Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released. Key Terms epithelium : A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs. Types of Epithelial Tissue Epithelial tissue is classified by cell shape and the number of cell layers.
Learning Objectives Classify epithelial tissue by cell shape and layers. Key Takeaways Key Points There are three principal cell shapes associated with epithelial cells: squamous epithelium, cuboidal epithelium, and columnar epithelium.
There are three ways of describing the layering of epithelium: simple, stratified, and pseudostratified. Pseudostratified epithelium possesses fine hair-like extensions called cilia and unicellular glands called goblet cells that secrete mucus. This epithelium is described as ciliated pseudostratified epithelium.
Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult.
In keratinized epithelia, the most apical layers exterior of cells are dead and and contain a tough, resistant protein called keratin. An example of this is found in mammalian skin that makes the epithelium waterproof. Transitional epithelia are found in tissues such as the urinary bladder where there is a change in the shape of the cell due to stretching. Key Terms simple columnar : A columnar epithelium that is uni-layered.
Keratinized : To produce or become like keratin. Licenses and Attributions. CC licensed content, Shared previously. Simple squamous epithelium. Allows materials to pass through by diffusion and filtration, and secretes lubricating substances. Simple cuboidal epithelium. Simple columnar epithelium. Learning Objectives Describe the primary functions and characteristics of epithelial tissue.
Key Points Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another. Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released.
Key Terms epithelium : A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs. Functions of the Epithelium Epithelia tissue forms boundaries between different environments, and nearly all substances must pass through the epithelium.
In its role as an interface tissue, epithelium accomplishes many functions, including: Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma. Absorption of substances in the digestive tract lining with distinct modifications. Regulation and excretion of chemicals between the underlying tissues and the body cavity. As protection against desiccation, it undergoes a process known as cornification or keratinization.
As cells move toward the surface, they differentiate and eventually die, leaving an outermost layer of dead cells filled with keratin View Image. The absence of nuclei in this layer shows that it is devoid of live cells. In some slides, the keratinized region is gray, but occasionally it has been penetrated in places by red stain. Note the differences in morphology of the cells as they move toward the surface. You will learn the names of these layers when we study the skin.
In the lower strata, look for the layer of spinous cells the spines look like little lines between cells, and can be difficult to see View Image ; the spines are sites where desmosomes attach the cells to one another. The basement membrane looks like a pink line at the base of the epithelium, which is rather easily seen in places on this slide.
The basement membrane is not always this thick in other epithelia Note that "basal lamina" is a term that refers to an ultrastructural feature while "basement membrane" refers to a light microscopic feature. Ultrastructure refers to structures seen at the electron microscope level. Only some of these cells reach the free surface of the epithelium, where it is generally ciliated and contains goblet cells. Since this type of epithelium lines the respiratory tract, it is referred to as the "respiratory epithelium".
The cilia appear as hairlike projections at the cell apex. What substructures form the core of each cilium? Transitional epithelium is confined to the urinary tract and is adapted for extensibility and water impermeability, as when the bladder fills with urine.
We will compare the epithelium lining the lumen of a distended slide and a non-distended ureter slide In the non-distended ureter , note the shape of the large surface cells, which are frequently dome-shaped and may bulge out into the lumen.
Now, compare with the distended ureter this tissue was artificially over- distended to make a point about the capacity of this epithelium.
The surface cells have been stretched thin, as have the other layers, and there actually appear to be fewer layers, as the cells can slide past one another to a certain degree. While understanding the light microscopic structure of tissues and organs is important for interpreting pathological change, much of the really interesting biological side of medicine now involves understanding cell structure in more detail. Therefore, we feel you should be comfortable with interpreting electron micrographs by the time you complete your histology course, as they are becoming more important in diagnosis and many of the micrographs in your professional literature will be of this sort.
We assume that you already have a general knowledge of cell ultrastructure and can recognize the nucleus, mitochondria, cell membranes, endoplasmic reticulum and ribosomes. The Wheater's Atlas contains an excellent review of cell ultrastructure, as does the Pawlina text. Please review these structures in these sources and the electron micrographs listed below and in the other chapters. The basal bodies are centrioles and have 9 triplets of microtubules with no central pair.
The cytoplasm is full of intermediate filaments tonofilaments , some of which are attached to the desmosomal plaque. Under it, you can see connective tissue, which we will study next time. Note how thin the epithelium is. Shown is the epithelial lining cell of a proximal tubule in the kidney. VIew VIrtual EM Slide You can appreciate that this epithelium skin is stratified has multiple layers of cells and that the layers near the surface at the top of the micrograph have keratinized lost their nuclei and become a layer of keratin.
The spiny appearance of cells deeper in the epithelium can also often be seen in the light microscope. They are points of cell-cell attachment, made more obvious by shrinkage during preparation. What junctions are found here? This is the lining of the esophagus, where it is no longer necessary to have an outer keratinized layer to protect against desiccation, as it was for skin.
Simple epithelial cells compose linings in blood vessels , kidneys, skin, and the lungs. Simple epithelium aids in diffusion and osmosis processes in the body. Stratified epithelium consists of epithelial cells stacked in multiple layers. These cells typically cover exterior surfaces of the body, such as the skin.
They are also found interiorly in portions of the digestive tract and reproductive tract. Stratified epithelium serves a protective role by helping to prevent water loss and damage by chemicals or friction. This tissue is constantly renewed as dividing cells on the bottom layer move toward the surface to replace older cells.
Pseudostratified epithelium appears to be stratified but is not. The single layer of cells in this type of tissue contain nuclei that are arranged at different levels, making it appear to be stratified.
All cells are in contact with the basement membrane. Pseudostratified epithelium is found in the respiratory tract and the male reproductive system. Pseudostratified epithelium in the respiratory tract is ciliated and contain finger-like projections that help to remove unwanted particles from the lungs.
Endothelial cells form the inner lining of the cardiovascular system and lymphatic system structures. Endothelial cells are epithelial cells that form a thin layer of simple squamous epithelium known as the endothelium.
Endothelium makes up the inner layer of vessels such as arteries , veins , and lymphatic vessels. In the smallest blood vessels, capillaries and sinusoids, endothelium comprises the majority of the vessel. Blood vessel endothelium is contiguous with the inner tissue lining of organs such as the brain, lungs, skin, and heart. Endothelial cells are derived from endothelial stem cells located in bone marrow. Endothelial cells are thin, flat cells that are packed closely together to form a single layer of endothelium.
The bottom surface of endothelium is attached to a basement membrane, while the free surface is usually exposed to fluid. Endothelium can be continuous, fenestrated porous , or discontinuous. With continuous endothelium, tight junctions are formed when the cell membranes of cells in close contact with one another join together to form a barrier that prevents the passage of fluid between the cells. Tight junctions may contain numerous transport vesicles to allow the passage of certain molecules and ions.
This can be observed in the endothelium of muscles and gonads.
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