Kosheekaa
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In the pursuit of knowledge, scientists often arrive at a crossroads where their opinion contrasts on a topic. With time, research reveals more insights that ultimately unite the differing opinions. However, some issues remain a subject of discussion, resulting in more confusion rather than clarity. Endothelium vs epithelium is one such topic. Both cells line the cavities, forming a protective barrier. These similarities cannot discount the difference between them. This article is an attempt to understand the background of this confusion and detail the differences between the both.
Epithelium and Endothelium
The epithelium is composed of epithelial cells and covers the outer surface of the body and its inner cavities. This layer is present at the interface of tissue and the external environment like skin, respiratory tract, eyes, gastrointestinal tract, etc. Endothelium comprises endothelial cells and line the inner surface of blood vessels and the heart.
History: How the Confusion Began
Among both epithelium and endothelium, the former were discovered first. The epithelium was defined as the layer that covers the inner cavities and outer surfaces. Endothelial cells, on the other hand, only line the blood vessels. However, the debate ensued as to whether both the layers are really different. Although endothelium has its few distinguishing features, it was still only an inner lining. In the era, when endothelium was identified, the inner lining was categorized as epithelium. Therefore, endothelium was originally referred to as epithelium instead of a separate entity. To account for the differences between both layers, the endothelium was named "false epithelium" or a "specialized form of epithelium". That’s why some scientists still associate these names with endothelium despite the revelation of many distinguishing features over time.
Endothelial vs Epithelial Cells
Both form the protective barrier in the body's cavities but have distinct characteristics listed below.
Origin: Epithelial cells originate from all the three germ layers- endoderm, mesoderm, and ectoderm whereas endothelial cells have mesodermal origin.
Location: The primary difference between endothelium and epithelium is their location. The former is present in the vessels, whereas the latter is more ubiquitous in nature and present throughout the body.
Structure of Layer: Epithelial cells can form single or multiple layers of epithelium, whereas endothelial cells form only a single layer of endothelium. Epithelium in certain tissues also have projections (cilia, microvilli) which are absent in endothelium.
Permeability: Tight junctions closely connect the epithelial cells to form impermeable epithelial sheets. They restrict the transport across the layer. However, epithelium of few tissues such as lungs, intestine, kidney exhibit the solute transport. Therefore, epithelium is a selective barrier that can modify its permeability depending on the tissue type.
Endothelium exhibits more permeability by allowing the passage of water and some solutes across the vessels. In some physiological conditions they also permit cellular migration across the vessels. Their permeability also alters in different tissues, ranging from low to very high permeability. For example, spleen endothelium has pores and gaps for movement of large molecules and cells across the vessels.
Cell Morphology: Epithelial cells adopt squamous, cuboidal, or columnar shapes whereas endothelial cells show flattened shape. In two-dimensional in vitro culture, epithelial cells adopt a polygonal morphology whereas endothelial cells have a cobblestone structure.
Functions: The primary function of epithelial cells is to form a protective barrier that blocks the network of pathogens and foreign particles. However, depending on the tissue type, it can also have secretory and absorptive functions. For example, in glands, epithelium secretes hormones, enzymes, and mucus, while in the intestine and kidney, it absorbs nutrients. Additionally, they also possess sensory receptors for temperature, taste, and touch that offer sensory perception of the stimuli.
Endothelium also serves as a protective barrier. Additionally, they regulate vascular tone, inflammation, angiogenesis, and coagulation by altering vessel diameter, secreting chemokines, releasing growth factors, and promoting platelet adhesion, respectively.
Clinical Relevance: Epithelium has exposure to the environment and, therefore, is more susceptible to infection and diseases. It has a role in cancer, eczema, psoriasis, inflammatory bowel diseases, etc.
Endothelium regulates blood flow and therefore has association with cardiovascular disorders, inflammatory disorders, blood vessel injury (including stroke and ischemia), atherosclerosis disorders, and coagulation disorders. Additionally, they are responsible for vessel formation in cancer and wound repair. Therefore, cancer therapy and wound repair treatments target angiogenesis.
ConclusionEpithelium and Endothelium
The epithelium is composed of epithelial cells and covers the outer surface of the body and its inner cavities. This layer is present at the interface of tissue and the external environment like skin, respiratory tract, eyes, gastrointestinal tract, etc. Endothelium comprises endothelial cells and line the inner surface of blood vessels and the heart.
History: How the Confusion Began
Among both epithelium and endothelium, the former were discovered first. The epithelium was defined as the layer that covers the inner cavities and outer surfaces. Endothelial cells, on the other hand, only line the blood vessels. However, the debate ensued as to whether both the layers are really different. Although endothelium has its few distinguishing features, it was still only an inner lining. In the era, when endothelium was identified, the inner lining was categorized as epithelium. Therefore, endothelium was originally referred to as epithelium instead of a separate entity. To account for the differences between both layers, the endothelium was named "false epithelium" or a "specialized form of epithelium". That’s why some scientists still associate these names with endothelium despite the revelation of many distinguishing features over time.
Endothelial vs Epithelial Cells
Both form the protective barrier in the body's cavities but have distinct characteristics listed below.
Origin: Epithelial cells originate from all the three germ layers- endoderm, mesoderm, and ectoderm whereas endothelial cells have mesodermal origin.
Location: The primary difference between endothelium and epithelium is their location. The former is present in the vessels, whereas the latter is more ubiquitous in nature and present throughout the body.
Structure of Layer: Epithelial cells can form single or multiple layers of epithelium, whereas endothelial cells form only a single layer of endothelium. Epithelium in certain tissues also have projections (cilia, microvilli) which are absent in endothelium.
Permeability: Tight junctions closely connect the epithelial cells to form impermeable epithelial sheets. They restrict the transport across the layer. However, epithelium of few tissues such as lungs, intestine, kidney exhibit the solute transport. Therefore, epithelium is a selective barrier that can modify its permeability depending on the tissue type.
Endothelium exhibits more permeability by allowing the passage of water and some solutes across the vessels. In some physiological conditions they also permit cellular migration across the vessels. Their permeability also alters in different tissues, ranging from low to very high permeability. For example, spleen endothelium has pores and gaps for movement of large molecules and cells across the vessels.
Cell Morphology: Epithelial cells adopt squamous, cuboidal, or columnar shapes whereas endothelial cells show flattened shape. In two-dimensional in vitro culture, epithelial cells adopt a polygonal morphology whereas endothelial cells have a cobblestone structure.
Functions: The primary function of epithelial cells is to form a protective barrier that blocks the network of pathogens and foreign particles. However, depending on the tissue type, it can also have secretory and absorptive functions. For example, in glands, epithelium secretes hormones, enzymes, and mucus, while in the intestine and kidney, it absorbs nutrients. Additionally, they also possess sensory receptors for temperature, taste, and touch that offer sensory perception of the stimuli.
Endothelium also serves as a protective barrier. Additionally, they regulate vascular tone, inflammation, angiogenesis, and coagulation by altering vessel diameter, secreting chemokines, releasing growth factors, and promoting platelet adhesion, respectively.
Clinical Relevance: Epithelium has exposure to the environment and, therefore, is more susceptible to infection and diseases. It has a role in cancer, eczema, psoriasis, inflammatory bowel diseases, etc.
Endothelium regulates blood flow and therefore has association with cardiovascular disorders, inflammatory disorders, blood vessel injury (including stroke and ischemia), atherosclerosis disorders, and coagulation disorders. Additionally, they are responsible for vessel formation in cancer and wound repair. Therefore, cancer therapy and wound repair treatments target angiogenesis.
Earlier studies considered endothelial and epithelial cells to be similar with a few differences. However, mounting evidence has demonstrated that the situation is actually quite the opposite. They are different, sharing a few similarities. The distinguishing features regarding the origin, structure, permeability, and functions indicate that both cell populations are inherently different. Although scientists perceive them as different, there are still many who refer to them as epithelium. It might create misconceptions, leading many to contribute the features of the epithelium to the endothelium. Therefore, it is vital to consider the differences between the two layers before making assumptions that can affect the research data. The scientific community's position may become clearer in the future with more solid evidence. Kosheeka is a good choice for studying these cells because it isolates them from various tissues and species and then tests and characterizes them thoroughly.
