General Information

The Lumen

A lumenThe inside space of a hollow organ or structure. refers to the interior space or cavity within a tubular structure, organ, or vessel. It is the hollow center or channel through which substances can pass. Because a lumen is an open space, it appears as an open white space on histological slides.  Finding the lumen (or lumina – plural) will always lead to the discovery of an epithelial tissue.

Apical and Basal Surfaces

The basal surface also called the basal lamina or basement membrane, is the bottom surface of epithelial cellsThe basic structural and functional units of life. that attaches to the underlying connective tissue. It consists of a thin layer of extracellular matrix, primarily composed of proteinsLarge molecules made of amino acids with various functions in the body. like collagenA structural protein in the dermis that provides strength and elasticity. and laminin, which provide structural support and anchor the epithelial cells to the underlying connective tissue. The basal surface acts as a selective barrier, regulating the passage of moleculesGroups of atoms bonded together. between the epithelium and the connective tissue. It also plays a role in cell signaling, differentiation, and tissue organizationThe structured arrangement of biological systems..

The apical surface, also known as the free surface, is the exposed or top surface of epithelial cells that faces the external environment, a body cavity, or a lumen (hollow space within an organ). This surface may have specialized structures such as microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption. (tiny projections) or ciliaHair-like projections on the surface of some cells that move fluids or particles. (hair-like structures) that serve various functions such as absorption, secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine., or movementA fundamental property of life involving motion of the body or its parts. of substances. The apical surface often interacts with substances or materials passing over or through the epithelial layer.

Avascular

​”Avascular” refers to a condition or characteristic of tissues or organs that lack a direct blood supply. In avascularTo be devoid of blood capillaries. Epithelial tissue is avascular, kind of like a cap of dead cells tissues, blood vessels, such as capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues., are absent or minimal, leading to limited or no blood circulation within the tissue. Since blood vessels play a crucial role in delivering oxygen, nutrients, and immune cells to tissues, avascular tissues may have lower metabolic activity and slower rates of healing compared to tissues with a rich blood supply. 

Innervation

When a tissue or structure is not innervated, it means that it does not receive nerve fibers or signals from the nervous systemThe organ system that controls body functions using electrical and chemical signals.. This can have implications for sensory perception, motor function, and the ability to respond to stimuliChanges in the environment that are detected by sensory receptors.. 

Cell Shapes

Epithelial tissue has three main cell shapes and two exceptions.  These main shapes are: squamous (egg-shaped, like fried egg shape), cuboidal, and columnar.  These different cell shapes reflect the diverse functions of epithelial tissue in protecting, absorbing, secreting, and transporting substances in various organs and systems throughout the body.

Layering

Epithelial cells can be found in one layer, called simple, or in multiple layers, called stratified.  Of course, there are two exceptions to this rule as well.  These classifications based on layering help describe the organization and function of epithelial tissues in different parts of the body. The number of cell layers in an epithelium correlates with its specific role in protection, absorption, secretion, or transport. If a tissue is a simple epithelium, it most likely functions in absorption or secretion.  If an epithelium is stratified, it most likely functions in some type of protection, allowing the layers of cells against the lumen to slough off with whatever it is that is moving through the lumen.