Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

General Part
Structure of the Human Body
Tissues - Epithelial Tissue - Morphofunctional Classification of Epithelial Tissue Types

According to this Classification, epithelia are divided into lining (covering) epithelia (Fig. 5) and glandular epithelia. Lining epithelia, in turn, are subdivided into simple and stratified. If the Cells of the epithelial layer are arranged in a single row, such epithelium is called simple; if they form several rows, it is termed stratified. Epithelia are considered simple when all their cells are in contact with the basement membrane. If the width of the cells in a Simple Epithelium exceeds their height, it is referred to as simple squamous epithelium (from the Greek squama meaning scale). When the width and height of the cells are approximately equal, the epithelium is called simple cuboidal, and if the height of the epithelial cells significantly exceeds their width, it is termed simple columnar or prismatic. Simple Cytology/practical/33.html">Pseudostratified columnar epithelium contains cells of various shapes and heights, which causes their nuclei to be located at different levels, forming several rows. This type of epithelium comprises basal cells that appear triangular in cross-section; their nuclei form the bottom row. Intermediate rows are formed by the nuclei of intercalated epithelial cells and mucus-secreting goblet cells. The top row is formed by the nuclei of ciliated cells, which bear cilia on their apical poles. Many epithelia consist of multiple Cell layers, of which only the lowest (basal) layer is attached to the basement membrane.

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Fig. 5. Schematic representation of lining epithelium types:

1 - simple squamous; 2 - simple cuboidal; 3 - simple columnar; 4 - simple pseudostratified columnar; 5 - stratified squamous non-keratinized; 6 - Stratified squamous keratinized; 7 - transitional when the organ wall is stretched; 8 - transitional when the organ is collapsed

The classification of Stratified Epithelium is based on the shape of its superficial cells. If they are columnar, the epithelium is called stratified columnar; if cuboidal, stratified cuboidal; and if flat, stratified squamous. Among the various types of epithelia in humans and mammals, stratified squamous is the most widespread. If the superficial layers of this epithelium undergo keratinization, it is called stratified keratinized squamous epithelium; if the keratinized layer is absent, it is termed stratified non-keratinized squamous epithelium.

A special variety of stratified epithelium is transitional epithelium, which is characteristic of the Urinary Tract. It comprises three cell types: basal, intermediate, and superficial. When the wall of an organ (such as the Urinary Bladder) is stretched, the epithelium becomes relatively thin. When the organ collapses, the upper PARTS OF THE intermediate cells shift upward, the superficial cells round off, and the overall thickness of the epithelium increases.

Glandular Epithelium (glands) consists of cells or Organs that synthesize specific products (secretions), accumulate them, and eliminate the End products of dissimilation from the body. Glands that release substances into the external environment (onto the Skin surface or mucous membranes) are called exocrine. Conversely, glands that secrete specific products into the internal environment of the body (Blood, Lymph, or tissue fluid) are termed endocrine. Glands are subdivided into unicellular and multicellular. Multicellular exocrine glands differ from multicellular Endocrine glands by the presence of a duct to transport the secretion.

Multicellular exocrine glands are classified into simple and complex. Simple glands have an unbranched duct, whereas complex glands feature a branched duct. Depending on the shape of their secretory units, simple glands can be alveolar (with spherical secretory units) or tubular. In Sweat Glands, the tubular secretory units are coiled into a glomerulus (ball). Complex glands can be alveolar, tubular, or tubuloalveolar. When the terminal secretory units branch out, such glands are described as "branched." The Structural Features of the MAIN TYPES OF exocrine glands are illustrated in Fig. 6.

Epithelial Tissues originate from various embryonic primordia; therefore, from an evolutionary standpoint, Epithelial Tissue is a composite group of tissues. Thanks to the research of Academician N. G. Khlopin, his students, and followers, a phylogenetic classification of epithelia was established, distinguishing the following types:

- ectodermal epithelium, developing from the ectoderm;

- entodermal epithelium, forming from the entoderm;

- nephrodermal epithelium, derived from the intermediate mesoderm;

- celodermal epithelium, derived from the intermediate mesoderm;

- ependymoglial epithelium, originating from the neural primordium;

- angiodermal epithelium (vascular epithelium, endothelium¹), arising from the mesenchyme.

Fig. 6. Structure OF THE main types of exocrine glands (diagram): 1-3 - simple unbranched glands; 4-7 - simple branched glands; 8,9 - complex branched glands