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Mesenchyme tissue differentiates into
chondroblasts and begins secreting the molecules that form the extracellular
matrix (ECM). Mesenchymal stem cells (MSCs) are undifferentiated, meaning they
can give rise to different cell types. Under the appropriate conditions and at
sites of cartilage formation, they are referred to as chondrogenic cells.
During cartilage formation, undifferentiated MSCs are highly proliferative and
form dense aggregates of chondrogenic cells at the center of chondrification.
These condrogenic cells then differentiate to chondroblasts, which will then
synthesize the cartilage ECM.
The extracellular matrix consists of ground
substance (proteoglycans and glycosaminoglycans) and associated fibers, such as
collagen. The chondroblasts then trap themselves in lacunae, small spaces that
are no longer in contact with the newly created matrix and contain
extracellular fluid. The chondroblast is now a chondrocyte, which is usually
inactive but can still secrete and degrade the matrix depending on the
The majority of body
cartilage is synthesized from chondroblasts that are largely inactive at later
developmental stages compared to earlier years (pre-pubescence). The division
of cells within cartilage occurs very slowly.
Therefore, growth in cartilage is
usually not based on an increase in size or mass of the cartilage itself.
Remodeling of cartilage is predominantly affected by changes and rearrangements
of the collagen matrix, which responds to tensile and compressive forces
experienced by the cartilage. Cartilage growth thus mainly refers to matrix
deposition, but can include both growth and remodeling of the ECM.
fetal development, the greater part of the skeleton is cartilaginous. This
temporary cartilage is gradually replaced by bone (endochondral ossification),
a process that ends at puberty. In contrast, the cartilage in the joints
remains permanently unossified during life.
cartilage has limited repair capabilities because chondrocytes are bound in
lacunae and cannot migrate to damaged areas. Also, because cartilage does not have a blood supply, the deposition of new matrix is slow.
Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last few years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.
These include marrow
stimulation techniques, including surgeries, stem cell injections, and grafting
of cartilage into damaged areas.
However, due to the extremely slow growth of cartilage and its avascular properties, regeneration and growth of cartilage post-injury is still very slow.
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