Joint & Cartilage. 03 PDF

Uskudar University School of Medicine Joints Structures and Cartilage Histology and Embryology Asım Savlu M.D. TYPICAL SYNOVIAL JOINT Longitudinal section through a diarthrosis with growing bones of a rodent knee, showing the position near the boundaries of the capsu...

Uskudar University School of Medicine Joints Structures and Cartilage Histology and Embryology Asım Savlu M.D. TYPICAL SYNOVIAL JOINT Longitudinal section through a diarthrosis with growing bones of a rodent knee, showing the position near the boundaries of the capsule (C) of the epiphyseal growth plate (E) where endochondral ossification occurs. Also shown are the articular cartilage (A) and the folds of synovial membrane (SM), which extend prominently into the joint cavity from connective tissue of the capsule for production of synovial fluid Fıgure 8‐19 Copyright © McGraw‐Hill Companies Diarthroses or synovial joints. Longitudinal section through a diarthrosis with growing bones of a rodent knee, showing the position near the boundaries of the capsule (C) of the epiphyseal growth plate (E) where endochondral ossification occurs. Also shown are the articular cartilage (A) and the folds of synovial membrane (SM), which extend prominently into the joint cavity from connective tissue of the capsule for production of synovial fluid. X10. Fıgure 8‐19 Copyright © McGraw‐Hill Companies Synovial membrane. The synovial membrane is a specialized connective tissue that lines capsules of synovial joints and contacts the synovial fluid lubricant, which it is primarily responsible for maintaining. The synovial membrane projects folds into the joint cavity (JC) and these contain many small blood vessels (V). The joint cavity surrounds the articular cartilage (AC). X100. Mallory trichrome Fıgure 8‐20 Copyright © McGraw‐Hill Companies Synovial membrane. Higher magnification of the fold showing a high density of capillaries and two specialized types of cells called synoviocytes. Contacting the synovial fluid at the tissue surface are many rounded macrophage‐like synovial cells (type A) derived from blood monocytes. These cells bind, engulf, and remove tissue debris from synovial fluid. These cells often form a layer at the tissue surface (A) and can superficially resemble an epithelium, but there is no basal lamina and the cells are not joined together by cell junctions. Fibroblast‐like (type B) synovial cells (B) are mesenchymally derived and specialized for synthesis of hyaluronan that enters the synovial fluid, replenishing it. X400. Fıgure 8‐20 Copyright © McGraw‐Hill Companies Synovial membrane. Among the macrophage‐like and fibroblast‐like synovial cells are collagen fibers and other typical components of connective tissue. Surface cells have no basement membrane or junctional complexes denoting an epithelium, despite the superficial resemblance. Blood capillaries are fenestrated, which facilitates exchange of substances between blood and synovial fluid. Fıgure 8‐20 Copyright © McGraw‐Hill Companies Tendons and Ligaments : Dense regular connective tissue. (a) Micrograph shows a longitudinal section of dense regular connective tissue in a tendon. Long, parallel bundles of collagen fibers fill the spaces between the elongated nuclei of fibrocytes. X100. H&E stain. (b) The electron micrograph shows one fibrocyte in a cross section of tendon, revealing that the sparse cytoplasm of the fibrocytes is divided into numerous thin cytoplasmic processes extending among adjacent collagen fibers. Fıgure 5‐22 X25,000. Copyright © McGraw‐Hill Companies  Fibroblasts. (a) Fibroblasts typically have large active nuclei and eosinophilic cytoplasm that tapers off in both directions along the axis of the nucleus, a morphology often referred to as “spindle‐shaped.” Nuclei (arrows) are clearly seen, but the eosinophilic cytoplasmic processes resemble the collagen bundles (C) that fill the ECM and are difficult to distinguish in H&E‐ stained sections. Fıgure 5‐3 Copyright © McGraw‐Hill Companies  Assembly of type I collagen. Shown here are the relationships among type I collagen molecules, fibrils, fibers, and bundles. 1. Rodlike triple‐helix collagen molecules, each 300 nm long, self‐assemble in a highly organized, lengthwise arrangement of overlapping regions. 2. The regular, overlapping arrangement of subunits continues as large collagen fibrils are assembled. 3. This structure causes fibrils to have characteristic cross striations with alternating dark and light bands when observed in the EM. 4. Fibrils assemble further and are linked together in larger collagen fibers visible by light microscopy. 5. Type I fibers often form into still larger aggregates bundled and linked together by other collagens. The photo shows an SEM view of type I collagen fibrils closely aggregated as part of a collagen fiber. Striations are visible on the surface of the fibrils. Fıgure 5‐11 Copyright © McGraw‐Hill Companies Articular cartilage. The large diagram shows a small region of articular cartilage in which collagen fibers run perpendicular to the tissue surface and then bend gradually, forming a broad arch parallel to that surface. The lower left diagram shows a 3D view of collagen fibers in articular cartilage. Proteoglycan aggregates bound to hyaluronic acid and collagen fill the space among the collagen fibers and bind a large amount of water, functioning as a biomechanical spring in articular cartilage. When pressure is applied, some water is forced out of the cartilage matrix into the synovial fluid. When pressure is released, water is attracted back into the interstices of the matrix. These water movements are brought about constantly by using the joint and are essential for nutrition of the cartilage and for facilitating the interchange of O2, CO2, and other molecules between the synovial fluid and the articular cartilage. Fıgure 8‐21 Copyright © McGraw‐Hill Companies Articular surfaces of a diarthrosis are made of hyaline cartilage that lacks the usual perichondrium covering. X40. H&E. Fıgure 8‐21 Copyright © McGraw‐Hill Companies a) There are three types of adult cartilage distributed in many areas of the skeleton, particularly in joints and where pliable support is useful, as in the ribs, ears, and nose. Cartilage support of other tissues throughout the respiratory tract is also prominent. The photomicrographs show the main features of (b) hyaline cartilage, (c) fibrocartilage, and (d) elastic cartilage. Dense connective tissue of perichondrium is shown here with hyaline and elastic cartilage. Copyright © McGraw‐Hill Companies Cartilage Cartilage is a tough, flexible form of connective tissue, characterized by an extracellular matrix (ECM) with high concentrations of GAGs and proteoglycans, which interact with collagen and elastic fibers. In the respiratory tract, ears, and nose, cartilage forms the framework supporting soft tissues. Provides shockabsorbing and sliding regions within joints and facilitates bone movements Guides development and growth of long bones, both before and after birth. Cartilage consists of cells called chondrocytes embedded in an extensive ECM. Chondrocytes synthesize and maintain ECM components and are located in matrix cavities called lacunae. Collagen, hyaluronic acid, proteoglycans, and various glycoproteins are the principal macromolecules present in all types of cartilage matrix. Cartilage Different functional requirements: three major forms of cartilage, each varying somewhat in matrix composition. Hyaline cartilage, the most common form, type II collagen is the principal collagen type. Elastic cartilage possesses abundant elastic fibers in addition to collagen type II. Fibrocartilage present in body regions subjected to pulling forces, is characterized by a matrix containing a dense network of coarse type collagen fibers. Cartilage In all three forms, cartilage is avascular and receives nutrients by diffusion from capillaries in adjacent connective tissue (perichondrium). Chondrocytes exhibit low metabolic activity. Cartilage also lacks lymphatic vessels and nerves. The perichondrium is a sheath of dense connective tissue that surrounds cartilage in most places, forming an interface between the cartilage and the tissues supported by the cartilage. The perichondrium harbors the cartilages vascular supply, as well as nerves and lymphatic vessels. Articular cartilage, which covers the surfaces of bones in movable joints lacks perichondrium and is sustained by the diffusion oxygen and nutrients from the synovial fluid. Cartilage HYALINE CARTILAGE Hyaline cartilage the most common of the three forms, is homogeneous and semitransparent in the fresh state. In adults hyaline cartilage is located in the articular surfaces of movable joints, in the walls of large respiratory passages (nose, larynx, trachea, bronchi), in the ventral ends of ribs, where they articulate with the sternum, in the epiphyseal plates of long bones, where it makes possible longitudinal bone growth. In the embryo, hyaline cartilage forms the temporary skeleton that is gradually replaced by bone. Cartilage HYALINE CARTILAGE Matrix The dry weight of hyaline cartilage is 40% collagen embedded in a firm, hydrated gel of proteoglycans and structural glycoproteins Proteoglycans cause the matrix to be generally basophilic. Most of the collagen in hyaline cartilage is type II collagen, although small amounts of several minor types are also present. Aggrecan (250 kD), with approximately 150 GAG side chains of chondroitin sulfate and keratan sulfate, is the most abundant proteoglycan of hyaline cartilage. Hundreds of these proteoglycans are bound noncovalently by link proteins to long polymers of hyaluronic acid, These proteoglycan complexes bind further to the surface of type II collagen fibrils Link proteins noncovalently bind the protein core of (b) A diagram of the transitional area between the proteoglycans to the linear hyaluronic acid perichondrium and the cartilage matrix. Fibroblast‐like molecules.The chondroitin sulfate side chains of the progenitor cells in the perichondrium give rise to larger proteoglycan electrostatically bind to the collagen chondroblasts, which divide and differentiate as chondrocytes. fibrils, forming a cross‐linked matrix. The circled area These functional cells produce matrix components and exist in is shown larger in the lower part of the figure. lacunae embedded in the matrix. Staining differences are Physical properties of these matrix components apparent between the matrix immediately around each lacuna, produce a highly hydrated, pliable material with called the territorial matrix, and that more distant from great strength. Approximately 75% of the wet lacunae, the interterritorial matrix. Collagen is more abundant Fıgure 7‐2 Copyright © McGraw‐Hill Companies weight of hyaline cartilage is water in the interterritorial parts of the matrix. Fıgure 5‐17 Copyright © McGraw‐Hill Companies Cartilage HYALINE CARTILAGE Matrix Water bound to GAGs in the proteoglycans constitutes up 60%‐80% of the weight of fresh hyaline cartilage. Important component of cartilage matrix is the structural multiadhesive glycoprotein chondronectin. Like fibronectin in other connective tissues, chondronectin binds specifically to GAGs, collagen type II, and integrins, mediating the adherence of chondrocytes to the ECM. Cartilage HYALINE CARTILAGE CHONDROCYTES At the periphery of the cartilage, young chondrocytes (or chondroblasts) have an elliptic shape, with the long axis parallel to the surface. Deeper in the cartilage, they are round and may appear in groups of up to eight cells that originate from mitotic divisions of a single chondrocyte and are called isogenous aggregates. Cartilage cells and the matrix often shrink during routine histologic preparation, resulting in both the irregular shape of the chondrocytes and their retraction from the matrix. In living tissue, and in properly prepared sections, the chondrocytes fill the lacunae completely. Because cartilage is devoid of blood capillaries, chondrocytes respire under low‐oxygen tension. Hyaline cartilage cells metabolize glucose mainly by anaerobic glycolysis to produce lactic acid as the end product Cartilage HYALINE CARTILAGE Chondrocyte synthesis of sulfated GAGs and secretion of proteoglycans are accelerated by many hormones and growth factors. A major regulator of hyaline cartilage growth is pituitary‐derived growth hormone or somatotropin. PERICHONDRIUM Except in the articular cartilage of joints, all hyaline cartilage is covered by a layer of dense connective tissue, the perichondrium, which is essential for the growth and maintenance of cartilage. The perichondrium consists largely of collagen type I fibers and fibroblasts. Among these fibroblasts in the inner layer of the perichondrium are progenitor cells for CHONDROBLASTS that divide and differentiate into chondrocytes. (a) The upper part of the photo shows the more acidophilic (b) The thin region of hyaline cartilage shown here has perichondrium (P), an example of dense connective tissue perichondrium (P) on both sides and shows larger lacunae consisting largely of type I collagen. There is a gradual containing isogenous groups of chondrocytes (C) within the transition and differentiation of cells from the perichondrium matrix (M). to the cartilage, with elongated fibroblastic cells becoming Such groups of two, four, or more cells are produced by larger and more rounded chondroblasts and chondrocytes (C) mitosis; the cells will separate into individual lacunae as they located within spaces or lacunae surrounded by the matrix (M) begin to secrete matrix. X160. H&E. secreted by the cells. X200. H&E. Fıgure 7‐3 Copyright © McGraw‐Hill Companies Cartilage ELASTIC CARTILAGE Elastic cartilage is essentially similar to hyaline cartilage except that it contains an abundant network of elastic fibers in addition to collagen type II which give fresh elastic cartilage a yellowish color. Demonstration of the elastic fibers usually requires stains such as orcein or resorcin fuchsin. Elastic cartilage is found in the auricle of the ear, the walls of the external auditory canals, the auditory (eustachian) tubes, the epiglottis, and the cuneiform cartilage in the larynx. Elastic cartilage in these locations includes a perichondrium similar to that of most hyaline cartilage. Formation of elastic fibers. (a) Initially, a developing fiber consists of many 10‐nm‐ diameter fibrillin microfibrils composed of molecular subunits secreted by fibroblasts and smooth muscle cells. (b) Elastin is deposited on the scaffold of microfibrils, forming growing, amorphous composite structures. The elastin molecules are also secreted by the fibroblasts and quickly become cross‐linked into larger assemblies (c) Elastin accumulates and ultimately occupies most of the electron‐dense center of the single elastic fiber shown here. Fibrillin microfibrils typically remain at the fiber surface. Collagen fibrils, seen in cross section, are also present surrounding the elastic fiber. All X50,000. Fıgure 5‐14 Copyright © McGraw‐Hill Companies The chondrocytes (C) and overall organization of elastic cartilage are similar to those of hyaline cartilage. Stains for elastin, however, reveal many dark‐staining elastic fibers in the matrix (M), in addition to the major components found in hyaline matrix. Elastic fibers provide greater flexibility to this form of cartilage. The section in part b includes perichondrium (P) that is also similar to that of hyaline cartilage, (a) X160. Hematoxylin and orcein. (b) X100. Weigert resorcin‐fuchsin. Fıgure 7‐4 Copyright © McGraw‐Hill Companies Cartilage FIBROCARTILAGE Fibrocartilage takes various forms but is essentially a combination of hyaline cartilage and dense connective tissue with gradual transitions between these tissues It is found in intervertebral discs, in attachments of certain ligaments, and in the pubic symphysis. Chondrocytes of fibrocartilage produce matrix containing type II collagen. In some fibrocartilage this matrix around the chondrocytes is very sparse. Regions with chondrocytes and hyaline matrix are separated by other regions containing bundles of type I collagen and scattered fibroblasts The relative scarcity of proteoglycans makes the matrix of fibrocartilage more acidophilic than that of hyaline or elastic cartilage. Intervertebral discs of the spinal column are composed primarily of fibrocartilage and act as lubricated cushions and shock absorbers preventing adjacent vertebrae from being damaged by abrasive forces or impacts. Held in place by ligaments, each disc has two major components the peripheral annulus fibrosus rich in bundles of type I collagen and the central nucleus pulposus with a gel‐like matrix Fibrocartilage varies in different organs, but is essentially (b) At higher magnification in a small region of a mixture of hyaline cartilage and dense connective intervertebral disc, the axially arranged aggregates of tissue. chondrocytes (C) are seen to be surrounded by small (a) A section of pubic symphysis shows lacunae with amounts of matrix and separated by larger regions with isolated and grouped chondrocytes (C) surrounded by dense collagen (D) and a small number of fibroblasts matrix (M) and separated'in some areas by dense regions with elongated nuclei (arrows). X250. hematoxylin. (D) containing more concentrated acidophilic type I collagen. Fıgure 7‐5 Copyright © McGraw‐Hill Companies