How does an embryo produce stem cells

What types of stem cells are there?

Embryonic stem cells (ES cells) are not found in the adult body. They only occur in a very early phase of embryonic development, the blastocyst. This is a stage of development that a human embryo goes through on the 5th day after fertilization. At this early stage, no organs have formed, it is just a kind of cluster of cells (see Figure 2).

In order to preserve embryonic stem cells, the outer cell layer of the blastocyst must be destroyed so that the remaining cells of the inner cell mass (embryoblast) can be isolated and transferred to cell cultures in the laboratory (see Figure 3). This inner cell mass, i.e. the isolated cells, have the potential to develop into every cell type in the human body under controlled conditions, but the embryonic stem cells can no longer form a complete organism.

This property is known asPluripotency(lat. roughly multi-ability); in contrast to the totipotent, unicellular embryos (= original cell), which are always at the very beginning of individual development. Pluripotent stem cells are characterized by the ability to produce all three cotyledons (ectoderm, endoderm, mesoderm). You are therefore not yet restricted to a specific type of tissue (see Figure 4).

Research is carried out worldwide with various animal as well as human (human) embryonic stem cells (hES cells). In the laboratory it is possible to produce certain cell types, e.g. nerve cells, from embryonic stem cells by adding certain growth factors. This is known as differentiation (see Figure 4). In addition, the cells can be multiplied in the laboratory and kept in cell culture. Blastocysts that were produced for artificial insemination (in vitro fertilization) but are no longer used for implantation are usually used to produce hES cells. In Germany, the manufacture of hES cells in the laboratory is prohibited. However, it is possible to import hES cells from other countries for research under strict conditions.

Another possibility of producing embryonic stem cells is so-called therapeutic cloning, which is based on a somatic cell nuclear transfer (SCNT). In the case of a donated egg cell, the nucleus is removed and the nucleus of a somatic cell (body cell) is used instead. The cells obtained by this method are also pluripotent. So far, the process has only been used on animal cells. Further information on cell nucleus transfer can be found on the website of the German Reference Center for Ethics in the Natural Sciences.

Figures 2 and 3 were kindly provided by the German Reference Center for Ethics in the Biosciences.