4.2: Epithelial Tissues
- Page ID
- 124530
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The epithelial tissues, or epithelia (pl.), cover external surfaces (e.g., skin) or line surfaces of hollow structures (e.g., bladder and body cavities). Glands are also classified as epithelial tissues because most glands develop from epithelia. In addition to providing protection by covering or lining surfaces, epithelial tissues also perform absorption, secretion, excretion, and filtration based on the shapes and layers of the epithelial cells in the tissue.
Epithelial tissues generally share the following characteristics:
- Cells are bound tightly together by specialized junctions to form sheets or membranes.
- Epithelial tissue has one side that is exposed to space called the apical surface (Figure \(\PageIndex{2}\)) while the other side that is attached to connective tissue underneath is called the basal surface.
- Epithelial tissue is attached to the connective tissue through the basement membrane (Figure \(\PageIndex{2}\)).
- Epithelial tissue is avascular which means it doesn’t have its own blood supply. Nutrient and waste disposal in these tissues rely on diffusion from connective tissue which is vascular.
- Epithelial cells are capable of regeneration because they are usually subjected to harsh environments or friction.
Figure \(\PageIndex{3}\) shows these characteristics in pseudostratified ciliated columnar epithelial tissue found in the respiratory tract.
Figure \(\PageIndex{2}\) ABCs of Epithelial Tissue: From the exterior toward the interior the structures of an epithelium are: Apical surface, Basal surface and Basement membrane, Connective Tissue of the lamina propria. (Image and caption credit: "ABC's of an Epithelium" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, modified from original by Cancer Research UK via Wikimedia Commons.)
Figure \(\PageIndex{3}\) Histology of Pseudostratified Columnar Epithelium @ 400x. Although it appears to have multiple layers because of the posltion of the nuclei, all cells attach to the basement membrane. (Image and caption credit: "Histology of Pseudostratified Columnar Epithelium" by Jennifer Lange, is licensed under CC-BY-NC-SA 4.0, image provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022).
Classifying Epithelial Tissue
The epithelia are classified based on two criteria: the number of layers (arrangement) and cell shape.
Classifying Epithelial tissue by Arrangement
Epithelia tissue are classified by two types of arrangement based on how many layers of cells there are. The arrangements are: simple and stratified
- Simple: the epithelial tissue consists of one layer of cells attached to the basement membrane.
- Stratified: the epithelial tissue consists of two or more layers of cells.
- Pseudostratified: describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer.
Classifying Epithelial tissue by Shape
Epithelial tissue are also classified by the shapes of the cells within the layers. The shapes are squamous, cuboidal and columnar.
- Squamous: flat, sometimes described as having a "fried egg" appearance.
- Cuboidal: square or cube-like
- Columnar: vertical rectangle or column-like
Figure \(\PageIndex{4}\) Cells of Epithelial Tissues: Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. (Image credit: "Cells of Epithelial Tissue" by OpenStax is licensed under CC BY 4.0)