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| Epithelial Tissue |
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| Fig 5.51 Epithelial Tissue (Epithelium) |
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| It is normally found as a covering either externally or internally. It is the simplest and the least specialised animal tissue. It is described as avascular, since a direct blood supply is absent. The tissue depends upon the other underlying tissues for its nourishment. The epithelial tissue can originate from any of the three germ layers-ectoderm, mesoderm or endoderm. |
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| The cells which form the tissue are known as epithelial cells. |
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 The cells always have a definite shape. They are either polygonal or cuboidal (isodiametric) or rectangular. Very rarely are the cells irregular. |
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 The cells are compactly arranged on a thin, structureless basement membrane which is secreted by the cells themselves. |
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 Due to the compact arrangement, intercellular spaces are usually absent. However, sometimes small intercellular spaces may be present filled with a cementing substance. |
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 The cells are characterised by the presence of a large amount of cytoplasm. It may be clear and transparent or granular. |
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 The cells are always uninucleate. The nucleus is large and prominent. |
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 The cells are capable of undergoing simple mitotic divisions. |
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| Based on the arrangement of cells, epithelium can be distinguished into three types: |
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 Simple or unilaminar epithelium, where the cells are arranged in a single layer on a basement membrane. |
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 Stratified or multilaminar epithelium, where the cells are arranged in more than one layer on a basement membrane. |
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 Pseudo-stratified epithelium, where the cells are arranged in a single layer on a basement membrane. However, there is a false appearance of more than one layer due to a difference in the height of the cells and the position of their nucleii. |
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