45. Bone and Cartilage 2024

What is cartilage composed of and what makes it resilient

Specialized connective tissue with type II collagen fibrils and highly hydrated proteoglycan ground substance

What percentage of cartilage weight is water and why

60–80%, allowing for shock absorption and diffusion

What type of collagen is dominant in cartilage

Type II collagen, providing shape and tensile strength

Why does cartilage appear as a solid but is permeable to nutrients

High water content and proteoglycans facilitate diffusion in avascular tissue

What is the function of the perichondrium

Dense CT layer providing new chondroblasts for growth and repair

What fiber type predominates in the perichondrium

Type I collagen

What cells in the perichondrium contribute to cartilage repair

Mesenchymal stem cells that become chondroblasts

What are isogenous groups in cartilage

Clusters of chondrocytes formed by mitotic division within lacunae

What glycosaminoglycans are found in cartilage ground substance

Hyaluronan, chondroitin sulfate, and keratan sulfate

What proteoglycan is most important in cartilage

Aggrecan, essential for load-bearing in joints

Why is cartilage avascular and what does this mean for healing

Lacks blood supply, making repair limited especially without perichondrium

Why is cartilage aneural

It lacks nerve fibers, contributing to low pain sensation unless surrounding tissues are involved

What are the three types of cartilage

Hyaline, elastic, and fibrocartilage

Where is hyaline cartilage found

Ventral ribs, trachea, bronchi, nasal cartilages, thyroid and cricoid cartilages

What are three special forms of hyaline cartilage

Articular cartilage, fetal skeleton, and epiphyseal plate

What distinguishes elastic cartilage histologically

Elastic fibers visible with Verhoeff’s stain or similar

Where is elastic cartilage found

Epiglottis, external ear, Eustachian tube, and small laryngeal cartilages

What is the function of elastic cartilage

Provides flexible support

What makes fibrocartilage histologically distinct

Type I collagen, no perichondrium, mix of hyaline cartilage and dense CT

Where is fibrocartilage found

Intervertebral discs, menisci, and tendon/ligament insertions

Why does damaged cartilage have limited repair

Due to avascularity, chondrocyte immobility, and limited proliferation

What condition is associated with aging cartilage

Osteoarthritis due to decreased chondrocyte repair and mitotic activity

What happens to hyaline cartilage in aging

It may calcify and be replaced by bone, especially in trachea and costal cartilages

What is the mineral component of bone matrix

Calcium phosphate in the form of hydroxyapatite

What type of collagen predominates in bone

Type I collagen

What is the periosteum and its function

Outer dense CT layer covering bone that contains osteoprogenitor cells for growth and repair

What is the endosteum and what cells does it contain

Inner cellular layer lining marrow cavity; contains osteoprogenitor and bone lining cells

What are osteoprogenitor cells

Partially differentiated stem cells that divide and become osteoblasts

What are osteoblasts

Bone-forming cells that secrete osteoid and later become osteocytes or bone lining cells

What do osteoblasts produce

Osteoid, the unmineralized bone matrix

What are osteocytes

Mature bone cells residing in lacunae, connected by canaliculi, acting as mechanosensors

How do osteocytes communicate

Via gap junctions in canaliculi

What are bone lining cells

Quiescent osteoblasts on bone surfaces, regulating calcium flow and supporting osteocytes

Where are bone lining cells called endosteal cells

When they line the marrow cavity

What are osteoclasts

Large multinucleated cells responsible for bone resorption

What are Howship’s lacunae

Resorption bays where osteoclasts are found

What is the ruffled border of osteoclasts

Region adjacent to bone where resorption occurs

What is the sealing zone in osteoclasts

Cytoplasmic ring demarcating resorption area

What is the basolateral region in osteoclasts

Region for exocytosis of digested bone material

What are the two main types of bone tissue

Compact (cortical) and cancellous (spongy, trabecular)

What is compact bone

Dense outer bone forming the surface of bones

What is cancellous bone

Lattice-like inner bone forming trabeculae and housing marrow

What is lamellar bone

Mature bone with organized sheets of matrix

What is woven bone

Immature bone with randomly arranged collagen

What is an osteon

Structural unit of compact bone with concentric lamellae surrounding a central canal

What does the central canal of an osteon contain

Blood vessels and nerves

What are Volkmann canals

Transverse canals in bone carrying vessels and nerves between osteons

What are the two types of bone formation

Intramembranous ossification and endochondral ossification

Where does intramembranous ossification occur

Flat bones of skull, face, mandible, and clavicle

What tissue does intramembranous ossification begin in

Well-vascularized primitive connective tissue

What cells initiate intramembranous bone formation

Mesenchymal cells transforming into osteoblasts

What do osteoblasts produce in intramembranous ossification

Eosinophilic osteoid matrix

What reshapes bone during intramembranous ossification

Coordinated activity of osteoblasts and osteoclasts

Where does endochondral ossification occur

Most bones of the extremities and weight-bearing axial skeleton

What type of cartilage is the precursor in endochondral ossification

Hyaline cartilage

What is the function of the bone collar

Impedes oxygen and nutrient diffusion, initiating matrix changes

What changes occur in cartilage during endochondral ossification

Chondrocyte hypertrophy, alkaline phosphatase production, and matrix calcification

What causes chondrocyte death in endochondral ossification

Calcification of the cartilage matrix

What invades the cartilage during endochondral ossification

Capillaries and osteoprogenitor cells

What is the primary ossification center

First site of bone formation in the diaphysis

When do secondary ossification centers form

At or after birth in the epiphyses

What structure separates primary and secondary ossification centers

Epiphyseal plate

What is the role of the epiphyseal plate

Site of longitudinal growth by chondrocyte proliferation

When does the epiphyseal plate close

Around age 20, marking the end of growth

What is the epiphyseal line

Bone remnant of the closed epiphyseal plate

What is the key difference between cartilage calcification and bone mineralization

Calcification is temporary and precedes resorption; mineralization is permanent in bone

What type of joint is a synovial joint

Diarthrosis, allowing free movement

What forms the outer layer of a synovial joint capsule

Dense connective tissue

What forms the inner layer of a synovial joint capsule

Synovial membrane (specialized connective tissue)

What does the synovial membrane produce

Synovial fluid

What is the function of synovial fluid

Lubricates the joint and nourishes articular cartilage

What kind of epithelium lines the synovial membrane

None, it is not a true epithelium and lacks a basal lamina

What supports exchange between blood and joint cavity

An abundant capillary bed in the synovial membrane

What are the two types of synovial membrane cells

Type A and Type B

What is the origin and function of Type A synovial cells

Derived from monocytes; regulate inflammation like macrophages

What is the function of Type B synovial cells

Fibroblast-like; produce synovial fluid