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There are several kinds of connective tissue. All are variations on a common theme, in that they are combinations of cells and extracellular fibers and fluids that are strong, resilient, and capable of repairing themselves.
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The most common type of cell found in connective tissue is the Fibroblast. A cell that can adopt many shapes, depending on its activity, the Fibroblast produces the two major classes of extracellular material found in connective tissue: fibers and Ground Substance. Fibers are strands of proteinaceous macromolecules that make connective tissues strong, resilient, and elastic; ground substance is a complex, viscous fluid that makes up the Matrix in which the fibers and cells are embedded.
The three main types of fibers in connective tissue are Collagenous Fibers, reticular fibers, and elastic fibers. Of these, Collagenous Fibers are the strongest and also the most common. While the strength and durability of leather is well established, few realize that leather consists of interwoven Collagenous Fibers - fibers that originate from the Dermis of the skin of large mammals. Strong though leather is, the Collagenous Fibers from which it is made are even stronger in the living animal. Collagenous Fibers possess a formidable tensile strength; they can withstand pulling forces of up to 300 kg/cm ? ? ? without rupturing. Under those conditions, the Collagenous Fibers will stretch very little - 3% at most. The properties of Collagenous Fibers are derived from the fibrous protein Collagen from which they are assembled.
Because Collagenous Fibers stretch so little and connective tissue so often needs to be elastic, the elastic Fiber has evolved. When stretched, an Elastic Fiber can return to its original length. Elastic fibers, which are also made by fibroblasts, have two components-a fibrous core and an amorphous covering. The amorphous covering, made of the protein Elastin, is thought to be responsible for the capacity of elastic fibers to recoil after being stretched. It is no accident, then, that connective tissues subjected to deformation often contain both kinds of fibers, Collagenous Fibers for strength and elastic fibers for elasticity.
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The third class of connective tissue Fiber, the reticular Fiber,
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often distributed in a spiderweb or “reticular” fashion, were called reticular fibers after their configuration. They are common to organs of the immune system, in which they form the connective tissue framework for the spleen and lymph nodes. More recently, electron microscopists have shown that reticular fibers are actually made of Collagen fibrils. (A Fibril is a tiny structure, visible by electron microscopy but too small to be seen by light microscopy. Many fibrils, when packed together, make up a Fiber). Within a given reticular Fiber, the Collagen fibrils that compose its core are covered by a Glycoprotein coat that resembles material of the Basement Membrane. In organs such as the spleen and lymph node, the reticular fibers are surrounded by thin cytoplasmic extensions of reticular cells - cells, quite similar (perhaps identical) to fibroblasts, that are thought to elaborate the reticular fibers. In ordinary histologic preparations not specially stained to detect them, reticular fibers are impossible to distinguish from Collagenous Fibers.
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 dense irregular connective 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.
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Connective tissue cells are varied. Most abundant is the Fibroblast, which seems to be capable of secreting all of the types of fibers. Mesenchyme cells, too, are abundant. Mesenchyme is embryonic connective tissue; Mesenchyme cells, therefore, are embryonic connective tissue cells. Despite their embryonic nature, Mesenchyme cells persist in the connective tissues of the adult. Similar in appearance to undifferentiated fibroblasts, these Mesenchyme cells are capable of developing into whatever type of connective tissue cell is required at the moment. They are thought to be multipotent and can differentiate into fibroblasts, cartilage cells, bone cells, and, on occasion, smooth muscle cells.
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Dense Regular Connective Tissue: The Tendon
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Tendons and related ligaments, which hold bones together, are made of Dense Regular Connective Tissue that consists of a small population of fibroblasts in a field of parallel Collagenous Fibers.
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