First off, I'll have you know, that a tissue is a group of cells that have a common embryonic origin. Their may be different types of cells that all come together to perform a common function. They can be solid, like bones; semisolid, like fats; or liquid, like blood. Tissue cells are separated by nonliving, intercellular materials they secrete. Tissues, as I mentioned already, have a common origin from the embryonic cell which has three layers, endoderm, mesoderm and ectoderm.
There are four main types of tissues: epithelial, connective, nervous and muscular. Each of these four tissues has many derivatives, each with their own function, shape and arrangement.
Epithelium tissue's origin is from all three embryonic layers. It is the covering and lining tissue of all outer openings and areas exposed to the environment. Glandular (it has glands) epithelium secrets pieces of the gland. These glands include thyroid, adrenal and sweat glands. There are five characteristics that make epithelial tissue its very own. 1. Polarity - there is always one side that isn't touching anything else. 2. Cellularity and specialized contacts - each has it's own function. 3. Supported by connective tissue - it's basically glued to the body by the connective tissue. 4. It's avascular - there aren't any blood vessels in the epithelium. 5. Regeneration - since it lines like every cavity and ALL of our skin, it's constantly being sloshed off, which means it must go through mitosis, like always.
Epithelial cells are classified in two ways and always have both in their name.
The first way is by shape. They can be:
when identifying shape, one must look at the most superficial layer. some layers may look like another shape when it is deeper.
- squamous (squished)
- cuboidal (cube-like)
- columnar (columns)
- transitional (they really don't have a standard shape)
- simple (only one layer)
- stratified (many organized layers)
- psudostratified (looks like many layers, but in reality it's not)
the simple layers...
simple squamous epithelium
description: this layer, the simple squamous, is a single layer of flattened cells with disk shaped nuclei in the center of the cell.
function: this layer is really great because it is what allows nutrients to flow easily by methods of diffusion and filtration when protection doesn't matter. This layer also secretes lubricating substances in serosae (a smooth membrane consisting of a thin layer of cells which secrete serous fluid).
location: parts of the body that don't need much protection include the kidneys, the air sacs in our lungs, the lining of our hearts, blood vessels and lymphatic vessels. as well as the lining of the ventral body cavity.
simple cuboidal epithelium
description: the simple cuboidal cells are cubelike that are in a single layer. they have a central nucleus that is large and spherical.
function: the only function of the simple cuboidal cells are secretion and absorbtion
location: they are found in the kidney tubules; the ducts and secretory portions of small glands and the lovely ovary surface.
simple columnar epithelium
description: This interesting layer is made up of a single layer of column shaped cells. they may or may not be ciliated. this layer might also contain goblet cells (the ones that secrete mucus)
function: The simple columnar epithelium are good for absorption, secretion of mucus, enzymes and other stuff.
location: The ones that are not ciliated, line the digestive tract all the way from the stomach to the anal canal, the gall bladder and the excretory ducts of some glands. The ciliated kind line small bronchi, the uterine tubes and some regions of the uterus.
the stratified layers...
stratified squamous epithelium
description: this is my favorite type of tissue. The stratified squamous epithelium is a really thick membrane composed of lots of layers. The basal layers look like cuboidal cells (because they are active and working on mitosis) but when they die and head toward the surface, that's when we identify them and say that they are squamous.
function: The function of the stratified squamous epithelium is to protect the underlying tissues that could get hurt. The top ones just fall off it we scratch our arm or drink something or go poo, but because we have living ones underneath that constantly undergo mitosis, we're still alive and kickin'.
location: the nonkeretinized type lines the mouth, esophogus and the vagina. The keretinized type is the outermost layer of our skin! by the way, just remember that in this layer, there aren't any blood vessels, so if you bleed, then you've scraped farther than this layer - kinda cool right?
stratified cuboidal epithelium
description: this cube shaped epithelium usually appears in two layers.
function: it has one and only function: protection.
location: It is present in the largest ducts of sweat glands, mammary glands and salivary glands.
stratified columnar epithelium
description: the stratified columnar epithelium is usually several layers thick, and like the stratified squamous tissue, the stratified columnar tissue is also cuboidal at the basal layers.
function: Just two functions here: protection and secretion.
location: this tissue is rather rare. it is only found in the male urethra, and maybe a few large ducts.
the random layers...
pseudostratified columnar epithelium
description: This "random layer" really does belong with the single layered cells, but I felt it deserved to go somewhere else. The cells differ in height. Some of them don't even reach the surface. Their nuclei are not similar. The cells may contain goblet cells and cilia.
function: The pseudostratified columnar epithelium are good for secretion, and propulsion of that mucus.
location: If they are lame (because they are the unciliated kind), they're probably found in the sperm-carrying ducts of the guys in our society. If they're pretty cool (they have cilia) you'll be able to find them in the trachea, as well as most of the upper respiratory tract.
description: this tissue looks like both squamous and cuboidal tissues. some of the basal cells might look cuboidal or columnar. the surface cells might be dome shaped or squamous shaped.
function: this tissues allows organs to stretch
location: this tissue lines the ureters, the bladder and part of the urethra.