Connective Tissue: Introduction

connective tissue: introduction
connective tissue are responsible for providing and maintaining the form of organs throughout the body. functioning in a mechanical role, they provide a matrix that connects and binds other tissues and cells in organs and gives metabolic support to cells as the medium for diffusion of nutrients and waste products.
structurally, connective tissue is formed by three classes of components: cells, fibers, and ground substance. unlike the other tissue types (epithelium, muscle, and nerve), which consist mainly of cells, the major constituent of connective tissue is the extracellular matrix (ecm). extracellular matrices consist of different combinations of protein fibers (collagen, reticular, and elastic fibers) and ground substance. in addition to its major structural role, molecules of connective tissue serve other important biological functions, such as forming a reservoir of factors controlling cell growth and differentiation. the hydrated nature of much connective tissue provides the medium through which nutrients and metabolic wastes are exchanged between cells and their blood supply.
1- cells of connective tissue
a variety of cells with different origins and functions are present in connective tissue. fibroblasts originate locally from undifferentiated mesenchymal cells and spend all their life in connective tissue other cells such as mast cells, macrophages, and plasma cells originate from hematopoietic stem cells in bone marrow, circulate in the blood, and then move into connective tissue where they remain and execute their functions. white blood cells (leukocytes) are transient cells of most connective tissues they also originate in the bone marrow and move to the connective tissue where they reside for a few days, then usually die by apoptosis. table 1 shows the main types and functions of connective tissue cells

cell type representative product or activity representative function
fibroblast, chondroblast, osteoblast, odontoblast production of fibers and ground substance structural
plasma cell production of antibodies immunologic (defense)
lymphocyte (several types) production of immune competent cells immunologic (defense)
eosinophilic leukocyte participation in allergic and vasoactive reactions, modulation of mast cell activities and the inflammatory process immunologic (defense)
neutrophilic leukocyte phagocytosis of foreign substances, bacteria defense
macrophage secretion of cytokines and other molecules, phagocytosis of foreign substances and bacteria, antigen processing and presentation to other cells defense
mast cell and basophilic leukocyte liberation of pharmacologically active molecules (eg, histamine) defense (participate in allergic reactions)
adipocyte storage of neutral fats energy reservoir, heat production

2- ground substance is a highly hydropinghilic, viscous complex of an ionic macromolecules (glycosaminoglycans and proteoglycans) and multiadhesive glycoproteins (laminin, fibronectin, and others) that stabilizes the ecm by binding to receptor proteins (integrins) on the surface of cells and to the other matrix components. is a colorless, transparent, gel-like material in which the cells and fibers of connective tissue are embedded. ground substance serves as a lubricant, helps prevent invasion of tissues by foreign agents, and resists forces of compression.
3- fibers: are long, slender protein polymers present in different proportions in different types of connective tissue. the three main types of connective tissue fibers are collagen, reticular, and elastic fibers. collagen and reticular fibers are both formed by the protein collagen, and elastic fibers are composed mainly of the protein elastin. these fibers are distributed unequally among the types of connective tissue and the predominant fiber type is usually responsible for conferring specific properties on the tissue.
1- collagen fibres
collagen fibres are the dominant fibre type in most connective tissues. the primary function of collagen fibres is to add strength to the connective tissue. the thickness of the fibres varies from ~ 1 to 10 µm. longitudinal striations may be visible in thicker fibres. these striations reveal that the fibres are composed of thinner collagen fibrils (0.2 to 0.5 µm in diameter). each of these fibrils is composed of microfibrils, which are only visible using electron microscopy.
2- reticular fibres: reticular fibres are very delicate and form fine networks instead of thick bundles. they are usually not visible in histological sections but can be demonstrated by using special stains. for example, in silver stained sections reticular fibres look like fine, black threads - coarse collagen fibres appear reddish brown in the same type of preparation.
because of their different staining characteristics, reticular fibres were initially thought to be completely different from collagen fibres. we now know that reticular fibres consist of collagen - although the main type of tropocollagen found in reticular fibres, type iii, is different from that of the coarse collagen fibres. reticular fibres give support to individual cells, for example, in muscle and adipose tissue.
3- elastic fibres
elastic fibres are coloured in fresh tissues - they are light yellow - but this colouration is only visible if large amounts of elastic fibres are present in the tissue, for example, in the elastic ligaments of the vertebral column. special stains are necessary to show elastic fibres in tissue sections.
connective tissue has been classified into several categories based on the manner of packing of the fibers and the ratio of cells to fibers. connective tissue that features many densely packed fibers going in many different directions is known as tissue. found in such places as the dermis of the skin and the submucosa of the gut, dense connective tissue is well suited to binding epithelial sheets to underlying tissues.
connective tissues that are subjected to the exertion of heavy forces in one direction, such as the forces that pull on tendons and ligaments, are quite different. they have densely packed collagenous fibers oriented parallel to one another. this type of connective tissue, known as dense regular connective tissue, has many fibers and relatively few cells. the paucity of cells, most of which are fibroblasts, may account for the slow healing of torn tendons. dense irregular connective they have densely packed collagenous fibers oriented in different direction.
connective tissue in which there are relatively few fibers and lots of cells is loose connective tissue. found in such places as the lamina propria of the gut, loose connective tissue is composed of fibers that run in many directions and form an open, watery, cellular meshwork. loose connective tissue often contains collagenous fibers, elastic fibers, and reticular fibers woven together in a variety of ways.