Although they are externally very different, internally, an elephant, a sunflower, and an amoeba are all made of the same building blocks. From the single cells that make up the most basic organisms to the trillions of cells that constitute the complex structure of the human body, each and every living being on Earth is comprised of cells. This idea, part of the cell theory, is one of the central tenants of biology. Cell theory also states that cells are the basic functional unit of living organisms and that all cells come from other cells. Although this knowledge is foundational today, scientists did not always know about cells. The discovery of the cell would not have been possible if not for advancements to the microscope. Interested in learning more about the microscopic world, scientist Rob ert Hooke improved the design of the existing compound microscope in 1665. His microscope used three lenses and a stage light, which illuminated and enlarged the specimens. These advancements allowed Hooke to see something wondrous when he placed a piece of cork under the microscope. Hooke detailed his observations of this tiny and previously unseen world in his book, "Micrographia." To him, the cork looked as if it was made of tiny pores, which he came to call "cells" because they reminded him of the cells in a monastery. In observing the cork's cells, Hooke noted in "Micrographia" that, "I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular… these pores, or cells,…were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this…" Not long after Hooke's discovery, Dutch scientist Antonie van Leeuwenhoek detected other hidden, minuscule organisms — bacteria and protozoa. It was unsurprising that van Leeuwenhoek would make such a discovery. He was a master microscope maker and perfected the design of the simple microscope (which only had a single lens), enabling it to magnify an object by around 200 to 300 times its original size. What van Leeuwenhoek saw with these microscopes was bacteria and protozoa, but he called these tiny creatures "animalcules." Van Leeuwenhoek became fascinated. He went on to be the first to observe and describe spermatozoa in 1677. He even took a look at the plaque between his teeth under the microscope. In a letter to the Royal Society, he wrote, "I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving." In the 19th century, biologists began taking a closer look at both animal and plant tissues, perfecting cell theory. Scientists could readily tell that plants were completely made up of cells due to their cell wall. However, this was not so obvious for animal cells, which lack a cell wall. Many scientists believed that animals were made of "globules." German scientists Theodore Schwann and Mattias Schleiden studied cells of animals and plants respectively. These scientists identified key differences between the two cell types and put forth the idea that cells were the fundamental units of both plants and animals. However, Schwann and Schleiden misunderstood how cells grow. Schleiden believed that cells were "seeded" by the nucleus and grew from there. Similarly, Schwann claimed that animal cells "crystalized" from the material between other cells. Eventually, other scientists began to uncover the truth. Another piece of the cell theory puzzle was identified by Rudolf Virchow in 1855, who stated that all cells are generated by existing cells. At the turn of the century, attention began to shift toward cytogenetics, which aimed to link the study of cells to the study of genetics. In the 1880s, Walter Sutton and Theodor Boveri were responsible for identifying the chromosome as the hub for heredity — forever linking genetics and cytology. Later discoveries further confirmed and solidified the role of the cell in heredity, such as James Watson and Francis Crick's studies on the structure of DNA. The discovery of the cell continued to impact science 100 years later, with the discovery of stem cells, the undifferentiated cells that have yet to develop into more specialized cells. Scientists began deriving embryonic stem cells from mice in the 1980s, and in 1998, James Thomson isolated human embryonic stem cells and developed cell lines. His work was then published in an article in the journal Science. It was later discovered that adult tissues, usually skin, could be reprogrammed into stem cells and then form other cell types. These cells are known as induced pluripotent stem cells. Stem cells are now used to treat many conditions, such as Alzheimer's and heart disease. The discovery of the cell has had a far greater impact on science than Hooke could have ever dreamed in 1665. In addition to giving us a fundamental understanding of the building blocks of all living organisms, the discovery of the cell has led to advances in medical technology and treatment. Today, scientists are working on personalized medicine, which would allow us to grow stem cells from our very own cells and then use them to understand disease processes. All of this and more grew from a single observation of the cell in a cork.