Classification of Layers
Classification of Epithelial cells is critical due to their vital and diverse role in the human body. Found from your head to your toes, these cells come in numerous shapes, sizes, and populations. Thus, Epithelial tissue has been divided into two major types of layering patterns and three main shapes. The two layering patterns are:
- Simple Epithelium
- Stratified Epithelium
Simple Epithelium is defined as having a single layer of cells and the nuclei of these cells being present on the same level within each cell. Simple Epithelium is found in areas that are usually protected from the external environment and areas of diffusion. Places such as internal compartments of the body are a good example of this
Stratified Epithelium is defined as having two or more layers or almost anything that is not simple epithelium. With more cells come more protection so these areas of epithelium are often found in places of chemical and mechanical stress.
Classification of Shape
The next way to classify Epithelium is by its shape. There are three major shapes that they can be classified into. These are:
- Squamous Epithelium- Flat in appearance
- Cuboidal Epithelium- Cube-shaped
- Columnar Epithelium- Cells are longer than they are wide (like Columns)
Simple Squamous Epithelium is considered extremely delicate and is usually located in the mesothelium (lining of cavities) and endothelium (lining of heart and blood vessels). Their main function is often to reduce friction as well as absorb or secrete materials such as nutrients and water.
Stratified Squamous Epithelium is often found in places of high mechanical, chemical, and even microbial stress (Pathogens). Some areas that include Stratified Squamous Epithelium are the surface of the skin, cheeks of the mouth, esophagus, vagina, anus, and more. You may be asking how places like the cheeks of your mouth and the skin are the same types of cells. Though they both are Stratified Squamous Epithelium, they have a significant difference. The difference is that one group of cells may produce keratin while others do not. Cells that produce keratin are called keratinized epithelium and ones that do not are called nonkeratinized epithelium.
Simple Cuboidal Epithelium functions to secrete and absorb materials. They have very limited protection due to the amount of them. They are located in the thyroid gland, kidney tubules, and ducts. These epithelia are vital for the function of the tubes of the kidney in regards to maintaining homeostasis.
Stratified Cuboidal Epithelium is extremely rare. When found they often function in secretion and absorption. The places this type of cell is found are in the sweat glands. As well as absorption and secretion, these cells provide more protection than their simple counterpart.
Simple Columnar Epithelium functions to protect, absorb, and secrete. They are often found in the linings of the intestinal tract (intestines, gallbladder, uterine tubes, and collecting ducts)
Stratified Columnar Epithelium functions to protect areas of the body from mechanical, chemical, and microbial stress. Located in the Pharynx, epiglottis, anus, salivary glands, mammary glands, and urethra these cells tend to deform after the second layer. The first layer of Columnar Epithelium is usually perfect in shape. As you move deeper into the tissue the cells become deformed in shape. This does not mean that these cells are not columnar, however.
Pseudostratified and Transitional Epithelium
With most things in Biology and science, there are exceptions and outliers to the normal circumstances. Epithelial tissue has multiple outliers. The two major ones are Transitional Epithelium and Pseudostratified Epithelium.
Transitional Epithelium is located in areas that permit expansion and recoil after stretching. This includes places like the bladder and ureters. This type of epithelium consists of many different layers as well as an oddly shaped cuboidal cell type. When this layer of cell expands the cells layer will become stretched out and look as if the cells got wider and shortened in height.
Pseudostratified Epithelium is mainly ciliated columnar epithelium. The nuclei of these cells are located at different levels than each other, unlike normal epithelial cells. These cells maintain the function of protection and secretion. They are located in the lining of nasal cavities, the male reproductive system, and the trachea.
One of the many functions of epithelia is the ability to create secretions. Cells that can produce secretions are also known as Glandular Epithelia and contain gland cells. Glands can be classified using three main criteria:
- Type of Secretion
- Exocrine glands (Serous, Mucous, Mixed)
- Endocrine glands (Hormones)
- Structure of the gland
- Number of cells and their shape
- Strucutre of the duct
- Mode of Secretion
- Eccrine secretion
- Apocrine secretion
- Holocrine secretion
Types of Secretion
Within the types of secretions, there are two main categories. The first is the Exocrine glands. Exocrine glands are glands that create secretions that pass through ducts and to the epithelial surface. Exocrine glands can further be categorized into three types of secretions/products made by the gland. These are:
- Serous Glands
- Mucous Glands
- Mixed Exocrine glands
Serous glands produce a watery fluid that contains enzymes. This fluid is also called serous fluid and contains enzymes like lysozymes and alpha-amylase. These glands can usually be found in the mouth to secrete fluid and help break down nutrients.
Mucous glands produce a special type of glycoprotein called mucin (mucus). The goal of mucins is to create a gel-like formation that can lead to numerous effects. Things such as lubricating the cell and forming chemical barriers are the main goals.
Mixed Exocrine Glands secrete both mucins and serous fluid.
The next type of secretion is Endocrine secretion. These glands’ secretions enter the lymphatic fluid or blood. Secretions that enter bodily fluids are typically known as hormones. These glands can be found all over your body and function to help maintain homeostasis and many other vital functions.
Structure Of The Gland
The Structure of a Gland can come in almost any shape, form, and the amount you can think of. For this reason, the structure of a gland is classified based on:
- The number of cells and their shape
- The structure of the ducts
If a gland is composed of a single cell, this gland is known as a Unicellular Gland. These cells typically secrete mucins and based on their location they are called one of two things. If this cell is found in the trachea it is known as a Goblet cell. If it is found in the salivary glands, it is known as a Mucous Cell. These cells can be found between epithelial cells.
If a gland is composed of multiple cells, these are known as Multicellular glands. These cells also secrete mucins and often secrete secretory sheets that cover a larger surface area. A common place to find multicellular glands are within the stomach. In the stomach, multicellular glands produce the lining using mucin to help protect against harsh enzymes. These cells can be found lined up next to each other on the apical surface.
Glandular Epithelia have three other major structures that are based on the shape of the tube the cells make. All of these glands are multicellular but, they form deeper into the tissue creating a duct where secretions are produced. This is not like Goblet cells or Mucous cells that produce secretions directly into the lumen or cavity. The three major shapes include:
- Tubular Glands
- Alveolar (acinar) Glands
- Tubuloalveolar (tubuloacinar) Glands
Tubular glands are cells arranged in a straight or coiled tube. These glands are extremely basic and usually secrete enzymes and materials for the digestive system and sweat. Specifically, Intestinal glands are a good example of straight tubular glands. Eccrine sweat glands are a good example of a coiled tubular gland.
Alveolar or Acinar glands form blind pockets. There is usually a duct that extends into the tissue and a group of glandular cells creates a bulb-shaped blind pocket. Sebaceous glands are examples of this type of gland.
The final type of gland is the Tubuloalveolar or Tubuloacinar gland. This gland combines the structure of a tubular and alveolar gland into one. They only come in compound forms and form glands of the respiratory passages, pancreas, and salivary glands.
Structure Of The Ducts
The ducts of a gland are the passageways that carry secretory products from the cells to the location it needs to go. Based on the structure of these passageways, you can categorize glands into two groups, Simple glands, and Compound glands.
A Simple gland is a gland that has no branching. This means that the ducts have only one way to travel. It is almost like having a town with one road. A Compound gland is a gland with repeated branches. This means that the duct diverges into different ducts creating a network. This would be like having a network of roads in a neighborhood.
Methods Of Secretion
Glandular Epithelia have many different ways of how they secrete their products. Based on where the cell is in the body and what it is producing can have a major role in the method of secretion. There are three main methods:
- Eccrine secretion
- Apocrine secretion
- Holocrine secretion
Eccrine secretion uses the process of Exocytosis to release its products. It will first package its product into a secondary vesicle. This secondary vesicle will carry the product to the apical surface and release the package through Exocytosis. This form of secretion is the most common in all cells and is used to produce things like your saliva and mucins from Goblet cells.
Apocrine secretion is the process of secreting products via the apical portion of the cell by shedding. The secretory vesicles will travel to the apical portion of the cytoplasm. Here, the cytoplasm will be shed from the rest of the cell and broken down in the lumen or cavity. This breakdown results in the release of secretory products from the vesicles. After, the gland cells will go through a period of regrowth before they can shed again. Lactiferous cells in the Mammary gland are good examples of this.
Holocrine secretion is the process of releasing the secretory product via bursting cells. During this secretory process, the entire cell will be packaged with secretory products. The cell will then burst and release the products. This process usually occurs in stratified epithelium because new cells need to replace the cells that have burst. Sebaceous glands near your hair follicles produce oil using this method.