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| Simple Epithelium |
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| Here, the cells are arranged in a single layer. |
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| Simple epithelium is classified into the following types based on the shape of the cells. |
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| Squamous epithelium |
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| fig 5.52 Squamous Epithelium |
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| It is composed of flat, polygonal cells that are compactly arranged on a basement membrane. Intercellular spaces are completely absent. Every cell has a large amount of cytoplasm, which is either clear or transparent or granular. The nucleus is large, oval and is situated in the centre of each cell. |
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| Since the arrangement of the cells resembles that of tiles on a pavement, it is commonly called as "pavement epithelium". |
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| It is the most common type of epithelium found widely distributed in the body. It is found lining the lumen of blood vessels where it is known as endothelium. In the unused spaces in the body, it is known as mesothelium. It also occurs in the lining of the buccal cavity, in the alveoli of lungs and in the Bowman's capsule of the nephrons. |
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| Cuboidal epithelium |
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| fig 5.53 Cuboidal Epithelium |
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| In this type, the cells are isodiametric. The length and the breadth are more or less equal. The cells are compactly arranged on a basement membrane. Intercellular spaces are absent. The cells contain granular cytoplasm and a single large nucleus situated in the centre. |
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| Cuboidal epithelium is found commonly in some glands and their ducts. It is also found in the sense organs. E.g., Retina of the eye. |
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| Columnar epithelium |
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| fig 5.54 Columnar Epithelium |
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| In columnar epithelium, the cells are rectangular and elongated. The length of each cell is about 2 or 3 times the breadth. The cells are compactly arranged on a basement membrane. However, a few intercellular spaces are present, filled with a cementing substance. The cells enclose granular cytoplasm and a single large oval nucleus, which is found more towards the basement membrane. |
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| Columnar epithelium is found lining the alimentary canal, from oesophagus to anus. It is also found in some glands and their ducts and in the nephrons. In the absorptive surfaces like nephrons of kidney and villi of intestine, columnar epithelial cells show fine projections called microvilli. |
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| Columnar epithelium with microvilli, is known as brush-border epithelium. |
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| Simple epithelium may sometimes become modified for some specific functions. Two such modifications are very common, namely, ciliated epithelium and glandular epithelium. |
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 Ciliated epithelium is always a modification of columnar epithelium. Here, the cells are characterized by the presence of numerous hair-like outgrowths called cilia, at their free ends. The cilia help in transportation and also function as a filtering mechanism. The functioning of the cilia requires moisture. Hence, ciliated epithelium is found only in moist places of the body. It is present in the regions like the nasal passage, the trachea and the reproductive ducts. |
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 Glandular epithelium is a modification of either cuboidal epithelium or columnar epithelium. Here, the cells become specialised for producing some specific secretions. The structure formed by the glandular epithelium for this purpose along with the other tissues is known as a gland. |
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| The glands are mainly of two types: |
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| a) Unicellular gland in which, only one cell in the entire epithelium becomes specialized for producing a secretion. For example: Mucous producing goblet cells in the small intestine. |
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| b) Multicellular gland in which, all the cells in the epithelium become specialised for producing the secretion. |
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| The multicellular glands can be classified into two types: |
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| a) Exocrine glands in which a duct is present for transporting the secretions. e.g. Liver, sweat gland. |
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| b) Endocrine glands or ductless glands in which a duct is absent. Hence, the secretions are transported by blood. e.g. Pituitary gland, thyroid gland. |
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