Cartilage - Lecture #13

Histogenesis (Generation and Growth) of Hyaline Cartilage) - occur in the following sequence:

""Mesenchymal cells"" (stellate, or "star"-shaped cells) retract their cell processes and become round-shaped cells. These cells form ""chondrification centers"" which are future cartilage sites. Mesenchymal cells then differentiate into ""chondroblasts"" which synthesize ""extracellular matrix (ECM)"". As they deposit more and more of this matrix in their surroundings, they become trapped in lacunae ("bubbles" within the matrix). Think of "Swiss cheese": • "cheese" = cartilage matrix • "bubbles" = lacunae • "olive in bubble" = chondrocyte in lacuna When the chondroblasts become trapped in the ECM, they are called ""chondrocytes"". Chondrocytes may later divide (by mitosis) to form ""isogenous groups"", which consist of 2 - 4 (or more) chondrocytes per group. Isogenous groups are progeny of a single cell. These new cells synthesize ECM and become isolated from each other by the newly- synthesized matrix

Histogenesis of the Perichondrium

""during development, mesenchymal cells surrounding the cartilage differentiate into fibroblasts (cells that form fibers). Fibroblasts form the dense collagenous CT, the FIBROUS outer layer of the perichondrium b. perichondrium is a CT capsule that covers ONLY hyaline and elastic cartilage""

Hyaline Cartilage - general

1. Is the most common type of cartilage 2. In the fresh state, it has a glassy, whitish-blue appearance

Fibrocartilage

1.is a transitional form between dense connective tissue and hyaline cartilage. 2. has little amorphous matrix, it contains type I collagen (acidophilic) 3. lacks a perichondrium 4.located where tough support or tensile strength is necessary, and serves as a shock absorber in the following structures: a. intervertebral discs (annulus fibrosus) b. pubic symphysis c.articular discs (articular disc in TMJ) d. menisci of the knee joint e. attached to bone (at junction between tendons and bones) 5. chondrocytes occur in groups or parallel rows, alternating with the thick bundles of collagen (collagen bundles are parallel to the direction of the stress on the tissue) 6.fibroblasts secrete proteoglycans. They become surrounded by matrix and become chondrocytes

Elastic Cartilage

1.is very similar to hyaline cartilage 2. not only does it contain type II collagen (fibrils of which are invisible with routine staining methods), but in addition, it contains elastic fibers in its ECM and in the fibrous layer of its perichondrium 3. it has a more limited distribution in the body than hyaline cartilage 4. is located in only a few places: the pinna of the ear, internal and external auditory tubes, the epiglottis and larynx 5.has more, and larger chondrocytes than hyaline cartilage 6.elastic fiber bundles are more prominent in the territorial matrix 7.it is much more flexible and elastic than hyaline cartilage 8.is yellow in the fresh state 9.its matrix does not calcify with aging

Cartilage Growth - Interstitial Growth

Cartilage grows by two ways: interstitially and appositionally a.interstitial growth occurs ONLY i. during the early stages of cartilage formation ii. in articular cartilage (which does not have a perichondrium, thus NO APPOSITIONAL growth here) iii. in epiphyseal plates (permits lengthening of bone) iv. deep within the cartilage, that is, cells and matrix are added to the inside of the tissue. This type of growth is possible because cartilage matrix is distensible

Carilage Introduction

Cartilage is classified as a specialized connective tissue because it is firm but pliable. Like other connective tissues, it contains cells, fibers and extracellular matrix. Cartilage-forming cells called chondroblasts produce the molecules (proteins, proteoglycans, and glycosaminoglycans) of the extracellular matrix as well as the fibers embedded in this matrix. When these cells become trapped in the matrix that they produced, they are called chondrocytes. Like epithelial tissue, cartilage is AVASCULAR, that is, it does not have its own blood vessels or lymphatic vessels and thus relies on the diffusion of nutrients from surrounding tissues for nourishment. Furthermore, it lacks nerve fibers. Cartilage is classified according to the main fiber type in its matrix which gives each type of cartilage its distinctive morphological and functional characteristics. Based on this, there are 3 types of cartilage: hyaline, elastic and fibrocartilage.

Osteoarthritis - Osteoarthritis (G. osteo, "bone"; arthron, "joint"; itis, "inflammation").

In a normal joint, hyaline cartilage covers and cushions the articular surfaces of bones. It provides a slippery surface that allows the articulating bones to glide during movement,and serves as a shock absorber. As one ages though, a number of risk factors may cause excessive "wear and tear" of the articular cartilage of certain joints, mainly in the weight-bearing joints of the hip and/or knee. The joints of the cervical spine, shoulder, low back and the distal joints of the fingers may be affected. The breakdown of hyaline cartilage at these joints causes a disorder called osteoarthritis. Osteoarthritis is the most common type of arthritis. Osteoarthritis is a "degenerative joint disease" that is most prevalent in older individuals. It is characterized by the breakdown of the hyaline cartilage that covers the articular surfaces of bones. As an individual ages, the GAGs in the extracellular matrix are structurally modified and are accompanied by a decrease in water content. The cartilage gradually becomes thinner, develops tiny fissures and eventually completely wears away, leaving bone rubbing against bone at the joint. This friction causes sanding down of the articulating bones, releasing tiny bone fragments that float in the fluid lubricating the joint. Since bone is a living tissue that has nerve fibers, osteoarthritis is a very painful condition. Swelling, stiffness, and a reduction in the range of motion at the affected joint follow.

Histogenesis of the Perichondrium - layers of perichondrium.

a) outer fibrous layer ---consists of dense collagenous connective tissue with type I collagen, fibroblasts, blood vessels and nerves. The blood vessels deliver nutrients and oxygen that diffuse to the underlying cartilage cells. This FIBROUS layer of the perichondrium is protective and supportive b) inner cellular layer -- consists of chondrogenic cells that divide, differentiate into chondroblasts and form ECM. As a result, cartilage grows on the surface of anexisting cartilage, known as appositional growth. This CELLULAR layer of the perichondrium allows cartilage growth ii. overall, the perichondrium is protective and permits cartilage growth and maintenance

Hyaline Cartilage - Location

a. ""in places where it maintains a lumen / space open"" (prevents it from collapsing) and provides a semiflexible support i. ""present in the nose, larynx, trachea, and bronchi"" b. at articular surfaces of bones i.covers the ventral ends of the ribs articulating with the sternum (the ribs, the articular hyaline cartilage, and the sternum form a semi-flexible union) ii. covers the articulating surfaces of bones of moveable joints (such as the shoulder, elbow, hip, and knee joints) to reduce friction and serve as a shock absorber or "cushion" between the articulating bones c. (at epiphyseal "growth" plates of growing bones) 4.Its smooth, glossy surface allows friction-free gliding between bones of movable joints

Hyaline Carilage Matrix -

a. contains proteins, mainly type II collagen (delicate fibers are invisible with routine staining), collagen makes up about 40% of dry weight. Tension-resisting collagen fibrils form a 3D network b. contains proteoglycans, glycoproteins, and extracellular fluid

Histophysiology of Hyaline Cartilage

a. is avascular (NO arteries, veins, or lymphatic vessels) b.does not have nerve fibers (imagine what would happen if there were sensory nerve fibers in the hyaline cartilage covering the articular surfaces of bone, like those of the knee joint) c.nutrients and oxygen diffuse through the ECM from nearby tissues; this limits the thickness of cartilage (if cartilage gets too thick, those cells furthest away from nutrients will die) d.resists forces (compression, tension), therefore it is suitable for joints. e. chondrocytes continuously monitor matrix composition. As matrix molecules break down, chondrocytes synthesize new molecules to replace and remodel the matrix f. hyaline cartilage tends to calcify with aging

Cartilage Cells - 3 Types

chondrogenic, chondroblasts, chondrocytes

Hyaline Cartilage Matrix - components (general)

contains proteoglycans, glycoproteins and ECF. hyaluronan molecules, capsullar ( pericellular space) matrix, territorial matrix, interterritorial matrix, fibronectin,

Hyaline Cartilage Matrix - general (d-f)

d. 60-80% of wet weight is intercellular water bound by proteoglycan aggregates e.extracellular matrix makes up over 95% of cartilage volume f.proteoglycan aggregates become trapped in the 3D matrix formed by the collagen fibrils producing the distinctive biomechanical characteristics of hyaline cartilage

Cartilage Cells - chondroblasts

differentiate from a) mesenchymal cells (in chondrification center) b) chondrogenic cells (in inner perichondrium) ii. RER, protein synthesis, GA, are basophilic iii. form matrix and fibers of cartilage

3 Types of Cartilage

hyaline, elastic, fibrocartilage

Cartiage cells - chondrocytes

i. are "grown-up" chondroblasts ii. have a large nucleus, and a prominent nucleolus iii. RER, GA, glycogen in cytoplasm iv. can go back to being chondroblasts v.are trapped in a lacuna (an isolated cavity or "bubble") within the cartilage matrix vi. monitor matrix composition and synthesize necessary molecules to maintain cartilage matrix

Hyaline Cartilage Matrix - hyaluronan molecules

i. each hyaluronan molecule has many proteoglycan monomers attached to it a) a proteoglycan monomer consists of about 180 GAG's attached to a core protein ii. many hyaluronan molecules are interwoven with a network of collagen fibrils in the matrix to form a 3D-mesh giving cartilage its resilience and enabling it to serve as a shock absorber

Hyaline Cartilage Matrix - Fibronectin

i. is an adhesive glycoprotein ii.it assists chondroblasts and chondrocytes to adhere to the ECM

Hyaline Cartilage Matrix - capsular (pericellular) matrix

i. is the intensely-stained thin layer of matrix immediately around the lacuna a) it contains the highest concentration of sulfated proteoglycans, hyaluronan and glycoproteins b) it contains fine collagen fibers which i)form a woven capsule around each chondrocyte

Hyaline Cartilage Matrix - interterritorial matrix

i. represents the majority of the matrix ii.fills the space around the territorial matrix j.cartilage matrix consists of 60 - 80% water, bound to proteoglycans. Its highly hydrated matrix permits diffusion of nutrients derived from surrounding tissues

Cartilage Cell - chondrogenic

i. rise from mesenchymal cells ii. can differentiate into a) chondroblasts (cartilage-forming cells) b) osteoprogenitor cells (bone-forming cells)

Cartilage Growth - Appositional Growth

i.in most cartilages ii. throughout the lifespan of the cartilage iii. at the surface of an existing cartilage, that is, cells and matrix are added to the outer surface of the cartilage The ability of hyaline cartilage to grow interstitially and appositionally while providing a semi-rigid support, makes it ideal as a "cartilage model of a future bone" during bone development

Hyaline Cartilage Matrix - territorial matrix

i.is the lighter-staining matrix that surrounds the isogenous group ii. contains a lower concentration of sulfated proteoglycans than the capsular matrix and is thus lighter-staining iii. contains collagen