Cells and Tissues Explained

What is a cell?

A cell is the basic structural and functional unit of life. It is a tiny, membrane-bound compartment that contains the machinery needed to grow, take in nutrients, turn those nutrients into energy, and reproduce. Human cells vary in size, but most are between 10 and 30 µm in diameter – roughly one-tenth to one-third the width of a human hair (≈70 µm).

What is a tissue?

A tissue is a group of similar cells that work together to perform a specific function. The cells in a tissue share a common shape, organization, and often the same type of extracellular matrix (ECM) that holds them together. Tissues are the building blocks of organs.

How cells become tissues – step by step

1. Cell specialization – During development, stem cells receive signals that turn on particular genes. A muscle stem cell, for example, activates genes that produce actin and myosin proteins, giving the cell contractile ability.
2. Cell adhesion – Specialized proteins on the cell surface, such as cadherins, bind to matching proteins on neighboring cells. This creates a stable physical link.
3. Extracellular matrix production – Cells secrete fibrous proteins like collagen that form a scaffold outside the cells. The ECM provides structural support and transmits mechanical forces.
4. Organization into layers or bundles – Depending on the function, cells arrange themselves in sheets, tubes, or clusters. Skin epidermis forms a flat, multi-layered sheet; blood vessels form a tube.
5. Functional integration – The coordinated activity of the cells and the ECM produces the tissue’s overall function, such as barrier protection, transport, or force generation.

Worked example: a tiny patch of epithelial tissue

Imagine a square patch of skin that is 5 mm on each side. The surface is covered by a single layer of flat epithelial cells that are each 20 µm wide.

• Convert the patch size to micrometres: 5 mm = 5 000 µm.
• Divide the length by the cell width: 5 000 µm ÷ 20 µm = 250 cells along one edge.
• Because the patch is square, the total number of cells in the layer is 250 × 250 = 62 500 cells. Each of those 62 500 cells adheres to its neighbours via cadherin proteins and sits on a thin basal lamina (a type of ECM). Together they form a continuous barrier that prevents water loss and blocks pathogens.

Worked example: muscle fibers in a fingertip

Consider a cylindrical fingertip muscle that is 1 cm long and 5 mm in diameter. Muscle fibers (cells) in skeletal muscle are roughly 30 µm in diameter.

• Convert the diameter to micrometres: 5 mm = 5 000 µm; radius = 2 500 µm.
• Cross-sectional area of the muscle: π × (2 500 µm)² ≈ 19 600 000 µm².
• Cross-sectional area of one fiber: π × (15 µm)² ≈ 707 µm².
• Estimated number of fibers: 19 600 000 µm² ÷ 707 µm² ≈ 27 700 fibers. These fibers are bundled together, each wrapped in a thin connective sheath (endomysium). The bundles are then wrapped in larger sheaths (perimysium and epimysium) that together form the functional muscle tissue capable of generating force.

The four basic tissue types in the human body

Tissue type	Primary cells	Typical function	Example
Epithelial	Squamous, cuboidal, columnar cells	Protection, absorption, secretion	Skin epidermis, lining of the gut
Connective	Fibroblasts, adipocytes, blood cells	Support, binding, transport, storage	Tendons, bone, blood
Muscle	Myocytes (skeletal, cardiac, smooth)	Contraction, movement	Biceps brachii, heart wall
Nervous	Neurons, glial cells	Signal transmission, processing	Brain cortex, peripheral nerves

Common beginner mistakes

Mistake	Why it is wrong
Thinking a tissue is the same as an organ	An organ is made of several different tissues working together. For example, the stomach wall contains muscle tissue, connective tissue, and lining tissue.
Assuming all cells in a tissue are identical	Even within a single tissue, cells can have subtle variations. In the liver, hepatocytes near a blood vessel receive more oxygen than those farther away.
Ignoring the extracellular matrix	The ECM is not “empty space”; it carries signals, stores growth factors, and determines tissue stiffness.
Believing that a single cell can perform the whole function of a tissue	Most tissue functions arise from the coordinated activity of many cells. A single nerve cell can transmit an impulse, but a sensory organ needs many such cells plus supporting tissue.
Confusing the four basic tissue types	Mixing up epithelial with connective tissue leads to wrong conclusions about function; epithelial cells form barriers, while connective tissue provides structural support.

Where the concept is used (standard textbook examples)

1. Skin epidermis – The outermost layer of the skin is a stratified squamous epithelium. It protects the body, prevents dehydration, and sheds dead cells.
2. Blood – Blood is a connective tissue in which cells (red blood cells, white blood cells, platelets) are suspended in a liquid extracellular matrix called plasma. The tissue transports oxygen, immune cells, and clotting factors.
3. Skeletal muscle – Muscle fibers (cells) bundle together into fascicles, which are wrapped in connective tissue layers (endomysium, perimysium, epimysium). The hierarchy allows precise force generation.
4. Intestinal lining – A simple columnar epithelium lines the small intestine, providing a large surface area for nutrient absorption while the underlying connective tissue supplies blood vessels and nerves.
5. Peripheral nerve – Bundles of neurons (axons) are surrounded by connective tissue sheaths (endoneurium, perineurium, epineurium) that protect and insulate the signal-conducting cells.

Check yourself

1. Question: What two main components hold a tissue together? Answer: Cell-cell adhesion proteins and the extracellular matrix.
2. Question: If a 2 mm-wide square of epithelium has cells that are 10 µm wide, how many cells are in the layer? Answer: Convert 2 mm to 2 000 µm; 2 000 µm ÷ 10 µm = 200 cells per side, so 200 × 200 = 40 000 cells.
3. Question: Name one mistake beginners make when describing tissues. Answer: They may confuse a tissue with an organ, thinking they are the same structure.

Understanding cells and tissues is the first step toward grasping how organs and systems operate. For a broader view of the human body, see the Pulse course.