Chapter 5. Connective tissue Connective tissues form the allow for a tight coiling of the molecule. structural and architectural framework of Collagen molecules have a high the body. In a connective tissue the cells concentration of hydroxylated proline are widely scattered and separated by and lysine, which is unique to collagen. an extracellular matrix. Connective Individual collagen molecules join tissues are composed of three major together to form collagen fibrils, which components: cells, fibers, and ground has an increase in strength similar to a substance. Specific connective tissues braided rope. Multiple fibrils will will be classified based on the types and sometimes aggregate in thicker bundles concentrations of these basic to form collagen fibers . The high components. The extracellular matrix is concentration of proteins in collagen composed of ground substance and fibers stain pink in H&E stained fibers. The ground substance includes sections. Collagen fibers of the papillary proteoglycans and glycosaminoglycans layer of the dermis are thin and dissolved in tissue fluid. These hydrated organized in a loose manner. Collagen molecules help to give the matrix a gel- fibers of the reticular layer of the dermis like consistency. The matrix viscosity are thicker and have a denser, more and rigidity is determined by the amount irregular appearance. When using a and type of glycosaminoglycans. trichrome stain, collagen fibers will Glycosaminoglycans associate with core appear bluish-green. proteins to make proteoglycans. Figure 5-1. Ground substance. Fibers: Figure 5-2. Collagen Collagen fibers have great tensile strength and are highly resistant to stretching, weight for weight stronger than steel. Collagen is the most abundant protein in the body. Collagen fibers are formed by the interaction of individual collagen molecules (3 polypeptide chains wrapped in a triple- helix) in a staggered manner to form a structure similar to a braided rope. Collagen is composed of a high- concentration of amino acid glycine to TABLE 5.1: Types of Collagen Type I most abundant and widespread large fibers and fiber bundles Type II thin fibrils cartilage matrix Type III like I, but more glycosylated thin fibrils surround and support soft tissues. Type IV doesn't form fibers or fibrils. in basal lamina Type V placental basement membranes blood vessels. Elastic fibers are very flexible and In H&E preparations, elastic fibers stretchable, able to stretch to 150% of cannot be distinguished. In aldehyde- their length and return without breaking. fuchsin stained tissues, elastic fibers will Elastic fibers are found in connective appear purple. tissues where these properties are required to allow organs to expand and return to their normal shape. Elastic fibers are convey the ability of a connective tissue such as a mesentery to stretch in many directions. This is illustrated by some fibers being stretched (appearing straight) will fibers in a different orientation are relaxed (appearing almost coiled). Figure 5-4. Appearance of reticular fibers. Reticular fibers are similar to collagen fibers but thinner and more glycosylated (mainly type III collagen and glycoproteins). Reticular fibers form a fine network to support tissues of soft organs, such as lymphoid organs, hematopoeitic organs, and the liver. To visualize reticular fibers it is necessary Figure 5-3. Elastic fibers in a loose connective to stain with silver salts so that the tissue. reticular fibers appear black. Reticular fibers form a delicate scaffold which Elastic fibers may also be found in often allows motile cells to move about sheets in regions of the body that need to the tissue, such as within lymph nodes. stretch extensively such as in the walls of muscular and elastic arteries. Further Fixed cells: examples of connective tissues with Fibroblasts are the major class elastic fibers include interalveolar septa, of fixed or resident connective tissue bronchi and bronchioles of the lungs, cells. Fibroblasts remain in the and vocal ligaments. Elastic fibers are connective tissue where they synthesize, much thinner than collagen fibers. secrete, and maintain the connective Elastic fibers will appear a blue-black or tissue. These cells have elongated nuclei reddish-purple with an elastic fiber stain. and their cytoplasm often cannot be observed because the cell is normally identify a macrophage, it is often pressed against the collagen and are the necessary to see some phagocytosed same color as the collagen fibers. material within the cell. Lymphocytes are circulating white blood cells found throughout many connective tissues, especially in regions likely to see foreign antigens Figure 5-5. Fibroblast. such as the connective tissue (lamina propria) of the small intestine. Wandering cells: Lymphocytes are cells with small dark Wandering or immigrant cells are (very heterochromatic) nuclei with very cells which come form the blood system. little cytoplasm. A region of lymphocyte Some cells, such as lymphocytes, move infiltration will appear very dense and through the connective tissue and return basophilic. to the circulatory system. other cells, such as macrophages, differentiate and take up permanent residence in the connective tissue. Figure 5-6. Macrophage. Macrophages are a phagocytic connective tissue cell. These cells are Figure 5-7. Lymphocytes. derived from monocytes, which leave the blood stream and differentiate into Some lymphocytes (B lymphocytes) will macrophages within the connective differentiate into plasma cells, which are tissue. They are lighter staining cells actively synthesizing and secreting with an irregular shaped or bumpy antibodies in response to a foreign euchromatic nucleus, and abundant antigen. Plasma cells are more numerous lysosomes. Macrophages are long- during immune responses. These cells lived (weeks to years) and are actively have an eccentric nucleus (pushed over involved in the phagocytosis of bacteria, to one side). The nucleus has a clumped tumor cells, foreign antigens, and the heterochromatin pattern called either remodeling of tissues. To conclusively cartwheel or clock-face. The cytoplasm is normally slightly basophilic with a Neutrophils in the connective tissue are clear zone of Golgi complex near the the bodies first line of defense against nucleus. bacteria. Mast cells regulate the permeability of capillaries. These round or oval cells have an oval nucleus, considerable cytoplasm packed with numerous basophilic granules, and are often observed near capillaries. ,These cells release heparin and histamine from the basophilic granules in response to an allergen or antigen binding to specific Figure 5-9. Neutrophil. IgE antibodies along the surface of the mast cell. Types of connective tissues: Connective tissue proper A loose or areolar connective tissue , such as a mesentery, is composed of relatively few collagen and elastic fibers, abundant ground substance and a limited number of cells. The cells are mainly fibroblasts. A loose cellular connective Figure 5-8. Mast cell. tissue has the same basic pattern of collagen fibers, elastic fibers, and Eosinophils, basophils, and fibroblasts; but also has a greater neutrophils are circulating granulocytes population of migrating cells such as (a class of white blood cells), which are lymphocytes which give the connective found in the connective tissue as well as tissue a more cellular appearance. A within the circulatory system. loose cellular connective tissue is normally found as the lamina propria Eosinophils have a bi-lobed under the epithelial lining of the nucleus and many eosinophilic digestive tract. cytoplasmic granules. The increased presence of eosinophils usually indicates In a dense irregular connective an allergic reaction in response to tissue , the fiber components are much parasitic infections. larger but appear in a somewhat random or woven manner. There is little ground Basophils also have a lobed substance and few cells. A good nucleus, but are filled with basophilic example of a dense irregular connective cytoplasmic granules. Basophils are tissue is the reticular layer of the dermis. most commonly found at sites of The irregular pattern formed by the thick inflammation. collagen bundles allow for strength in many directions. Therefore a dense Neutrophils normally have a irregular connective tissue is also found lobed nucleus and cytoplasmic granules. lining the capsules of many organs such as the kidney, spleen, and liver. In a dense regular connective tissue , the fibers are packed together and oriented in one direction. The fibers do not have to be perfectly straight and parallel to one another. In a relaxed tendon, the dense regular connective tissue may have a slightly wavy appearance, with all the collagen fibers running in the same general direction. A dense regular connective tissue will be Figure 5-10. White Adipocytes. found in tendons, ligaments, and the cornea. Brown adipose tissue (brown fat) is a multilocular fat, having A reticular connective tissue is multiple smaller lipid droplets in the cell composed of primarily reticular fibers, rather than a single lipid droplet as in which form a supportive network for soft white fat and a spherical nucleus. Brown organs. A reticular connective tissue has adipocytes have many mitochondria, and an anastomosing appearance with more it is the mitochondrial cytochromes open spaces than other types of which give the cells a reddish-brown connective tissues. A reticular color. Regions of brown adipocytes connective tissue is found in often have a glandular appearance. hematopoeitic organs, lymphoid organs, Brown fat is involved in heat production and the liver. and is important in thermoregulation in infants. Adipose tissue: White adipose tissue (white fat) is a specialized connective tissue for the storage of lipids. White adipose tissue appears as a vacuolated, pale region resembling chicken wire. Individual fat cells (adipocytes) have a non-staining central region as the single large lipid droplet has been removed in tissue processing. The nucleus is pushed to the edge of the cell and a pale ring of cytoplasm surround the site of the lipid droplet giving the cell a signet ring appearance. White fat is important for energy storage, shock absorption, thermal insulation, and body contouring. Figure 5-11. Brown Adipocytes. It is located in the hypodermis and throughout other loose supportive tissues.