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Robert Hook refined the design of the compound microscope around 1665 and published a book titled Micrographia which illustrated his findings using the instrument.

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In the 1800s, biologists began taking a closer look at both animal and plant tissues. What they learned advanced cell theory. Scientists could easily tell that plants were completely made up of cells. That's because plant cells have a layer on the outside called a 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. Schwann studied animal cells and Schleiden studied plant cells. These scientists found key differences between the two cell types. They had the idea that cells were the simplest units of both plants and animals.

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A scientist named Rudolf Virchow made an important discovery in 1855. He found that all new cells are made by existing cells. They copy themselves.

Later, scientists began to focus on genes. Genes tell the body how to grow and work. Chromosomes are like threads that carry genes and sit inside cells. In the 1880s, Walter Sutton and Theodor Boveri discovered the purpose of chromosomes. They are responsible for passing down genes from one generation to the next.

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The discovery of the cell continued to be important for science in the 1900s. Scientists discovered stem cells. Stem cells are simple cells. They still have to develop into cells with more specific jobs. This means they can grow into many different parts of the body. They could become part of your skin or your heart, for example. Stem cells are now used to treat many conditions such as heart disease.

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A new structure in human cells has been discovered by researchers at Karolinska Institutet in Sweden in collaboration with colleagues in the UK. The structure is a new type of protein complex that the cell uses to attach to its surroundings and proves to play a key part in cell division.

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No one discovered cells