Tissues, organs and organ systems
A tissue is a group of cells with a common origin that work together to perform a specific function. Organising cells this way is the first level of division of labour in an animal body, and every structure is built from just four tissue types:
- Epithelial tissue
- Connective tissue
- Muscular tissue
- Neural tissue
An organ stands one level higher, because it is made of more than one type of tissue, and those tissues act together towards a shared purpose. Organs whose jobs are related are grouped into an organ system. The degree of complexity is not the same in all animals: different animals show different levels of organisation in their organ systems, and greater complexity brings a sharper division of labour and higher physiological efficiency.
What makes an epithelium an epithelium
Epithelial tissue, or simply epithelium, is defined by one feature above all others: it always has a free surface, and that surface faces either a body fluid or the outside environment. The cells are compactly packed, with very little intercellular matrix separating them.
An epithelium rests on a basement membrane, a non-cellular layer that separates it from the tissue lying beneath. Epithelia are avascular, so nourishment must reach them from the tissue below that membrane. Two structural classes exist: simple epithelium, a single layer of cells, and compound epithelium, two or more layers.
Cell junctions
In nearly all animal tissues, specialised cell junctions provide structural and functional links between neighbouring cells. Tight junctions stop substances from leaking across a tissue. Adhering junctions perform a cementing role, holding adjacent cells firmly together. Gap junctions connect the cytoplasm of neighbouring cells and permit rapid transfer of ions and small molecules, so they serve communication between cells.
Simple epithelium: the three cell shapes
Simple epithelium, being a single layer of cells, lines body cavities, ducts and tubes. Its three forms are told apart by the shape of the cell.
Simple squamous epithelium is made of thin, flattened cells with irregular boundaries. It lines the walls of blood vessels and the air sacs of the lungs, and this thinness is the whole point: it forms a diffusion boundary, allowing gases to diffuse across it, and it takes part in filtration.
Simple cuboidal epithelium is built of cube-like cells. It is found in the ducts of glands and in the tubular parts of the nephrons in the kidneys, and its functions are secretion and absorption.
Simple columnar epithelium consists of tall, slender cells whose nuclei sit at the base of each cell. It forms the lining of the stomach and the intestine, and it too carries out secretion and absorption.
When columnar or cuboidal cells bear cilia on their free surface, the tissue is called ciliated epithelium. The cilia beat to move particles or mucus in a definite direction over the surface. This is the lining of the bronchioles and of the fallopian tubes.
Glandular epithelium and the two kinds of glands
Some columnar or cuboidal cells become specialised for secretion, and these are called glandular epithelium. A single secretory cell acting alone forms a unicellular gland - the goblet cells scattered through the lining of the alimentary canal are modified columnar epithelial cells of precisely this kind. Where many secretory cells are gathered together, as in the salivary glands, the gland is multicellular.
Glands are then separated by how they deliver what they make. Exocrine glands secrete through ducts, and their products include mucus, saliva, earwax, oil, milk and digestive enzymes. Endocrine glands are ductless; their products are hormones, which are poured directly into the fluid that bathes the gland rather than being carried away by any tube. The pancreas is a mixed gland: the islets of Langerhans form its endocrine portion and release hormones into the blood, while the acini form its exocrine portion and release digestive enzymes through ducts.
Compound epithelium
Compound epithelium is made of two or more layers of cells, and the extra thickness changes what the tissue is good for. Its main function is protection against chemical and mechanical stress; it has only a limited role in secretion and absorption, which is the reverse of the balance struck by the simple epithelia.
It covers the dry surface of the skin and the moist surface of the buccal cavity, and it also forms the lining of the pharynx and the inner lining of the ducts of the salivary glands and of the pancreatic ducts. Each of these places meets abrasion or strong secretions, and the deeper layers keep replacing whatever is worn away at the surface.
NEET Location–Function Checkpoint
| Tissue or structure | High-yield site | Main role |
|---|---|---|
| Simple squamous epithelium | Alveoli and blood-vessel walls | Diffusion and filtration |
| Simple cuboidal epithelium | Kidney tubules and gland ducts | Secretion and absorption |
| Simple columnar epithelium | Stomach and intestine | Secretion and absorption |
| Ciliated epithelium | Bronchioles and uterine tubes | Moves material over the epithelial surface |
| Compound epithelium | Skin, buccal cavity and pharynx | Protection from abrasion and stress |
Junctions are not interchangeable
- Tight junction: limits leakage across the epithelial sheet.
- Adhering junction: mechanically cements neighbouring cells.
- Gap junction: permits ions and small molecules to move directly between adjacent cells for rapid communication.
[NEET Important] “Endocrine pancreas” means the islets of Langerhans, not the entire pancreas. Its acini are exocrine and release enzymes into ducts.