The cells of the cuboidal epithelium are equally tall as they are wide and have a central round nucleus. The simple cuboidal epithelium is found, e. They have large cytoplasmic content so, they perform important complex functions like absorption and secretion. They are found in exocrine glands, endocrine glands, and ducts of endocrine glands.
Image: Electron microscopic image of human jejunum epithelium cells with densely packed microvilli. The simple columnar epithelium consists of longitudinal-oval cells with a uniform nucleus, aligned in a row mostly located at the base of the cell. Their apical surface is often covered with microvilli to facilitate a strong absorption.
This epithelium type can be found in high secretary areas and absorptive areas, e. The classification of the stratified epithelium is based on the cell shape of the superficial layer. They are multilayered found in parts where protection from mechanical or chemical abuse is essential.
The stratified epithelium is classified into 3 different forms. The cell shape of the stratified, non-keratinized squamous epithelium changes from the basal toward the free surface and is divided into 4 sections:. This type of epithelium is found in the mucosa of the oral cavity and esophagus, as well as the vagina, and the eye corneal epithelium. Image: Epidermis-structure diagram labeled. The outermost cell layers of the epithelium consist of flattened cells with no nuclei and cytoplasm, converting into scales.
They are called the stratum corneum, and their purpose is to mechanically protect underlying tissue from dehydration. The above structure shows an example of the typical skin epithelium—the epidermis. It is the outermost layer of the 3 layers that make up the skin; the inner layers are the dermis and hypodermis.
Image : Mucous membrane of the urinary bladder with urothelium. The urothelium consists of a basal layer, several intermediate cell layers, and an umbrella cell layer. Umbrella cells superficial cells are large and often have 2 nuclei. The crusta, a very dense network of cytoplasm, is located beneath its apical membrane. The plasma membrane consists predominantly of rigid plaques containing uroplakin transmembrane proteins.
Transitional epithelium is primarily found in the efferent urinary tract, i. The important characteristic of this epithelium type is that their cells do touch the basal membrane, but not all of them reach the free surface. The cells that reach the free surface belong to the columnar type. The cells that do not reach the free surface rest on the basal lamina and have a round nucleus.
The term pseudostratified is derived from the appearance of this epithelium. Because the cell nuclei appear at different heights, it conveys the erroneous impression that there is more than 1 layer of cells. The non-ciliated pseudostratified epithelium is found, e. Images : Summary of epithelial tissue cells. Associations of particularly differentiated and specialized epithelial cells are called glandular epithelium and consequently located in the glandular tissue. They are specialized in producing substances such as enzymes, hormones, sweat, oil, mucus, etc.
The elaboration and release are carried out by external stimuli or constitutively into ducts on the surface of the blood. Based on the specialization of the glandular epithelium, we distinguish between exocrine and endocrine glands. The classification is based, among other factors, on the number and location of secreting cells and the type of secretion.
Image : Exocrine glands are classified by their structure. Exocrine glands secrete their products through a duct onto the inner or outer surface of the body. They have their ducts and extensive blood supply and are in control of hormones and nerves.
Examples are salivary glands, sweat glands, tear glands, and mammary glands of the breast. Describe the structural characteristics of the various epithelial tissues and how these characteristics enable their functions. Epithelial tissue primarily appears as large sheets of cells covering all surfaces of the body exposed to the external environment and lining internal body cavities.
In addition, epithelial tissue is responsible for forming a majority of glandular tissue found in the human body. Epithelial tissue is derived from all three major embryonic layers.
The epithelial tissue composing cutaneous membranes develops from the ectoderm. Epithelial tissue composing a majority of the mucous membranes originate in the endoderm. Epithelial tissue that lines vessels and open spaces within the body are derived from mesoderm. Of particular note, epithelial tissue that lines vessels in the lymphatic and cardiovascular systems is called endothelium whereas epithelial tissue that forms the serous membranes lining the true cavities is called mesothelium.
Regardless of its location and function, all epithelial tissue shares important structural features. First, epithelial tissue is highly cellular, with little or no extracellular material present between cells. Second, adjoining cells form specialized intercellular connections called cell junctions.
Third, epithelial cells exhibit polarity with differences in structure and function between the exposed, or apical , facing cell surface and the basal surface closest to the underlying tissue. Fourth, epithelial tissues are avascular; nutrients must enter the tissue by diffusion or absorption from underlying tissues or the surface. Last, epithelial tissue is capable of rapidly replacing damaged and dead cells, necessary with respect to the harsh environment this tissue encounters.
The cells of an epithelium act as gatekeepers of the body, controlling permeability by allowing selective transfer of materials across its surface. All substances that enter the body must cross an epithelium. Many epithelial cells are capable of secreting mucous and other specific chemical compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes and cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles.
Epithelial cells are typically characterized by unequal distribution of organelles and membrane-bound proteins between their apical and basal surfaces. Structures found on some epithelial cells are an adaptation to specific functions. For example, cilia are extensions of the apical cell membrane that are supported by microtubules. These extensions beat in unison, allowing for the movement of fluids and particles along the surface.
Such ciliated epithelia line the ventricles of the brain where it helps circulate cerebrospinal fluid and line the respirtatory system where it helps sweep particles of dust and pathogens up and out of the respiratory tract.
Epithelial cells in close contact with underlying connective tissues secrete glycoproteins and collagen from their basal surface which forms the basal lamina. The basal lamina interacts with the reticular lamina secreted by the underlying connective tissue, forming a basement membrane that helps anchor the layers together.
Cells of epithelia are closely connected with limited extracellular material present. Three basic types of connections may be present: tight junctions, anchoring junctions, and gap junctions Figure 4.
Epithelial cells are held close together by cell junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions. A Tight junction restricts the movement of fluids between adjacent cells due to the presence of integral proteins that fuse together to form a firm seal.
Tight junctions are observed in the epithelium of the urinary bladder, preventing the escape of fluids comprising the urine. An anchoring junction provides a strong yet flexible connection between epithelial cells. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes hold neighboring cells together by way of cadherin molecules which are embedded in protein plates in the cell membranes and link together between the adjacent cells.
Hemidesmosomes , which look like half a desmosome, link cells to components in the extracellular matrix, such as the basal lamina. While similar in appearance to desmosomes, hemidesmosomes use adhesion proteins called integrins rather than cadherins.
The lining of the esophagus is an example of a non-keratinized or moist stratified epithelium. Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra. Privacy Policy. Skip to main content. Organization at the Tissue Level.
Search for:. Epithelial Tissue. Characteristics of Epithelial Tissue The human body consists of four types of tissue: epithelial, connective, muscular, and nervous.
Learning Objectives Describe the primary functions and characteristics of epithelial tissue. Key Takeaways 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. 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 contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.
Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed Figure. Cell shapes can be squamous flattened and thin , cuboidal boxy, as wide as it is tall , or columnar rectangular, taller than it is wide. Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina.
Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary. Simple Epithelium The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells.
Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs.
Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium.
In simple cuboidal epithelium , the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands. In simple columnar epithelium , the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells.
Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract.
Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.
Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end.
The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia. Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. View the University of Michigan WebScope to explore the tissue sample in greater detail.
A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space.
Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body.
Another kind of stratified epithelium is transitional epithelium , so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces.
As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. Figure summarizes the different categories of epithelial cell tissue cells.
Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium?
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