cartilage.pdf

Histology Of Cartilage Dr.Abdulhussain Sahib  Objectives a. Important features. b. Function, types. c. Sites. d. The special characteristics of each type. e. Growth and regeneration.  Characteristics of cartilage Cartilage is a special form of connective tissue t...

Histology Of Cartilage Dr.Abdulhussain Sahib  Objectives a. Important features. b. Function, types. c. Sites. d. The special characteristics of each type. e. Growth and regeneration.  Characteristics of cartilage Cartilage is a special form of connective tissue that also develops from the mesenchyme. Cartilage consists of : Cells(chondrocytes and chondroblasts ) Extracellular matrix.  In contrast to connective tissue, cartilage is nonvascular (avascular) and receives its nutrition via diffusion through the extracellular matrix. Cartilage exhibits tensile strength, provides firm structural support for soft tissues, allows flexibility without distortion, and is resilient to compression.  There are three main types of cartilage in the body(based on the amount and types of connective tissue fibers that are present in the extracellular matrix): 1. Hyaline cartilage. 2. Elastic cartilage. 3. Fibrocartilage.  Cells(chondrocytes and chondroblasts ) Chondroblasts is embryonic cartilage producing cells. The chondrocytes lie in spaces (or lacunae) present in the matrix. Chondrocytes synthesize and maintain all ECM components At first the cells are small and show the features of metabolically active cells. The nucleus is euchromatic. Mitochondria, endoplasmic reticulum (er) and Golgi complex are prominent.  As the cartilage cells mature they enlarge, often reaching a diameter of 40 μm or more. The nuclei become heterochromatic and organelles become less prominent. The cytoplasm of chondrocytes may also contain glycogen and lipids.  Ground substance The ground substance of cartilage is made up of complex molecules containing proteins and carbohydrates (proteoglycans). The carbohydrates are chemically glycosaminoglycans (GAG) which include: Chondroitin sulphate.  Keratin sulphate. Hyaluronic acid. The core protein is aggrecan. The proteoglycan molecules are tightly bound.,along with the water content, these molecules form a firm gel that gives cartilage its firm consistency.  Another important component of cartilage matrix is the structural multiadhesive glycoprotein chondronectin. Like fibronectin in other connective tissues, chondronectin binds specifically to GAGs, collagen, and integrins, mediating the adherence of chondrocytes to the ECM.  Fibers of cartilage Fibers of cartilags are collagen and elastic fibers according to the type of cartilage. The collagen fibers present in cartilage are (as a rule) chemically distinct from those in most other tissues. They are described as type II collagen. Fibrocartilage and the perichondrium, contain the normal type I collagen.  Perichondrium Except in the articular cartilage of joints, all hyaline cartilage is covered by a layer of dense connective tissue, the perichondrium, which is essential for the growth and maintenance of cartilage. The outer region of the perichondrium consists  largely of collagen type I fibers and fibroblasts,  an inner layer adjoining the cartilage matrix also contains mesenchymal stem cells which provide a source for new chondroblasts that divide and differentiate into chondrocytes.  Hyaline cartilage Hyaline cartilage, the most common of the three types, is homogeneous and semitransparent in the fresh state,and covered by perichondrium except at epiphyses and articular cartilage) The extracellular matrix is homogeneous, with type II collagen and aggrecan Major cells are chondrocytes and chondroblasts. Typical arrangement of chondrocytes ------- Isolated or in small isogenous groups  Towards the center of a mass of hyaline cartilage the chondrocytes are large and are usually present in groups (of two or more). The groups are formed by division of a single parent cell. The cells tend to remain together as the dense matrix prevents their separation. Groups of cartilage cells are called cell-nests (or isogenous cell groups).  Towards the periphery of the cartilage, the cells are small and elongated in a direction parallel to the surface. Just under the perichondrium the cells become indistinguishable from fibroblasts. Immediately around lacunae housing individual chondrocytes, and around cell nests the matrix stains deeper than elsewhere giving the appearance of a capsule. This deep staining matrix is newly formed and is called the territorial matrix or lacunar capsule. In contrast the pale staining matrix separating cell nests is the interstitial matrix. In adults hyaline cartilage is located in: 1.The articular surfaces of movable joints. 2.The walls of larger respiratory passages (nose, larynx, trachea, bronchi). 3.The ventral ends of ribs, where they articulate with the sternum. 4.The epiphyseal plates of long bones, where it makes possible longitudinal bone growth.  In the embryo, hyaline cartilage forms the temporary skeleton that is gradually replaced by bone.  In growing children long bones consist of a bony diaphysis (corresponding to the shaft) and of one or more bony epiphyses (corresponding to bone ends or projections). Each epiphysis is connected to the diaphysis by a plate of hyaline cartilage called the epiphyseal plate. This plate is essential for bone growth.  Main functions 1. Provides smooth, low-friction surfaces in joints. 2. Structural support for respiratory tract  Calcification of hyaline cartilage is often seen in old people. The costal cartilages or the large cartilages of the larynx are commonly affected. In contrast to hyaline cartilage, elastic cartilage and fibrocartilage do not undergo calcification. Although articular cartilage is a variety of hyaline cartilage, it does not undergo calcification or ossification.  Elastic cartilage Elastic cartilage is essentially similar to hyaline cartilage except that it contains an abundant network of elastic fibers in addition to a meshwork of collagen type II fibrils, which give fresh elastic cartilage a yellowish color. With appropriate staining the elastic fibers usually appear as dark bundles distributed unevenly through the matrix. More flexible than hyaline cartilage, and readily recovers its shape after being deformed.  Elastic cartilage is found in : The auricle of the ear. The walls of the external auditory canals. The auditory (Eustachian) tubes. The epiglottis, and the upper respiratory tract. Elastic cartilage in these locations includes a perichondrium similar to that of most hyaline cartilage. Throughout elastic cartilage the cells resemble those of hyaline cartilage both physiologically and structurally.  Fibrocartilage Fibrocartilage is essentially a mingling of hyaline cartilage and dense connective tissue. It is found in : Intervertebral discs) act as lubricated cushions and shock absorbers preventing damage to adjacent vertebrae from abrasive forces or impacts(. In attachments of certain ligaments. In the pubic symphysis. ****all places where it serves as very tough, yet cushioning support tissue for bone.  Chondrocytes of fibrocartilage occur singly and often in aligned isogenous aggregates, producing type II collagen and other ECM components, although the matrix around these chondrocytes is typically sparse. Areas with chondrocytes and hyaline matrix are separated by other regions with fibroblasts and dense bundles of type I collagen which confer extra tensile strength to this tissue.  The relative scarcity of proteoglycans overall makes fibrocartilage matrix more acidophilic than that of hyaline or elastic cartilage. There is no distinct surrounding perichondrium in fibrocartilage. In a small region of intervertebral disc, the axially arranged aggregates of chondrocytes (C) are seen to be surrounded by small amounts of matrix and separated by larger regions with dense collagen and scattered fibroblasts with elongated nuclei (arrows).  Cartilage formation, growth, & repair All cartilage forms from embryonic mesenchyme in the process of chondrogenesis. The first indication of cell differentiation is the rounding up of the mesenchymal cells, which retract their extensions, multiply rapidly, and become more densely packed together. In general the terms “chondroblasts” and “chondrocytes” respectively refer to the cartilage cells during and after the period of rapid proliferation.  At both stages the cells have basophilic cytoplasm rich in RER for collagen synthesis. Production of the ECM encloses the cells in their lacunae and then gradually separates chondroblasts from one another. During embryonic development, the cartilage differentiation takes place primarily from the center outward; therefore the more central cells have the characteristics of chondrocytes, whereas the peripheral cells are typical chondroblasts. The superficial mesenchyme develops as the perichondrium.  Once formed, the cartilage tissue enlarges both by interstitial growth, involving mitotic division of preexisting chondrocytes, and by appositional growth, which involves chondroblast differentiation from progenitor cells in the perichondrium. In both cases, the synthesis of matrix contributes greatly to the growth of the cartilage. Appositional growth of cartilage is more important during postnatal development, Interstitial growth in cartilaginous regions within long bones is important in increasing the length of these structures. In articular cartilage, cells and matrix near the articulating surface are gradually worn away and must be replaced from within, because there is no perichondrium to add cells by appositional growth.  Except in young children, damaged cartilage undergoes slow and often incomplete repair, primarily dependent on cells in the perichondrium which invade the injured area and produce new cartilage. In damaged areas the perichondrium produces a scar of dense connective tissue instead of forming new cartilage. The poor capacity of cartilage for repair or regeneration is due in part to its avascularity and low metabolic rate.