How to use this page
This journey is designed to be done over about a week — you’ll come back three times. Each stop takes 30–40 minutes and has the same three beats:
- A little reading to set the scene.
- A little interaction to test what you just read (H5P activities embedded in your course page).
- A little confirmation — a mini self-check to prove you’ve got it.
When you finish all three stops, you’re ready for the lab session (or the epithelial tissue practical, if you’re enrolled in the remote section A cut or slice of the body or an organ for study.).
Stop 1 – Foundations
Why this stop matters: Everything else builds on these. If you can name the three simple epithelia and say where they live, you already know half the week.
What is epithelial tissue, anyway?
Epithelial tissues are one of four human body tissue types, alongside muscular, connective, and nervous tissues. They form linings in blood vessels, the bladderA muscular organ that stores urine before excretion., brain ventricles, the heart, and sweat glands. The lumenThe inside space of a hollow organ or structure. is the interior space or cavity within a tubular structure, organ, or vessel. On histological slides, the lumen appears as an open white space — and finding the lumen reliably leads you to the epithelial tissue. The basal surface (also called the basement membrane) anchors epithelial cellsThe basic structural and functional units of life. to underlying connective tissue through a thin extracellular layer containing collagenA structural protein in the dermis that provides strength and elasticity. and laminin. The apical surface is the exposed top layer facing the external environment or body cavities — often featuring microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption. or ciliaHair-like projections on the surface of some cells that move fluids or particles. for 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.. Two more things to know about epithelial tissue: it’s avascularTo be devoid of blood capillaries. Epithelial tissue is avascular, kind of like a cap of dead cells (no blood vessels of its own — nutrients diffuse in from the connective tissue below), and it’s generally non-innervated. This is why epithelium has slower metabolismThe sum of all chemical reactions in the body. and slower healing than well-vascularized tissues.
Quick reference:
Asvascular
Not innervated
Always against a lumen
How we classify it
We name every epithelium using just two features, in this order:
- Number of cell layers — simple (one layer) or stratified (many).
- Shape of the surface cells — squamous (flat), cuboidal (cube), columnar (tall), or transitional (shape-shifting).
So “simple squamous” = one layer of flat cells. “Stratified squamous” = many layers, flat on top. That’s the whole formula.
Meet the simple three
Simple squamous
One layer of flat cells. Think: gas/fluid exchange.
Simple squamous epithelium is a single layer of flat, scale-like cells. The cells are thin and closely packed, enabling efficient diffusionPassive movement of molecules from areas of high to low concentration. and filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre. Imagine a fried egg viewed from above — that’s the top-down look. From the side, the cells look like a thin line with flattened nucleiClusters of neurons in the CNS responsible for processing information. bulging slightly. Cells scraped from your cheek look like the top-down view; histological cross-sections are harder to recognize at first.
Layering: one cell layer. The tips of the cells overlap ever so slightly, like pancakes pushed to one side.
Function: gas, liquid, and small-molecule diffusion. The thin structure enables rapid exchange. In blood vessels, simple squamous cells allow O₂ and CO₂ to transfer into interstitial fluidThe fluid surrounding cells within tissues.. In lungs, they line alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood., giving gases a very short distance to travel.
Locations: alveoli, blood vessels (endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f — continuous with the heart’s endocardium), and serous membranesThin tissues that line body cavities and secrete fluid..
Specializations: endothelial cells may contain fenestrations (small pores) that facilitate substance exchange.
ID hint: A whisper-thin line of flattened nuclei. If you see flat nuclei and nothing else, you’re in the right place.
Simple cuboidal
One layer of cube-shaped cells. Think: secretion and absorption.
Simple squamous epithelium is a single layer of flat, scale-like cells. The cells are thin and closely packed, enabling efficient diffusion and filtration. Imagine a fried egg viewed from above — that’s the top-down look. From the side, the cells look like a thin line with flattened nuclei bulging slightly. Cells scraped from your cheek look like the top-down view; histological cross-sections are harder to recognize at first.
Layering: one cell layer. The tips of the cells overlap ever so slightly, like pancakes pushed to one side.
Function: gas, liquid, and small-molecule diffusion. The thin structure enables rapid exchange. In blood vessels, simple squamous cells allow O₂ and CO₂ to transfer into interstitial fluid. In lungs, they line alveoli, giving gases a very short distance to travel.
Locations: alveoli, blood vessels (endothelium — continuous with the heart’s endocardium), and serous membranes.
Specializations: endothelial cells may contain fenestrations (small pores) that facilitate substance exchange.
ID hint: A neat row of dice, each with a round nucleusThe control center of the cell that contains DNA and directs cellular activities. in the middle.
Simple columnar
One layer of tall cells. Think: absorption (small intestine).
Simple columnar epithelium is a single layer of tall, column-shaped cells. The cells are taller than they are wide, with nuclei typically located near the baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). of the cell. This shape allows efficient absorption and secretion.
Layering: one layer.
Function: absorption, secretion, and protection. Columnar cells line most of the gastrointestinal tract — especially the small intestine, where they absorb nutrients. In the stomach, gastric gland cells secrete mucus and gastric juice. Where the GI tract isn’t absorbing or secreting, simple columnar still provides a protective barrier against mechanical damage and pathogens.
Locations: stomach, small intestine, large intestine, bronchiolesSmall airways branching from the bronchi that lead to alveoli; lack cartilage and control airflow wi, uterine (fallopian) tubes, and the uterusThe muscular organ where a fertilized egg implants and develops..
Specializations: microvilli on the apical surface to expand absorptive surface area, and cilia in locations like the uterine tubesAlso called fallopian tubes, they transport the oocyte to the uterus. to move contents along.
ID hint: Tall cells, nuclei all lined up near the base, with a fuzzy brush borderA dense layer of microvilli on the surface of certain cells, increasing surface area for absorpti on top and the occasional chalice-shaped goblet cell.
List of terms
- section
- bladder
- lumen
- cells
- collagen
- microvilli
- cilia
- secretion
- movement
- avascular
- metabolism
- diffusion
- filtration
- nuclei
- interstitial fluid
- alveoli
- endothelium
- serous membranes
- nucleus
- base
- bronchioles
- uterus
- uterine tubes
- brush border