Cells with similar characteristics of form and function are grouped together to form a tissue as was discussed in Chapter 7. A tissue is a collection of similarly specialized cells that will then form into organs. Tissue types are categorized according to four basic histological types. These basic histological tissue types include epithelial, connective, muscle, and nerve tissue (Table 8-1, see Table 7-1). These basic tissue types have subcategories that serve specialized functions. It is during prenatal development that embryonic cell layers differentiate into the various basic embryological tissue types: ectoderm, mesoderm, and endoderm that will later form the basic histological tissue types of the body (see Table 3-4).

Most tissue of the body can be renewed as the individual cells die and are removed from the tissue. The turnover time is the time it takes for the newly divided cells to be completely replaced throughout the tissue. The turnover time differs for each of the basic tissue types, as well as for specific regions of the oral cavity. A complete understanding of turnover time may be the future basis for how aging and disease processes in the body are fought, including those occurring in the oral cavity.

Epithelium generally consists of closely grouped polyhedral cells surrounded by very little or no intercellular substance or tissue fluid (Figure 8-1). This tissue is capable of rapid turnover. In fact, epithelium is highly regenerative because its deeper germinal cells are capable of reproduction by mitosis (see Table 7-2). Epithelial cells usually undergo cellular differentiation as they move from the deeper germinal layers to the surface of the tissue.

Epithelial cells are tightly joined to one another by intercellular junctions in the form of desmosomes, except in the more superficial layers. The epithelial cells are also tightly joined in some cases to nearby noncellular surfaces by hemidesmosomes, such as with the basement membrane (see Figures 7-5 and 7-6).

The basement membrane is located between most epithelium and deeper connective tissue, such skin and oral mucosa, and is produced by both the epithelium and the adjoining connective tissue.

Epithelium is avascular, having no blood supply of its own. Cellular nutrition consisting of oxygen and metabolites is obtained by diffusion from the adjoining connective tissue, which is usually highly vascularized, providing its own source of nutrition.

Epithelium can be classified into two main categories based on their arrangement into layers of cells: simple and stratified. Simple epithelium consists of a single layer of epithelial cells. The further classification of tissue involves different types of epithelial cells according to cellular shape; they can be simple squamous, simple cuboidal, or simple columnar (Table 8-2).

TABLE 8-2

Types of Epithelial Cells

CELLS WITH DESCRIPTION/EXAMPLES	MICROSCOPIC APPEARANCE IN TISSUE∗
Squamous cells
Flattened cells with cell height much less than cell width i.e., blood vessel lining (endothelium)
Cuboidal cells
Cube-shaped cells with approximately equal cell height and cell width i.e., salivary gland
Columnar cells
Rectangular or tall cells in which cell height exceeds cell width i.e., salivary gland ducts

^∗Note that these epithelial cells are shown within simple epithelium. Diagrams from Stevens A, Lowe J: Human Histology, ed 3, Mosby, St Louis, 2005.

Simple squamous epithelium consists of flattened platelike epithelial cells, or squames, lining blood and lymphatic vessels, heart, and serous cavities, as well as interfaces in the lungs and kidneys (see Table 8-2). The special term endothelium is used to refer to the simple squamous epithelium lining of these vessels and serous cavities.

Simple cuboidal epithelium consists of cube-shaped cells that line the ducts of various glands, such as certain ducts of the salivary glands. Simple columnar epithelium consists of rectangular or tall cells, such as in the lining of other salivary gland ducts, as well as the inner enamel epithelium, whose cells become enamel-forming ameloblasts (see Figures 6-9 and 11-6).

Epithelium can also be classified as pseudostratified columnar epithelium (Figure 8-2). This epithelium falsely appears as multiple cell layers when viewed under low-power microscopic magnification, because the cells’ nuclei appear at different levels (see Table 8-1). However, in reality, under higher microscopic magnification, cells of different heights are seen. Thus, this is a type of simple epithelium because all the cells line up to contact inner the basement membrane, but not all reach the outer surface of the tissue. Pseudostratified columnar epithelium lines the upper respiratory tract, including the nasal cavity and paranasal sinuses (see Figure 11-19). This type of epithelium may have cilia or be nonciliated at the tissue surface.

Stratified epithelium consists of two or more layers of cells, with only the deepest layer lining up to contact the basement membrane (see Table 8-1). It is important to note that only the cellular shape of the most superficial or surface layer is used to determine the classification of stratified epithelium. Thus, stratified epithelium can consist of cuboidal, columnar, or squamous epithelial cells, or a combination of types, as seen in a transitional epithelium.

Most epithelium in the body is stratified squamous epithelium, which includes the superficial layer of the skin and oral mucosa (see Figures 8-1 and 8-7 and Chapter 9). Only the most superficial layers of this tissue are flat cells, or squames; the deeper cells vary from the deeper cuboidal to the more superficial polyhedral. Interdigitation of the outer epithelium with the deeper connective tissue, having a basement membrane between them, appear on two-dimensional microscopic sections as the rete ridges (or rete pegs).

Stratified squamous epithelium can be keratinized or nonkeratinized. Keratin (see Chapter 7) is a tough, fibrous, opaque, waterproof protein that is impervious to pathogenic invasion and resistant to friction. Keratin is produced during the maturation of the keratinocyte epithelial cells as they migrate from near the basement membrane to the surface of the keratinized tissue, which occurs in certain regions of the oral mucosa found in the oral cavity (see Figure 9-11).

Another example of keratinized stratified squamous epithelium is epidermis, which is the superficial layer of the skin (see Figures 8-1 and 8-7). The epidermis overlies a basement membrane and the adjoining deeper layers of connective tissue (dermis and hypodermis, respectively; discussed later). The skin has varying degrees of keratinization depending on the region of the body; areas such as the palms of the hands and bottom of the feet have thicker layers of keratin, which form calluses. However, the keratin is less densely packed in both the skin and oral cavity, as compared with the densely packed hard keratin of the nails and hair.

These overall higher turnover times vary slightly for the different types of epithelium. However, the epithelium of the oral mucosa generally has a higher turnover time than the epidermis of the skin (see Table 9-6). More specifically, within the oral cavity, the epithelium that lines the cheek tissue (buccal mucosa) has a higher turnover time (14 days) than the epithelium that covers the skin (27 days). This difference becomes apparent when dental professionals sadly note a traumatic superficial injury to the facial skin lasting for weeks, and, at the same time, happily observe the quicker healing in the cheek area after the patient accidentally bites the superficial oral mucosa.

Thus the differences of turnover time are especially noted during repair or healing of the tissue after injury. Immediately after an injury to either the skin or oral mucosa, a clot from blood products forms in the area, and the inflammatory response is triggered by the blood’s white blood cells (WBCs) as they migrate into the tissue (Figure 8-3). If the source of injury is removed, tissue repair can begin within the next few days. The epithelial cells at the periphery of the injury will lose their desmosomal intercellular junctions and migrate to form a new epithelial surface layer beneath the clot.

Thus, a clot is very important in repair of the epithelium and must be retained in the first days of repair because it acts as a guide to form a new surface. A clot stays moist in the oral cavity but dries out on the skin (called a scab when on the skin). Later, after the epithelial surface is repaired, the clot is broken down by enzymes because it is no longer needed. Repair of the epithelium is a process that is also tied to repair in the deeper connective tissue (discussed later).