Four primary tissues
Every organ is built from just four tissue types: epithelial, connective, muscle and nervous. Learning to recognise them by structure is one of the most useful habits in anatomy.
Epithelial tissue
Epithelium covers surfaces, lines cavities and forms glands. Its cells sit tightly together on a basement membrane, with very little material between them. It is avascular, meaning it has no blood vessels, and gets nutrients by diffusion from the connective tissue beneath. Epithelia are named with two words:
- Number of layers: simple (one layer) or stratified (several).
- Cell shape: squamous (flat), cuboidal (cube-like) or columnar (tall).
Thin simple squamous epithelium allows quick diffusion, so it lines the air sacs of the lungs and the inside of blood vessels. Stratified squamous epithelium resists wear and forms the outer skin. Simple columnar epithelium, often with mucus-secreting cells, lines much of the digestive tract. Transitional epithelium stretches in the urinary bladder.
Connective tissue
Connective tissue is defined by its extracellular matrix: scattered cells surrounded by abundant fibres and ground substance. Types include loose connective tissue (packing around organs), dense regular tissue (parallel collagen fibres forming tendons and ligaments), dense irregular tissue (the deep skin layer), adipose (fat storage and insulation), cartilage (cells called chondrocytes in lacunae), bone, and blood.
Muscle and nervous tissue
Skeletal muscle is striped (striated), multinucleate and voluntary. Cardiac muscle is striated, branched, and joined by intercalated discs. Smooth muscle is non-striated and lines the walls of hollow organs. Nervous tissue contains neurons, which conduct electrical signals, and neuroglia, which support, insulate and protect them.
Worked walk-through: naming a tissue
Imagine a slide showing a single layer of very flat cells with thin, disc-shaped nuclei, lining a tiny air sac.
- Is the space between cells small or large? Small, so this is epithelium, not connective tissue.
- How many layers? One, so simple.
- What shape are the cells? Flat, so squamous.
- Name it: simple squamous epithelium.
- Check function: gases must cross quickly, and a single flat cell is the shortest possible path.
Now compare a slide with many layers of cells whose upper cells are flattened. Same method: tightly packed cells make it epithelium, several layers make it stratified, and flat surface cells make it squamous. The result, stratified squamous, suits a surface that is rubbed and scraped. Notice how the name itself describes the layers and shape.