Tailbud
Question Received:
Response:
Why does the human embryo have a tail?
5th March 2000
"Why" questions are especially difficult to
answer, but often the most interesting! The answer in this case is probably
"evolution", but we shall need to understand some of the background issues first
to see why.
Yes, the human embryo has an obvious tail region during the period of
development in which the neural tube and limb buds are forming. It first appears
in the 4th week after fertilisation, becomes most prominent during the 5th week
(see the illustration alongside), and then gradually becomes less and less
noticeable over the following weeks until by week 13 only the barest trace of a
tail can be seen as a slight elevation at the base of the spine.
That is not to say that the tail usually disappears without a trace - even in the adult there is a small triangular bone called the coccyx attached to the lower tip of the sacrum which is a reminder of the original tail. The coccyx is formed by the fusion of 3 to 5 (usually 4) small vertebrae during development. The spinal cord does not extend all the way down to the coccyx in the adult, reaching only as far as the intervertebral disc between lumbar 1 and lumbar 2 vertebrae. A pair of coccygeal nerves can be traced from the tip of the spinal cord, down through the vertebral canal, and out into the perineal region via the coccyx.
However, when the tail bud first appears in the embryo it is the site where the last part of the spinal cord is formed, and it is only later in development, during the fetal period, that differential growth of the vertebral column (which grows rapidly) and the spinal cord (which grows more slowly) results in the tip of the spinal cord retreating to a higher position in the vertebral canal. Thus, even though a tail is no longer visible externally by the time of birth, during development of the embryo the tail region makes an important contribution to the setting up of key structures such as the spinal cord. In addition to the coccyx, it is probably also contributing to development of the sacrum and related pelvic structures. (Very rarely, a baby is born with a "tail". This is usually composed only of inessential soft tissues and can be readily removed.)
The neural tube has been mentioned earlier. This is the precursor to the central nervous system, going on to form the brain and spinal cord. Most of the neural tube is created by the folding of an originally flat plate of ectodermal cells (the neural plate) into a cylindrical tube which then separates from the overlying ectoderm. While this process occurs, an important set of cells called the neural crest is formed where the neural folds meet, and the crest cells then migrate into the surrounding regions of the embryo. Here they produce a wide range of cell types, including sensory nerve cells, autonomic motor neurons, myelin-producing cells, skeletal tissues in the face and neck region, and pigment cells in the skin, to name just a few. The walls of most of the digestive tract become colonised by neural crest cells, which then set up the peripheral neural networks that control the tract. In the tail region of the embryo, however, a different pattern of development takes place. Instead of a neural plate that folds to form a tube, the last part of the tube is formed by the aggregation of a solid cord of cells which then form a central lumen which connects with the already-formed region of the neural tube. Thus, the last part of the spinal cord is formed by a different process. Generally this works well, but just occasionally the transition from the first to the second process gets into difficulty and a variety of birth defects can result. These include diplomyelia (duplication of the spinal cord), diastematomyelia (the presence of a bony spur between two hemicords), and some types of spina bifida. (This is something I have worked on - you may want to see Dryden 1980, for example. See also: Copp and Brook, 1989.)
There is a lot we do not yet understand about the development of the tail bud. For example, does it give rise to any neural crest cells, and if so, where do they end up? There are conditions in which the last part of the digestive tract is not properly connected up with the nervous system (Hirschsprung’s disease), and this is probably a consequence of the failure of neural crest cells to arrive at the right place at the right time. It is not clear yet whether those cells should have come from more cranial regions in the embryo where the neural plate folds to form the neural tube, or from the tail region where the secondary form of neurulation takes place.
So far we have looked mainly at "how" tail bud development occurs. Now to the "why" part of your question. Vertebrate embryos all seem to go through a similar stage of development during the time that the neural tube is being formed. There are inter-species differences before neurulation, and increasing differences after this stage as the species-specific characteristics unfold, but for this stage all vertebrate embryos seem to share common developmental processes (De Robertis et al, 1994). They all develop a tail region during this stage, and as noted above that tail region contributes to normal development whether or not the species retains a pronounced tail into adulthood. So it looks as if evolution has resulted in the selection of developmental mechanisms that can work effectively within a range of species at a particular stage of development - they are embryological common denominators.
References
Copp, A.J., and Brook, F.A. (1989) Does lumbosacral spina bifida arise by failure of neural folding or by defective canalisation? Journal of Medical Genetics, 26, 160-166.
De Robertis, E.M., Fainsod, A., Gont, L.K., and Steinbeisser, H. (1994) The evolution of vertebrate gastrulation. In: The evolution of developmental mechanisms, edited by M. Akam, P. Holland, P. Ingham, and G. Wray. Cambridge: The Company of Biologists Limited (pp 117-124).
Dryden, R. (1980) Duplication of the spinal cord: a discussion of the possible embryogenesis of diplomyelia. Developmental Medicine and Child Neurology, 22, 234-243.