`star` `color{green}("Squamous Epithelium:")`

● The `color{brown}("squamous epithelium")` is made of a `color{violet}("single thin layer")` of flattened cells with `color{violet}("irregular boundaries.")`

● They are found in the `color{violet}("walls of blood vessels")` and `color{violet}("air sacs of lungs")` and are involved in a functions like forming a `color{violet}("diffusion boundary.")`

`star` `color{green}("Cuboidal Epithelium:")`

● The `color{brown}("cuboidal epithelium")` is composed of a single layer of `color{violet}("cube-like cells.")`

● This is commonly found in `color{violet}("ducts of glands")` and tubular parts of `color{violet}("nephrons in kidneys")` and its main functions are `color{violet}("secretion")` and `color{violet}("absorption.")`

● The `color{violet}("epithelium of proximal convoluted tubule (PCT)")` of nephron in the kidney has `color{brown}("microvilli")`.

`star` `color{green}("Columnar epithelium:")`

● The `color{violet}("columnar epithelium")` is composed of a single layer of tall and `color{violet}("slender cells.")`

● Their `color{violet}("nuclei")` are located at the `color{violet}("base.")`

● Free surface may have `color{violet}("microvilli.")`

● They are found in the `color{violet}("lining of stomach")` and intestine and help in `color{violet}("secretion")` and `color{violet}("absorption.")`

`star` `color{green}("Ciliated epithelium:")`

● If the `color{brown}("columnar or cuboidal")` cells bear cilia on their free surface they are called `color{brown}("ciliated epithelium.")`

● Their function is to move particles or `color{violet}("mucus")` in a specific direction over the `color{violet}("epithelium.")`

● They are mainly present in the inner surface of `color{violet}("hollow organs")` like `color{violet}("bronchioles")` and `color{violet}("fallopian tubes")`

● Some of the `color{violet}("columnar or cuboidal")` cells get specialised for `color{violet}("secretion")` and are called `color{brown}("glandular epithelium.")`

● They are mainly of two types:

`star` `color{brown}("Unicellular:")` consisting of `color{violet}("isolated glandular cells")` (goblet cells of the alimentary canal)

`star` `color{brown}("Multicellular:")` consisting of `color{violet}("cluster of cells")` (salivary gland).

● On the basis of the mode of `color{violet}("pouring")` of their secretions, glands are divided into two categories namely
`color{brown}("exocrine")` and `color{brown}("endocrine glands.")`

● `color{brown}("Exocrine glands")` secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products.

● These products are released through `color{violet}("ducts or tubes.")`

● In contrast, `color{brown}("endocrine glands")` do not have `color{violet}("ducts")`.

● Their products called `color{violet}("hormones")` are `color{violet}("secreted directly")` into the fluid bathing the gland.

● `color{violet}("Compound epithelium")` is made of more than one layer (multi-layered) of cells and thus has a limited role in `color{violet}("secretion")` and `color{violet}("absorption.")`

● Their main function is to provide protection against `color{violet}("chemical")` and `color{violet}("mechanical stresses.")`

● They cover the `color{violet}("dry surface")` of the skin, the moist surface of `color{violet}("buccal cavity, pharynx, inner lining of ducts")` of `color{violet}("salivary glands")` and of `color{violet}("pancreatic ducts.")`

● All cells in `color{violet}("epithelium")` are held together with little `color{violet}("intercellular material.")`

● In nearly all `color{violet}("animal tissues,")` `color{brown}("specialised junctions")` provide both `color{violet}("structural")` and `color{violet}("functional links")` between its `color{violet}("individual cells.")`

● Three types of `color{violet}("cell junctions")` are found in the `color{violet}("epithelium")` and other `color{violet}("tissues.")`

● These are called as `color{violet}("tight, adhering")` and `color{violet}("gap junctions.")`

● `color{brown}("Tight junctions")` help to stop substances from leaking across a tissue.

● `color{brown}("Adhering junctions")` perform cementing to keep neighbouring cells together.

● `color{brown}("Gap junctions")` facilitate the cells to communicate with each other by connecting the `color{violet}("cytoplasm")` of adjoining cells, for rapid transfer of ions, `color{violet}("small molecules")` and sometimes `color{violet}("big molecules.")`