Epithelium Tissue
Epithelium Tissue
Histology: The study of tissues.
Teamwork
The human body contains about 100 trillion cells
However, no one cell can carry out all the necessary bodily functions by itself, so cells aggregate and form tissues.
Tissue→collection of cells with a similar structure and function.
Tissues
There are 4 primary tissue types in the human body:
Epithelial (covering/lining) tightly packed
Connective (support) cells barely touch. A lot of non-living material.
Muscle (movement) tightly packed together. Lots of protein.
Nervous (control) not packed but touch.
Epithelial Tissue
Covers the surface of the body, lines the body cavities, forms the external and internal lining of most organs, and constitutes the bulk of most glands.
Functions include:
Protection-ex. Skin
Secretion-ex. Pancreas
Sensory reception-ex. Slin
Absorption-ex. Small intestine
Filtration-ex. Kidneys
It’s found all over the place!
Characteristics of Epithelia
Composed almost entirely of sheets of close-packed cells–very little extracellular material.
Epithelial cells are often strongly connected to adjacent cells via tight junctions and desmosomes (protein).
Tight junctions are protein complexes that completely encircle a cell and thus connect it to all its neighboring cells and makes it impossible for anything to pass between them. Common in the lining of the stomach and intestines.
Desmosomes - “spot weld” That holds cells together and enables a tissue to resist mechanical stress. Common in the epidermis and cervix.
Polarity
Different areas of epithelial cells have different structures (If they have different structures, they can have different cells).
the area of an epithelial cell adjacent to the exterior or to a body cavity is the apical side and the opposite area is the basal side.
Always supported by a layer of connective tissue.
It’s known as the basement membrane (basal lamina).
No blood vessels - it’s avascular.
Innervated
Has nerves
We classify epithelia based on the number of cell layers present and the shape of the cells in the apical layer.
If there is only one layer of cells, the epithelium is simple. If there is more than one layer, the epithelium is stratified.
There are 3 cell shapes:
Squamous - flat, scale-like
Cuboidal - cube-shaped
Columnar - column shaped
Epithelium Types
Simple squamous
Simple cuboidal
Simple columnar
Stratified squamous
Stratified cuboidal
Stratified columnar
Pseudostratified columnar
Transitional
Simple Squamous Epithelium
Microscopic Appearance
Single layer of flattened cells, shaped like fried eggs with a bulge where the nucleus is located.
Nucleus is flattened in the plan of the cell.
Cytoplasm may be very thin and tough to see.
In surface view, cells have angular contours and nuclei appear round.
Locations:
Air sacs (alveoli) of lungs
Glomerular capsules of kidneys
Some kidney tubules
Lining of heart and all blood vessels
Here it is called the endothelium
Serous membranes
External linings of visceral organs
Functions:
Thinness allows material to rapidly diffuse or be transported through the epithelial layer.
Example: Gases diffuse across the thin air sac epithelium in the lungs.
Blood traveling to the lungs is high in CO2 and low in O2. After gas exchange in the lungs, the blood leaving is high in O2 and low in CO2.
Blood is filtered through the epithelium of the kidney capillaries.
This begins the process of removing waste products from the blood and modifying its ionic content.
Secreting of a lubricating fluid by the epithelium making up the serous membranes.
Simple Cuboidal Epithelium
Microscopic Appearance
Single layer of square or round cells.
Centrally-placed spherical nuclei.
Apical layer often has microvilli.
Microvilli are small extensions of the cell membrane that increase the surface area of the cell.
Locations:
Liver
Pancreas
Thyroid, salivary, and most other glands.
Most kidney tubules
Bronchioles (small tubes within the lungs)
Functions:
Absorption
Kidney tubules
Microvilli often present to increase the available surface area
Liver
Secretion
Kidney tubules
Liver
Pancreas
Thyroids, salivary, and most other glands.
Simple Columnar Epithelium
Microscopic Appearance
Single layer of tall, narrow cells.
Oval or sausage-shaped nuclei, vertically-oriented, usually located in the basal half of the cells.
Secretory vesicles sometimes visible in the apical portion of the cell.
Microvilli occasionally present
Cilia occasionally present
Cilia are hair-like extensions of the cell membrane that can move and sweep material across the cell surface.
Goblet cells often interspersed
Secrete a lubricating mucus.
Location
Inner lining of stomach, intestines, rectum.
Inner lining of gallbladder
Inner lining of uterus and uterine tubes.
Functions:
Absorption and secretion
Columnar cells in the small intestine have microvilli to increase the available surface area for the absorption of nutrients.
Movement of egg and embryo in uterine tube.
Hence the presence of cilia.
Secretion of mucus.
Lots of goblet cells in the large intestine so as to lubricate it and ease the passage of feces.
Pseudostratified Columnar Epithelium
Microscopic Appearance:
Looks like multi-layered, but it’s NOT!
All cells touch the basement membrane.
In stratified epithelia, only the bottom cells touch the basement membrane.
Cells are of varying heights which gives the appearance of stratification. Nuclei are at several levels.
Often has goblet cells interspersed.
Cells often have cilia.
Locations:
Respiratory tract from nasal cavity to bronchi (pharynx, larynx, trachea)
Ciliated
Goblet cells
Functions:
In the respiratory tract there are lots of mucus-secreting goblet cells
The mucus traps dust and bacteria.
Cilia “sweep” the bacteria-laden mucus up the respiratory tract towards the pharynx where it can be swallowed.
Smoking paralyzes cilia—smokers have to cough violently to expel their mucus. Then they die.
Stratified Squamous Epithelium
Microscopic Appearance:
Multiple cell layers with cells becoming flatter and flatter toward the surface.
In keratinized stratified squamous epithelium, the apical layers are layers of dead cells lacking nuclei and paced with the tough protein keratin.
Nonkeratinized stratified squamous epithelium lacks the layers of dead cells at the surface.
Locations of keratinized version:
Epidermis. Palms and soles of feet are typically heavily keratinized.
Locations of nonkeratinized version:
Lining of oral cavity and surface of tongue
Lining of esophagus
Lining of vagina and anal canal.
Functions:
Protection!
Keratinized version (a.k.a. Dry epithelium) protects against mechanical abrasion, water loss, and pathogen entry.
Keratin is very strong, waterproof, and is bacteriostatic (prevents bacteria from reproducing.
Non-keratinized version (a.k.a., wet epithelium) also protects from mechanical abrasion.
Eating food, swallowing, sexual intercourse, birth, defecation.
Stratified Cuboidal Epithelium
Microscopic Appearance:
2 or more layers of cells.
Surface layers are square or round (cuboidal).
Locations:
Some sweat glands ducts.
Ovarian follicle
Cells that surround the developing egg.
Functions:
Contribute to sweat secretion.
Secretion of ovarian hormones (example: estrogens)
Stratified Columnar Epithelium
Microscopic Appearance
2 or more layers of cells
Surface cells tall and narrow with basally located nuclei and basal cells are almost cuboidal
Locations:
Rare
Pharynx, epiglottis, and male urethra, vas deferens
Functions:
Often seen where 2 other tissue types meet
Structural integrity of gland ducts
Allows for a high volume of fluid or material to pass.
Transitional Epithelium
Microscopic Appearance:
Somewhat resemble]les stratified squamous epithelium, but the surface cells are rounded and often bulge above the surface.
Typically 5-6 cell layers thick when relaxed and 2-3 cell layers thick when stretched
Cells maybe flattened and thinner when 2 nuclei
Locations:
Lining the urinary tract.
Part of kidney, ureters, bladder, part of urethra.
Functions
Stretches to allow filling of the urinary tract.