- Slides: 19
Supporting connective tissue Cartilage, Bone and Joints Bone and cartilage, like all other connective tissues, consist of cells and extracellular matrix. It is the ground substance of the matrix which is most responsible for the conspicuous differences between bone and cartilage. The ground substance of bone is mineralized, making the bone rigid and strong, but brittle. The ground substance of cartilage is not mineralized but is more like very firm Jello®, making cartilage stiff and incompressible but more flexible and resilient than bone. In both cartilage and bone, the microscopic appearance of ground substance tends to obscure the presence of collagen. However, as in most other connective tissues (other than blood), collagen is a principal fibrous component which confers tensile strength. In spite of their solidity, both bone and cartilage are capable of growth.
Cartilage § Is a connective tissue contains: Cell, fibers and Ground substance § The ground substance is composed primarily of chondromucoids containing abundant of chondroitin sulfate § does, unlike other connective tissues, not contain vessels or nerves. § is surrounded by a layer of dense connective tissue, the perichondrium. § Cartilage is rather rare in the adult humans, but it is very important during development because of its firmness and its ability to grow rapidly. In developing humans, most of the bones of the skeleton are preceded by a temporary cartilage "model". Cartilage is also formed very early during the repair of bone fractures. § Nourishment by diffusion from capillaries into adjacent C. T.
Types of cartilage Hyaline cartilage Elastic cartilage Fibrocartilage
• HYALINE CARTILAGE has a clear or glass-like appearance when stained with haematoxylin & eosin. The cartilage-forming cells, chondroblasts, begin to secrete the components of the extracellular matrix of cartilage. The extracellular matrix consists of, ground substance (hyaluronan, chondroitin sulfates and keratan sulfate) and tropocollagen, which polymerises extracellularly into fine collagen fibres. Tropocollagen type II is the dominant form in collagen fibres of almost all types of cartilage. § As the amount of matrix increases the chondroblasts become separated from each other and are, from this time on, located isolated in small cavities within the matrix, the lacunae. Concurrently the cells differentiate into mature cartilage cells, chondrocytes.
HYALINE CARTILAGE The matrix appears structureless because the collagen fibres are too fine to be resolved by light microscopy (~20 nm), and because they have about the same refractive index as the ground substance. Collagen accounts for ~ 40% of the dry weight of the matrix. The matrix near the isogenous groups of chondrocytes contains larger amounts and different types of glycosaminoglycans than the matrix further away from the isogenous groups. This part of the matrix is also termed territorial matrix or capsule. § In H&E stained sections , it has a clear or glass – like appearance. The territorial matrix is more basophilic, i. e. it stains darker. The remainder of the matrix is called the interterritorial matrix. Fresh cartilage contains about 75% water which forms a gel with the components of the ground substance. Cartilage is nourished by diffusion of gases and nutrients through this gel. § Locations: "C" rings in the trachea, nose, articular ends of bones, fetal skeleton Function: precursor to bone, support
HYALINE CARTILAGE The hyaline cartilage is more highly magnified. The chondrocytes (A) are located in lacunae (C). The matrix (B) contain collagen fibers that are so fine they are not visible in tissue preparations.
ELASTIC CARTILAGE corresponds histologically to hyaline cartilage, but, in addition, elastic cartilage contains a dense network of delicately branched elastic fibres. § Elastic cartilage is contained within the bracket (up) This tissue is more highly magnified (down). The chondrocytes (A) are contained in lacunae (C). The matrix (B) contains abundant elastic fibers. These fibers give great flexibility to this tissue. § Locations: ear, auditory canal, epiglottis Functions: flexible support
FIBROCARTILAGE (INTERVERTEBRAL DISC) is a form of connective tissue transitional between dense connective tissue and hyaline cartilage. Chondrocytes may lie singly or in pairs, but most often they form short rows between dense bundles of collagen fibres. In contrast to other carstilage types, collagen type I is dominant in fibrous cartilage. Stained with haematoxylin & eosin. § Locations: is typically found in relation to joints (forming intra-articular lips, disks and menisci) and is the main component of the intervertebral disks. merges imperceptibly into the neighbouring tissues, typically tendons or articular hyaline cartilage. , pubic symphysis. § It is difficult to define the perichondrium because of the fibrous appearance of the cartilage and the gradual transition to surrounding tissue types. § Functions: supports, withstands compression
Bone § Bone is organized as flat plates and cylinders, § Bones support and protect body tissues and organs: they also work with muscles to perform movement § The bone matrix is mineralized (calcium) and hardened, but includes little holes or lacunae for the osteocytes (bone producing cells).
BONE CELLS § § § § Osteocytes are small, inactive cells, seemingly isolated from one another in individual lacunae. Inconspicuous cell processes extend out through tiny canaliculi and provide gapjunctional contact among neighboring osteocytes as a means of communication and nutrient supply. Osteoblasts (bone-forming cells) are small cuboidal cells, usually found lying adjacent to one another upon lamellae they have just secreted. – Osteoblasts lay down new bone lamellae. They are active in bone development and also in bone remodelling. – Populations of osteocytes and osteoblasts may be interchangeable, reflecting different stages in the activity of individual cells. That is, active osteoblasts that become enclosed in bone may adopt the resting osteocyte form, while osteocytes which are released from their bony matrix (by osteoclast activity) may become active osteoblasts. Osteoclasts (bone-removing cells) are large cells with multiple nuclei, each one typically sitting alone within a small hollow (Howship's lacuna) that it has just made by "eating" away the adjacent bone matrix. – Osteoclasts remove preexisting bone. They are active in bone development and also in bone remodelling. – Osteoclasts are more closely related to macrophages than to osteocytes or osteoblasts. They form a distinct cell population derived from the same precursor cells as macrophages. NOTE that osteocytes, osteoblasts and osteoclasts can be identified not only by their appearance as cells but also by their position in relation to the adjacent bone lamellae. Osteocytes are isolated within lacunae, separated from other cells by intervening bone matrix. Osteoblasts occur along a surface which parallels the lamellae that they are producing. Osteoclasts occur in small hollows (called Howship's lacunae) which they have eroded into the surface. The edges of these hollows typically cut across the underlying lamellae.
Types of bones A) Compact bone: appears to be very solid and makes up the shafts and long bones and the outer regions of all bones B) Spongy bone (canaliculous): occurs inside bones and includes large, marrow filled spaces
Mature bones: Most mature bones are more-or-less hollow. The outer bone is called cortical or compact bone, while the inner marrow cavity may be criss-crossed by thin strands of bone called trabeculae. § All mature bone is formed in layers called lamellae. Within the lamellae are small spaces, or lacunae, in which osteocytes reside. Neighboring lacunae are interconnected by thin channels, or canaliculi. §
Compact bone In compact bone, the lamellae are organized into sets of concentric rings of inorganic matrix, the lamellae (B). Between the lamellae are bone cells, the osteocytes (C) located in lacunae. Nutrients diffuse from cell to cell through the canaliculi (D). called osteons or Haversian systems. Each osteon includes a central channel, the Haversian canal, which contains a blood vessel. The lamellar systems (osteons) of bone are continually being remodelled Well-formed Haversian systems are best displayed on the slides of "ground bone". Ground bone.
LAMELLAR (MATURE) BONE Stained with thionin and picric acid 1 - Haversian system (osteon) 2 - Haversian canal 3 - interstitial system (older, partially resorbed, Haversian system) 4 - outer cortical lamellae (cortical bone) 5 - periosteum 4 5 4 1 2 3
Structure and vascularization of bone Image from Purves et al. , Life: The Science of Biology, 4 th Edition, by Sinauer Associates (www. sinauer. com) and WH Freeman (www. whfreeman. com). Blood is a connective tissue of cells separated by a liquid (plasma) matrix.
Bone slide preparations § Ground bone: Matrix details (especially canaliculi) § H&E-stained decalcified bone: calcium is removed from the bone before sectioning and staining, in which details of cells are visible. Collagen in the matrix is left intact by the decalcification process, so the bone matrix appears eosinophilic (i. e. , pink in H&E-stained material). Matrix details (especially canaliculi) are less apparent than in ground
WOVEN (IMMATURE) BONE Stained with haematoxylin and eosin 1 - intercellular bone matrix 2 - osteocytes 3 - periosteum 1 3
OSTEOCYTES Stained with haematoxylin & eosin