Tale of the tail: Disappearing act before birth

By: BRADLEY J. FIKES - Staff Writer | Saturday, February 17, 2007 7:38 PM PST ∞

Yes, you have a tail. Or, to be precise, you and every other human had a tail. Not that anyone would remember it: several-week-old embryos aren't known for their memories. Long before birth, your tail was resorbed into your developing body.

In very rare cases, a vestige of this tail sometimes remains in newborns. Usually harmless and easily removed, human tails are more of a curiosity than anything else.

Why should a tail grow in a human embryo at all, only later to disappear? The explanation from science is that these are vestiges of tails that grew in nonhuman ancestors. On rare occasions, a remnant of the tail persists through embryological development, although this tail of course has no function.

As a general rule, embryos of different species resemble each other more closely than do the adults of their species. The extremely young human embryo has what are called "gill arches," that resemble a fish's gills. These transform into jaw, face and throat tissue. Likewise, the embryos of some baleen whales produce tooth buds, then resorb them.

Normal human embryos develop a tail with about 10 vertebrae, which gradually shrinks over a period of a few weeks. By the eighth week, the tail is usually gone. Sometimes, a remnant persists, and this is what gives rise to human tails.

Doctors classify human infant tails as either "pseudotails" or "true tails." Pseudo tails are disorganized masses of tissue often caused by tumors. True tails contain well-organized connective, muscle and nervous tissue, blood vessels, and sweat and sebaceous skin glands. Rarely, these true tails will also contain cartilage and can even move.

Normally, all that's left of the tail by birth is a series of shrunken, often fused, vertebrae. They number five to three, but usually four, vertebrae. These are collectively called the coccyx, or tailbone. Unlike the other vertebrae, the spinal cord doesn't enter through the coccyx.

The word coccyx, by the way, comes from the Greek word for "cuckoo," referring to its resemblance to a cuckoo's beak. The coccyx performs several functions, mainly as an anchor for muscles and ligaments. It provides attachment for muscles such as the gluteus maximus and functions as a shock absorber.

Despite being concealed within the body, the coccyx is vulnerable to fracturing under stress. Sometimes, the damage and resultant pain is so severe that the cocxyx is removed.

And that's the end to the tale of the tail.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.

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