Tissuelevel

At the tissue level, cells organize into coordinated groups called tissues, performing specific functions essential to the organism’s physiology. Tissues are composed of cells with similar structure and function and are supported by extracellular matrix and cell–cell junctions. This level sits between the cellular and organ levels in the hierarchy of biological organization.

In animals, the main tissue types are epithelial, connective, muscle, and nervous tissue. Epithelial tissue lines surfaces and forms glands; connective tissue provides structural support, protection, and storage; muscle tissue enables movement through contraction; and nervous tissue transmits electrical signals to coordinate activities. These tissues combine to form organs such as the stomach, heart, and brain.

In plants, tissue-level organization includes dermal, ground, and vascular tissues. Dermal tissue protects the plant and helps regulate gas exchange; ground tissue performs photosynthesis, storage, and support; vascular tissue, consisting of xylem and phloem, conducts water, minerals, and sugars. Plant cells differentiating into simple tissues (parenchyma, collenchyma, sclerenchyma) and complex tissues create the functional architecture of roots, stems, leaves, and reproductive organs.

Developmentally, tissues arise through processes of differentiation and morphogenesis. In animals, germ layers give rise to the major tissue types, while in plants, continuous activity of meristems generates new tissues during growth. The tissue level is central to understanding physiology, development, pathology, and regenerative biology, including efforts to engineer artificial tissues for medical use.