The development of our spine: Growing a tail, then losing it again

In the earliest weeks of human development, we grow a tail. The tail’s growth and disappearance is a glimpse into the delicate balance of growth and regression during our creation in the womb.

This process, called neurulation, forms the spinal cord and surrounding tissues. Researchers found that the process in humans shares similarities with animals but also shows distinct differences.

The tail grows
New research involving 108 human embryos has provided fresh insights into how the human tail forms and later regresses.

The tail forms very early on, between weeks 3 and 7 after conception. The tail gradually lengthens and narrows into a pointed structure.

Unlike in mice, where the spinal cord develops rapidly and uniformly, the research showed that humans follow unique timelines and structures. For example, humans take seven hours to form a spinal segment, compared to two hours in mice.

The tail disappears
Around six weeks after conception, the fetus´ tail starts to shrink and eventually disappears.

The regression of the tail coincides with a "burst" of programmed cell death, a natural process called apoptosis, which eliminates specific cells in the tail's core tissues.

This balancing act between cell survival and programmed death is regulated by molecules that help sustain the stem cells responsible for tail elongation. Once these molecules decrease, the process halts, and the tail fades away.

Curiously, this regression mirrors a similar process seen in rodents, with one key difference: humans lose their tails entirely, while mice retain a shortened version.

Why it matters
Besides being a curious thing that we have tails in the womb - the development of the human tail is crucial for forming the lower spinal cord, which controls leg movement, sensation, and vital organ functions like bladder and bowel control.

If this process goes wrong, it can lead to neural tube defects. It may cause serious congenital neural tube conditions like spina bifida, where the bony spine does not close correctly and the babies are born with exposed spinal nerves.

The temporary human tail includes tissue layers that have the potential to form vertebrae. This raises questions about whether the human embryonic tail qualifies as a true tail or remains a remnant of evolution. Regardless, its short existence offers key insights into how our bodies form.

This brief life of the human tail reveals not only our evolutionary past but also the remarkable complexity of how the human body develops.

Would have been interesting to try and wag a tail - just for a day ...

About the scientific paper:

First author: Chloe Santos, UL
Published in: eLife, December 2024
Link to paper: https://elifesciences.org/articles/88584.pdf