The cartilage

Cartilage
Introduction: Cartilage is a type of connective tissue .It exhibits tensile strength, provides frim structural support for soft tissues, allows flexibility without distortion, and is resilient to compression. Cartilage has all the common feature of connective tissue, which include ground substance, fiber and cells called chondrocytes and chondroblasts.

Cartilage is differs from bone in several ways. For one, the primary cell types are chondrocytes as opposed to osteocytes. Chondrocytes are first chondroblasts cells that produce the collagen extracellular matrix (ECM) and then get caught in the matrix. They lie in spaces called lacunae with up to eight chondrocytes located in each. Chondrocytes rely on diffusion to obtain nutrients as, unlike bone, cartilage is avascular, meaning there are no vessels to carry blood to cartilage tissue. This lack of blood supply causes cartilage to heal very slowly compared with bone.
Ground substance: or matrix produced and maintained by chondrocytes . matrix is solid and firm but somewhat flexible . The base substance of cartilage is chondroitin sulfate, and the microarchitecture is substantially less organized than in bone. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself.
Articular cartilage function is dependent on the molecular composition of its ECM, which consists mainly of proteoglycans and collagens. The 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.

Ground substance: contains the following component:
A-proteoglycans: containing side chains of glycosaminoglycan s (GAGs), specifically chondroitin sulfate &keratin sulfate. Diffusion from blood vessels in surrounding connective tissue.
B- Glycoproteins: including chonrdonectin and chondrocalcin (a calcium binding protein).
C- No mineral (inorganic) component, because cartilage is not mineralized.
D- Water (tissue fluid) - high degree of hydration (75%). E.g. joints,
Fiber
a-Type I collagen: is found in fibrocartilage
b- Type II collagen: is found in hyaline and elastic cartilage.
Cells
a- Chondrogenic cells are found in the perichondrium , where they undergo mitosis and differentiate into chondroblasts .
b- Chondroblasts arise from chondrogenic cells and undergo mitosis and synthesize the cartilage matrix and extracellular matrix.
c- Chondrocytes reside in lacunae . They form isogenous group that are surrounded by a territorial matrix that stains basophilic due to the higher local concentration of chondroitin sulfate .Chondrocytes may undergo mitoses .
No blood or lymphatic vessels or nerves penetrate cartilage .the cell receive their nourishment from the surrounding connective tissue by diffusion through the matrix .
Injured cartilage heals slowly .


Perichondrium :
The cartilage is covered externally by a dense connective tissue sheath known as perichondrium except articular cartilage and fibro cartilage fibrous sheath is called the perichondrium.
Most of hyaline and elastic cartilage is surrounded by a peripheral layer of vascularized , dense irregular connective tissue called perichondrium, it has an outer fibrous layer(vascular ) and the inner cellular layer is chondrogenic (cellular).It has cells which can regrow cartilage to some extent if the cartilage is damaged.
Function
Firm support with some flexibility bear mechanical stress .
Properties of cartilage
- High tensile strength withstand pressure
-low metabolic rate.
-capacity of continued growth
-avascular diffusion from capillaries.
Types of cartilage tissue
Cartilage is classified based on amount and types of fibers that are present in the matrix to:
Hyaline cartilage
Hyaline cartilage is the common type is the most widespread cartilage type and, in adults, it forms the articular surfaces of long bones, the rib tips, the rings of the trachea, and parts of the skull. This type of cartilage is predominately collagen (yet with few collagen fibers), and its name refers to its glassy appearance.
In the embryo, bones form first as hyaline cartilage before ossifying as development progresses. Hyaline cartilage is covered externally by a fibrous membrane, called the perichondrium, except at the articular ends of bones; it also occurs under the skin (for instance, ears and nose).It contains no nerves or blood vessels, and its structure is relatively simple.
Hyaline cartilage have chondrocytes in lacunae throughout matrix arranged appear either singly or in group called isogenous groups (clusters of chondrocytes derived from one cell by mitotic division).
-Extracellular matrix has type II collagen: proteoglycans chonrdonectin.
-Territorial matrix: surrounds each chondrocyte.
- Inner territorial: includes type III collagen fibers.

Hyaline cartilage surrounds by perichondrium which consist of dense connective tissue except at articular surface .

Elastic cartilage
Elastic or yellow cartilage contains elastic fiber networks and collagen fibers. The principal protein is elastin. Elastic cartilage is histologically similar to hyaline cartilage but contains many yellow elastic fibers lying in a solid matrix. These fibers form bundles that appear dark under a microscope. They give elastic cartilage great flexibility so it can withstand repeated bending. Chondrocytes lie between the fibers. Elastic cartilage is found in the epiglottis (part of the larynx) and the pinnae (the external ear flaps of many mammals, including humans).


Fibrocartilage
Fibrous cartilage has lots of collagen fibers (Type I and Type II), and it tends to grade into dense tendon and ligament tissue. White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. No perichondrium has row of chondrocytes in lacunae separated by parallel bundles of collagen fibers type II. It owes its flexibility and toughness to the fibrous tissue, and its elasticity to the cartilaginous tissue. It is the only type of cartilage that contains type I collagen in addition to the normal type II. Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs, menisci, and the temporal mandibular joint.






Cartilage Growth
Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue. Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules (aggrecan and collagen type II) that form the extracellular matrix. Following the initial chondrification that occurs during embryogenesis, cartilage growth consists mostly of the maturing of immature cartilage to a more mature state. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself.

A chondrocyte: A chondrocyte, stained for calcium, showing its nucleus (N) and mitochondria (M). 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.
Repair
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage is difficult to heal. Also, because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue.