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Cartilage N. Swailes, Ph.D. Department of Anatomy and Cell

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Cartilage N. Swailes, Ph.D. Department of Anatomy and Cell Module 1.6: Cartilage N. Swailes, Ph.D. Department of Anatomy and Cell Biology Rm: B046A ML Tel: 5-7726 E-mail: [email protected] Required reading Mescher AL, Junqueira’s Basic Histology Text and Atlas, 12th Edition, Chapter 7: pp114-120 Ross MH and Pawlina W, Histology: A Text and Atlas, 6th Edition, Chapter 7: pp198-216 Learning objectives 1) Describe the components and organization of a typical cartilage (hyaline) and outline the mechanical properties each imparts to the tissue. 2) Comment on the similarities and differences between the cellular and extracellular components of cartilage and the other basic connective tissues you have seen. 3) Comment on the blood supply and waste removal to/from cartilage. 4) Classify, compare and contrast the organization, location and function of the four different types of cartilage. How is each suited to the location it occupies? 5) Outline the processes of cartilage formation (chondrogenesis) and discuss its growth and repair. 1 | Page Swailes Part A: General Organization of Cartilage A1. Introduction Cartilage is a specialized connective tissue. It is composed of cells called chondrocytes that sit in lacunae (spaces) within the extracellular matrix that they maintain. The main type of cartilage is called hyaline cartilage. Extracellular matrix A2. General Organization of Cartilage The ECM of cartilage is composed of collagen fibers embedded in a proteoglycan rich ground substance. These components make the ECM solid but pliable. A3. Cartilage Extracellular Matrix: Collagen Collagen There are four types of cartilage specific collagens Figure: Chart showing the composition of hyaline that form a 3-D meshwork of fibrils within all cartilage, the most common type of cartilage in the body. cartilages types. There are other types of collagen associated with cartilage in small amounts that are not listed. It should be noted that Type I collagen is found in large quantities in fibrocartilage. Collagen type Function in extracellular matrix Type II Does not form fibers, therefore not designed to resist strong tensile forces. (most abundant) Instead withstands forces accompanying intermittent pressure. Type XI Regulates the size of Type II fibrils. Type X Organizes the collagen fibrils into a 3-D meshwork. This lattice is crucial for allowing cartilage to resist compressive forces. Type VI Forms the region of extracellular matrix around chondrocytes Involved in attaching these cells to the extracellular matrix. Type I Found in large quantities in fibrocartilage only. Forms dense irregular bundles that withstand strong tensile forces. 2 | Page Swailes A4. Cartilage Extracellular Matrix: Ground Substance Ground substance Ground substance is an amorphous gel-like substance within which the cells and fibers of connective tissues are embedded. In cartilage it is composed of: Keratan sulfate i. Aggrecan molecules Aggrecan is a proteoglycan that has a very strong Chondroitin affinity for water (hydrophilic). A proteoglycan sulfate consists of non-branching polysaccharide chains called glycosaminoglycans (GAGs) that are bound to a protein core. Each aggrecan molecule contains many chondroitin sulfate and keratan sulfate chains (GAGs). Protein ii. Hyaluronan (hyaluronic acid) core Is a special GAG that does not bind to a protein core, instead it is a long and linear molecule. In cartilage it binds many aggrecan molecules along its length to form proteoglycan aggregates. iii. Proteoglycan aggregates The proteoglycan aggregates bind to the collagen fibrils in the ECM and become trapped within the 3-D hexagonal lattice of fibers. This arrangement of hydrophilic proteoglycan aggregates fills the spaces within a collagen meshwork and provides cartilage with its unique ability to cushion and resist compressive forces. iv. Chondronectin Chondronectin is a cartilage specific glycoprotein. Remember the Hyaluronan glycoprotein laminin and how it was involved in anchoring the epithelium to the basal lamina? In a similar way to laminin, the glycoprotein chondronectin binds the cartilage cells (chondrocytes) to the collagen fibrils of the ECM in cartilage. Chondrocyte Hyaluronan Collagen fibril (Type II) Aggrecan molecule (proteoglycan) A5. Cartilage cells Cartilage also contains cells that synthesize and become embedded in the ECM. There are two major cell types found in cartilage: Chondrocyte • Chondroblasts Lacuna Chondroblasts are located in the perichondrium, an area of dense irregular Isogenous group connective tissue that surrounds all types of cartilage (except articular cartilage). These cells form a stem cell population that divide and secrete the ECM (fibers and ground substance). Once a chondroblast has synthesized enough ECM to become surrounded in it own secretions it matures to form a chondrocyte. • Chondrocytes Chondrocytes are mature cartilage cells. They Chondroblast are mitotic during growth, but this becomes limited in adult tissue. Those cells that have undergone division within the matrix are organized into clusters of cells called p Inactive isogenous groups. As the newly divided cell produces matrix it becomes surrounded and the cell clusters are pushed further apart. The erichondrium spaces within the matrix that the chondrocytes occupy are called lacunae. Fibroblast A6. Perichondrium Cartilage is surrounded by a dense irregular connective tissue called the perichondrium (except articular and fibrocartilage). It is essential for appositional growth and maintenance of the Chondrocyte underlying cartilage. During active appositional growth (see later) it is ii composed of: Chondroblast i. An outer fibrous perichondrium composed of dense irregular Type I collagen bundles and fibroblasts i (predominates in non-growing cartilage). ii. An inner chondrogenic layer of cells that divide to form chondroblasts which Fibroblast 4 | Page differentiate to form chondrocytes. A7. Blood Supply to Cartilage Cartilage is avascular which poses a number of problems: 1. How do chondrocytes receive nutrients and remove waste products? i. Chondrocytes respire under low oxygen tension and metabolize glucose by anaerobic glycolysis to produce lactic acid. ii. The large ratio of proteoglycan to collagen fibers in Matrix cartilage allows diffusion more easily through the H O 2 expands ECM from blood vessels within the perichondrium. iii. The hydrophilic nature of the proteoglycan rich ECM draws water and nutrients into the cartilage. When compressive forces are applied to the cartilage, this forces water (and waste products out) just like water can be sucked up and squeezed out of a sponge. 2. How does an avascular cartilage grow with the body? The limitations of getting nutrients to a cellular tissue without a direct blood supply limit the thickness to which cartilage can Compression grow. 3. How does an avascular cartilage repair itself if damaged? H2O It can’t (more later!) A8. Cartilage Function The 3-D hexagonal lattice created by the meshwork of collagen fibers is filled with hydrophilic proteoglycans. When fully hydrated, the large water content of the ECM acts as a shock absorber and resists the compressive forces that are applied to its surface. Part B: Types of cartilage B1. Hyaline cartilage Appearance Hyaline cartilage has a glassy appearance in life [Gr. Hyalos = glassy]. Composition and organization Note that the general organization previously was based on hyaline cartilage. The only exception is the specialized hyaline cartilage “articular cartilage”, which lacks a perichondrium. Locations It is the most common form of cartilage found: - in the respiratory tract - lining articular surfaces - in the ribs (costal cartilages) - in the epiphyseal plates (see Bone Development) - forming the temporary skeleton of the fetus (see Bone Development ) 5 | Page B2. Articular cartilage Appearance Articular cartilage is a specialized form of hyaline cartilage that lines the surfaces of joints. It is different from the other types of hyaline cartilage in the body because it lacks a perichondrium on all aspects. It is a remnant of the original cartilage model of the developing bone. Composition and organization Articular cartilage has four zones: 1. Tangential zone Chondrocytes are flattened and surrounded by organized Type II collagen fibers arranged in fasicles parallel to the free surface. 2. Transitional zone Chondrocytes are round and randomly distributed within the ECM. Collagen fibers are less well organized in this region. 3. Radial zone Chondrocytes are small, round and organized into short columns perpendicular to the free surface. The collagen fibers are aligned perpendicular to the long axis of the bone. 4. Calcified zone This region has a calcified matrix and is seperated from the radial zone by a calcified line or tidemark. Chondrocytes above this line are the source of cells for growth and renewal. 1 1 2 2 3 3 4 Tidemark Bone 6 | Page 4 B3. Elastic cartilage Chondrocyte Appearance Distinctive appearance due to the high elastin fiber Elastin fibers content of the ECM. Composition and organization Similar to hyaline cartilage except the ECM contains a large quantity of branching elastin fibers interspersed among the collagen fibers. As a result, this type of cartilage is highly flexible and subsequent deformation results in recoil to its original shape and position. Note that elastic cartilage has a perichondrium. Locations Elastic cartilage is found in the: - auricle of the ear - auditory tubes - epiglottis of the larynx B4. Fibrocartilage Perichondrium Appearance Intermediate to dense connective
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