Cartilage and Bone Flashcards
Cartilage
supporting connective tissue
no nerves, blood vessels, or lymphatics
composition of ECM:
- firm – tissue can bear mechanical stresses without permanent distortion
- resilient – ability to resume original shape after deformation
- smooth – acts as a cushion
Function of Cartilage
provides framework to soft tissues
withstands tensile & compressive forces & helps facilitate bone movement
involved in development & growth of long bones
Chondrocytes
cells of cartilage; occur singly or in isogenous groups & located within cavity called lacunae
synthesizes and maintains ECM
most cartilage surrounded by perichondrium – nutrients
Perichondrium
2 layers important for growth & cartilage maintenance
outer fibrous layer: consists of collagen type I fibers & fibroblasts
inner cellular layer: contains chondrogenic cells which differentiate into chondroblasts
ECM of Cartilage
mainly consists of type II collagen fibers
ground substance rich in GAGs linked to core protein forming proteoglycan monomer called aggrecan
aggrecan links to hyaluronic acid – attracts water
composition of ECM makes cartilage suited to withstand tensile & compressive forces
3 types of Cartilage
hyaline
fibrocartilage
elastic cartilage
Hyaline Cartilage
most abundant, ECM contains type II collagen fibers, gelatinous
fn: resists compression; provides flexible support; reduces friction between bony surfaces in articulation
location: end of bones, respiratory passages, attaching to ribs, embryonic skeleton
covered by perichondrium except articular cartilage
glassy
Elastic Cartilage
similar to hyaline cartilage, but has additional rich network of elastic fibers in ECM
more abundant & larger chondrocytes comparted to hyaline cartilage
fn: strength & elasticity; resilient & maintains shape
location: epiglottis, external ear, auditory tubes
has perichondrium
Fibrocartilage
characterized by thick type I collagen fibers that alternate with parallel groups of columns of chondrocytes & little ground substance, type II collagen fibers also present
no perichondrium
fn: resists tension & compression & provides cushioning
location: pubic symphysis, intervertebral discs, menisci of knee, and entheses
Interstitial Growth of Cartilage
a. condensation of embryonic mesenchyme; pre-cursor for all types of cartilage
b. mesenchymal cells differentiate into chondroblasts
c. chondroblasts separate from one another as they produce ECM, which swells with water and become islated in their lacunae
d. multiplication of chondroblasts within the matrix gives rise to isogenous groups; isolated chondroblasts become quiescent & maintain matrix as chondrocytes
Appositional Growth of Cartilage
mesenchymal cells at the surface of developing cartilage differentiate into fibroblasts; secrete collagen type I fibers & form the outer fibrous layer of perichondrium
inner cellular layer of perichondrium has mesenchymal cells that differentiate into chondroblasts secrete matrix onto the surface of the newly forming cartilage in a process called appositional growth
Bone
supporting connective tissue; characterized by hard, mineralized ECM containing osteocytes
Bone matrix is solid, but porous; microscopic channels permit diffusion of metabolites & gases
richly supplied by nerves, blood vessels, & lymphatics
surfaces lined by layers (endosteum & periosteum); both involved in remodeling and repair
Function of Bone
protects internal organs
involved in movement
supports soft tissues & provides attachment point for muscles
storage of minerals & fats
produces blood cells
Osteoblasts
bone building cells located on surface of bone matrix; typically form a single layer of cuboidal cells
communicated with other osteoblasts & osteocytes via gap junctions
fn: produce bone matrix called osteoid; osteoid later calcifies
once surrounded by ECM osteoblasts become osteocytes
Osteocytes
mature bone cells; do not divide
entrapped in lacunae; processes extend through canaliculi
fn: maintain surrounding bone tissue
Osteoclasts
large, multinuclear cells derived from several monocytes; display ruffled border with activity
concentrated in endosteum
fn: secrete lysosomal enzymes & acids for osteolysis
ECM Bone
Organic matter (osteoid): 1/3 of matrix
- organic components impart tensile strength and flexibility to bone tissue
Inorganic matter: 2/3 matrix
- osteoid is mineralized in the process of matrix formation by inorganic matter; primarily calcium & phosphate in the form of hydroxyapatite crystals
- mineralization of osteoid imparts compressional strength to bone tissue
Classification of Bones
classified based on their shape: flat, long, irregular, short, sesamoid
Long Bones
characterized by a shaft (diaphysis) & 2 ends (epihyses)
metaphysis extends from the diaphysis to the epiphyseal line
meduallary cavity forms inner portion of the bone and is filled with marrow
external surface lined by periosteum and internal surface lined by endosteum
Compact & Spongy Bone
compact → surface of bone; arranged in osteons, also referred to as cortical or dense bone
spongy → interior of bone; open network of trabeculae, also referred to as trabecular or cancellous
compact & spongy bone have the same matrix but different 3D arrangement
Compact Bone
Functional unit of compact bone is the osteon which runs parallel to the diaphysis
each osteon consists of a central canal, concentric lamellae, osteocytes, & canaliculi
Spongy Bone
no osteons; tissue is arranged as trabeculae
trabeculae composed of parallel lamellae with osteycytes between
trabeculae oriented along lines of stress & resists compression from many directions
Canaliculi open at the surface of the trabeculae; gases & nutrients diffuse through tissue in canaliculi to exchange with BVs in red marrow
Bone Formation
intramembranous ossification: develops from mesenchyme
endochondral ossification: bone replaces cartilage model
both types of ossification involve bone matrix forming initially as woven/immature bone & then converting to mature bone
Intramembranous Ossification
- Mesenchymal cells become highly vascularized & cluster & then differentiate into osteoblasts
multiple ossification centers develop within thickened mesenchyme
- osteoid secreted by osteoblasts becomes mineralized
- developing immature/woven bone grows outward forming trabeculae & mesenchyme at periphery becomes periosteum
bone growth & angiogenesis occur in parallel
- continued growth & remodeling results occurs & lamellar bone replaces trabeculae of immature/woven bone
Endochondral Ossification
- Development & Growth of Cartilage Model
mesenchymal cells aggregate & develop into chondroblasts
model grows interstitially & appositionally
- Periosteum & Bone Collar Form
perichondrium around diaphysis converts to periosteum;intramembranous ossification produces bone collar
Diaphyseal chondrocytes & cartilage degenerate; matrix calcifies
- Develop of Primary Ossification Center
blood vessels invade diaphysis bringing mesenchymal cells; these differentiate into osteoblasts & blood forming cells which populate the primary ossification center
- Development of Medullary Cavity
osteoclasts breakdown some spongy bone which forms the cavity
- Development of Secondary Ossification Centers
blood vessels invade epihyses bringing mesenchymal cells; spongy bone forms
- Formation of Articular Cartilage & Epiphyseal Plate
hyaline cartilage remains at epiphyseal plate until early adulthood; responsible for lengthwise growth of bone
Bone Growth
involves interstitial growth of cartilage & endochondral ossification
Zone of Resting Cartilage/Reserve
- scattered inactive chondrocytes; no FN in growth
- anchors plate to epihysis
Zone of Proliferating Cartilage
- larger chondroytes in stacks divide; undergo interstitial growth
Zone of Hypertrophic Cartilage
- large, maturing chondrocytes, rate of cell divison slows; walls of lacunae get hinner
Zone of Calcified Cartilage
- chondrocytes die bc matrix calcifies
Zone of Ossification
- walls break down between lacunae & BVs & osteoprogenitor cells invade from medullary cavity
- osteoprogenitor cells differentiate into osteoblasts & osteoclasts
Increasing Bone Diameter
as bone elongates, it also enlarges in diameter by appositional growth; osteoprogenitor cells of periosteum differentiate into osteoblasts
simultaneously osteoclasts resorb bone at inner surface
