Epithelial tissue consists of different layers. One is a superficial layer of closely packed sheets of cells covering the external surface of the body. Another, the dermis, is the connective tissue layer of the skin underlying the epithelial tissue. A much thinner layer of epithelial tissue also lines the internal cavities of the body and the tubes that drain glands and carry blood throughout the body. Most epithelium has the capability of cell renewal by mitosis of the basal cells, and the rate of renewal is dependent on the location of the epithelium in the body. For example, human buccal mucosa (Fig. 2-1) renews in 10 to 14 days, whereas the junctional epithelium of the gingiva renews in 4 to 6 days.

The dermis is closely associated and adherent to the epidermis and has an interdigitating relationship with it in some areas. Because epithelium does not contain blood vessels, the skin depends on vessels located in the connective tissue of the dermis. The vessels are in close proximity, nourishing the skin and playing an important part in its function of thermal regulation. Nerves also exist in the dermis, and some penetrate the epithelial layer to function as receptors. Sweat glands, hair follicles and their associated sebaceous glands, and erector pili muscles are located in the dermis and subcutaneous tissue. Ectoderm is the source of the epithelial lining of some internal organs, but not of all epithelial-lined surfaces. For example, the epithelial lining of the alimentary canal is of endodermal origin. The epithelial lining of the peritoneal cavity and the endothelial lining of blood vessels are from mesoderm. Epithelium is described according to cell shape and cell arrangement in one or more layers. Some cells form a single layer known as simple epithelium. Epithelium with all cells in contact with the basal lamina, but not with the surface, is known as pseudostratified. The type consisting of several cell layers with only the basal cell layer in contact with the basal lamina is known as stratified epithelium (Fig. 2-2). Further modifications are based on cell shape. For example, the surface cells may be flattened, as in keratinized stratified squamous epithelium of the palm of the hand (see Fig. 2-2).

Epithelial membranes function in one or more of the absorptive processes: contractility, digestion, secretion, excretion, protection, and sensation. Table 2-1 illustrates the classification of epithelia by cell type, cell shape, cell modifications, characteristics, and location.

Table 2.1

Classification of epithelia

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Cell type	Cell shape	Cell modifications	Characteristics	Location
Simple
1. Squamous a. Endothelial b. Mesothelial 2. Cuboidal 3. Columnar 4. Pseudostratified	Spindle Oval to polygonal Cube Rodlike Rodlike with thin section		Cilia may appear Microvilli, cilia may appear Cilia, stereocilia	Lines heart, blood, lymph vessels Lines pleural, pericardial, peritoneal cavities Kidney, glands, respiratory passages Most glands, small intestines, respiratory passages Respiratory passages, male reproductive organs
Stratified
1. Squamous 2. Columnar 3. Transitional	Polyhedral Columnar cells on cuboidal or columnar on columnar Cube to pear		Intercellular bridges Distension causes cell flattening	Covering of the body, mouth, pharynx, vagina Oropharynx, larynx Urinary passages, bladder

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Images from Gartner LP, Hiatt JL: Color textbook of histology, ed 3, St. Louis, 2007, Saunders.

Neural tissue is a second type of tissue. Nerve tissue arises from neuroepithelial cells, which are highly organized areas for reception and correlation. The nervous system carries out numerous functions with only two principal types of cells, which are the neurons and the neuroglia. Neurons are the nerve cells that receive and conduct impulses and regulate muscle and gland activity. Neuroglial cells are the supporting cells of the nervous system. Each neuron consists of three parts. The first part is the cell body or perikaryon, which contains the nucleus and the cytoplasm. The cytoplasm contains a chromatophilic substance or rough endoplasmic reticulum (RER). The function of the RER, as in other cells, is protein synthesis. Sensory ganglia are accumulations of cell bodies outside the CNS swellings associated with, for example, each of the spinal nerves containing unipolar or pseudounipolar ganglion cells held together with a connective tissue capsule. These ganglia are called dorsal root ganglia of the spinal cord (Fig. 2-3). Some ganglia associated with the cranial nerves include the trigeminal, facial, glossopharyngeal, and hypoglossal cranial nerves (Fig. 2-4).

Proteins travel from the perikaryon by axoplasmic transport into the second part of the neuron, the axon, which is a long, thin singular process that varies in length from a few millimeters to several feet or more. The axon conducts nerve impulses away from the nerve cell body. It terminates by branching into axon terminals, or synaptic terminals. Most axons outside the CNS are protected and insulated by a myelin sheath, which is a multilayer of phospholipid produced by the neurilemma (Schwann cells) (Fig. 2-5). The third component of the neuron is the dendrite; usually there are multiple dendrites, which receive impulses and conduct these impulses to the cell body (Fig. 2-6).

Neuroglial cells carry out the functions of support. These are 5 to 50 times more numerous than neurons. Neuroglial cells protect and support nerve cells, and some are even phagocytic, meaning that they ingest bacteria.

Connective tissue varies in its proportion of cells, fibers, and intercellular substance and its location in the body. Connective tissue proper is classified as loose (Fig. 2-7), dense (Fig. 2-8), or loose connective tissue with special properties. It functions in tissue support, and protection of the body parts, in areas of fluid exchange and storage of adipose (fat) tissue. The ligaments that attach bones and the tendons that attach muscles to bones are forms of dense connective tissue.

Cartilage

The three types of cartilage are hyaline (Fig. />

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