#3 Tissues, Part 2 - Epithelial Tissue

Creating order where there would otherwise be total mayhem.

Because you and pretty much every other animal is made up of incredibly complex, feisty, fidgety systems that need to be kept apart to some extent if they're going to get anything done.

Body needs order for it to function. It can't have your liver all up in your brain, or squished between your kidneys.

Without epithelial tissue, you'd essentially be a mushy pile of unarticulated goo.

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<Proper Epithelium & Glandular Epithelium>

Proper Epithelium,
which covers and lines your outer and inner body

Glandular Epithelium,
which forms glands and secretes hormones and other substances

Your primary epithelium protects your whole body, inside and out.

It covers the surface of your body when it combines with connective tissue to create skin, but it also lines your body cavities, and coats the internal and external walls of many of your organs.

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<We're All Just Tubes!>

The epithelium does all this to protect your deeper layers of tissue from injury of infection - like for example, by lining your stomach with epithelial cells that produce mucus, so that you don't digest yourself along with your lunch.

AVASCULAR
Not associated with or supplied by blood vessels.

The shape of the individual cells, and the number of layers that they form in.

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<Cell Shapes: Squamous, Cuboidal, or Columnar>

Squamous cells are flat. Their name means "scale", and they look kind of squished, like fish scales.

Cuboidal cells are cube-ish shaped, about as tall as they are wide. They absorb nutrients and produce secretions, like sweat. Their nucleus is pretty circular.

Columnar cells are tall and thick and look like columns, and they cushion underlying tissues. And as if they were cuboidal cells that got stretched tall, their nuclei also are stretched into an ellipse.

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<How Form Relates to Function>

The shape of each kind of epithelial cell correlates with its function.

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<Layering: Simple or Stratified>

A simple epithelium has only one layer of cells.

A stratified type has multiple layers set on top of each other, like the bricks and mortar of a wall.

A pseudostratified epithelium is mostly just one layer, but the cells can be different shapes and sizes, and the nuclei can be at lots of different levels, so it looks sort of messy and multilayered, even though it really isn't.

The tissue's first name as its number of layers, and its last name as the shape of its cells.

A simple squamous epithelium refers to a single layer of flat, scale-like cells, like the lining of the air sacs deep in your lungs.

A stratified cuboidal tissue would have layers of cube-shaped cells, like the linings of the ducts that leak swat and spit.

Epithelial tissue regenerates really quickly.

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<Epithelial Cells: Apical & Basal Sides>

The apical or upper side, is exposed to either the outside of your body, or whatever internal cavity it's lining.

The basal side, or inner surface, is tightly attached to the basement membrane, a thin layer of mostly collagen fibers that helps hold the epithelium together, and anchors it to the next-deeper layer- your connective tissue.

Allowing for some level of absorption, filtration, and excretion of substances.

The tissue lining your small intestines, for instance, is what allows you to absorb nutrients through diffusion and active transport, so that's pretty important.

Your glands are also made up largely of epithelial tissue, so it also plays a big role in facilitating all of your secretions - from sweat and mucus, to hormones and enzymes.

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<Glandular Epithelial Tissue Forms Endocrine & Exocrine Glands>

This glandular epithelium forms two different kinds of glands - your endocrine glands,the ones that secrete hormones right into your bloodstream or to nearby cells, and your exocrine glands, the type that secrete their juices into tubes or ducts that lead to the outside of the body, or the inside of your tube, rather than right into the blood.