Cartilage
Cartilage is a resilient and smooth type of connective tissue. It appears semi-transparent and non-porous in its natural state. A tough fibrous membrane called perichondrium usually covers this material. In tetrapods, it protects the ends of long bones at joints as articular cartilage. This substance forms structural components for the rib cage, neck, and bronchial tubes. It also makes up intervertebral discs between spinal vertebrae. Other taxa like chondrichthyans rely on it for much of their skeleton. The material is not as hard or rigid as bone. Yet it remains stiffer than muscle or tendon.
In embryogenesis, the skeletal system derives from the mesoderm germ layer. Chondrification begins when condensed mesenchyme tissue differentiates into chondroblasts. These cells secrete molecules such as aggrecan and collagen type II to form the extracellular matrix. All vertebrates use cartilage as the main skeletal tissue during early ontogenetic stages. Osteichthyans see many cartilaginous elements ossify later through endochondral processes. Growth after initial formation involves maturing immature cartilage to a more mature state. Cell division within cartilage occurs very slowly. Thus growth relies mostly on matrix deposition rather than size increase. Non-coding RNAs act as epigenetic modulators affecting chondrogenesis throughout this process.
Mechanical properties of articular cartilage in load-bearing joints have been studied extensively at macro scales. These properties include responses to frictional, compressive, shear, and tensile loading. Cartilage displays viscoelastic properties due to its interstitial fluid content. A confined compression test overcomes obstacles posed by free-moving fluid. In creep mode, tissue displacement measures time under constant load. Relaxation mode measures force over time under constant displacement. Equilibrium displacement can take hours to reach under common loading conditions. The aggregate modulus typically ranges from 0.5 to 0.9 MPa for articular cartilage. Young's Modulus usually falls between 0.45 to 0.80 MPa. Permeability values range from 10^-15 to 10^-16 m^4/Ns. Higher permeability allows fluid to flow out of the matrix more rapidly.
Osteoarthritis affects the whole joint but heavily impacts articular cartilage. The cartilage covering bones thins until it completely wears away. This results in bone against bone contact within the joint. Reduced motion and pain follow this degradation process. Osteoarthritis is considered the result of wear and tear rather than a true disease. It affects joints exposed to high stress levels. Chondroitin sulfate or glucosamine sulfate supplements claim to reduce symptoms. Little good evidence supports these claims however. Increased expression of inflammatory cytokines causes aberrant changes in chondrocyte function. Matrix metalloproteinases mediate excess catabolic activity. Achondroplasia involves reduced proliferation of chondrocytes during infancy. Costochondritis inflames rib cartilage causing chest pain. Spinal disc herniation compresses adjacent nerves causing back pain.
Chondrocytes are bound in lacunae so they cannot migrate to damaged areas. Cartilage damage remains difficult to heal due to this limitation. Hyaline cartilage lacks blood supply making new matrix deposition slow. Surgeons have elaborated procedures to postpone the need for joint replacement. A tear of the meniscus can often be surgically trimmed to reduce problems. Complete healing after injury is hindered by specific inflammation involving M1/M2 macrophages. Biological engineering techniques generate new cartilage using cellular scaffolding materials. Freeze-thawed PVA hydrogels serve as base material for such purposes. These gels exhibit biocompatibility and wear resistance over time. Two-year implantation of PVA hydrogels in rabbits showed intact properties without degradation. Such materials offer shock absorption and lubrication superior to polyethylene-based options.
Cartilaginous fish like sharks, rays, and chimaeras possess skeletons composed entirely of cartilage. This tissue also exists among some arthropods such as horseshoe crabs. Marine snails and cephalopods contain similar structures within mollusks. Sabellid polychaetes or feather duster worms feature cartilage supporting their tentacles. Branchial cartilage of Limulus polyphemus is a vesicular cell-rich type. Endosternite cartilage forms close to ventral nerve cords expressing ColA and SoxE. Cephalopod cranial cartilage shows resemblance to vertebrate hyaline cartilage. Octopus vulgaris and Sepia officinalis serve as models for these studies. Odontophore structures support the radula in gastropods like Busycotypus canaliculatus. Vascular plants sometimes bear the name cartilaginous though they contain no actual cartilage.
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Common questions
What is cartilage and what does it look like?
Cartilage is a resilient and smooth type of connective tissue that appears semi-transparent and non-porous in its natural state. A tough fibrous membrane called perichondrium usually covers this material.
When does chondrification begin during embryogenesis?
Chondrification begins when condensed mesenchyme tissue differentiates into chondroblasts. These cells secrete molecules such as aggrecan and collagen type II to form the extracellular matrix.
How strong is articular cartilage compared to bone or muscle?
The aggregate modulus for articular cartilage typically ranges from 0.5 to 0.9 MPa while Young's Modulus falls between 0.45 to 0.80 MPa. This substance is not as hard or rigid as bone yet remains stiffer than muscle or tendon.
Why can cartilage damage heal slowly or not at all?
Chondrocytes are bound in lacunae so they cannot migrate to damaged areas and hyaline cartilage lacks blood supply making new matrix deposition slow. Complete healing after injury is hindered by specific inflammation involving M1/M2 macrophages.
Which animals have skeletons made entirely of cartilage?
Cartilaginous fish like sharks, rays, and chimaeras possess skeletons composed entirely of cartilage. Other taxa like chondrichthyans rely on it for much of their skeleton.