Objectives
- Identify basic kinds of epithelium
- Identify types of cells and layers of epithelium
- Identify different types of glands
Epithelium Overview
Epithelium is the first of the four basic tissue types that we will be studying. The other three are connective tissue, muscular tissue, and nervous tissue. Epithelium is a boundary tissue that covers free surfaces and lines cavities in the body. Epithelium has the following characteristics:
- Apical surface is nonadhesive
- Rests on and firmly attached to a basement membrane (basal lamina)
- Cells are closely apposed by cellular junctions
- Has apical-basal polarization, e.g., basal nuclei and apical secretion granules in some cells
- May have apical modifications, e.g., microvilli or cilia
- Avascular– no blood vessels in the epithelium proper. Oxygen and nutrients diffuse to epithelium from blood vessels in the deep connective tissues.
Epithelium classifications:
By layer-
- Simple (single layer of cells resting on a basement membrane)
- Stratified (multilayered with the most basal layer resting on a basement membrane)
By shape/height-
- Squamous– flattened with width of cell greater than height. The nucleus takes up most of the cell, little cytoplasm can be seen.
- Cuboidal– cubed-shape with height, width, and depth about equal. Nuclei are usually centrally located.
- Columnar– taller than wide. Nuclei are typically basally located.
Naming the epithelium: use the shape of the cells at the most apical surface followed by the layer classification. For example: epithelium of multiple layers with the most apical layer having flattened cells would be named “stratified squamous” but if only a single layer of flattened cells, then “simple squamous“. Stratified epithelium typically has more cuboidal-shaped basal cells resting on the basement membrane but it is named by the cells of the apical surface.
Exceptions:
- Transitional epithelium (aka urothelium) is only found in the urinary track. It is stratified with thick apical cells that bulge into the lumen known as “umbrella cells”.
- Pseudostratified epithelium may appear stratified but all cells rest on the basement membrane even through they may not reach the apical surface.
- In some areas of the body, you may see a mixture of cell heights such as a mixture of cuboidal and columnar. You may also see a transition along a tube structure of cells becoming taller or shorter depending on the structure.
Definitions of Terms Commonly Used in Discussions of Epithelium
- Apical– the superficial portion of an epithelium. It faces the free surface or the lumen- opposite of basal.
- Basal– the deep portion of an epithelium. It rests on the basement membrane- opposite of apical.
- Lumen– the hole or deep cavity in a tubular organ. The small intestine, blood vessels, and the central nervous system all have a lumen. The lumen is a direct result of the nonadhesive character of the apical surface epithelial cells
- Endothelium- the simple squamous epithelium that lines the entire cardiovascular system
- Mesothelium– the simple squamous epithelium that lines the serous body cavities (thoracic cavities, pericardial cavity, peritoneal cavity, tunica vaginalis testis [surrounds testes-really an extension of the peritoneal cavity])
- Keratinized epithelium– contains a layer of squamous cells filled with keratin filaments which stain darker than non-keratinized cells. These cells are also degenerated and typically contain no nuclei. Used when describing stratified squamous epithelium.
>Epithelium Microcopy Slides
Use the following slides to practice identifying epithelium. Don’t worry about identify organs and major structures, just work on distinguishing the different types of epithelium discussed above.
In this slide we see a typical neurovascular bundle you may find anywhere in the body. Nerves, veins, and arteries often run together surrounded by connective tissue. For now, we will focus on looking at epithelium lining blood vessels. Endothelium is the term used to describe epithelium lining blood vessels and the heart. Endothelium is made up of simple squamous epithelium. The basement membrane can be difficult to see or not be seen at all in many light microcopy images with a typical hematoxylin and eosin stain as shown above. Hematoxylin stains nucleic acids blue/purple and eosin stains the cytoplasm a pinkish color. The bright red cells seen in the center of the vein and artery without nuclei are red blood cells.
Once you click on the specimen slide, it will guide you through different areas of the slide of importance. As you explore the slide, you’ll notice other tissue types such as connective tissue, adipose tissue, nerve fibers, and smooth muscle surrounding the endothelium. For now, just focus on looking for simple squamous epithelium.
This slide contains a coronal section of an infant kidney. The outer, curved region is the natural anatomical surface. Blood travels through the kidney to the cortex region where it begins the filtration process. You will find simple squamous epithelium lining any blood vessels and simple cuboidal epithelium lining the surrounding ducts. If you look closely at the cuboidal cells, their apical surface may have a fuzzy appearance indicating microvilli, also known as the brush border. The brush border is used to increase surface area of these cells for transport of molecules as urine is being concentrated. As you move towards the center, or medulla, you will notice tubules in longitudinal and crossection. Here, you will see more simple cuboidal epithelium and some tubules lined by simple squamous epithelium indicating the thick and thin limbs of the loops of Henle. Capillaries can also be seen carrying red blood cells and lined by endothelium. Don’t worry about identifying different structures of the kidney, just focus on identifying epithelium.
This slide contains a partial section of the gallbladder. A complete section would be closed off, like a circle. Notice the lumen of the gallbladder. You should look for the simple columnar epithelium lining the lumen. You can also scroll through other parts of the slide and find more endothelium lining blood vessels. You may also see some simple squamous epithelium on the very edge of the gall bladder, this is a mesothelial layer from visceral peritoneum which partially covers the gall bladder.
The next section contains slides with more types of epithelium and also glands to identify.
Glands Overview
In this section, we will continue identifying different types of epithelium and also look for different glands. Glands are collections of cells which secrete product either onto the surface of the body (exocrine) or into the cardiovascular system (endocrine). Exocrine glands contain secretory epithelial cells, supportive connective tissues, a blood supply, and a duct that conveys secretions from the epithelial cells to the surface of the body (skin, oral cavity, lumen of GI tract). Endocrine glands contain secretory cells, connective tissues, and a blood supply that also receives the secretion. Individual cells that secrete a product into a lumen are sometimes called unicellular glands, e.g., goblet cells secrete mucus into the lumen of the respiratory system and GI tract.
Classification is done using morphology (shape) of the structure:
- Simple gland– a single, tube-like structure, not branched
- Compound gland– multiple branches
- Alveolar or Acinar– secretory portion expands distally (like a grape).
- Tubular– secretory portion forms a tube
- Tubuloalveolar or Tubuloacinar– a mix of tube and distal expanding structures. Most large exocrine glands (salivary, pancreas) form this shape.
Secretory cells of alveoli may secrete protein-rich (serous acini) product or mucous-rich (mucous acini) product. The differences in these cells is visible in a typical H&E stain. Serous acini cells have round, basal nuclei with a slightly darker-stained cytoplasm due to the staining of granules carrying the protein product. Mucous acini tend to have basal, flattened nuclei with a washed-out appearing cytoplasm. Mucous product doesn’t pick up the eosin dye. Glands can be described as serous, mucous, or mixed.
>Epithelium and Gland Slides
This slide contains a crossection of part of the small intestines (duodenum) and the pancreas. Use this slide to observe the simple columnar epithelium lining the lumen of the duodenum. You will see a mixture of tall columnar absorptive cells with microvilli interspersed with lighter, staining cells- goblet cells which produce mucous. The invaginating structures of tissue you see that project into the lumen are known as villi. Just deep to the villi are Brunner glands, coiled tubular glands that secrete bicarbonate and mucous. When you view the pancreas, you will find compound tubuloalveolar glands. Notice the alveoli are made up of serous cells which secrete digestive enzymes.
This slide contains 2 sections of trachea, one has been flattened out while the other maintains the natural c-shape. The trachea is a tube that runs through the neck and thorax for movement of air. Examples of hyaline cartilage can be identified which provide flexibility while maintaining an open airway. You should focus on the luminal surfaces of the trachea sections to observe the pseudostratified, ciliated, columnar epithelium with goblet cells. Think back to the definition of pseudostratified- all cells’ basal surface are resting on the basement membrane but not all cells reach the apical surface. The shorter, darkly-stained cells are basal cells which undergo mitosis to replace the epithelial cell population. The cilia seen on the taller cells are motile cilia (9 + 2 microtubule arrangement) and move mucous through the airway. Goblet cells are lighter stained and appear in a goblet shape. The trachea also has compound tubuloalveolar glands that are serous and mucous (mixed glands) found between the epithelium and cartilage layers.
The epiglottis is tissue made up of cartilage, connective tissue (CT), epithelium, and glands. It covers the opening of the trachea (airway) when swallowing to prevent food and fluid from entering the respiratory system. Use this slide to identify the surface epithelium, in some places it is not fully in tact. You can also find vessels, glands and ducts.
>More Practice
In this section of lung tissue, you can see lots of air space and cellular structures. The outer surface of the lung is covered with visceral peritoneum, called mesothelium in histology. The tissue of the lung contains airways lined by epithelium and vasculature. Review the slide to practice identifying epithelium in this tissue. Answers are located below.
This is a section of the esophagus. Use the slide to identify the epithelium lining the lumen and the glands deep to the epithelium layer. This slide will be used to identify other basic tissues in future lessons.
This is a section of thick skin which can be found from the palmar surface of hands or the plantar surface of feet. Use the slide to locate the epidermis where the epithelium is located and identify the type of epithelium.
>More Practice Answers
GWU 38 Lung:
1. Simple squamous epithelium
2. Orientation, see next
3. Pseudostratified columnar, ciliated epithelium (aka respiratory epithelium)
4. Simple squamous epithelium
GWU 49 Esophagus:
1. Stratified squamous, non-keratinized
2. Duct- stratified cuboidal
2. Gland- mixed but mainly mucous
GWU 26 Thick Skin
- 1. Stratified squamous keratinized epithelium. You will learn all of the layers of this epithelium in the Skin Histology Lesson.
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