Epithelial Tissue:  The epithelium is a layer of cells that constitute the lining for all external (skin) and internal body surfaces and cavities, including a specialized type of epithelial tissue known as endothelium that lines the interior surface of blood vessels.  Epithelial tissues function to protect organs by regulating the secretion, absorption, excretion, filtration and diffusion of all substances as well as serve to regulate the responses to sensory and mechanical stimuli.  Epithelial cells are packed tightly and in such organized structures that there is very little intercellular material between adjacent cells.  Typically epithelial layers have an apical surface, the one surface that is free and not in contact with other cells.  Directly opposite to the apical surface is the basal surface that is attached to the basement membrane.  The basement membrane is the anchor between the epithelial layer and the underlying connective tissue and is made up of a mixture of carbohydrates and proteins such as collagens, laminins and integrins.  The tight junctions between individual epithelial cells and the attachment to the basement membrane make tissue dissociation difficult.  In fact, incomplete or inefficient tissue dissociation is one of the major hurdles encountered in primary cell preparations that often lead to low yields or poor cellular quality.

Connective Tissue:  As discussed previously the basement membrane anchors the epithelial tissue to a layer of specialized tissue called connective tissue.  In general connective tissue serves to provide structural support and bind or “connect” all types of tissue.  Connective tissue exists in many different forms and is characterized by large amounts of extracellular matrix with few cells scattered throughout this matrix.  The connective tissue matrix is made up of collagenous, elastic and reticular fibers and cells that include fibroblasts, adipocytes, histiocytes, lymphocytes, monocytes, eosinophils. neutrophils, macrophages, mast cells, and mesenchymal cells. 

Extracellular Matrix:  The extracellular matrix is the non-living viscous substance in which the cells and fibers of connective tissues are embedded.  The extracellular matrix is primary composed of glycoproteins (collagens, laminins, fibronectin and elastins) and proteoglycans (chondroitin, heparan, keratan sulfates and hyaluronic acid).  Most vertebrate cells can survive only if they are anchored to the extracellular matrix through specialized integrin proteins. 

Collagen fibers: The collagen fibers are present in varying concentrations in virtually all connective tissues. Measuring 1-10 µm in thickness, they are unbranched, wavy, and contain repeating transverse bands at regular intervals.  Biochemically, native collagen is a major fibrous component of extracellular connective tissue (skin, tendon, blood vessels, bone).  Fibroblasts and possibly other mesenchymal cells synthesize the collagen subunits and release them into the extracellular matrix where they undergo enzymatic processing and aggregate into collagen fibers.  The collegen interchains are often cross-linked via the hydroxyprolyl residues that serve to stabilize the collagen complex and make the fibers insoluble and resistant to hydrolytic attack by most proteases.  The abundance of collagen fibers and the degree of cross-linking tend to increase with advancing age, making cell isolation more difficult from adult tissues.