Connective Tissue and Connective Tissue Proteins

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.

Collectively, connective tissue’s three main roles include immunological protection, nutritional support, and mechanical support in the body.

The most common resident cell in ordinary connective tissue is the fibroblast. These will secrete collagen and other aspects of the extracellular matrix. Fibroblasts are not immature cells as the name implies but are mature connective tissue cells. These are the cells that form scars after an injury. Chondroblasts in cartilage and osteoblasts in bone are related types of cells.

The extracellular matrix in connective tissue is made from ground substance or fibers. Ground substance is mainly water, although there are other proteins and carbohydrates in the substance. The effect of glycosaminoglycans, glycoproteins, and proteoglycans is to make ground substance similar to gelatin. The fibers, collagen and elastin, are commonly seen as well. Reticular fibers are the same thing as collagen but are more delicate fibers in connective tissue.

As mentioned, there are many different types of collagen but just a few types are commonly seen. Type I collagen is seen in ordinary fibrous connective tissue. Type II collagen is seen in cartilage. Type III collagen is in reticular fibers. Type IV collagen is found in smooth and skeletal muscle. Type VII is seen in basement membranes. There are different disorders that affect different types of collagen.

The functions of connective tissue include the transportation of metabolites and nutrients, mechanical support, and immunological defense. It is extremely important in tissue repair, as is seen when scar forms. Specialized connective tissue can engage in hematopoiesis, heat generation, and energy storage.

Collagen is the main protein in connective tissue. It is the most abundant protein in the entire animal kingdom. Most collagen is types I, II, and III, although there are 16 different types. They form collagen fibrils. Not all collagen is made by fibroblasts; some is made by epithelial cells. Collagen is not stretchy so it keeps connective tissue mechanically stable.

Collagen is made as a triple helix. Figure 23 shows what the collagen fibril looks like molecularly:

Figure 23.

The three-stranded collagen molecule packs side-by-side. Collagen fibrils have significant tensile strength so that it can be stretched a great deal without being broken. Altogether, the collagen molecules form a fiber, as is seen in tendons. Type 1 collagen is so strong that it is stronger than steel.

Collagen starts out as procollagens, made typically in the rough endoplasmic reticulum. It gets processed in the Golgi apparatus and is secreted into the extracellular space through exocytosis. The protein becomes tropocollagen and is a triple helix. The molecule gets completed in its construction outside of the cell.

As mentioned, there are different structures that are made from collagen. Type I is the major collagen in connective tissue, while type II is seen in collagen. Type II collagen has smaller fibrils that are oriented more randomly than in type I collagen. It forms a rigid substance as is typical of cartilaginous tissues. Type II collagen is linked with type 9 collagen to make the cartilaginous matrix.