Lesson 8 - Cartilage Notes PDF

_____________LESSON 8 _____________ CARTILAGE I. INTRODUCTION The cartilage is a specialized form or subtype of connective tissue consisting of cells, chondrocytes, and fibers embedded in a ground substance gel-like. Together with bone, it forms the group of so-called supporting connective tissues....

_____________LESSON 8 _____________ CARTILAGE I. INTRODUCTION The cartilage is a specialized form or subtype of connective tissue consisting of cells, chondrocytes, and fibers embedded in a ground substance gel-like. Together with bone, it forms the group of so-called supporting connective tissues. Cartilage is avascular and also lacks nerves and lymphatic vessels. Therefore, it is nourished by diffusion, thus limiting its thickness to the possibilities of physical diffusion. In addition, it has great tensile strength since the extracellular substance is made up of collagen and/or elastic fibers. As cartilage can grow rapidly, but maintaining the degree of firmness, it is the ideal tissue to form the skeleton of the embryo. Most of the skeleton is formed on cartilaginous casts that are later replaced by bone. In postnatal life, cartilage is less abundant, but it is essential in the growth of the long bones of the young animal and persists in the adult on the articular surfaces of the long bones. II. TYPES According to the amount of extracellular substance and abundance and type of fibers (collagen and/or elastic) there are three types of cartilage: - Hyaline cartilage, the most abundant and characteristic (we will study it as a model), contains type II collagen fibers in its extracellular matrix. - Elastic cartilage, contains type II collagen and abundant elastic fibers in its extracellular matrix. - Fibrocartilage, contains type I collagen in its extracellular matrix. II. 1. HYALINE CARTILAGE (Figures 1, 2, 3, 4 and 5) The hyaline cartilage is found in the adult in the articular surfaces of bones, in the trachea and the bronchi, ribs and bones of the nose and is the skeleton in the embryo. In the young animal it is found in the epiphyseal plate of the growing bones. Macroscopically is semi-transparent and bluish white. 1 1. Histogenesis of cartilage It originates from the mesenchymal connective tissue. The first indication of cartilage formation in the embryo is the appearance of clusters of undifferentiated and stellate mesenchymal cells. These cells increase in size, lose their processes and synthesize extracellular substance and type II procollagen. Are called chondroblasts and the chondroblasts accumulations are called centers chondrification. The superficial mesenchyme will form the perichondrium. Chondroblasts have lots of rough endoplasmic reticulum cisterns and a very evident Golgi complex. As the extracellular matrix increases, cells become spherical and are isolated from each other in compartments called lacunae. At this time, they are called chondrocytes. Cartilage growth takes place by two different mechanisms. The chondrocytes in their lacunae undergo several mitotic divisions, and after each division they secrete new ground substance and collagen fibers that separate the two daughter cells. This process leads to cartilage expansion from inside and is known as interstitial growth (Figure 1A). Perichondrium B A Perichondrium Figure 1. Interstitial (A) and appositional (B) growth of cartilage tissue. At the same time, the mesenchyme surrounding the cartilaginous outline differentiates into the perichondrium (Figure 1) which has 2 layers. The one adjacent to the cartilage is composed of chondroblasts and is called the cellular or chondrogenic layer. These cells synthesize some components of the ground substance and collagen fibers, becoming chondrocytes. This form of cartilage growth is called appositional growth (Figure 1B). The ability of the chondrogenic layer to produce cartilage persists into adult life but is latent until the need for new cartilage is created. The outermost layer or fibrous layer of the perichondrium is made up of unordered type I collagen fibers and fibroblasts. 2 2. Microscopic structure The hyaline cartilage is externally surrounded by the perichondrium (whose morphological characteristics we have already seen in the previous section). B A Figure 2. Chondroblast (A) and chondrocyte (B) diagrams. The chondrocytes of mature hyaline cartilage vary in size. As a consequence of appositional growth, those below the perichondrium are small and their lacunae are elliptical with their longest shafts are parallel to the surface (Figure 1). Inside the cartilage, where interstitial growth occurs, the cells are larger and acquire a polyhedral shape (Figure 1). These cells are found in lacunae called chondroplasts that contain 1, 2, 4 or 6 chondrocytes. These cells are called nests or isogenous groups of Renaut and are the progeny of a single chondrocyte which has undergone several mitotic divisions during the interstitial growth. These groups are surrounded by basophilic chondroprotein and some collagen fibers that form the capsule (Figure 3). Both structures, the isogenous group and the capsule, give rise to the structural unit of cartilage, the chondron (Figure 3). Capsule: ground substance and collagen fibers Chondrocyte Chondrocyte in the chondroplast Capsule = territorial matrix ChondronChondrocyte and chondroplast plus capsule Lacuna = Chondroplast Figure 3. Schematic representation of the chondrocyte, the chondroplast and the chondron. Chondrocytes have a spherical nucleus with one or more nucleoli and in the live cartilage the cells fill lacuna. In the histological sections, the cell surface is observed separated from the wall of the lacunae, the retraction suffered by these cells is due to the fixation and processing of the tissue (Figure 1). Active chondrocytes have an abundant rough endoplasmic reticulum and an evident Golgi complex. Old chondrocytes accumulate glycogen and lipids and in the slides the cells appear vacuolized (Figure 2). 3 The main constituents of the extracellular matrix of hyaline cartilage, called the cartilaginous matrix, are type II collagen and proteoglycans. The collagen fibrils are very thin, measuring 10-20 nm and are arranged in three-dimensional networks. In addition, different patterns of collagen distribution have been observed in the cartilaginous matrix, suggesting that the orientation of the fibers is adapted to resist stress. There are also other types of collagen in small amounts, such as IX, X and XI, although the exact role they play is unknown. Proteoglycans consist of a central axis of protein and cross connections of glycosaminoglycans. The most important of the latter are chondroitin sulfate and keratan sulfate. Long molecules of hyaluronic acid are attached to one end of the axial protein. Proteoglycan complexes occupy the interstices of the collagen fibril network. The collagen frame maintains tissue shape and resists tensile forces, while proteoglycan aggregates provide a hydrated viscous gel that absorbs compressive forces. The cartilaginous matrix is intensely basophilic, PAS+ and metachromatic. Surrounding each lacuna is the territorial matrix, which is more basophilic than the rest of the cartilaginous matrix and measures about 50 µm. It appears that the chondrocyte and the territorial matrix work together in the polymerization of collagen and proteoglycans. The interterritorial matrix is clearer due to the greater abundance of collagen fibers (interterritorial fibers). In addition, there is a capsule of collagen fibers, 1 to 3 µm thick, that surrounds the lacunae in which the chondrocytes are found and is called pericellular capsule. It seems that this capsule has a protective function for the chondrocytes, which are exposed, like the rest of the cartilage, to tension and compression movements. The nutrition of the cartilage is carried out by diffusion through the capillaries of the perichondrium. I I.2. ELASTIC CARTILAGE (Figure 4) It is found in areas where elasticity and at the same time a certain rigidity is needed, such as the ear, the inner ear and the epiglottic, cuneiform and corniculate cartilages of the larynx. In addition to all the components of cartilage, it has a dense network of elastic fibers. It presents more and larger chondrocytes and less abundant matrix than hyaline cartilage. II.3. FIBROCARTILAGE (Figure 4) It is less frequent and is found mainly in the intervertebral discs. In addition, the muscles that make up the atria and ventricles of the dog heart are linked by fibrocartilage. The most important thing about this type of cartilage is the presence of type I collagen mixed with areas of hyaline cartilage. It lacks a perichondrium. The chondrocytes are arranged in rows and the matrix is 4 acidophilic due to the predominance of collagen fibers. It presents great resistance to traction and compression forces. B A Figure 4. Hyaline cartilage (A), elastic cartilage (B) and fibrocartilage (C). 5 C