Epithelial Tissue

Epithelial tissues come in three basic types: squamous, cuboidal and columnar. These three types of tissue are seen in either simple (only one cell layer thick) or stratified (many cells in thickness) arrangements. We will begin with the simple epithelia, as they are easier to visualize, and then touch on pseudostratified columnar epithelium before we move on to the stratified squamous epithelium. Please note that, whereas stratified cuboidal and columnar epithelia do exist, they are rare and will not be discussed here.

The Simple Epithelial Tissue Types
Simple Squamous Epithelium:

In this silver stained, whole mount of a simple squamous epithleium, we see it as it would look from a "top" view. These cells are large, but quite thin, and have a prominent, protruding nucleus. A good analogy to their shape is the sunny-side-up fried egg. It has also been called "pavement epithelium, because it can look like like paving stones as seen from above (although paving stones don't have something similar to the nucleus, so I like the fried egg analogy better). This epithelium is called mesothelium when it is found in certain locations within the body such as the linings of the peritoneal, pleural and pericardial cavities. Other places simple squamous epithelia can be found include; the glomerulus of the kidney, the walls of capillaries, and the alveoli of the lungs, to name a few. Here they are forming a thin sheet-like layer which allows for minimal resistance to diffusion. Here we see simple squamous epithelia as seen in cross section. This photograph is of the peripheral portion of the chick embryo at about 33 hours of incubation. Even though there are several layers of cells visible, they are all distinct and separate simple squamous epithelial layers. In other words, they are each a different structure, each of which is only one cell layer thick. One of the nuclei of a squamous cell is shown (arrows), which demonstrates how the nuclei can "bulge out" along the simple squamous epithelial layer. The embryonic layers seen here are, from top to bottom: i. ectoderm, ii. somatic mesoderm, iii. splanchnic mesoderm and iv. endoderm (here an example of a simple cuboidal epithelium).

Simple Cuboidal Epithelium: Simple Columnar Epithelium:

These are collecting ducts in a section the medulla of a mammalian kidney. Each duct is lined by simple cuboidal epithelium, where the height of the cells is approximately the same as the width (and depth). This type of epithelium is thicker than simple simple squamous epithelium, so it does not allow for passive diffusion as readily. There are also tubules and capillaries composed of simple squamous epithelia visible.

This is a cross section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). Since columnar cells are quite thick, they do not readily allow passive diffusion. As a result, these cells use active transport to move nutrients through them from the intestine to the blood. This is what we commonly call "absorption." To help with this, they have numerous microvilli on their apical (lumenal) surface, which increases their surface area to allow for greater absorption. These are seen as a darker pink staining border at their lumenal edge. The black arrow is indicating a goblet cell. These cells show a triangular basal nucleus and a large amount of clear apical cytoplasm. The cytoplasm stains clear due to a large amount of mucus within it that the cell secretes to protect the intestinal cells from digestion by the digestive enzymes (your body wants to digest the food, not itself!).

The Simple Epithelial Tissue Types
Simple Squamous Epithelium:

In this silver stained, whole mount of a simple squamous epithleium, we see it as it would look from a "top" view. These cells are large, but quite thin, and have a prominent, protruding nucleus. A good analogy to their shape is the sunny-side-up fried egg. It has also been called "pavement epithelium, because it can look like like paving stones as seen from above (although paving stones don't have something similar to the nucleus, so I like the fried egg analogy better). This epithelium is called mesothelium when it is found in certain locations within the body such as the linings of the peritoneal, pleural and pericardial cavities. Other places simple squamous epithelia can be found include; the glomerulus of the kidney, the walls of capillaries, and the alveoli of the lungs, to name a few. Here they are forming a thin sheet-like layer which allows for minimal resistance to diffusion.	Here we see simple squamous epithelia as seen in cross section. This photograph is of the peripheral portion of the chick embryo at about 33 hours of incubation. Even though there are several layers of cells visible, they are all distinct and separate simple squamous epithelial layers. In other words, they are each a different structure, each of which is only one cell layer thick. One of the nuclei of a squamous cell is shown (arrows), which demonstrates how the nuclei can "bulge out" along the simple squamous epithelial layer. The embryonic layers seen here are, from top to bottom: i. ectoderm, ii. somatic mesoderm, iii. splanchnic mesoderm and iv. endoderm (here an example of a simple cuboidal epithelium).

Simple Cuboidal Epithelium:	Simple Columnar Epithelium:

These are collecting ducts in a section the medulla of a mammalian kidney. Each duct is lined by simple cuboidal epithelium, where the height of the cells is approximately the same as the width (and depth). This type of epithelium is thicker than simple simple squamous epithelium, so it does not allow for passive diffusion as readily. There are also tubules and capillaries composed of simple squamous epithelia visible.	This is a cross section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). Since columnar cells are quite thick, they do not readily allow passive diffusion. As a result, these cells use active transport to move nutrients through them from the intestine to the blood. This is what we commonly call "absorption." To help with this, they have numerous microvilli on their apical (lumenal) surface, which increases their surface area to allow for greater absorption. These are seen as a darker pink staining border at their lumenal edge. The black arrow is indicating a goblet cell. These cells show a triangular basal nucleus and a large amount of clear apical cytoplasm. The cytoplasm stains clear due to a large amount of mucus within it that the cell secretes to protect the intestinal cells from digestion by the digestive enzymes (your body wants to digest the food, not itself!).

The Pseudostratified Epithelial Tissue Type
Pseudostratified Columnar Epithelium:
Cross section through 33 hour chick

Here we see a cross section through the 33 hour chick, which shows the neural tube, mesenchyme and various other labelled structures. The slide on the right focuses on the pseudostratified columnar epithelium associated with the neural tube.

Ectoderm
Neural Tube
Neurocoele
Mesenchyme

Foregut
Heart
Extraembryonic Coelom
Splanchnopleure

Here we see a close up of the cranial neural tube region of the 33 hour chick. When the neural tube first forms, it is composed of pseudostratified columnar epithelium. A pseudostratified epithelium consists of a single layer of tall cells with nuclei at varying heights within the cells. Since their nuclei are at differing heights within the cells (unlike a simple columnar epithelium where nuclei are roughly at the same height within the cells), it appears as though there are multiple layers of cells. In reality, though, pseudostratified columnar epithelium consists of only one layer of cells.

Ectoderm
Pseudostratified Columnar Epithelium (of the Neural Tube)

Neurocoele
Mesenchyme

The Stratified Epithelial Tissue Type
Stratified Squamous Epithelium:

Satisfied(!) squamous epithelium - oh baby!

Here, in a section of frog skin, we see the epidermis (outer part of the skin) showing a typical stratified squamous epithelium and the dermis showing connective tissue. The cells of the basal layer of the epidermis (closest to the dermis) are cuboidal to columnar in shape. These cells are actively mitotic, producing new cells that get pushed upward into the overlying layers. As these cells are pushed up, they become flatter and longer (just as if you were to squash something), taking on the typical squamous shape. When the cells reach the top, they are sloughed off and replaced by cells from below. The dermis which underlies the epidermis is composed of a dense, irregular connective tissue, which we will see again later.

Epidermis (Stratified Squamous Epithelium)
Dermis

The Stratified Epithelial Tissue Type
Stratified Squamous Epithelium:

Here, in a section of frog skin, we see the epidermis (outer part of the skin) showing a typical stratified squamous epithelium and the dermis showing connective tissue. The cells of the basal layer of the epidermis (closest to the dermis) are cuboidal to columnar in shape. These cells are actively mitotic, producing new cells that get pushed upward into the overlying layers. As these cells are pushed up, they become flatter and longer (just as if you were to squash something), taking on the typical squamous shape. When the cells reach the top, they are sloughed off and replaced by cells from below. The dermis which underlies the epidermis is composed of a dense, irregular connective tissue, which we will see again later. Epidermis (Stratified Squamous Epithelium) Dermis

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Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.
Last page revision: June 2012