locations in Adults:

    • Spongy bone of skull, vertebrae, ribs, sternum, scapula, hip bones

    • Present in epiphyses of certain long bones.

Yellow Bone Marrow Characteristics

  • Typically found in the medullary cavity of long bones.

  • Can convert back to red bone marrow if there is a need for increased blood cell production.

Bone Shapes

  • Categories:

    • Long

    • Short

    • Flat

    • Irregular

    • Pneumatized

    • Sesamoid

    • Sutural

  • Bone shape and function are interrelated.

Bone Shape Characteristics

  • All bones consist of compact bone on the outer surface, spongy bone inside, except for long bones.

Long Bones Definition

  • Longer in one axis than another; can vary in size from the femur (large) to the phalanx (small).

Short Bones Characteristics

  • Cube-shaped bones with compact bone encasing spongy bone.

    • Examples: Carpal bones (wrist) and tarsals (ankle).

Flat Bones Explanation

  • Comprise two compact bone layers enclosing spongy bone in between.

  • Function: Provide protection to underlying soft tissues.

    • Examples: Cranium, sternum, ribs, scapula.

Irregular Bones Definition

  • Complex shapes not categorized as long, short, or flat.

    • Examples: Vertebrae, sphenoid bone, ethmoid bone.

Pneumatized Bones Characteristics

  • Hollow bones or those with numerous air pockets.

    • Example: Ethmoid bone (cranial).

Sesamoid Bones Definition

  • Small, round, flat bones that develop within tendons.

    • Largest example: Patella (kneecap).

    • Function: Decrease stress and friction on tendons, enhancing joint movement.

Sutural Bones

  • Located in the joints (sutures) of the skull; also known as Wormian bones.

  • Characteristics: Small, flat, irregularly shaped bones developing from separate ossification centers.

Bone Markings

  • Definition: Surface features on bones, aiding forensic analysis and archeological studies by determining size, sex, age, and general appearance from skeletal remains.

Types of Bone Markings

  • Categories:

    1. Elevations and Projections

    2. Processes (bumps) where tendons and ligaments attach

    3. Processes at joints

    4. Depressions

    5. Openings.

Elevations, Projections, and Processes

  • Definition: Protrusions on bones (for muscle attachments).

  • Examples:

    • Ramus: Extension forming an angle with the rest of the bone.

Processes for Tendon and Ligament Attachment

  • Various types include:

    • Trochanter: Large, rough projection on femur

    • Crest: Prominent ridge

    • Spine: Pointed process

    • Line: Low ridge

    • Tubercle: Small, round projection

    • Tuberosity: Rough projection.

Processes at Joints

  • Types:

    • Head: Expanded articular end of an epiphysis

    • Neck: Narrow connection between epiphysis and diaphysis

    • Facet: Small, flat articular surface

    • Condyle: Smooth, rounded articular process

    • Trochlea: Grooved articular process shaped like a pulley.

Depressions in Bone Markings

  • Examples:

    • Sulcus: Narrow groove

    • Fossa: Shallow depression.

Openings in Bone Markings

  • Types:

    • Sinus/Antrum: Air-filled chamber within a bone

    • Meatus/Canal: Passageway for blood vessels/nerves

    • Fissure: Deep furrow or slit

    • Foramen: Round passageway for blood vessels/nerves.

Bone Development: Endochondral Ossification

  • Definition: Most bones form within cartilage during fetal development.

    • At about 5 weeks, a hyaline cartilage model is created, growing by appositional and interstitial growth.

    • Transformation begins at around 8 weeks.

Stages of Endochondral Ossification

  1. As the cartilage model grows, the central matrix calcifies and chondrocytes die.

  2. The perichondrium becomes periosteum, and its cells differentiate into osteoblasts forming a bone collar.

  3. Capillaries and osteoblasts invade the center, breaking down calcified cartilage and forming a medullary cavity filled with spongy bone (primary ossification center).

  4. Osteoblasts from the primary ossification center move into cartilage of metaphyses forming columns of bone.

  5. Metaphyseal cartilage continues to grow outward while the diaphysis lengthens.

  6. Secondary ossification centers form in the epiphyses, filling with spongy bone (timing varies).

  7. The epiphyseal cartilages remain, allowing for further lengthening until epiphyseal closure occurs at maturity, when replaced by epiphyseal lines.

Appositional Bone Growth

  • Definition: Expands the diameter of bone.

  • Process: Involves osteoblasts building new bone on the surface and osteoclasts breaking down bone in the medullary cavity.

Factors Regulating Bone Growth

  • Nutrients:

    • Minerals: Calcium, Phosphate, Magnesium

    • Vitamins: D, A, C

  • Hormones:

    • Parathyroid hormone and calcitonin

    • Growth hormone, thyroxine, sex hormones.

Vitamin D Pathway

  • Production begins in the skin and is activated in the liver and kidneys. This active form aids in calcium and phosphate absorption and bone mineralization.

Hormonal Influence on Bone Growth

  • Parathyroid hormone promotes bone breakdown to raise blood calcium levels, while calcitonin encourages bone formation.

Clinical Correlation: Rickets

  • Description: A condition leading to soft bones in children due to inadequate calcium deposition, often from vitamin D deficiency or poor sunlight exposure.

  • Symptoms: Bone pain and deformities such as lateral leg bending. Treatment includes Vitamin D and/or calcium supplementation.

Functions of Bone**

  • Supports the body

  • Acts as an attachment site for muscles

  • Facilitates movement

  • Protects vital organs (brain, heart, lungs, etc.)

  • Stores/releases minerals (98% of body’s calcium in bones)

  • Stores fat

  • Produces blood cells in marrow (red and white blood cells, platelets).