A Basic Structural Unit (BSU) is essentially a building block. In anatomy, the basic structural unit of the body is the cell. All living organisms have cells, which start as the zygote — the single cell at the beginning of life, after a spermatazoon fertilizes an oocyte. In humans, the body has more than 200 different cell types. The human mouth contains bone cells, epithelial cells, endothelial cells, muscle cells, nerve cells, and cartilage cells. These make up four broader categories of tissue in the body: nerve tissue, muscle tissue, connective tissue, and nervous tissue. This means that the oral cavity involves nearly all the different types of tissue and basic structural units of the body. The innermost part of the BSU, or the nucleus, contains the genetic (DNA) information for the organism. Mitochondria provide energy to the cell to perform bodily functions, and the cell membrane functions as an outer wall.