Flowers have long been prized for their natural beauty and almost guarantee a positive reception when given as a romantic gift. However, it appears that the chemical responsible for floral smells (‘bourgeonal’) is also linked with love and romance in an intriguing and unexpected way.
As humans our sense of smell is predominantly used to enrich and inform our experience of the world around us. This can be both pleasurable (i.e. smelling a flower) or functional allowing us to avoid harm (i.e. smelling to find whether or not food is safe to eat). In most animals smell serves a similar function. However, many animals have the added ability to detect chemical signals known as pheromones. Many species use sex pheromones to drive reproductive behaviour; sometimes relying on such signals to communicate important information about an individuals reproductive state or sexual potency.
The existence of an equivalent pheromone detection system in humans is highly disputed. This is mainly because the genes responsible for making pheromone detectors in humans are inactive, meaning pheromone receptors are not produced in the human nose. Furthermore, humans do not possess the organ (the vomeronasal organ) used to detect these ‘sex signals’. But does this mean that odor plays no role in the way we reproduce?
We experience smell through a series of specialised proteins known as ‘odor receptors’ (ORs). Although we normally associate these receptors with the nose, a healthy body of evidence has now identified these smell detecting proteins in many other areas of the body. Most bizarrely, they appear to exist in sperm cells! So, several questions come to mind; why do sperm need to smell, what can they smell, and could this be how we use smell for sexual communication?
It appears that certain ORs, found both in the nose and in sperm cells, respond to the floral compound ‘bourgeonal’ – the smell we associate with flowers. Therefore it would seem that male reproductive cells can smell flowers! This leads to an obvious question: What functional importance does the ability to detect floral odors play in sperm cell physiology and human reproduction?
The answer to this question may seem even stranger: Research suggests that the presence of floral scent detectors in sperm may be used to help them navigate towards an egg.
Sperm are motile cells that move via the presence of a ‘tail’. They may move towards the egg via a process known as chemotaxis; where cells direct their movement based on the relative concentration of chemicals around them. Experimental evidence has identified that sperm cell movement is directed towards areas of high concentrations of bourgeonal. Does this therefore suggest that, contrary to popular belief, eggs do not wait passively for the sperm to arrive but in fact produce floral chemicals that in turn attract the sperm to them? This would indicate that human female reproductive cells have acquired the capacity to produce chemical attractants, which may function to increase the probability of successful fertilisation by sperm. Interestingly, the ability for sperm cells to smell can also physically influence their swimming behaviour. It is known that the the swimming speed of sperm can be defined by the presence of particular chemicals. The presence of floral chemicals increase swimming speeds and directed movements. So, in a particularly remarkable fashion, it appears that sperm cells can detect floral odours and use these for navigation.
This research may soon radically transformed the way we understand sperm-egg communication, inferring that female eggs may produce floral odors which attract sperm for fertilisation. Most importantly, such research could provide the basis for novel strategies in the manipulation of human reproduction, offering advances in contraception and fertility treatments.
Infertility is often caused by a deficit in the number and quality of sperm; so bourgeonal could be used in IVF to enhance the swimming ability and targeting of sperm cells to eggs. Furthermore, the development of a drug which can block bourgeonal compounds could be used as an alternative to hormone manipulation strategies, as a new and effective contraceptive. However, the future of this research ultimately lies in the identification of a female-produced floral ‘scent’, which would provide the first empirical evidence for the use of pheromone sex signals in human reproduction.
Post by: Isabelle Abbey-Vital