The mast cell, found in connective tissue proper, has many cytoplasmic granules. In contrast, fixed macrophages are permanent residents of their tissues. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers.
The macrophage cells are an essential component of the immune system, which is the body’s defense against potential pathogens and degraded host cells. The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue. The mesenchymal cell is a multipotent adult stem cell. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. The brown adipocytes store lipids as many droplets, and have high metabolic activity. There are two basic types of adipocytes: white and brown. As you might expect, a fibrocyte, a less active form of fibroblast, is the second most common cell type in connective tissue proper.Īdipocytes are cells that store lipids as droplets that fill most of the cytoplasm. Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix.
The most abundant cell in connective tissue proper is the fibroblast. In fluid connective tissue, in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. Supportive connective tissue-bone and cartilage-provide structure and strength to the body and protect soft tissues. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance.
Connective tissue proper includes loose connective tissue and dense connective tissue. The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix ( (Figure)). The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. The major component of the matrix is a ground substance often crisscrossed by protein fibers. The matrix plays a major role in the functioning of this tissue. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix.
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