Tissue

Tissue derives from the French word ‘tissue’, which means something that is ‘woven’. In multi cellular higher organisms like animals and plants, the tissue comprises cells of similar structure that work together to perform a specific function. In biology, tissue is a cellular hierarchical organizational level between cells and organs. Similar tissues comprise ‘organs’, e.g., the brain, heart, lung, etc. All the organs together make up the entire body. In animals, four types of tissues exist: epithelial, muscle, connective, and nervous. Epithelial tissues work as a covering on the outside of the inner walls of the organs. They act as a barrier to guard the organ against external pathogens, injury, environment, and fluid loss. Muscle tissue is a specialized tissue that allows muscles to contract. Cardiac, skeletal, and smooth muscles are the three types of muscle tissue. Connective tissue is a gel comprising cells and a ground substance that surrounds cells. Nervous tissue is made up of neurons that help the transmission of nerve impulses.

In plants, tissues are divided into ground, epidermal, and vascular. In ground tissue, photosynthesis manufactures nutrients and is stored as reserve nutrients. Epidermal tissue is the plant body's outermost layer, regulating transpiration and water absorption. The xylem and phloem are the main components of vascular tissue, which internally transport nutrients and fluids.

Histology is the term used for studying human and animal tissues; histopathology is the study of tissues related to the disease. Plant tissues are studied in plant anatomy and physiology. Developments in electron microscopy, immunofluorescence, and freeze-fracture have improved the detailed examination of tissues. These tools can examine the appearance of tissues in health and disease, which substantially enhance the medical diagnosis and prognosis of diseases related to tissue. [M. Anwar Hossain]