Germ or sex cells: DNA content, research, clinical applications

Germ or sex cells have many important implications for health and research.

In today’s post the goal is to provide a helpful, comprehensive overview of these remarkable cells. Where does research on these cells stand today? It’s an exciting time.

What are sex cells?  | Are sex cells haploid or diploid?  | Why are germ cells important? | Sex cells in research

sex cells, germ cells
Sex or germ cell development in testes and ovary, left and right respectively. These cells include precursor or stem cells that make sperm (as we see on the left in male seminiferous tubules) and the sperm themselves as well as eggs in follicles in different stages of development in the ovary.

What are sex cells?

I’m talking about reproductive cells.

Sex cells include sperm for males and eggs for females but also the more primitive stem cells that make them. Some of the latter are called primordial germ cells. Male sex cells are also sometimes called spermatozoa, from which the shorter”sperm” is derived. Female sex cells are more formally oocytes (literally “egg cells”) or ova in biomedical research.

Taken together, we researchers most often call these sex cells by the name “germ cells” or gametes.

The “germ” name here does not reflect germs (as in pathogens). Instead it reflects the meaning of “germ” as in the origin of something else. Like the “germ of an idea.”

Are sex cells haploid or diploid?

The DNA content is kind of a big deal in these cells.

Sperm and eggs are haploid, meaning they only have half the amount of DNA as everyday regular cells of the body. They just have one copy of each of the 23 types of human chromosomes. We call this “1N”. In contrast, everyday somatic cells have two copies making 46 total chromosomes so they are 2N.

The haploid nature of sex cells makes good sense because fertilization brings one of each of them and their DNA together to make a resulting diploid one-cell embryo. That 2N embryo will then divide and all the resulting cells (except future germ cells) will be diploid.

Unlike the gametes themselves, the more immature stem and precursor cells that give rise to sperm and eggs are diploid. For example, spermatogonia, which ultimately give rise to sperm, are diploid. They divide through mitosis and then give rise to other cells that ultimately  go through a process called meiosis, wherein daughter cells end up haploid.

So if someone asks, “how much DNA do sex cells have?” The answer is they can be either haploid (1N) or diploid (2n) depending on which specific cells we are talking about here.  Some of these cells are even transiently 4N before the next two meiotic divisions.

Why are germ cells important?

Germ cells make sexual reproduction possible.

There are many other reasons why these special cells are important. If they carry mutations, for example, those will likely be passed along during development to offspring.

If germ cells have other kinds of problems they can cause infertility.

Sex cells can also give rise in rare cases to tumors, typically called germ cell tumors.

In even rarer occasions embryonic or abnormal germ cells that inappropriately remain in the body after development can later cause other tumors called teratoma. 

Sex cells in research

Scientists are studying germ cells in many kinds of research. Some of this relates to human reproduction and development. Other projects are focused on infertility or cancer. For instance, if we can understand why sometimes male sex cells are dysfunctional then we might be able to improve fertility. The same goes for female sex cells as well.

Interestingly, new research suggests that powerful stem cells called pluripotent stem cells might be able to make both male and female germ cells. It works in mice and the process might work in humans.  Some work in the lab suggests human stem cells can make potentially functionally human germ cells too.

This is a striking development because we now have technology to make pluripotent stem cells called iPS cells from just about anybody. If, in turn, specific people’s iPS cells can be made into sex cells, then those in theory might be used for assisted reproduction. This could take the place of standard reproduction or IVF in cases where there is infertility.

We don’t know if that would be safe though. The only way to be sure would be to try it in people and then if that doesn’t work it could be disastrous.

Unfortunately, this new idea of making human germ cells to be used for reproduction from iPS cells also has the potential to be misused too. One possibility is human cloning.

The video from The Whitehead above really hammers on the point that sex or germ cells are immortal in a sense.

Overall, sex cells are some of the most fascinating cells in the body.

1 thought on “Germ or sex cells: DNA content, research, clinical applications”

  1. I was looking at the blog written about February 2020 about stem cell treatment for GVHD, the comments seemed to have wrapped up in March 2020. there have been new developments in that field that should be mentioned, and the main prolific commentary seemed to be (Allen & Tanya) which I found strange and inaccurate, just thought I’d let you know.

    I am in communication with City of Hope and that research institution is considering a clinical trial using allogeneic MSCs to measure the efficacy of treating cancer survivor patients with GVHD, as well as tissue damage, in a translational regenerative medicine application.

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