There are 3200 cells in every 2.54 cm of skin. An adult has about 654.38+000 billion cells. But human cells will decrease with age. Human cells are the basic unit of human structure and physiological function, and the foundation of growth and development. Human cells have various shapes, such as spheres, squares and cylinders. Its size varies greatly. Most cells are only a few microns in diameter, and some can reach 100 micron or more. Although the shapes and sizes of cells are different, their structures are basically the same.