tissues

Tissues are groups of cells that share a common structure and function. In multicellular organisms, tissues form the building blocks of organs and organ systems. They arise from specialized cells and extracellular matrices and are characterized by their cellular composition, organization, and the way they interact with surrounding tissues.

In animals, the major tissue types are epithelial, connective, muscle, and nervous tissue. Epithelial tissue lines surfaces, protects underlying structures, and forms glands. Connective tissue supports and binds tissues together and often contains an extracellular matrix rich in fibers and ground substance, as in bone, cartilage, blood, and adipose tissue. Muscle tissue contracts to produce movement and includes skeletal, smooth, and cardiac forms. Nervous tissue contains neurons and supporting glial cells and transmits electrical impulses that coordinate body activities.

In plants, tissues are organized into meristematic tissues that divide actively and give rise to growth, and permanent tissues with specialized functions. Permanent tissues include simple tissues such as parenchyma, collenchyma, and sclerenchyma, and complex tissues such as xylem and phloem, which transport water, nutrients, and photosynthates. Dermal tissue forms the outer protective layer, while ground tissue carries out photosynthesis, storage, and metabolic processes.

Tissues are studied in histology, the branch of anatomy that examines microscopic structure and organization. The arrangement of tissues into organs enables diverse biological tasks, from protection and support to transport, communication, and response to the environment.