Why are embryonic germ cells considered stem cells?

Question

In a class that I'm taking we were presented 3 types of stem cells.

1. Adult stem cells which come from bone marrow
2. Embryonic stem cells which come from embryos
3. Embryonic germ cells which come from testes

I understand that adult stem cells are a bit more specialized than embryonic stem cells, but I'm confused why embryonic germ cells are considered stem cells. From Wikipedia, the definition of stem cells is:

Undifferentiated biological cells that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells

But from my limited understanding of biology embryonic germ cells can't specialize into any other cells unless they fuse with another gamete, so why would they be considered stem cells? What am I missing here?

Answer 1

The difference in designation is the timing of the foundation of the cell line and the tissue that it was sourced from.

Embryonic Stem Cells are harvested from the inner cell mass of a Blastocyst around day 5 post fertilization. This is the first or second generation of cells to have started to differentiate, but they still have Pluripotency, which means that they can differentiate into any one of the three germ line cell types. [1][2]

Those three germ cell types are:

1. Endoderm
2. Mesoderm
3. Ectoderm

Embryonic Germ Cells are "cultured from primordial germ cells obtained from the gonadal ridge and mesenchyma of 5- to 9-week fetal tissue...." [1] These cells are multipotent, meaning that they will usually only be able to give rise to mesenchymal derived cell lines.[3][4]

There are also differences in replicative ability. ES cells are for the most part immortal cells able to be maintained in in-vitro culture for long periods of time, while EG cells have a life cycle of about 70 to 80 cell divisions before they reach quiescence.[1] If I had to venture a guess as to why this was the case, I would say that EG cells have likely already reached a stage of differentiation where they are repressing or down regulating the expression of Telomerase. ES cells will be expressing telomerase, so they will not experience telomere shorting due to the 3' end replication problem during DNA synthesis in S phase of mitosis.

There are other cell types of Adult Stem Cells other than Hemopoietic Stem Cells, so I am not sure why your course is making the distinction. It could be that in practice, we have only really successfully used Hemopoietic Stem Cells in treatments for diseases by doing bone marrow transplants. You would likely have to have your instructor clarify this point for you.

Answer 2

Stem cells in embryos are the basis for the development of the organism i.e once the gametes are fused you have a produced a cell that will replicate and differentiate into every cell that body is composed

Any cell that differentiates into another cell can be defined as a stem cell. Yes, the testes only produced sperm cells but a localized group of stem cells will almost always be differentiated into the same type of cell over the course of the organisms lifetime.

In reality a stem cell is no different than any other stem cell as they are all pluripotent. What makes stem cells different are the location in the body and the differentiation signals that are prevalent there.

For example the hematopoietic stem cells in bone are responsible for producing all types of blood cells due, while the stem cells in the crypt of the villi lining the intestinal tract differentiate into absorptive epithelial cells and mucus secreting goblet cells.

To summarize, a stem cell's differentiation is based on signals. The type of signals are ultimately determined by the function of the particular location that the stem cells reside in. (It is more complex than just "signals" but if you decide to delve into a bit of genetics and biochemistry it will make more sense).

Answer 3

Adult stem cells don't always come from bone marrow. In any or most adult tissues still have stem cells exist. Stem cell doesn't only mean it can specialize into other cells. Any cell, if it has self-renew ability and never grows old, can be called stem cells. So, for men's germ-line, even it has limit differentiation ability, maybe just could turn into sperm, but it has self-renew ability and never grows old. The is the reason that some old man, over 100 years old, they still can have a newborn....