The concept of a cell started with microscopic observations of dead cork tissue by Robert Hooke in 1665. Hooke coined the term "cell" based on the resemblance of the small subdivisions in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the first person to observe the living and moving cells under a microscope. In the century that followed, the theory that cells represented the basic unit of life developed.
Multicellular organisms develop from a single fertilized egg cell into a complex organism containing trillions of cells. During this developmental process, early, undifferentiated cells differentiate and become specialized in their structure and function. These different cell types form specialized tissues that work in concert to perform all of the functions necessary for the living organism. Different cells express different genes, which means they use their genes to make certain proteins but not others, depending on their internal condition and cues that their parent cells have picked up from their surroundings, primarily signals from other cells in the body.
There are about 200 types of cells in the human body with varied shapes, such as squamous cells, cuboidal cells, columnar cells. A structural skin cell may be shaped like a flat plate (squamous) and live only for a short time before it is shed and replaced. Packed tightly into rows and sheets, the squamous skin cells provide a protective barrier for the cells and tissues that lie beneath. On the other hand, nerve cells may be shaped like a star, sending out long processes up to a meter in length and may live for the entire lifetime of an organism.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.1: Studying cells
