The Microscope

• The first microscopes were developed in the late 1500s and early 1600s, but it wasn't until Holland's Anton van Leeuwenhoek developed a microscope capable of seeing bacteria in the mid-1800s that a series of scientists identified cells as the building blocks of all organic matter. The importance of the microscope and its ability to observe cells was fully recognized. Scientists realized that understanding cells -- how they divided and how they became differentiated -- would be crucial to understanding life and its mechanisms.
  
  

Blood Cell Types

• In 1908, the Russian scientist Alexander Maximov concluded from his microscopic studies that all blood cells come from only one type. He called this parent cell a stem cell, and theorized that certain signals from the body or the blood itself must be causing these stem cells to differentiate into the different types of cells present in the blood.
  
  

Bone Marrow Stem Cells

• It wasn't until the early 1960s that the theory of the existence of stem cells received more proof, this time from the research of two Canadian scientists, Ernest McColloch and J.E. Till. By injecting bone marrow cells into mice that had been irradiated, then recording the appearance of new nodules on the spleens of the mice, they were able to prove that the bone marrow cells functioned as the long-postulated stem cells would -- they were self-renewing.
  
  

Embryonic Stem Cells

• In 1981, Martin Evans and Matthew Kaufman, working at the University of Cambridge, and Gail R. Martin, at the University of California at San Francisco, were able to successfully extract cells from the embryos of mice cells that were capable of morphing into different types of cells and that could renew themselves perpetually and without ever further differentiating. Martin named these cells embryonic stem cells.
  
  

Human Embryonic Stem Cells

• The last step in discovering stem cells was to successfully derive and maintain a human stem cell line from embryonic tissue. A cell biologist from the University of Wisconsin-Madison, James Thomson, accomplished this in 1998 by growing the first human embryonic stem cell line from the tissue of leftover embryos at a fertility clinic.