Scientists derive most embryonic stem cells from embryos that develop from eggs. These eggs have been fertilized in vitro, usually in a special clinic. With the donors’ consent, they are then donated for research purposes. They never come from eggs fertilized in the conventional way.

Growing embryonic stem cells in a lab

The process of growing cells in a lab is called cell culture. Stem cells are generated by transferring cells to a plastic laboratory culture dish. This dish contains a culture medium, which looks like broth.

The cells divide in the dish. Embryonic stem cell lines are not produced each time because this process is not very efficient. If the plated cells manage to multiply sufficiently to populate the dish, they are moved to a few fresh culture dishes. This process is repeated multiple times over a period of months.

Passages

Each of these cycles is referred to as a passage. The original cells yield millions of embryonic stem cells once the cell line is established. Embryonic stem cells that look genetically normal and have proliferated in a cell culture for more than six months without differentiating are referred to as an embryonic stem cell line.

Batches of cells can be frozen and transported to other laboratories for further experimentation at any time.

Identifying embryonic stem cells

Experts haven’t agreed on a standard test battery to identify these cells and their basic properties. Among the specific techniques applied to determine the presence of transcription factors are Nanog and Oct4. Transcription factors are a key part of the processes of cell differentiation and embryonic development because they help turn genes on and off at the right time. Both of the above techniques help keep the stem cells in an undifferentiated state, in which they can replenish themselves.

Growing the stem cells over several months ensures that they are capable of self-renewal and long-term growth. To assess whether the chromosome number has changed or they are damaged, the chromosomes are examined under a microscope.