Cartilage lecture

What type of cells form the pre-bone models of appendages?

mesenchyme cells (5.5 week embryo)

What follows mesenchyme condensation?

• chondroblasts differentiate from mesenchyme cells and secrete matrix

• no joint cavities are present

• full fledged cartilaginous bone models will develop (9.5 week embryo)

What is the process of bone formation?

• a cuff of bone starts to forms around the midshaft of the cartilage model; progenitor cells differentiate into osteoblasts instead of chondroblasts

• the cuff forms a bone collar around the diaphysis of a developing bone (primary center of ossification)

• cartilage cells become hypertrophic and die as cartilage matrix calcifies

• periosteal cells then migrate into the cavity

How does hand development look for a 20 week fetus?

• the bones of the hand are ossifying and are in proportion to those of an adult individual

• thin layers of bone are deposited at the periosteal and endosteal surfaces

• osteoclasts start to remodel

syndactyly

condition where digits to not form normally

• happens when there is a disruption in embryonic development

apert’s syndrome

condition where separations points between bones do not fully separate

where is cartilage commonly found?

• joints

• ears

• nose

What is cartilage?

• specialized connective tissue that has an extracellular matrix (ECM) which allows tissue to withstand mechanical stress

• consists of chondroblasts which produce the ECM

• once completely surrounded by matrix, chondroblasts become chondrocytes

• chondrocytes reside in lacunae (cell nest)

• 95% is ECM and consists of ground substance (proteoglycans and GAGs), collagen and elastic fibers

Characteristics of cartilage

• firm but pliable

• avascular, lacks nerves and lymphatic vessels

• provides support and scaffold for bone formation

• surrounded by sheath of dense connective tissue, periochondrium

What is the name of the connective tissue that surrounds cartilage?

perichondrium

chondrogenesis

• differentiation of mesenchyme into chondroblasts

• mesenchymal cells first condense and then differentiate outward

• secretion of matrix separates the chondroblasts which are now chondrocytes

• chondrocytes apppear in the center

• interstitial (growing in length) and appositional growth (growing in width)

Perichondrium

• made of 2 layers

  • inner cellular layer = source of cartilage cells

  • outer layer = fibrous, has fibroblasts which make collagenous fibers and blood vessels

• functions in growth and repair of cartilage

• makes up perimeter of elastic and hyaline cartilage

• fibrocartilage and articular cartilage both lack perichondrium

What are the 3 types of cartilage

• hyaline cartilage: most common

• elastic cartilage: elastic, does not calcify

• fibrocartilage

Hyaline cartilage

• chondrocytes reside in lacunae (cell nest)

• found in tracheal rings, bronchi, larnyx, nose, articular surfaces (long bones) and ends of ribs (costal cartilages)

• primarily type II collagen

• has role in bone development and growth

  • growth occurs at epiphyseal growth plate

  • site of interstitial growth (lengthening of long bones)

  • in adult bones, hyaline cartilage is only found at articular surfaces

• forms growth plates at proximal and distal ends of long bones (endochondrial ossification)

Hyaline cartilage growth and repair

• growth occurs by appositional or interstitial growth

• calcifies before replacement by bone

• repair is limited because it’s avascular

• repair only occurs by perichondral activity in young individuals

• aging process involves calcification

elastic cartilage

• matrix contains elastin + everything in hyaline cartilage, less mineral

• located in external ear, external auditory meatus, eustachian tube, epiglottis, parts of larynx

• chondrocytes are large and hypercellular

• surrounded by perichondrium

Fibrocartilage

• intermediate between cartilage and dense fibrous CT, large bundles of collagen fibers

• alternates layers of hyaline cartilage and thick layers of dense CT (chondrocytes and type I/II collagen)

• acts as shock absorber - fibers oriented in direction of functional stresses (resistance to compression and shear forces)

• limited distribution in body: intervertebral discs, mandibular condyle, symphysis pubis, sternoclavicular joint, menisci of knees