Home to epithelia!
Simple epithelia serve many roles in various body locales. As components of serous and synovial membranes, simple epithelia secrete fluids that lubricate tissues to minimize friction as organs or other body structures rub against one another. Other simple epithelia line body tracts as protective, absorptive, or secretory cells. All glands of the body are constructed of epithelial cells as are the ducts that connect the exocrine types to body surfaces. As linings in the alveoli(air sacs), kidneys, and blood vessels, simple squamous types assist in diffusion, osmosis and filtration phenomena. As linings and covering on all external and internal body surfaces, epithelia serve as the "first line of defense" against microbial invasions.
Simple Squamous Epithelia
Simple squamous epithelia consist of only a single layer of flattened cells resting on connective tissues. These flattened cells are specifically found in thin barriers where exchange of nutrients, wastes, or respiratory gases occurs.
Here is a labelled image of tubules! Compare the squamous cells with cuboidal types.
Simple squamous epithelia are found in:
alveoli and capillaries of lungs where gas exchange occurs
kidney glomerulus and tubules where filtration and diffusion processes form urine
capillaries where diffusion and osmosis occur
ventral body cavities as mesothelium of serous membranes
all vessels and the heart as endothelium
Simple Cuboidal Epithelium
Due to larger cytoplasmic volumes, cuboidal cells can undertake more complex functions such as absorption and secretion. Most secretory cells of glands are cuboidal epithelial cells. In addition, the ducts of most exocrine glands are lined by cuboidal cells as are most of the tubules in the kidneys.
Simple cuboidal epithelia are found in:
Simple Columnar Epithelium
Simple columnar epithelia are the ultimate cells for absorption and secretion. With the largest cytoplasmic volumes of all epithelia, these cells possess the organelle density and energy reserves to engage in the most complex and efficient secretory or absorptive functions. Simple columnar epithelia with microvilli line the small intestine where 90% of absorption from the digestive tract occurs. Ciliated types are found in small bronchioles of the respiratory tract and in the fallopian tubes of the female reproductive tract. In these locales, the cilia aid in the movement of mucous or reproductive cells . The unicellular gland or goblet cell, is a specialized columnar cell of mucous membranes that secretes mucous for protection.
Simple columnar epithelium is found in:
ducts of exocrine glands
larger tubules or collecting ducts of the kidney
stomach, small intestine, and large intestine
smaller respiratory tubes or bronchioles
As the name implies, pseudostratified epithelium is "falsely stratified". Cells are columnar but tall and thin. All cells rest on the basement membrane. The unique appearance of pseudostratified epithelia occurs because the tall, thin cells intertwine. Nuclei appear at various levels but there is no distinct layering.
Pseudostratified epithelia are most prevalent in the upper or lower respiratory tract as ciliated types. Non-ciliated pseudostratified epithelium can be found in the ducts of larger glands or the male urethra but these may be transition zones between epithelial types. The cilia of respiratory pseudostratified epithelia beat in a rhythmic manner to propel mucous along the surfaces of cells. This mucous layer, a product of goblet cells, entraps dust, debris, and microbes inhaled into the tract. The resultant movement of mucous from the lower tract via ciliary movements is called the "ciliary escalator".
Lets take a look at ciliated, pseudostratified epithelia from the nasal mucosa and trachea! One locale in the nasal mucosa possessed an abundance of goblet cells.
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