The most versatile stem cells occur earliest in life. As a fertilized human egg divides, it first becomes a solid ball of cells, the morula. Next, about five days after fertilization, it becomes a hollow ball, the blastocyst. The cells of the outer layer of the blastocyst eventually form part of the placenta. Inside the ball is a small clump of cells, the inner cell mass, that will form all the tissues in the body. When isolated from blastocysts created by in vitro fertilization (IVF) and grown in culture, these are the cells known as embryonic stem cells (ES cells).

The floating blastocyst takes another day or so to attach to the wall of the uterus and begin to draw nutrients from it. The basic structure of the placenta forms in about three weeks.

Well before then the embryonic cells are already “too old” to make ES cells. They have already become committed to more restricted fates. The initial flat sheet of embryonic cells folds and twists and grows to form a recognizable embryo, with a rudimentary head and a central cavity surrounded by three "layers" of cells. The cavity lengthens to form the gut. Cells in the innermost layer, the endoderm, make the lining of the gut and associated organs like the pancreas and liver. Cells in the middle layer, the mesoderm, form pretty much everything else on our inside: muscle, bone, heart, and kidneys, and all the connective tissues in between. Those cells in the outermost layer, the ectoderm, become skin and brain.

By about eight weeks after fertilization, all the major structures and tissues of a human body, including the heart and even the eyelids, are in place, although they've still got a lot of developing to do. The result, less than 4 centimetres (1.5 inches) long, is now called a fetus.

As pregnancy continues, stem cells become more and more specialized. Fetuses and adults have many types of stem cells, but each type generally makes fewer different kinds of cells than stem cells from earlier stages in development. The so-called bronchoalveolar stem cells found in the lungs in adults, for example, make only cells found in particular parts of the lung.

Credit: Jenny Nichols / Institute for Stem Cell Research