How is not going through puberty linked to not having a sense of smell ?

I was diagnosed at the age of 23 with Kallmann syndrome. Up to that point I was always dismissed as a “late starter” or “late bloomer” when I asked why I has not started puberty yet.

When I saw the endocrinologist at the Royal Free Hospital in London UK, one of the first questions he asked was “could I smell”. He was the first doctor ever to ask me this question. I knew I could not smell but can not remember ever thinking about it much and never for a moment linking it to my lack of puberty. In all my previous appointments with doctors I never bothered to mention it either.

So what does link not being able to smell and not starting puberty ?

It is a fascinating story I think.

It all starts very early in the development of the foetus, between the 10th and 14th week of foetal development. The key is the movement of nerve cells or neurones. As the foetal brain develops there is a lot of movement as cells and tissues come together to form organs.

The sense of the smell comes from olfactory nerve cells and they have to form a structure called the olfactory bulb in order to work correctly and produce the sense of smell. Puberty and reproductive function relies on nerve cells that release a hormone called GnRH which should be located within the hypothalamus deep within the brain.

It so happens that during early development the olfactory nerves and GnRH releasing neurones originate in the same place and have to travel along the same pathway to their final destination; the olfactory bulb and the hypothalamus respectively.

In Kallmann syndrome the movement of these neurones is blocked, either because the pathway they are supposed to move through has not formed correctly or the proteins that are supposed to help them move are missing.

This means the olfactory nerves and GnRH releasing neurones are left stranded in their starting position. The olfactory nerves can not form the olfactory bulb so there is no sense of smell and the GnRH releasing neurones do no reach the hypothalamus so puberty and the reproductive cycle can not start.

The distances involved in this migration are so small, less than the width of a pin head, but the end result can be quite dramatic.