The basement membrane is part of the barrier necessary in the epithelium. This becomes important in tissues like the kidneys and brain, where barriers are necessary. The laminin and collagen networks in the kidneys allow for selective permeability of the epithelial lining. The same thing happens in the brain with the blood brain barrier and in the capillaries. In these cases, the basement membrane assists in controlling the intermembranous passage of molecules. Laminin is also important in the structure of hemidesmosomes. It interacts with the integrin on the cell surface and type VII collagen anchors the basement membrane to the underlying connective tissue, protecting the tissue from shear forces.
CONNECTIVE TISSUE AND CONNECTIVE TISSUE PROTEINS We’ve talked about connective tissue as being one of the four major types of tissues. It is an abundant tissue, acting as a framework for the other tissues of the body. It is important for communication between tissues, mechanical support, and transport within the body. It also has the possibility for inflammation because immune cells can be found within connective tissue. The key component of connective tissue is that it consists of individual cells that are not directly connected directly but are connected via the extracellular matrix. Connective tissue comes from mesoderm or mesenchyme within the embryonic body. Common cells of connective tissue include adipocytes, fibroblasts, and the immune cells, such as lymphocytes, macrophages, and mast cells. A main feature of connective tissue is the presence of ground substance (made of minerals, plasma, proteoglycans, glycosaminoglycans, and glycoproteins). Minerals are found in bone, while plasma is found in the blood fluid because both bone and blood are considered connective tissue. The main fiber is collagen, with elastin and reticular fibers found in lesser amounts. Fibroblasts make the collagen in connective tissue. As mentioned, there is ordinary connective tissue—adipose and fibrous tissue—and specialized connective tissue—lymphoid tissue, elastic tissue, bone, cartilage, and blood. These will differ in the cell types seen as well as in the type of protein seen in the tissue.
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