Cartilage: Characteristics and Composition

Cartilage Characteristics

• Tough and flexible tissue
• Avascular and aneural, therefore, incapable of self-repair after fracture
• Firm, rubbery, and resilient material
• Consists of cells and fibers embedded in a firm, unmineralized, gel-like intercellular matrix
• Nutrition occurs through diffusion from the perichondrium and synovial fluid in joint cavities

Cartilage Composition

• Cells: chondrogenic cells, chondroblasts, and chondrocytes
• Extracellular matrix (ECM): fibers and amorphous ground substance
• Fibers: mainly type II collagen, providing strength, and also elastic fibers, providing flexibility and elasticity
• Amorphous ground substance: mucopolysaccharides, GAGs, proteoglycans, and glycoproteins

Perichondrium

• A sheath of dense irregular connective tissue that surrounds cartilage, except for fibrocartilage.
• It forms the interface between cartilage and tissues supported by cartilage.
• Contains blood vessels, nerves, and lymphatic vessels
• Provides nutrition to cartilage through diffusion
• Important for appositional growth and maintenance of cartilage
• Two layers: an outer fibrous layer with connective tissue fibers, fibroblasts, and blood vessels, and an inner cellular layer containing chondrogenic cells that differentiate into chondroblasts

Perichondrium Functions

• Prevents permanent distortion due to mechanical stress
• Bears mechanical stress and supports soft tissues
• Provides a sliding surface for joints
• Facilitates bone movements as a shock absorber and sliding area
• Aids in the development and growth of long bones before and after birth

Cartilage Formation (Chondrogenesis)

• Mesenchymal cells form aggregations and differentiate into chondroblasts.
• Chondroblasts lose their cytoplasmic processes, become rounded and produce ECM
• These cells become entrapped in the ECM and are then called chondrocytes

Cartilage Growth

• Appositional growth: new chondrocytes form from the inner part of the surrounding perichondrium, leading to an increase in width; starts later than interstitial growth and continues through adolescence
• Interstitial growth: growth within the tissue through the formation of new chondrocytes within pre-existing chondrocytes by division; leads to an increase in length; significant during early phases of cartilage formation, when cartilage is young and pliable

Cartilage Regeneration

• Damaged cartilage undergoes slow and incomplete repair.
• Perichondral cells invade injured and damaged areas to generate new cartilage.
• This process is usually slow and often incomplete, except in young children.
• If extensively damaged, the perichondrium produces scar tissue of dense CT instead of forming new cartilage.
• Poor regenerative capacity is due to its avascular nature

Classification of Cartilage

• Three types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage

Hyaline Cartilage

• Most common type of cartilage
• Translucent, resilient and white
• Found in nearly all fetal skeletons
• Covered by perichondrium except in articular cartilage in joints
• Location: articular ends of bones in movable joints, costal cartilages, cartilage of the nose, larynx (thyroid, cricoid, arythenoid), trachea, bronchi, and epiphyseal plate of growing long bones involved in endochondral ossification
• Function: provides flexibility, support, reduces friction, and absorbs shock

Histology of Hyaline Cartilage

• Lined by perichondrium that includes dense CT, an inner chondrogenic layer, and an outer fibrous layer
• Cells include: chondrocytes in lacunae, arranged in clusters of 2–4 cells as isogenous groups, surrounded by an amorphous cartilaginous matrix
• Matrix includes: ground substance, collagen fibers (mainly type II), and a firm hydrated gel of proteoglycans and structural glycoproteins that stain basophilic and display a similar refractive index
• Different regions of ECM: capsular matrix, territorial matrix, and inter-territorial matrix

Clinical Correlations of Hyaline Cartilage

• Prone to calcification during aging
• Limited ability to repair
• Osteoarthritis: related to aging and injury to cartilage, leading to chronic joint pain and deformity; affects weight-bearing joints such as the hip and knee

Elastic Cartilage

• Yellowish in appearance
• Location: external ear (auricle, pinna), walls of external acoustic meatus, Eustachian tube (auditory tube), and certain larynx cartilages (epiglottis, corniculate, cuneiform cartilage)
• Not calcified during aging

Histology of Elastic Cartilage

• Lined by perichondrium
• Chondrocytes in lacunae are seen as isogenous groups, abundant and larger than those in hyaline cartilage
• ECM: contains collagen fibers of type II, a dense network of fine branching elastic fibers, and is less susceptible to degeneration

Fibrocartilage

• Opaque and resilient
• Location: articular discs of the temporomandibular joint (TMJ) and sternoclavicular joint, menisci of the knee joint, and attachments of certain tendons and ligaments
• Function: offers resistance to traction force and provides great tensile strength

Histology of Fibrocartilage

• Absence of perichondrium
• Chondrocytes (single or in isogenous aggregates) are arranged axially or parallelly in long rows, separated by coarse collagen fibers of type I

ECM of Fibrocartilage

• More acidophilic because of the richer collagen type I content
• A firm dense matrix consisting of Type I collagen fibers forming thick, coarse bundles, and some type II collagen

Cartilage Comparisons

• Hyaline cartilage: chondrocytes and chondroblasts, mostly in isogenous groups, Type II collagen, has perichondrium (except articular cartilage surfaces), undergoes A (appositional) & I (interstitial) growth, endochondral ossification and ageing-related calcification
• Elastic cartilage: chondrocytes & chondroblasts, mostly in isogenous groups, Type II collagen & elastic fibers, has perichondrium, undergoes A & I growth, and no calcification
• Fibrocartilage: chondrocytes and fibroblasts, small and sparsely arranged in parallel rows, Type II and I collagens, No perichondrium, undergoes I growth only, and fibrocartilaginous callus calcification in bone repair

Bone Characteristics

• Highly vascularized
• Highly mineralized
• Has a nerve supply
• Covered by a protective periosteum of dense CT
• Tough and resilient

Bone Functions

• Provide a framework to give form and support for the body
• Offer areas for attachment of muscles and ligaments
• Act as levers for movements in locomotion
• Protect vital organs
• Responsible for production blood cells by marrow (haematopoiesis)
• Serve as a storage site for minerals, such as calcium and phosphate ions
• Play a role in the transmission of weight and force
• Act in sound transduction in the middle ear (auditory ossicles)

Bone Composition

• Cells: osteogenic cells, osteoblasts, osteocytes, and osteoclasts
• Intercellular substance consists of organic (1/3) and inorganic (2/3) and other (blood vessels, lymphatics, nerves)
• Organic: type I collagen fibers and amorphous mucopolysaccharides
• Inorganic: hydroxyapatite (calcium and phosphate ions)

Bone Classification

• Shape: long, short, flat, irregular
• Division: axial, appendicular
• Development: intramembranous ossification, endochondral ossification
• Structure: compact/cortical, cancellous/spongy
• Maturity: woven (immature), lamellar (mature)
• Others: sesamoid, penumatic, accessory

Irregular Bone

• Sesamoid bone: nodule of bone found embedded in certain tendons in areas exposed to severe pressure/friction. e.g. patella
• Pneumatic bone: bone with air cavities and evaginations of mucosal linings of nasal cavities, middle ear, mastoid antrum e.g. maxilla, ethmoid bone
• Accessory bone: not regularly present but can be found in certain bones that normally ossify from several centres and and don't unite with main bone mass e.g. supernumerary digits, sutural bone

Axial Skeleton

• Consists of the skull, hyoid bone, vertebral column, and thoracic cage
• Forms the vertical axis/center of the body

Appendicular Skeleton

• Bones that attached to the axial skeleton
• Limb bones: upper and lower limb bones
• Girdle bones: pectoral and pelvic girdle bones

Bone Development

• Intramembranous ossification: development occurs in membrane when mesenchymal cells transform into osteoprogenitor cells which become osteoblasts and then osteocytes. Examples include the skull (frontal, pariental), and mandible
• Endochondral ossification: bone formation in hyaline cartilage from mesenchymal cells that become chondroblasts which mature into chondrocytes, forming the cartilage model. Cartilage dies off and is replaced by calcified osteoprogenitor cells that become osteoblasts, and then osteocytes. Examples include long bones (except the clavicle), and short bones

Compact Bone

• Spongy bone is uniform in structure
• Made of Haversian systems
• Hard and dense, and resembles ivory
• Lamellae in circular pattern in its outer part (cortical bone)
• Protection & support is provided

Spongy Bone

• Spongy bone has an irregular shape and consists of a network of bony trabeculae & spicules with intervening bone marrow cavities
• Spongy bone has a lower density and its lamellae are in a parallel pattern in the inner core of bone (medullary bone)
• Support & blood cell production

Bone Maturity

• Woven (immature) bone: has an irregular arrangement and interlaces collagen fibers and lamellae without an organized pattern in the developing fetus
• Lamellar (mature) bone: regular arrangement and has layers (lamellae) of calcified interstitial substance/bone matrix in adults

Structure of Long Bone

• Diaphysis (shaft): thick-walled tube, mainly compact bone; inner/central medullary cavity filled with bone marrow (yellow/red marrow); external surface covered by periosteum; site of primary ossification center
• Epiphyses (2 ends): wider than shaft; mainly cancellous bones with articular surfaces of articular cartilage; site of secondary ossification center
• Metaphis: transitional zone between diaphysis and epiphysis; where cartilaginous epiphyseal disc is in growing age; growth zone of long bone

Osteon

• Is a structural unit of a compact bone
• Consists of: a Haversian canal, or a concentric layer of bone tissue around canals containing blood vessels and a nerve
• Lacunae: small cavities occupied by osteocytes uniformly placed along lamellae
• Canaliculi: small narrow spaces with radiating fine passages
• Cement line: thin dense external bone layer

Haversian Canal

• Oval is shape
• runs parallel to the long axis of bone
• contains blood vessels and nerves

Lamellae

• Lie parallel with outer and inner lining of bone
• arranged in concentric pattern around Haversian canal
• Lie between Haversian systems

Volkmann's Canal

• These lie obliquely to Haversian canals
• Connect haversian canals
• Provide passages for vascular anastomosis

Marrow Cavity

• Red (haematopoietic) marrow consist of developing cells, blood sinuses, and blood forming tissues,
• Yellow (fatty) marrow consists of fat cells

Endosteum

• A thin layer of CT covering marrow cavity
• Lines the Haversian canal and all the internal cavities of bone
• Contains flattened osteoprogenitor cells

v & vi. Endosteum & Periosteum

• Nutrition of osseous tissues, supply of osteoblasts for bone repair and growth
• Contains flattened osteoprogenitor cells the can morph toosteoblasts

Periosteum

• Offers a a thick layer of dense CT covering external surfaces of compact bone
• Its outer fibrous layer includes collagen bundles, fibroblasts, and connective tissues
• Includes Sharpey's fibres that protrude bone matrix and bind periosteum to bone

Histology of growing part of long bone

• Epiphyseal plate is area of elongation
• Epiphyseal closure occurs at age 15-18 years old in females and 18-22 years old in males when longitudinal growth stops
• There are 5 zones of epiphyseal cartilage

5 Zones of Epiphyseal Growth

• Resting zone with typical hyalin cartilage
• Proliferative zone where chondrocytes form columns parallel to long axis of bone
• Hypertrophic zone consisting of large chondrocytes
• Calcified cartilage zone: - cartilage matrix becomes calcified & chondrocytes regenerate again
• Ossification zone - endochondral bone tissues appears with blood capillarie"

Blood Supply of Long Bone

• Epiphyseal vein & artery, Metaphyseal vein & artery supply epiphysis & metaphysis and red bone marrow
• Periosteal artery & vein enter diaphysis through the perforating (Volkmann's) canal
• A large blood supply near the the nutrient foramen enter the bone and divide into proximal and distal branches to supply blood to spongy bone

Bone Remodelling

• It is an ongoing replacement of old bone tissue
• Involves bone resorption and bone deposition

Resorption

• Removal of mineral & collagen fibres from bone by osteoclast (occurs bone breakdown)
• Results in destruction bone ECM and lengthening the bone

Deposition

• Addition of mineral & collagen fibres to bone by osteoblasts (occurs bone building)
• Results in formation bone ECM to create and maintain shape of bones

Factor's Affecting Bone Remodelling

• Nutrition: eating diet with proteins, Vitamins D, C, A, K, B12, and Minerals like calcium & phosphate ions
• Physical & mental stress
• Hormones like parathyroid, calcitonin, thyroid, pituitary growth and glucocorticoids

Hormones that Affect Bone Remodelling

• Parathyroid: hormone that promote bone resorption (bone break down)
• Calcitonin: hormone inhibits bone reabsorption (bone building)
• Thyroid & pituitary growth hormone (GH): - hormone stimulates osteoblast activity for bone matrix synthesis
• Glucocorticoids for bone collagen by promoting calcium ion resorption to limit

Clinical Correlations

• Nutritional Deficiency: leads to rickets in children due to abnormal bone growth and osteomalacia in adults
• Physical and Mental Stress: result osteoporosis due to bone resorption
• Marfan Syndrome: - cause hereditary due to excessive cartilage formation

Rickets

• Cause is due to nutritional deficiency and leads
• Large forehead, odd shaped ribs and breast bones, large abdomen
• wide joints at elbow, stunned growth with spine curvature and wide ankles

Osteoporosis

• Bone mass increases by abnormal formation due to resorption

Fracture Healing

• Formation of a fracture hematoma
• Fibroblasts from the periosteum migrate and deposit collagen
• Cells differentiate into chondroblasts and fibrocartilaginous callus forms
• Osetogenic cells become produce spongy bone trabeculae to join the portions
• Fibrocartilage becomes new spongy bone known as bony cartilage

Bone Remodelling Cycle

1. Fracture occurs with blood flow stops to those cells at the side
2. Fibroblasts produce collagen for chondroblasts to form
3. Osteogenic cells produce spongy bone
4. Deadbone resorbed by osteoclasts and compact bone replaces spongy bone

Learn about cartilage, a tough and flexible tissue, its avascular and aneural nature affecting self-repair, composition of cells, extracellular matrix, and perichondrium's supportive role. Understand nutrition through diffusion and structural components like collagen and elastic fibers.