The four basic tissues

A tissue is a group of similar cells, together with their extracellular matrix, that performs a common function. Epithelial tissue consists of tightly joined cells with little matrix that cover surfaces, line cavities and form glands. Connective tissue has relatively few, scattered cells embedded in abundant extracellular matrix; it binds, supports, protects, stores energy and transports (bone, cartilage, fat and blood are all connective tissues). Muscle tissue consists of elongated cells specialised for contraction (skeletal, cardiac, smooth). Nervous tissue consists of neurons, specialised for rapid electrical and chemical signalling, and supporting glia.

Organs combine the four: the stomach wall, for example, has an epithelial lining, connective-tissue lamina propria and submucosa, smooth muscle layers and an intrinsic nerve plexus. Organs are often described as a functional parenchyma (e.g. hepatocytes, nephrons) supported by a connective-tissue stroma carrying vessels and nerves.

Histology routinely uses haematoxylin and eosin (H&E) staining. Haematoxylin, a basic (cationic) dye, stains acidic structures — DNA in nuclei, RNA in ribosome-rich cytoplasm, cartilage matrix — blue-purple (basophilic). Eosin, an acidic (anionic) dye, stains basic proteins — most cytoplasm, mitochondria, collagen — pink (eosinophilic). Special stains highlight particular components: PAS (glycogen, mucins, basement membranes), trichrome (collagen), silver (reticular fibres), Verhoeff/orcein (elastic fibres), Sudan/Oil Red O (lipid), immunohistochemistry (specific proteins).