Tissue
A tissue is a group of similar cells that work together to perform a specific function, such as muscle tissue or leaf epidermis.
Definition
A tissue is a group of cells that look alike and do the same job; cells team up to form tissues, and tissues team up to form organs. More fully, a tissue is a group of similar cells together with the material between them that performs a specialized function. Animals have four primary tissue types: epithelial (covering and lining), connective (support and binding, including bone and blood), muscle (movement), and nervous (signaling), while plants have dermal, vascular, and ground tissues. Each tissue is an ensemble of cells of shared embryonic origin along with their extracellular matrix, which in connective tissue may dominate the volume: epithelia are polarized sheets joined by tight junctions and anchored to a basement membrane, connective tissues embed cells in collagen, elastin, and proteoglycan matrices, muscle contains contractile actin-myosin arrays, and nervous tissue consists of neurons and supporting glia. Tissue identity is maintained by cell-cell signaling and adhesion molecules such as cadherins, and histology, the microscopic study of tissues, is central to diagnosing disease.
Example
Muscle tissue is made of many muscle cells pulling together, so when you bend your arm the muscle tissue in your bicep is shortening. The lining of your small intestine is epithelial tissue with a surface area of about $30\ \text{m}^2$ thanks to folds and finger-like villi, and xylem is a plant vascular tissue that moves water from roots to leaves, sometimes over $100\ \text{m}$ in tall trees. Bone is a connective tissue in which osteocytes occupy less than $10$ percent of the volume, the rest being a mineralized matrix of collagen fibers and hydroxyapatite crystals that gives bone both tensile and compressive strength, and cardiac muscle cells are joined by intercalated discs with gap junctions so the whole tissue contracts as a unit.
Key Insight
One cell cannot do much on its own, but thousands of the same kind working together can move a whole limb. Blood is a tissue even though it flows: it is a group of cells (red cells, white cells, platelets) in a shared material (plasma) with a shared function, transport. Tissue engineering and organ transplant rejection both hinge on the same fact, that cells recognize their tissue neighbors chemically, and a cell in the wrong tissue environment usually stops functioning or is destroyed.