Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 1.
"Table 1. Connective Tissues
Tissue Cells Fibers Location
loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia
dense, fibrous connective tissue fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments
cartilage chondrocytes, chondroblasts hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs
bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons
adipose adipocytes few adipose (fat)
blood and bone