"Cartilage and Bone" PDF - Medical Lecture Notes

CARTILAGE AND BONE Winston S. Abena, MD., FPASMAP, COSH Saint Paul University Philippines School of Medicine Cartilage & Bone are specialized CTs composed of 3 elements (cells, fibers, & ground substance/matrix) differ from other CTs in the rigidity of thei...

CARTILAGE AND BONE Winston S. Abena, MD., FPASMAP, COSH Saint Paul University Philippines School of Medicine Cartilage & Bone are specialized CTs composed of 3 elements (cells, fibers, & ground substance/matrix) differ from other CTs in the rigidity of their matrices ground substance is composed principally of chondromucoids, which are rich in chondroitin sulfates in cartilage; in bones, with inorganic salts principally calcium phosphate  Cartilage a specialized type of CT; provides physical rigidity & resilience where there is extreme pressure & abrasive forces it is a dense, firm but pliable tissue consisting of cells called chondrocytes, and extracellular fibers embedded in a firm amorphous, gel-like matrix the intercellular components predominate over the cells, which are isolated in small cavities within the matrix called lacunae (cells in these cavities are nourished by diffusion of metabolites through the aqueous phase of the matrix from capillaries in the surrounding CT).  there are no nerves or blood vessels, unlike other CTs colloidal properties of its matrix are important to the nutrition of its cells & are mainly responsible for its firmness & resilience (they retain these properties while rapidly growing) In early fetal life, it temporarily forms most of the skeleton. In postnatal life, it continues to play an important role in the growth in the length of the bones of the extremities throughout childhood.  When adult stature has been attained, all cartilage has been replaced by bone, except: the joint surfaces of long bones, the ventral ends of the ribs, the intervertebral disks of the spine, the tracheal rings, ears, nose & larynx. Collagenous or elastic fibers in the matrix increase the tensile strength & elasticity respectively. Classification: depends on the type & abundance of fibers within the matrix 1. Hyaline cartilage 3. Fibrocartilage 2. Elastic cartilage  HYALINE CARTILAGE appear as elastic, bluish gray, translucent as in fresh condition most common & the most characteristic type most widely distributed type divided into the following sub-types: a) adult hyaline c) fetal hyaline b) articular hyaline A. Adult Hyaline Cartilage found on: ventral ends of the ribs, in the cartilages of the nose, larynx, trachea & bronchi it is also a major component of the epiphyseal cartilage of growing long bones Perichondrium  a tough layer of dense fibrous CT that encloses this cartilage (except over articular surfaces)  these are mesenchymal cells which differentiated to fusiform cells & collagen fibers  the inner layer contains several potential cartilage-forming cells called chondroblasts The Trachea Low mag of hyaline cartilage *Chondroblast – flattened or fusiform CT cells are arranged parallel to the surface of the cartilage - this inner layer is termed “chondrogenetic layer of the perichondrium” (where appositional growth by cell enlargement occurs)  layers of cells are added during proliferation to the periphery of the center of chondrification - cells in the center continue to secrete additional hyaline matrix = moves the cells apart (chondrocytes)  division (interstitial growth) = cells occur in pairs or groups of four or six (this form of growth is transient; will remain quiescent throughout adult life) Perichondrium Chondroblasts Chondrocytes in cell nests  the outer layer – is a condensation of the surrounding areolar tissue; contains several blood vessels  Chondrocytes – derive their nutrition through osmosis & diffusion  the large fluid content of the matrix permits nutrients, dissolved gases & waste products to diffuse readily between the small blood vessels of the perichondrium & the more deeply placed chondrocytes. Cartilage Cells  Chondrocytes – appear irregularly shaped & shrunken away from the walls of their lacunae  are usually ovoid or spherical & each contains a large, spherical centrally placed nucleus with one or more nuclei.  its cytoplasm is finely granular & moderately basophilic, due to the presence of abundant free ribosomes & a relatively well-developed RER; it also contains large mitochondria, vacuoles, fat droplets, a prominent Golgi complex & variable amounts of glycogen TEM of chondrocyte  non-actively growing cartilage: less extensive ER, less prominent Golgi complex  it has an irregular cell surface with short processes that extend into depressions within the matrix = increases surface area & maintains cell nutrition  the product of interstitial growth produces cell nest or isogenous group of cells (group of chondrocytes within a single lacuna)  In the early stages of replacement of cartilage by bone in the process called endochondral ossification, the chondrocytes hypertrophy at the expense of the surrounding matrix.  The matrix is reduced to thin plates or spicules between the enlarged lacunae  this becomes sites of deposition of calcium phosphate  chondrocytes degenerate & their coalesced lacunae are invaded by blood vessels from the perichondrium, accompanied by cells that differentiate into bone-forming cells called osteoblasts.  Near the ends of the cartilage of the long bones (femur/tibia), the proliferating chondrocytes become arranged in longitudinal columns parallel to the long axis of the cartilage & form the cartilagenous epiphyseal plates between the epiphysis & the shaft of a developing bone. Secretion of Matrix Components:  Chondrocyte role: production of collagen & chondromucoprotein  the secretion of collagen precursors follows the same intracellular path as the secretion of protein by a glandular cell  Golgi complex – is the site of synthesis of the complex constituents of the extracellular matrix. Sugars & sulfate may go directly to the Golgi region where they are believed to be incorporated into polysaccharides released into the surrounding matrix.  Ribosomes – amino acids (proline & glycine)  protein + polysaccharides = proteoglycans  packaged into secretory vesicles for discharge Cartilage Matrix (Intercellular Substance)  appear homogenous in fresh condition & after ordinary fixation  it contains fine collagenous fibrils which are masked by a ground substance of similar refractive index; it makes up to 40% of its dry weight; collagen fibrils are mostly slender, 10 – 25 nm in width, lacking the cross banding characteristic of collagen; they are arranged in a feltwork in most of the matrix, but they become parallel with the surface in the subperichondrial region, gradually blending with the perichondrial fibers which are wider & cross banded. TEM stained to show GAGs in cartilage matrix  type II collagen predominates & occurs in cross-striated fibers (15 – 45 mm in dia.)  type IX, X, & XI – have been identified in cartilage in small amounts to stabilize the network of type II collagen fibers  Amorphous ground substance – deeply colored with the Periodic acid-Schiff (PAS) reaction for complex carbohydrates; has a marked affinity for basic dyes & stains metachromatically with toluidine blue  the proteoglycans are present in higher concentration than in any other CTs & they form a much firmer gel; many glycosaminoglycan molecules radiate from a core protein in a bottle-brush configuration  it contains proteoglycan complexes containing chondroitin 4-sulfate & chondroitin 6-sulfate, some hyaluronic acid & keratan sulfate  Chondrocytes also synthesize chondronectin – a large molecule involved in the adherence of the chondrocyte to the collagen fibers of the surrounding matrix  those that surround groups of isogenous cells usually stain more deeply than elsewhere; this basophilic stained rim is termed the capsular or territorial matrix – it often show concentric lines (due to periodic deposition of the substances which is a secretion of the cell itself)  the less basophilic matrix between the cell groups is called the interterritorial matrix  no fibers can be detected by polarizing microscope; can be demonstrated after digestion with trypsin or dilute alkalis; can be seen readily with EM  blood supply: blood vessels, lymphatics & nerves are absent; its large fluid content permits nutrients, dissolved gases, & waste products to diffuse readily between small blood vessels, the perichondrium, & the more deeply placed chondrocytes Degenerative Changes:  the cartilage loses its translucency & becomes less cellular; matrix shows less basophilia (due to loss of proteoglycans & increase in non-collagenous proteins)  Calcification – is the most important retrogressive change within cartilage - it is a normal process during bone formation wherein there is deposition of calcium salts  matrix becomes hard & brittle like a bone. - minute granules of calcium phosphate & calcium carbonate are deposited in the intercellular substance initially in the vicinity of the cells & later in the general matrix - when the intercellular matrix becomes calcified  the cells die (no more diffusion of nutrients)  Asbestos transformation – the cartilage is transformed into an asbestos like consistency; closely packed fibers (not collagenous) may be deposited in the matrix = silky appearance  may lead to softening of the matrix or even to formation of a cavity Regeneration & Transplantation:  Regeneration – is a slow process & occurs primarily by activity of the perichondrium. When cartilage is broken or injured, the wound is invaded by the perichondrial CT which gradually develops into a cartilage; depends on the presence of a perichondrium  Autografts – survive provided that they contain living cells & receive sufficient nutrition (homografts) B. Articular Hyaline Cartilage found between articulating surfaces perichondrium – absent, especially on surfaces that are in contact with the articulating bones chondrocytes – toward the center (bigger); cell families are present matrix – similar to adult hyaline cartilage - it is avascular (partly nourished by synovial fluid & blood vessels found in the surrounding areolar tissue) C. Fetal Hyaline Cartilage found in: entire skeletal system of the developing embryo, except the flat bones of the skull & face in the fetus, nearly all of the skeleton is first laid down as hyaline cartilage tissue in the formation of the bones. it is provided with fetal perichondrium – the outer layer containing plenty of small blood vessels; the chondrogenetic layer is thick with plenty of flattened cells.  cartilage cells – are not arranged in groups, but are scattered singly; they are very small & irregularly arranged considered to be the most cellular sub-type of hyaline cartilage matrix – similar to adult variety; devoid of blood vessels provides a framework for the endochondral or intracartilaginous type of ossification & represents the early stage in development of all types of cartilage  ELASTIC CARTILAGE appears yellow in fresh condition (due to the preponderance of elastic fibers); it is more opaque than hyaline cartilage; more flexible found in: external ear, wall of the external auditory & eustachian canal, & in some small cartilages of the larynx Elastic fibers – in the matrix branch & course in all directions to form a dense network of anastomosing & interlacing fibers - in the peripheral layers, are thin & wide meshed - in deeper portions, are thicker & closely packed Elastic cartilage – darker staining matrix  Matrix – also contain masked collagenous fibrils (subperichondrial region) Chondrocytes – less accumulation of fat & glycogen; fewer cell families - cell content is more or less similar to that of the adult hyaline type - they are housed in lacunae that are scattered singly or in pairs it is surrounded by a perichondrium; and growth occurs both interstitially & by apposition from the perichondrium it has a tendency to undergo fatty degeneration or metamorphosis; the fatty tissue formed differs from ordinary adipose tissue by being avascular; calcification occurs rarely its mechanical properties are dependent on the proteoglycans of its matrix  Elastic cartilage – elastin fibers in the matrix Perichondrium Chondroblasts Chondrocytes High mag of elastic cartilage  FIBROCARTILAGE found in: pubic symphysis, occurs in the intervertebral discs; in certain insertion of ligaments & tendons to bones where a tough support or tensile strength is required the tissue as a whole is acidophilic it can not be considered a modification of hyaline cartilage (unlike elastic); it may be considered a transitional form between cartilage & dense regular CT. it lacks a perichondrium Low mag of fibrocartilage between two pieces of bone  Chondrocytes – lie singly or in pairs, or are sometimes aligned in rows between bundles of type I collagen fibers - most of them are ovoid in shape; lodged in lacunae which are smaller in size & fewer in number than those in the other varieties Matrix – is scanty & quite inconspicuous, except in the immediate vicinity of the cells; it is thin & basophilic territorial type of matrix around the lacunae Blood supply – derived from the blood vessels of the surrounding tissues High mag of fibrocartilage – aligned cells with bundles of collagen  Intervertebral discs – consist largely of fibro-cartilage which is continuous above & below with the articular cartilage of the adjacent vertebrae & peripherally with the spinal ligaments. - in the center of each discs, is a gelatinous ellipsoid mass of variable extent known as Nucleus pulposus (a remnant of the embryonic notochord); it may contain fluid rich in hyaluronic acid & type II collagen fibers, & cellular debris - it is surrounded by a thick ring of fibrocartilage, called the annulus fibrosus, made up of multiple concentric lamellae of collagen fibers - its rupture & herniation of the nucleus pulposus into the spinal canal may be the cause of severe pain & other neurological symptoms. Diagram of intervertebral joints Intervertebral joint N = nucleus pulposa  Histogenesis of Cartilage develop from mesenchyme at the site of future cartilage formation in the embryo, the mesenchymal cells first withdraw their processes & multiply by mitosis, forming dense aggregations called protochondral tissue or center of chondrification  this is composed of closely adjacent spherical cells with nuclei of the cells very close together & in distinct cell boundaries  Chondroblasts – secrete around themselves a hyaline matrix; when newly formed, it stains darker & forms a capsule around the cell - periodic deposition of secreted material gives the capsule the appearance of having concentric lines (increase amount of matrix) - the cells become isolated in separate compartments or lacunae taking on the cytological characteristics of mature cartilage cells or chondrocytes *** they accumulate vacuoles, lipid & glycogen collagenous CT fibers appear within the matrix, masked by the hyaline ground substance in which they are embedded mesenchyme surrounding the enlarging cartilage is compressed & forms a fibrous envelope (perichondrium)  Growth or increase in the size of cartilage occur by 2 methods: 1. Interstitial (Endogenous) growth o expansion of cartilage from within occurs only in young cartilage, when the cartilage matrix is soft & yielding, continuing until the cartilage has reached its definite size & shape o chondrocytes divide by mitosis within their lacunae forming the cell nests or cell families containing 2 – 10 cells in a group; each daughter cell starts to deposit its own capsule until finally the cells of a family become separate rounded cartilage cells lodged in its own lacunae  o increase in the number of chondrocytes by mitosis (hyperplasia) brings about an increase in the size or length of the cartilage 2. Appositional (Exogenous) growth o a process in which new layers of cartilage are added to one surface o the growth takes place at the periphery resulting from activity within the inner chondrogenetic layer of the perichondrium o fibroblasts in the perichondrium multiply by division & some transform into cartilage cells & surround themselves with intercellular substance o growth brings about increase in width of the cartilage through hypertrophy or enlargement of cells  Bone and Ossification Bone/Osseous tissue – is a rigid form of CT constituting most of the skeleton of higher vertebrates - it consists of cells, fibers, & ground substance or matrix - its extracellular components are calcified making it hard & brittle (for its mechanical function of support); contains crystals of hydroxyapatite (Ca10[PO4]6[OH]2) – a complex calcium salt - it is an extremely dense CT distinguished by the calcification of its extracellular matrix - it provides internal support of the body & for attachment of muscles & tendons essential for locomotion - it encloses the blood-forming elements of the BM & protects vital organs of the cranial & thoracic cavities - an important metabolic function as site for the storage of calcium in the blood & other body fluids.  Functions: Bone Maintain body shape Protect internal organs, brain, spinal cord System of levers for the muscle system to act upon Mineral storage – Ca2+ and Phosphorus Blood formation: hematopoiesis Surface features for muscle insertions and origins Macroscopic Structure of Bones: 2 forms of Bones: 1. Cancellous or spongy (substantia spongiosa) 2. Compact (substantia compacta) Cancellous bone – consists of a 3-dimensional lattice of branching bony spicules or trabeculae enclosing a system of intercommunicating spaces that are occupied by BM. Compact bone – appears as a solid continuous mass containing spaces seen only with the aid of a microscope Cancellous Bone  In typical long bones (humerus or femur), the shaft (diaphysis) – consists of a thick-walled hollow cylinder of compact bone with a central medullary cavity containing BM. - the ends of long bones consists mainly of spongy bone covered by a thin, peripheral cortex of compact bone - the epiphysis or ends of long bones are separated from the diaphysis by a cartilagenous epiphyseal plate; columns of spongy bone unite this plate to the diaphysis in a transitional region called metaphysis Bone Anatomy Diaphysis Metaphysis Epiphysis – Proximal/Distal Epiphyseal line Periosteum Compact bone Spongy bone Articular Cartilage Medullary cavity Marrow Nutrient artery  Bones are invested by periosteum – a layer of specialized CT endowed with osteogenic potentialities, to form bones; there are certain areas where a periosteal covering is lacking, such as the ends of long bones that are covered by articular cartilage The marrow cavity of the diaphysis & the cavities of spongy bone are lined by a thin cellular layer that also possesses osteogenic properties called the endosteum In flat bones of the skull, the substantia compacta forms on both surface, thick layers referred to as inner & outer tables. The layer of spongy bone of varying thickness between these tables constitute the diploe.  MICROSCOPIC STRUCTURE OF BONES Structure of Bones in General: Bone matrix – composed of calcified substance made up of organic & inorganic elements deposited in lamellae or layers. - found embedded are collagenous fibers; they occur in the form of cross-striated fibers 500 – 700Ǻ in dia. varying in orientation resulting to alternating refractile & less refractile layers. - the sulfate content of the ground substance is lower than that of the hyaline matrix, rendering the bone matrix acidophilic in staining  - the inorganic elements of calcium, magnesium & sodium constitute the greater portion of the matrix - uniformly spaced cavities called lacunae containing the bone cells or osteocytes are found  these are radiating canaliculi that lodge the cytoplasmic processes of the cells (for passage of metabolites & nutritive materials) Bone cells: (4 types in actively growing bones) a) Osteoprogenitor cells c) Osteocytes b) Osteoblasts d) Osteoclasts Cells of Bone Tissue Osteogenic cells: A. Osteoprogenitor Cells  undifferentiated cells having the capacity for mitosis & for further structural & functional specialization  pale-staining oval or elongated nuclei; an inconspicuous acidophilic or faintly basophilic cytoplasm  found on the free bony surfaces, endosteum, periosteum, lining the haversian canals & at the epiphyseal plate of growing bones  they are active during the normal growth of bones & in adult life, may be activated during healing of fractures or repair of other forms of injury  they undergo division, transforming into osteoblasts (bone-forming cells) or unite giving rise to osteoclasts (bone-destroying cells)  Osteoprogenitor cells From mesenchymal stem cells Resting cell that can transform into osteoblasts Found on inside/outside surfaces of bone, line osteonal canals and blood vessels in bone “bone-lining cells” separate category in text, said to be from osteoblasts, help osteocytes Flattened, elongate nuclei B. Osteoblasts  responsible for formation of bone matrix  found on surfaces of developing bones  they are arranged during the active stage in an epithelioid layer of cuboidal or low columnar cells connected to each other by slender, short processes  nucleus is located at the end of the cell farthest from the bony surface; numerous elongated mitochondria; well developed Golgi apparatus; intensely basophilic cytoplasm (large content of ribonucleoprotein)  have a strong histochemical reaction for alkaline phosphatase; PAS reaction – shows small pink-staining granules in the cytoplasm (precursors of bone matrix)  EM: shows cell structures actively engaged in protein synthesis – extensive ER, ribosomes in the cytoplasmic matrix, well developed Golgi complex, small lipid droplets & membrane-limited dense bodies  synthesize type I collagen, glycoproteins & proteo- glycans of bone matrix, & some of its minor components (osteocalcin, osteonectin, & osteopontin)  they have surface receptors for certain hormones, vitamins & cytokines that influence their activity  with decline of their synthetic ability  become flattened against the bone, & their basophilia & content of ER are reduced  the release of matrix by osteoblasts  become enveloped by their own secretions  transformed into osteocytes  Osteoblasts Synthesize organic components of matrix (collagen, bone matrix proteins) Collagen forms osteoid: strands of spiral fibers that form matrix – not immediately calcified Influence deposit of Ca++, PO4 Active vs. inactive osteoblasts (flat/cuboidal) Estrogen, PTH stimulate activity Osteoblasts Photomicrograph of endochondral ossification. In the upper region is a row of osteoblasts with intense cytoplasmic basophilia, a feature to be expected in cells synthesizing a glycoprotein (collagen). Note an osteoblast being captured in the bone matrix (arrow). Between the layer of osteoblasts and the calcified bone matrix is a pale region made of non-calcified bone matrix called osteoid. C. Osteocytes  principal cells of fully formed bone residing in lacunae within the calcified interstitial substance  cell body is flattened according to the lenticular cavity or lacunae that it occupies; radiating from it are slender canaliculi that are occupied by the slender processes of the osteocytes  EM: processes of neighboring osteocytes are in contact at their ends; form gap junctions or nexuses at their sites of contact; bone cells appear to be in communication with one another & with the cells of low electrical resistance permitting flow of ions & small molecules  LM: nucleus & cytoplasm resemble osteoblasts except that the cytoplasm has less affinity to basic dyes & the Golgi region is less conspicuous  it is essentially an osteoblast that has become surrounded by bone matrix in its development  play an active role in the release of calcium from bone to blood; thus it participates in the homeostatic regulation of its concentration in the fluids of the body.  Osteocytes Mature bone cell enclosed in bone matrix In lacunae, but communicate with other cells Often shrink in preparation Quiescent, formative, resorptive (M: Marrow cavity, Os: osteoid, CB: calcified bone matrix, C: caniculi, L: lamellae) D. Osteoclasts  giant multinucleated cells, about 20 – 100 μm in dia., that are closely associated with areas of bone resorption (active agents in bone resorption); however they are only effective when they are directly adherent to mineralized bone matrix & the matrix is usually covered by a thin layer of unmineralized matrix called osteoid.  found in shallow concavities in the surface of bone called lacunae of Howship  EM: reveal the tendency of the nuclei to congregate near the outer surface which is smooth contoured; the side adjacent to the bone exhibits a radial striation & shown to have an infolded structure called “ruffled border”  **Around the circumference of the ruffled border, the membrane is very closely applied to the underlying bone, & the subjacent cytoplasm is unusually rich in actin filaments = sealing zone **there are multiple Golgi complexes & pairs of centrioles; the cytoplasm near the ruffled border contains many mitochondria, a few profiles of RER, & numerous lysosomes  their nuclei resemble those of osteoblasts; cytoplasm is slightly basophilic when stained with basic dyes  in routine sections, cytoplasm is usually eosinophilic & vacuolated  they secrete H⁺ into the underlying lacuna acidifying its contents to dissolve the bone mineral  they also secrete collagenase & other hydrolytic enzymes to digest the organic components of the matrix Bone: A. Spongy or Cancellous Bone  found in the diploe of the flat bones of the skull & face; in the middle or inner portion of all other bones; thin portion inside the diaphysis of long bone but constitutes a greater part of the epiphysis  simple in structure consisting of irregular, branching bony spicules or trabecula (lamellae) that form a network; these bony spicules are made up of layers of homogenous bone matrix consisting of organic & inorganic elements  its bone cells/osteocytes are embedded in the trabeculae or spicules  cells are small, ovoid in shape with fine cytoplasmic projections; each osteocytes is lodged in a lacuna which possesses minute radiating canaliculi; the cytoplasmic projections of the osteocytes extend into this small canaliculi  prenatal spongy bone – indistinct lamellae (irregular network); this is characteristic of rapid bone development & is referred to as “woven bone”  trabeculae are relatively thin, & are not usually penetrated by blood vessels; there are usually no Haversian systems; bone cells are nourished by diffusion from the surface thru the minute bony canaliculi that interconnect lacunae & extend to the surface  numerous small communicating marrow cavities or intertrabecular spaces vary greatly in size & shape; lined by a cellular membrane called endosteum which is composed of flattened cells that are potential bone & blood-forming cells  cavities are occupied by hemopoietic, myeloid tissue & some fat cells B. Compact Bone  found on the outer surface of all bones  shaft of diaphysis of long bone  spongy bone found within is very thin Periosteum  a dense, fibrous membrane covers all portion of the compact bone except those covered by articular cartilages  the outer layer is relatively dense CT containing blood vessels; inner layer is made up of loose fibroelastic CT containing flattened cells which are potential osteoblasts or bone-forming cells  Haversian canals – communicate with one another through the canaliculi; also communicate with the blood vessels of the periosteum & BM through the Volkmann’s canals; its lamellae contain alternating bright & dark concentric layers that result in the differing orientation of collagen fibers in the successive lamellae  immediately beneath are several lamellae that extend uninterruptedly around the circumference of the shaft = Periosteal/External/Basic Circumferential Lamellae  surrounding the central medullary cavity are several lamellae constituting the Endosteal/Inner Circumferential Lamellae  between the external & internal circumferential lamellae are the Haversian system  all these bony lamellae are composed of the solid, brittle calcerous bone matrix which contains calcium, magnesium, phosphate, & sodium; also inorganic matter such as calcium phosphate  the organic substance of the matrix is composed of collagen, osseo-mucoid, & osseo-albuminoid (found to be reduced in old age = brittle)  matrix is permeated by cross banded collagen fibers Bone cells  small flattened cells with fine cytoplasmic processes lodged within lacunae or cavities are found within & between the different bony lamellae  are placed in open & continuous communication with all the osteocytes, blood vessels of the marrow & the periosteum Blood Supply:  a vascular tissue; nutrition of bone is carried through the continuous communication of the bone canaliculi with the blood vessels located in & outside the bone  the dense bone matrix is devoid of blood vessels  2 categories of vascular channels: a) Haversian canals – the longitudinal channels in the centers of the Haversian systems/osteons containing one or two blood vessels - they are connected with one another & communicate with the free surface & marrow cavity through transverse & oblique channels called Volkmann’s canals b) Volkmann’s canals – are not surrounded by concentrically arranged lamellae but traverse the bone in a direction perpendicular or oblique to the lamellae - often larger than the Haversian canals Endosteum  a thin layer of squamous or flattened cells lining the walls of those cavities in the bone where myeloid tissue is lodged  all cavities of the bone, including the Haversian canals & the marrow spaces within spongy bone are lined by endosteum Structure of Thin Compact Bone:  they are found forming the outer surfaces of all flat bones (tables of the bones of the skull); rarely present are the Haversian systems  flat bones consists of an outer surface of thin layer of compact bone & a thin layer (diploe) of cancellous or spongy bones Distinguishing characteristics of Bone: a) abundant osteocytes or bone cells with cytoplasmic processes b) no prominent fibers; intercellular substance is apparently homogenous c) solid, hard & brittle ground substance d) blood vessels present in canals surrounded by bone matrix Spongy/Cancellous Bone: a) shows a simple & less organized architecture b) composed of anastomosing bony trabeculae or spicules c) shows a meshwork pattern with numerous cavities lined by endosteum & containing BM d) absence of Haversian systems Compact Bone: a) composed of dense & concentrically arranged bony trabeculae or lamellae b) appears more solid with fewer cavities c) shows a regular arrangement of lamellae d) presence of Haversian systems  Histogenesis of Bone (Ossification or Bone Development)  bone develops by replacement of a pre-existing CT  2 different methods: 1) Intramembranous ossification – when bone formation occurs directly in primitive CT 2) Intracartilaginous/Endochondral ossification – when it takes place in pre-existing cartilage, where the bulk of the cartilage must be first removed before bony deposition takes place - more concerned with cartilage resorption than deposition  bone deposition in the 2 methods are the same  first – bone is laid down as a network of trabeculae or primary spongiosa & is later converted to compact bone by filling of the interstices between trabeculae  when bone arises in tissues not belonging to the osseous system or in CTs without osteogenic properties, the condition is pathological & is termed Ectopic bone formation Intramembranous Ossification  flat bones of the skull & face (membrane bones)  bone develops directly from the mesenchyme without the intervention of cartilage formation  the mesenchyme consists of primitive CT cells, connected to one another by their processes, but without cytoplasmic continuity & of semifluid intercellular substance containing delicate collagenous fibers some cells differentiate into osteogenic or osteoprogenitor cells Mesenchymal cells condense into an ossification center they enlarge, assume a polyhedral form & the cytoplasm becomes more basophilic (osteoblasts) an acidophilic ground substance appears, forming thin bars of dense matrix (masking the CT fibers already present within the matrix)  increase in size & surrounds the cells (matrix is not calcified & constitute the organic component of bone matrix) = “osteoid” (resembling bone)/primary spongiosa matrix becomes calcifiable (as a result of osteoblastic activity); minerals are deposited in an orderly fashion as minute crystals in close association with the collagen fibers Cells secrete collagen osteoblasts with their processes are imprisoned by matrix being deposited around them; lacunae & canaliculi are formed. Because processes of adjoining cells make contact with each other, the canaliculi of adjoining lacunae connect with each other new osteoblasts arising by differentiation of osteoprogenitor cells maintain a layer of bone- forming cells at the surface of newly formed bone (mitosis)  bone increases in thickness initially, the bone consists of spicules & trabeculae, & it is spongy & of the woven type (early intramembranous bone) – collagen fibers run in all direction some of this spongy bone is replaced by compact bone as the areas between trabeculae are filled with concentric lamellar bone, creating inner & outer plates between the plates, spongy bone remains (diploe) & the spaces within it, the primary marrow cavities, are filled with richly vascularized CT which gradually becomes transformed into myeloid or hemopoietic tissue Bone formation occurs around blood vessels (V) V V *CT surrounding the growing bone condenses to form the periosteum/endosteum *osteoblasts of periosteum at the periphery also lay down material in successive layers of bony lamellae  forming the outer compact bone *bone destruction (osteoblast) is slower at the periphery of the bone  forming the plate of compact bone (table) *Haversian systems are rare since the bone is very thin = membrane bone *NO cartilage formation Stages of Intramembranous Ossification: 1. Formation of the mesenchymal membrane with enlargement & rounding of mesenchymal cells 2. Vascularization of the mesenchymal tissue 3. Differentiation of some primitive mesenchymal cells into osteoblasts 4. Condensation of matrix into dense acidophilic bars arranged in an anastomosing manner 5. Encasement of osteoblasts as the bar increases in over all size, but is not yet calcified. The tissue is termed osteoid tissue. 6. Formation of bony canaliculi & lacunae: cytoplasmic processes lays the mould for the canaliculi, while the cell body does the same for lacuna 7. Calcification of dense bars by osteoblastic activity. 8. Destruction of osseous tissue by osteoclasts with resulting formation of irregular anastomosing bony spicules & several marrow cavities termed the “diploe” or spongy bone. 9. Periosteal Ossification: Growth continues as layer upon layer of calcified matrix is added; at first, bone appears spongy or cancellous (Diploe); growth of lamellated concentric osteon systems on either side of diploe, gives rise to outer & inner resulting tables or peripheral compact bone. 10. Differentiation into hemopoietic tissue: Primitive mesenchymal tissue filling the spaces between the bony spicules or trabeculae differentiate into the hemopoietic tissue of the red BM. Intracartilaginous/Endochondral Ossification  involves the replacement of a cartilage by bone; observed in a long bone  the cartilage plate is only a provisional tissue as it is gradually eroded & replaced by bone  destruction of cartilage constitutes a pre-requisite to the formation of bone  the cartilage is replaced by bone except at the joint surfaces, but is a slow process not achieved until the bone has reached its full size & growth has ceased  it involves the bones of the entire skeletal system except the membrane bones (flat bones of the skull & face) Stages of Endochondral Ossification: 1. Fetal Hyaline Cartilage formation  arises from a mesenchymal blastema, it assumes more or less the shape of the future bone  inner layer of fetal perichondrium – contains several flattened oval cells that divide actively by mitosis; possess both osteogenic & hemopoietic potentials 2. Appearance of the ossification centers  in long bones, there usually appear 3 centers: one is located in the middle of the shaft (diaphyseal center) & one in the center of each epiphyses or end of the long bone (secondary center of ossification); the diaphy-center appears  ahead & is referred to as the primary center of ossification  establishment of a center of ossification is indicated by the enlargement of the chondrocytes at the middle of the shaft of the hyaline cartilage model; the cells hypertrophy in this region; glycogen accumulates in the cytoplasm & becomes vacuolated 3. Formation of the periosteal band or bone collar  the osteogenic potentials of cells in the perichondrium are activated; a thin layer of bone is deposited around the middle of the shaft  once the bone collar starts to grow, its perichondrium is referred to as fetal periosteum  bone collar – is thin-walled & short at first, but becomes progressively thick-walled & longer as the ossification progresses. It is closely adherent to the cartilage & forms a splint that assists in maintaining the strength of the shaft, which has been weakened by the dissolution of part of the cartilage; the perichondrium which surrounded the cartilage therefore becomes a periosteum 4. Absorption of Cartilage matrix  absorption of cartilage matrix occurs as the chondrocyte enlarge  this process occurs at a faster rate between cells of the same row or column  this is believed to be accomplished by the action of the alkaline phosphatase secreted by the enlarging cartilage cells  as the chondrocytes hypertrophy, there is an enlargement of their lacunae (areolae) while the intervening cartilage matrix is gradually reduced to thin septa & irregularly-shaped spicules  the enlarged cartilage cells begin to show signs of degenerative process (loss of chromatin) & appearance of fat within their cytoplasm  due to continuous absorption of the cartilage matrix along the columns of cells, there is fusion of the widening areolae & the formation of parallel cavities known as the primary of primitive marrow cavities; strips of unabsorbed cartilage matrix remain & are seen between these cavities  In the initial process of ossification, the chondrocytes undergo a regular sequence of changes representing the different stages of cytomorphosis. The different zones seen from the periphery of the process & which somewhat overlap each other, are as follows: a) Quiescent or Zone of Reserve Cartilage  composed of primitive hyaline cartilage present nearest to the ends of the bone; the cells are randomly arranged & the growth of zone is relatively slow b) Zone of Cell proliferation or Multiplication  cells are more or less aligned in rows which course parallel with the long axis of the growing bone  this is the active zone showing numerous mitosis  by division of cartilage cells & their distribution in rows, length of cartilage is increased more than its diameter as new matrix is formed c) Zone of Maturation & Hypertrophy  no further cell multiplication, but they mature & enlarge  cytoplasm contains considerable amounts of glycogen & alkaline phosphatase at this stage/highly vacuolated  the cartilage between adjacent cells within a row becomes even thinner d) Zone of Cartilage Calcification  this is a narrow zone; matrix between adjacent lacunae within a row has disappeared & begin to calcify  the process of calcification of remaining strips of cartilage matrix is necessary before ossification can take place; the calcified cartilage pieces are basophilic in staining appearing bluish-gray in stained specimens  prior to degeneration, the hypertrophied chondrocytes release membrane-enclosed vesicles (matrix vesicles) containing Ca⁺ ions into the surrounding connective matrix; these tiny vesicles & their contained Ca⁺ are thought to serve as sites for the calcification process e) Zone of Cartilage Removal & Bone Deposition (Zone of Provisional Ossification)/Zone of Degeneration  thin partitions between the lacunae within a row undergo dissolution (outer part of this zone)  this is accompanied by death of the cartilage cells & is associated by an erosive action that is associated with the blood vessels that grow in from the marrow cavity  longitudinal canals, filled with vessels & myeloid tissue are thus formed in tunnels surrounded by calcified cartilage matrix  there are numerous chondroclasts in this area  toward the marrow cavity, the calcified cartilage matrix between the newly formed tunnels becomes reduced in amount by resorption, but remnants of & their bony coverings are subsequently resorbed as the marrow cavity enlarges 5. Invasion of Periosteal buds of mesenchymal tissue & small blood vessels  from the inner layer of the fetal periosteum, periosteal or osteogenic buds grow inward towards the primary marrow cavities; these buds of fibrous tissue contain developing blood vessels, osteogenic & hemopoietic elements  they invade the calcified cartilage matrix, finding their way into the primitive marrow cavities  some cells of the osteogenic tissue differentiate into osteoblasts, forming a lining membrane or endosteum for the marrow cavities  the rest of the osteogenic tissue in the cavities constitute the fetal BM or myeloid tissue 6. Primary Bone Deposition  bony or osseous tissue deposited upon the strips of remaining calcified cartilage matrix by the osteoblasts that are brought in by the periosteal buds  results in the formation of primary bone that appears in the form of irregular spicules or trabeculae between primary marrow cavities  primary bone – appears acidophilic or pink because of its low content of chondroitin sulfate; some of the osteoblasts that get caught within the bony matrix & become fixed in their lacunae become the osteocytes; the rest of the osteoblasts continue laying down bony tissue resulting in the formation of wider strips of provisional or primary bone  the osteoblasts lay down bony material only on one side of the cells along their attached sides where they form a continuous layer resembling cuboidal epithelium  the over-all region of newly formed bone is known as the spongiosa on the basis of its spongy appearance; this is subdivided into: a) primary spongiosa (zone of provisional ossification) – a relatively short region beginning just below the level where the hypertrophic cartilage cells disappear & characterized by marrow spaces in fairly uniform width b) secondary spongiosa – a relatively long region with wide & irregularly contoured marrow spaces extending toward the center of the shaft (diaphysis) 7. Primary Bone Destruction  as soon as primary bone appears, the osteoclasts, which are also derived from the periosteal or osteogenic buds, start to function  they first appear in concavities called lacunae of Howship along the surface of the bony spicules or primary bone  due to the destructive activity of the osteoclasts, absorption of the primary bone continues; since bone destruction seems to be faster than bone formation in shafts of long bones, finally one large central medullary cavity is formed. This is also known as the secondary marrow cavity, which becomes lined by a flattened cellular membrane (endosteum)  it contains the osteogenic & hemopoietic elements constituting the primitive BM  proteolytic enzyme activity of the osteoclast rather than phagocytic activity accounts for absorption of the bone matrix 8. Periosteal Ossification  deposits of parallel & concentric bony tissue underneath the periosteum by the osteoblasts take place; this process is called periosteal ossification that results in a widening of the original periosteal bone band or collar  when layers of periosteal bone are deposited successively around the central medullary cavity, the osteoclasts again start to function as bone destroyers  within the compact bone matrix, there are formed longitudinal cavities that are parallel to the central medullary cavity; these are also invaded by the periosteal bud containing osteoblasts that immediately line these cavities; they start laying down concentric bony lamellae until the lumen is reduced to a small size  these cavities, containing blood vessels & a small amount of BM become the Haversian canals lined by endosteum; the concentric bony lamellae around them constitute the Haversian lamellae = Haversian systems (are not permanent structures as they are continuously destroyed by osteoclasts)  the areas of bony matrix between these Haversian systems constitute the Interstitial/ground lamellae  the bigger canals having a horizontal direction are formed by the passage inwards of the periosteal buds; they are not surrounded by concentric bony lamellae & they become the Volkmann’s canals  bone acquires growth in width 9. Periosteal & Endosteal lamellae formation  the inner layer of the periosteum begins to deposit successive uninterrupted bony lamellae constituting the periosteal or external circumferential lamellae  upon the wall of the central medullary cavity, the endosteum also lays down successive uninterrupted bony lamellae which becomes the endosteal or internal circumferential lamellae LM of a cancellous bone spicule  Diagram of compact bone (Haversian Systems) Compact Bone Haversian system (osteon) with central canal and osteocytes in lacunae connected by canaliculi  through the activity of the periosteum in periosteal ossification, increase in width of the bone takes place; increase in length is acquired through the extension of the endochondral ossification within the plate of fetal hyaline cartilage; from the center, it moves gradually towards the epiphysis, the epiphyseal line is pushed farther resulting in a lengthening of the bone  growth of bone occurs through both constructive & destructive processes Epiphyseal Ossification  occurs later than that of the shaft  at about the time of birth, secondary centers of ossification (epiphyseal centers) appear in both ends of most long bones; the sequence of changes in the cartilage of these centers is identical with that observed in the diaphysis  cartilage cells proliferate & hypertrophy; vascular osteogenic buds enter from the periphery; cartilage removal & bone deposition follow  ossification spreads peripherally in all directions until there is cartilage replacement by bone, except in 2 regions: cartilage remains over the fine end as articular cartilage, & as a plate between the diaphysis & epiphysis = epiphyseal plate or disc Low mag of an epiphyseal growth plate (square) of a long bone Long bone growth plate Proliferating Zone Hypertrophic Zone Calcifying Zone Epiphyseal growth plate cartilage  at the end of the growing period, proliferation of the cartilage cells slows & finally ceases; replacement of cartilage by bone continues resulting in obliteration of the epiphyseal plate; the closure of the epiphyses occurs at the age of 18 – 20 years  bone destruction here is not so marked; hence there is no formation of one single marrow cavity but rather thin & narrow anastomosing bone trabeculae with several small marrow cavities of the spongy or cancellous bone  inner layer of periosteum lays down a thin plate of compact bone around this spongy bone; this is made up of a few layers of successive & continuous concentric bony lamellae  growth in length of the femur takes place mainly at the distal epiphyses  growth of the tibia takes place mainly at the proximal end  Diagram of endochondral ossification  HISTOPHYSIOLOGY OF BONE  vitamins & hormones – play an important role in ossification & maintenance of bone *vitamin D deficiency – faulty absorption of Calcium from foods & a diminished Phosphate concentration in plasma; this results to rickets (children) - the epiphyseal discs become thicker & irregular, & the cartilage matrix & osteoid tissue fail to calcify - causes a diminution in Calcium content of the bone, a condition known as osteomalacia (adults) *vitamin C deficiency – results in a condition known as scurvy - characterized by an inability of tissues that originate from mesenchyma to produce & maintain fibers, & ground substance diminished - this causes destruction of osteocollagenous fibers & a production of organic matrix in bones - deficiency may lead to growth retardation & delayed healing of fractures *vitamin A deficiency – there is a diminution in the rate of skeletal growth & interference with the process of remodeling together with the balance between bone deposition & erosion  Hormones – greatly influences the growth & maintenance of bones *Growth hormone (anterior pituitary) – is essential for normal bone growth; lack (Dwarfism), excessive production (Gigantism) *Parathyroid hormone – regulates bone resorption controlling the release of calcium to the blood; its action is in direct opposition to that of thyrocalcitonin, which inhibits its resorptive activity - also acts upon the kidney to increase the rate of resorption of Calcium from the glomerular filtrate; its renal effect prevents a continual loss of calcium in the urine that would ultimately deplete the calcium stores in the bones - therefore, there is a balance between release & deposition of Calcium to maintain the level of calcium in the blood *Sex hormones (Gonads) – affects the appearance & closure of the secondary or epiphyseal ossification centers - skeletal maturation is accelerated & growth stunted (Precocious puberty); decline in hormone secretion disturbs the balance between bone deposition & bone resorption resulting in fragile bones (Aging) Bone Repair  fracture  hemorrhage (from torn vessels) & clotting  fibroblasts & capillaries invades the clot-forming granulation tissue (procallus)  transforms to dense fibrous tissue & later into cartilage (temporary callus), unites the fractured bones  osteoblasts – develop from the periosteum & endosteum, laying down spongy bone that replaces progressively the cartilage of the temporary callus similar to endochondral ossification = bony union  the bony callus, initially spongy, undergoes reorganization into compact bone & excess bone is resorbed Joints & Synovial Membranes  Joints/articulations – sites where 2 or more components of the skeleton (bone/cartilage) meet; CT structures that permit varying degrees of movement between the adjoining bones  Joint capsules – consists of 2 distinct layers: a) Inner synovial layer – cellular & secretes a colorless, viscid fluid of the joint cavity (synovial fluid – transudate of blood plasma to which the synovial cells add hyaluronate & lubricin, which contribute to its viscoelastic & lubricating properties); it is thrown into marked folds, frequently containing blood vessels & which project into the joint cavity; with age, these folds or villi increase in size & number - there are 2 cell types present: 1) type A synovial cells – situated at or near the free surface & have surface filapodia & numerous micropinocytotic vesicles; their cytoplasm contain numerous mitochondria & a prominent Golgi complex, but little ER; site of synthesis of hyaluronate & lubricin 2) type B synovial cells – more deeply situated & resemble fibroblasts; they have abundant tubular & cisternal ER - glycogen – is present in both synovial cell types b) External fibrous layer – made up of dense CT consisting of collagenous fibers & fibroblasts, macrophages, blood vessels, & many lymphatics Diagram of a synovial joint Synovial joint – C = articular cartilage; Cp = joint capsule; S = synovial membrane; E = ligament Synovial joint – AC articular cartilage (AC)  Osteoarthritis – wear and tear damage to the articular cartilage  Rheumatoid Arthritis – swelling of the synovial membrane (left) & destruction of articular cartilage (right) Bone Marrow  a reticular type of CT containing abundant blood vessels & different varieties of CT cells, especially the blood cells in their various developmental stages occupying the marrow cavities of bone  Classification: A. Red Bone Marrow (myeloid tissue) - this is the active hemopoietic tissue found in all the marrow cavities during fetal & early infantile life - as the individual matures, it becomes confined only to the marrow cavities of the spongy bones (ribs, sternum, vertebral bodies, epiphysis, & diploe) - it is composed of fibro-reticular CT abundantly supplied by sinusoidal blood vessels lined by a reticuloendothelium; scattered free within the reticular mesh are the blood-forming elements representing all the developmental stages of the different blood cells - tissue appears reddish in color due to the presence of these abundant blood cells & very little fat - marrow of practically all bones appear cellular & reddish during the first few years of life; fat cells begin to appear between the blood cells between the ages of 5 & 7 years (results to change from red to yellow variety); at 18 years old, the red BM is confined only in the regions of the vertebrae, sternum, ribs, bones of the skull, & in the epiphysis of the humerus & femur B. Yellow Bone Marrow - found in the central medullary cavity of the diaphysis of long bones in the adults - it is composed of a fibro-reticular CT with blood vessels, nerves & abundant fat cells crowding out the hemopoietic elements (imparts a yellowish color to the tissue) - does not play an active role in hemopoiesis