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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 near the lower end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basement membrane, although some do not reach the top surface. Pseudostratified columnar epithelium is found in the respiratory tract, where these cells have cilia.
Both simple and pseudostratified columnar epithelia often have goblet cells between the epithelial cells. A goblet cell is a mucous-secreting unicellular “gland” found between the columnar epithelial cells of mucous membranes. This is the only type of gland that is made from a single cell. ( [link] ).
View the University of Michigan WebScope at (External Link) 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 top layer of cells, closest to the free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The top cells are squamous, whereas the bottom layers contain either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Your 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 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 relaxed and has cuboidal shaped cells. As the bladder fills with urine, this epithelium stretches and the 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. [link] 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?
A gland is a structure made up of one or more cells modified to create and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an endocrine gland , a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).
The secretions of endocrine glands are called hormones. Hormones are released into the fluid around the cell, which then enter the bloodstream. The bloodstream delivers hormone to target organs throughout the body. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads (ovaries and testicles).
Exocrine glands release their contents through a duct A(tube)that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts.
Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine.
The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The duct is single in a simple gland but in compound glands is divided into one or more branches ( [link] ). In tubular glands, the ducts can be straight or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched gland, a duct is connected to more than one secretory group of cells.
In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basement membrane that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.
Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. Glands are classified both according to the type of secretion and by their structure.
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