Chapter 8-Bone-combined_e4553772aee626a15cf29ed4ff9b259f.pdf

Histology Bone Chapter 8 Dr. Sundus Shalabi, MD, PhD Faculty of Medicine Arab American University-Palestine [email protected] Outline  Bone Tissue It is a specialized connective tissue composed of calcified ECM (the bone matrix) and...

Histology Bone Chapter 8 Dr. Sundus Shalabi, MD, PhD Faculty of Medicine Arab American University-Palestine [email protected] Outline  Bone Tissue It is a specialized connective tissue composed of calcified ECM (the bone matrix) and the following three major cell types: Osteocytes which are found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi (L. canalis, canal) that extend into the matrix. Osteoblasts growing cells which synthesize and secrete the organic components of the matrix. Osteoclasts which are giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone tissue.  Bone Tissue As nutrients and metabolites are unable to diffuse through calcified matrix of the bone, the exchanges between osteocytes and blood capillaries depend on communication through spaces of the canaliculi. Lined by two layers: 1. Endosteum on the internal surface surrounding the marrow cavity. 2. Periosteum on the external surface. Because of its hardness, bone cannot be sectioned routinely. Bone matrix is usually softened by immersion in a decalcifying solution before paraffin embedding, or embedded in plastic after fixation and sectioned with a specialized microtome. Components of the bone  Bone Tissue Function : 1. Provides solid support for the body. 2. Protects vital organs such as those in the cranial and thoracic cavities, and encloses internal organs. 3. Medullary cavities contains bone marrow where blood cells are formed. 4. Serves as a reservoir of calcium, phosphate, and other ions. 5. Multiply the forces generated during skeletal muscle contraction.  Bone cells Osteoblasts Origin : mesenchymal stem cells. Location : located exclusively at the surfaces of bone matrix. Function : Production of the organic components of bone matrix (type I collagen fibers, proteoglycans, and matricellular glycoproteins such as osteonectin). Deposition of the inorganic components of bone also depends on osteoblast activity Fate : When their synthetic activity is completed, some osteoblasts differentiate as osteocytes entrapped in matrix-bound lacunae, some flatten and cover the matrix surface as bone lining cells, and the majority undergo apoptosis.  Osteoblasts Active osteoblasts are located exclusively at the surfaces of bone matrix. They are bound to the matrix by integrins, typically forming a single layer of cuboidal cells joined by adherent and gap junctions. During the processes of matrix synthesis and calcification, osteoblasts secrete matrix components at the cell surface in contact with existing bone matrix. Thus, producing a layer of unique collagen-rich material called osteoid between the osteoblast layer and the preexisting bone surface.  Bone mineralization Although The process of matrix mineralization is not completely understood. But steps can be described as the following: 1. Osteoblasts secrete osteocalcin, which together with various glycoproteins binds Ca2+ ions. 2. Osteoblasts also release membrane-enclosed matrix vesicles rich in alkaline phosphatase and other enzymes whose activity raises the local concentration of PO 4 ions. 3. This leads to increase concentration of Ca and phosphorus which accumulates in matrix vesicles that act as foci for hydroxyapatite crystals [Ca10(PO4)6(OH)2]. 4. These crystals grow rapidly by further accumulation of mineral and eventually produce a confluent mass of calcified material embedding the collagen fibers and proteoglycans. Bone Mineralization  Osteocytes The most abundant cells in bone Origin: Derived from osteoblasts as they differentiate into osteocytes enclosed singly within the lacunae spaced throughout the mineralized matrix. During this transition, the cells extend many long dendritic processes that pass-through canaliculi. The dendritic processes radiate from each lacunae. Osteocytes also communicate with one another and ultimately with nearby osteoblasts and bone lining cells via gap junctions at the ends of their processes. Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of interstitial fluid in the canaliculi. (a)TEM showing an osteocyte in a lacuna and two dendritic processes in canaliculi (C) surrounded by bony matrix. Many such processes are extended from each cell as osteoid is being secreted; this material then undergoes calcification around the processes, giving rise to canaliculi. (X30,000). Osteocytes exhibit significantly less RER, smaller Golgi complexes, and more condensed nuclear chromatin than osteoblasts (b) Photomicrograph of bone, not decalcified or sectioned, but ground very thin to demonstrate lacunae and canaliculi. The lacunae and canaliculi (C) appear dark and show the communication between these structures through which nutrients derived from blood vessels diffuse and are passed from cell to cell in living bone. (X400; Ground bone) (c) SEM of nondecalcified, sectioned, and acid-etched bone showing lacunae and canaliculi (C). (X400)  Osteocytes Function : 1. Act as mechanosensors detecting the mechanical load on the bone. Due to connections between osteocyte processes and nearly all other bone cells in the extensive lacunar-canalicular network. 2. Sense stress- or fatigue-induced microdamage and trigger remedial activity in osteoblasts and osteoclasts. 3. Maintain the calcified matrix and their death is followed by rapid matrix resorption. 4. Express many different proteins, including factors with paracrine and endocrine effects that help regulate bone remodeling.  Osteoclasts Very large, motile cells with multiple nuclei. Origin : fusion of bone marrow-derived monocytes. Development requires two polypeptides produced by osteoblasts: Macrophage-colony-stimulating factor (M-CSF). The receptor activator of nuclear factor-κB ligand (RANKL). In areas of bone undergoing resorption, osteoclasts on the bone surface lie within enzymatically etched depressions or cavities in the matrix known as resorption lacunae (or Howship lacunae).  Osteoclasts Function: Matrix resorption during bone growth and remodeling. In active osteoclasts, the surface facing bone matrix is folded into irregular projections forming ruffled border surrounded by clear cytoplasmic zone (rich in actin filaments that provide adhesion). In this adhesion zone osteoclasts secrete collagenases & protons (that produce acidic media ) to dissolve bone crystals (bone resorption). This process is usually controlled by parathyroid hormone and calcitonin. (a) Photo of bone showing two osteoclasts (Ocl) digesting and resorbing bone matrix (B) in relatively large resorption cavities (or Howship lacunae) on the matrix surface. An osteocyte (Oc) in its smaller lacuna is also shown. (X400; H&E)  An osteoclast’s circumferential sealing zone where integrins tightly bind the cell to the bone matrix. The sealing zone surrounds a ruffled border of microvilli and other cytoplasmic projections close to this matrix. The sealed space between the cell and the matrix is acidified to ~pH 4.5 by proton pumps in the ruffled part of the cell membrane and receives secreted matrix metalloproteases and other hydrolytic enzymes. Acidification of the sealed space promotes dissolution of hydroxyapatite from bone and stimulates activity of the protein hydrolases, producing localized matrix resorption. The breakdown products of collagen fibers and other polypeptides are endocytosed by the osteoclast and further degraded in lysosomes, while Ca2+ and other ions are released directly and taken up by the blood.  Bone matrix Formed of : Inorganic materials (ex:Calcium hydroxyapatite is most abundant, bicarbonate, citrate, Mg, K, and Na ions). Organic matter (all embedded in calcified matrix) as type I collagen, small proteoglycans and multiadhesive glycoproteins such as osteonectin. Calcium-binding proteins, notably osteocalcin, and the phosphatases released from cells in matrix vesicles promote calcification of the matrix. The association of minerals with collagen fibers during calcification provides the hardness and resistance required for bone function.  Periosteum & Endosteum External and internal surfaces of all bones are covered by connective tissue of the periosteum and endosteum. The periosteum is organized in 2 layers: 1. An outer fibrous layer : Dense connective tissue , containing mostly bundled type I collagen, but also fibroblasts and blood vessels. Binds to the bone with Bundles of periosteal collagen, called perforating (or Sharpey) fibers. 2. An inner layer which is more cellular and includes osteoblasts, bone lining cells, and mesenchymal stem cells referred to as osteoprogenitor cells.  Periosteum & Endosteum The endosteum covers small trabeculae of bony matrix that project into the marrow cavities. The endosteum also contains osteoprogenitor cells, osteoblasts, and bone lining cells, but within a sparse, delicate matrix of collagen fibers.  Bone types By cross section and by gross features there are 2 types of bone: 1. compact (cortical) bone, which is the dense area near the surface it represents 80% of the total bone mass. 2. cancellous (trabecular) bone (the other 20%) lies in deeper areas with numerous interconnecting cavities. Microscopically both compact and cancellous bones typically show two types of organization: 1. Mature lamellar bone, with matrix existing as discrete sheets. 2. Woven bone, newly formed with randomly arranged components.  Bone types Long bones are composed of : Epiphyses composed of cancellous bone covered by a thin layer of compact cortical bone. Diaphysis which is the cylindrical part is almost totally dense compact bone, with a thin region of cancellous bone on the inner surface around the central marrow cavity. Short bones (eg: bones of wrist and ankle) usually have cores of cancellous bone surrounded completely by compact bone. Flat bones that form the calvaria (skullcap) have two layers of compact bone called plates, separated by a thicker layer of cancellous bone called the diploë.  Lamellar Bone Most bones in adults whether compact or cancellous, is organized as lamellar bone. characterized by multiple layers or lamellae of calcified matrix. These lamellae are organized as parallel sheets or concentrically around a central canal. Lamellar Bone Osteon (or Haversian system): Refers to the complex of concentric lamellae, surrounding a central canal that contains small blood vessels, nerves, and endosteum. Between successive lamellae lie the lacunae, each with one osteocyte, all interconnected by the canaliculi containing the cells’ dendritic processes Processes of adjacent cells are in contact via gap junctions, and all cells of an osteon receive nutrients and oxygen from vessels in the central canal. Lamellar Bone Osteon (or Haversian system): The outer boundary of each osteon is a more collagen rich layer called the cement layer. The haversian canals communicate with the marrow cavity the periosteum and one another through transverse perforating canals (or Volkmann canals) that perforate the lamellae. Scattered among the intact osteons are numerous irregularly shaped groups of parallel lamellae called interstitial lamellae (these are osteons partially destroyed by osteoclasts during growth and remodeling of bone). Lamellar Bone Compact bone (eg, in the diaphysis of long bones) also includes parallel lamellae organized as multiple external circumferential lamellae immediately beneath the periosteum and fewer inner circumferential lamellae around the marrow cavity.  Bone remodeling Bone remodeling occurs continuously throughout life. In compact bone, remodeling resorbs parts of old osteons and produces new ones. Osteoclasts remove old bone and form small, tunnel-like cavities, Which are invaded by osteoprogenitor cells from the endosteum or periosteum and sprouting loops of capillaries. Osteoblasts develop, line the wall of the tunnels, and begin to secrete osteoid in a cyclic manner, forming a new osteon with concentric lamellae of bone. In healthy adults, 5%-10% of the bone turns over annually.  Woven bone Nonlamellar bone with random deposition of collagen type I. The first bone tissue to appear in embryonic development and in fracture repair. Usually temporary and is replaced in adults by lamellar bone. Has a lower mineral content (it is more easily penetrated by x-rays) Has a higher proportion of osteocytes than mature lamellar bone. The last 2 characteristics make the immature woven bone forms more quickly but has less strength than lamellar bone. Osteogenesis Intramembranous ossification: osteoblasts differentiate directly from mesenchyme and begin secreting osteoid. Endochondral ossification: a preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts, which then begin osteoid production. In both processes, woven bone is produced first and is soon replaced by stronger lamellar bone. During growth of all bones, areas of woven bone, areas of bone resorption, and areas of lamellar bone all exist contiguous to one another Intramembranous Ossification Most flat bones begin to firm this way. Occurs within condensed sheets (“membranes”) of embryonic mesenchymal tissue. Most bones of the skull and jaws, as well as the scapula and clavicle, are formed embryonically by intramembranous ossification Intramembranous Ossification Within the condensed mesenchyme, bone formation begins in ossification centers Osteoprogenitor cells arise, proliferate, and form incomplete layers of osteoblasts around a network of developing capillaries. Osteoid secreted by the osteoblasts calcifies. Irregular areas of woven bone forms with osteocytes in lacunae and canaliculi. Continued matrix secretion and calcification enlarges these areas leading to the fusion of neighboring ossification centers. Intramembranous Ossification woven bone matrix is replaced by compact bone that encloses a region of cancellous bone with marrow and larger blood vessels. Mesenchymal regions that do not undergo ossification give rise to the endosteum and the periosteum of the new bone. In cranial flat bones: lamellar bone formation predominates over bone resorption at both the internal and external surfaces. Internal and external plates of compact bone arise, while the central portion (diploë) maintains its cancellous nature. The fontanelles or “soft spots” on the heads of newborn infants are areas of the skull in which the membranous tissue is not yet ossified.  Intramembranous Ossification A section of fetal pig mandible developing by intramembranous ossification. Areas of typical mesenchyme (M) and condensed mesenchyme (CM) are adjacent to layers of new osteoblasts (O). Some osteoblasts have secreted matrices of bone (B), the surfaces of which remain covered by osteoblasts. Between these thin regions of new woven bone are areas with small blood vessels (V). (X40; H&E)  Intramembranous Ossification At higher magnification, another section shows these same structures, but also includes the developing periosteum (P) adjacent to the masses of woven bone that will soon merge to form a continuous plate of bone. The larger mesenchyme-filled region at the top is part of the developing marrow cavity. Osteocytes in lacunae can be seen within the bony matrix. (X100; H&E)  Endochondral Ossification Endochondral (Gr. endon, within + chondros, cartilage) ossification. Takes place within hyaline cartilage, shaped as a small version, or model, of the bone to be formed. This type of ossification forms most bones of the body and is especially well studied in developing long bones. Endochondral Ossification Bone collar produced by osteoblasts >>>> Impedes blood supply >>>> chondrocytes undergo hypertrophy> compressing matric>> initiates calcification Endochondral Ossification A small region of a primary ossification center showing key features of endochondral ossification. Compressed remnants of calcified cartilage matrix (C) are basophilic and devoid of chondrocytes. This material becomes enclosed by more lightly stained osteoid and woven bone (B) that contains osteocytes in lacunae. The new bone is produced by active osteoblasts (O) arranged as a layer on the remnants of old cartilage. (X200; Pararosaniline–toluidine blue)  Endochondral Ossification With the primary and secondary ossification centers, two regions of cartilage remain: Articular cartilage within the joints between long bones, which normally persists through adult life. The specially organized epiphyseal cartilage (also called the epiphyseal plate or growth plate), which connects each epiphysis to the diaphysis and allows longitudinal bone growth. Epiphyseal plate The epiphyseal cartilage is responsible for the growth in length of the bone and disappears upon completion of bone development at adulthood. Elimination of these epiphyseal plates (“epiphyseal closure”) occurs at various times with different bones and by about age 20 is complete in all bones, making further growth in bone length no longer possible. In forensics or through x-ray examination of the growing skeleton, it is possible to determine the “bone age” of a young person, by noting which epiphyses have completed closure.  Zones of epiphysial growth plate The zone of reserve (or resting) cartilage is composed of typical hyaline cartilage. In the proliferative zone, the cartilage cells divide repeatedly, enlarge and secrete more type II collagen and proteoglycans, and become organized into columns parallel to the long axis of the bone. The zone of hypertrophy contains swollen, terminally differentiated chondrocytes, which compress the matrix into aligned spicules and stiffen it by secretion of type X collagen. Unique to the hypertrophic chondrocytes in developing (or fractured) bone, type X collagen limits diffusion in the matrix and with growth factors promotes vascularization from the adjacent primary ossification center. In the zone of calcified cartilage, chondrocytes about to undergo apoptosis release matrix vesicles and osteocalcin to begin matrix calcification by the formation of hydroxyapatite crystals. In the zone of ossification, bone tissue first appears. Capillaries and osteoprogenitor cells invade the now vacant chondrocytic lacunae, many of which merge to form the initial marrow cavity. Osteoblasts settle in a layer over the spicules of calcified cartilage matrix and secrete osteoid, which becomes woven bone. This woven bone is then remodeled as lamellar bone. The growth plate (GP) shows its zones of hyaline cartilage with chondrocytes at rest (R), proliferating (P), and hypertrophying (H). As the chondrocytes swell, they release alkaline phosphatase and type X collagen, which initiates hydroxyapatite formation and strengthens the adjacent calcifying spicules (C) of old cartilage matrix. The tunnel-like lacunae in which the chondrocytes have undergone apoptosis are invaded from the diaphysis by capillaries that begin to convert these spaces into marrow (M) cavities. Endosteum with osteoblasts also moves in from the diaphyseal primary ossification center, covering the spicules of calcified cartilage and laying down layers of osteoid to form a matrix of woven bone (B). (X40; H&E) Higher magnification shows more detail of the cells and matrix spicules in the zones undergoing hypertrophy (H) and ossification. Staining properties of the matrix clearly change as it is compressed and begins to calcify (C), and when osteoid and bone (B) are laid down. The large spaces between the ossifying matrix spicules become the marrow cavity (M), in which pooled masses of eosinophilic red blood cells and aggregates of basophilic white blood cell precursors can be distinguished. Still difficult to see at this magnification is the thin endosteum between the calcifying matrices and the marrow. (X100; H&E) Summary of longitudinal bone growth Occurs by cell proliferation in the epiphyseal plate cartilage. At the same time, chondrocytes in the diaphysis side of the plate undergo hypertrophy, their matrix becomes calcified, and the cells die. Osteoblasts lay down a layer of new bone on the calcified cartilage matrix. Because the rates of these two opposing events (proliferation and destruction) are approximately equal, the epiphyseal plate does not change thickness, but is instead displaced away from the center of the diaphysis as the length of the bone increases.  Growth in the circumference of long bones. Does not involve endochondral ossification. Occurs through the activity of osteoblasts Appositional developing from osteoprogenitor cells in the periosteum. bone growth Begins with formation of the bone collar on the cartilaginous diaphysis. The increasing bone circumference is accompanied by enlargement of the central marrow cavity by the activity of osteoclasts in the endosteum.  Bone remodeling and repair Bone growth involves both the continuous resorption of bone tissue formed earlier and the simultaneous laying down of new bone at a rate exceeding that of bone removal. The sum of osteoblast and osteoclast activities in a growing bone constitutes osteogenesis or the process of bone modeling, which maintains each bone’s general shape while increasing its mass. The rate of bone turnover is very active in young children, where it can be 200 times faster than that of adults. In adults, the skeleton is also renewed continuously in a process of bone remodeling that involves the coordinated, localized cellular activities for bone resorption and bone formation. The constant remodeling of bone ensures that, despite its hardness, this tissue remains plastic and capable of adapting its internal structure in the face of changing stresses.  Bone Repair Bone normally has an excellent capacity for repair. Why? Because it contains osteoprogenitor stem cells in the periosteum, endosteum, and marrow and is very well vascularized. Bone repair after a fracture or other damage uses cells, signaling molecules, and processes already active in bone remodeling. Surgically created gaps in bone can be filled with new bone, especially when periosteum is left in place. The major phases that occur typically during bone fracture repair include initial formation of fibrocartilage and its replacement with a temporary callus of woven bone.  Bone fractures – Continued Metabolic Role of Bone- Calcium metabolism The skeleton serves as the calcium reservoir, containing 99% of the body’s total calcium in hydroxyapatite crystals. Calcium ions are required for the activity of many enzymes and many proteins mediating cell adhesion, cytoskeletal movements, exocytosis, membrane permeability, and other cellular functions. The concentration of calcium in the blood (9-10 mg/dL) and tissues is generally quite stable because of a continuous interchange between blood calcium and bone calcium. The principal mechanism for raising blood calcium levels is the mobilization of ions from hydroxyapatite to interstitial fluid, primarily in cancellous bone. Regulation of Calcium mobilization Parathyroid hormone (PTH) from the parathyroid glands raises low blood calcium levels by stimulating osteoclasts and osteocytes to resorb bone matrix and release Ca2+. The PTH effect on osteoclasts is indirect; PTH receptors occur on osteoblasts, which respond by secreting RANKL and other paracrine factors that stimulate osteoclast formation and activity. Calcitonin, produced within the thyroid gland, can reduce elevated blood calcium levels by opposing the effects of PTH in bone. This hormone directly targets osteoclasts to slow matrix resorption and bone turnover Joints Joints are regions where adjacent bones are capped and held together firmly by other connective tissues. The type of joint determines the degree of movement between the bones. Joints classified as synarthroses (Gr. syn, together + arthrosis, articulation) allow very limited or no movement and are subdivided into fibrous and cartilaginous joints, depending on the type of tissue joining the bones  Major subtypes of synarthroses: Synostoses involve bones linked to other bones and allow essentially no movement. In older adults, synostoses unite the skull bones, which in children and young adults are held together by sutures, or thin layers of dense connective tissue with osteogenic cells. Syndesmoses join bones by dense connective tissue only. Examples include the interosseous ligament of the inferior tibiofibular joint and the posterior region of the sacroiliac joints. Symphyses have a thick pad of fibrocartilage between the thin articular cartilage covering the ends of the bones. All symphyses, such as the intervertebral discs and pubic symphysis, occur in the midline of the body. Intervertebral discs Intervertebral discs are large symphyses between the articular surfaces of successive bony vertebral bodies. The annulus fibrosus, the outer portion. ▪ Consists of concentric fibrocartilage laminae in which collagen bundles are arranged orthogonally in adjacent layers. ▪ The multiple lamellae of fibrocartilage produce a disc with unusual toughness able to withstand pressures and torsion generated within the vertebral column. Nucleus pulposus: a gel-like body situated in the center of the annulus fibrosus. ▪ Allows each disc to function as a shock absorber. ▪ Consists of a viscous fluid matrix rich in hyaluronan and type II collagen fibers, but also contains scattered, vacuolated cells derived from the embryonic notochord, the only cells of that structure to persist postnatally. ▪ The nucleus pulposus is large in children, but these structures gradually become smaller with age and are partially replaced by fibrocartilage  Intervertebral disc Section of a rat tail showing an intervertebral disc and the two adjacent vertebrae with bone marrow (BM) cavities. The disc consists of concentric layers of fibrocartilage, comprising the annulus fibrosus (AF), which surrounds the nucleus pulposus (NP). The nucleus pulposus contains scattered residual cells of the embryonic notochord embedded in abundant gel- like matrix. The intervertebral discs function primarily as shock absorbers within the spinal column and allow greater mobility within the spinal column. (X40; PSH) Diarthroses Permit free bone movement. Such as the elbow and the knee. Generally, unite long bones and allow great mobility. Ligaments and a capsule of dense connective tissue maintain proper alignment of the bones. The capsule encloses a sealed joint cavity, containing a clear, viscous liquid called synovial fluid. The joint cavity is lined, not by epithelium, but by a specialized connective tissue called the synovial membrane that extends folds and villi into the joint cavity and produces the lubricant synovial fluid. 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), Diarthroses The synovial membrane may have prominent regions with dense connective tissue or fat. The superficial regions of this tissue however are usually well vascularized, with many porous (fenestrated) capillaries. Synovial membrane have cells typical of connective tissue proper and a changing population of leukocytes. Has two specialized cells with distinctly different functions and origins: Macrophage-like synovial cells. Fibroblastic synovial cells, Diarthroses Macrophage-like synovial cells, also called type A cells,. Are derived from blood monocytes and remove wear-and-tear debris from the synovial fluid. These modified macrophages, which represent approximately 25% of the cells lining the synovium. Regulate inflammatory events within diarthrotic joints. Fibroblastic synovial cells, or type B cells. Produce abundant hyaluronan and smaller amounts of proteoglycans. Much of this material is transported by water from the capillaries into the joint cavity to form the synovial fluid. The synovial fluid: Lubricates the joint. Reduce friction on all internal surfaces. Supplies nutrients and oxygen to the articular cartilage. (a) 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) (b) 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). 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) Synovial Membrane  Articular cartilage The collagen fibers of the hyaline articular cartilage are disposed as arches with their tops near the exposed surface which is not covered by perichondrium. This arrangement of collagen helps distribute more evenly the forces generated by pressure on joints. The resilient articular cartilage efficiently absorbs the intermittent mechanical pressures to which many joints are subjected.  Articular cartilage Articular surfaces of a diarthrosis are made of hyaline cartilage that lacks the usual perichondrium covering (X40; H&E). The End