The body structure is composed of systems that are made up of tissues that are also made up of cells (approximately 40 billions for an adult body). These components work in harmony for the proper functioning of the body.
A cell is the basic structural unit of living organisms. All living cells arise from other cells, either by division of one cell to make two, as in MITOSIS and MEIOSIS, or by fusion of two cells to make one, as in the union of the sperm and ovum to make the zygote in sexual reproduction.
A tissue is a group or layer of similarly specialized cells that together perform certain special functions. The body consists of several tissues classified according to its organ, e.g. connective tissue, epithelial tissue, fibrous tissue, etc.
An organ is made-up of tissues and an independent body part that performs specific function or functions. The body is composed of different organs like the heart, brain, liver, pancreas, lungs, kidney, etc.
A system is a set or series of interconnected or interdependent parts or entities that act together in a common purpose or produce results impossible by action of one alone.