Dialog in the Museum

“The nose has long been neglected by science. This has changed fundamentally in the last two decades,” emphasized Prof. Dr. Hanns Hatt from Ruhr University Bochum at the beginning of his lecture. The biologist, chemist, and medical physician numbers among the world’s leading scientists in the field of sensory physiology. Hatt and his research team succeeded for the first time in decoding a human olfactory receptor and clarifying its specific interaction with an odorant. He was also the first to discover that olfactory receptors are also found in many places in the human body other than the nose and fulfill important functions there. As part of the “Dialog in the Museum” lecture series, Hatt spoke about the importance of the sense of smell in human life and the effect of scents, and presented possible therapeutic approaches.

New molecular biological and genetic engineering methods have enabled scientists to describe the entire mechanisms of how scents act in the nose and convey the impression of smell in the brain, he added. Moreover, the frequent loss of smell and taste among COVID-19 patients has brought the sense of smell back to the focus of attention. “Smelling is essential to the sense of taste,” Hatt said. In humans, taste is one of the simplest, most primeval senses; this only takes place on the tongue, which is where the sensors for sweet, sour, salty, and bitter are located. But the most important requisite for eating and tasting is the nose – the sense of smell. If you hold your nose while you eat, you can’t tell a potato from a kohlrabi because you then can’t taste anything.

Smelling begins in the womb: We can smell even before we can hear or see. Embryos between the 24th and 26th weeks of pregnancy already have a perfectly developed olfactory system and “learn” scents in the womb; they perceive these for example through the food ingested by their mothers. The evaluation of scents is thus not genetically determined, but is conditioned or shaped by one’s cultural background and personal experience. The reason we all like vanilla, for example, is that its scent is also present in mother’s milk. The olfactory memory develops from an early age, Hatt pointed out: “Recollections of scents are the most stable that we have in our memories. Scents can take us back to our early childhood.” One scent that most people evaluate positively because it dates back to their childhood is that of apple pie; there is also a subjective “scent of home” (e.g. the smell of dung or the sea).

In this connection, Hatt explained how smelling works: The olfactory mucous membrane of the nose contains around 20 million olfactory cells, along with supporting cells responsible for their supply and embryonic stem cells that enable the olfactory system to renew itself about every four weeks. “The most important elements for the sense of smell are the receptors, which have only been known since the early 1990s: They have a binding site for odorants and function according to the lock-and-key principle,” Hatt said. All humans have the same 350 types of olfactory receptors, each of which is specialized for a particular scent, such as vanilla, violet, musk, etc. However, the activating scents are only currently known for about 30 percent of the receptors.

“Humans can distinguish between several million scents. But how can we perceive many more different scents with this relatively small number of receptors?” Hatt asked. This works in much the same way as the alphabet, with its endless possible combinations of letters. The “scent alphabet” has 350 letters, but “scent words” can comprise 150 letters or more. In nature, mixtures of scents are normally encountered. Hatt described the physiological process of smelling as follows: A scent is transported by inhaled air to the upper regions of the nose, where the scent molecule seeks out its corresponding olfactory cell according to the lock-and-key principle. The olfactory cell then transmits a nerve impulse to the olfactory brain or bulb (Bulbus olfactorius) and to the so-called glomeruli, of which there are also 350: Each glomerulus is specialized for one olfactory cell and collects its information. When a scent is detected, several glomeruli – depending on how many “letters” the scent comprises – send this information to the memory center, the hippocampus, where the “scent word” is assembled and stored. At the same time, the emotion associated with the scent at the time it is smelled is linked to it. This takes place in the emotional center of the brain, the limbic system. Because each person stores particular scents differently, Hatt continued, the same scent will trigger different sensations and assessments in different people.

“Each person produces a perfume that only he or she possesses – a distinctive, individual smell. This personalized body odor is determined by the genes. The more distinctive the genes, the more distinctive the individual smell.” The unique nature of these individual scents was exploited by the former GDR, for example, in the form of odor preserves: In the museum in the Stasi bunker near Leipzig, over 10,000 vacuum-packed cloths are stored, each with the perspiration odor of a former GDR citizen. The scents are so stable that a dog can still recognize a person today on the basis of these cloths.

Individual scents also influence the choice of partner. For example, women tend to choose men with a body odor different from their own. Since different body odors are also an indication of different genes, this helps achieve nature’s goal of ensuring a high degree of genetic mixing. Even people without a sense of smell can perceive odors: “Since scent receptors are not only found in the nose, but between 5 and 30 receptors are distributed throughout the body, odor effects can even be triggered in so-called odor-blind people: Scents find their way into the blood through the skin, in respiration, and via the gastrointestinal tract.” Hatt also pointed out that human sperm cells contain more than 20 different olfactory receptors, which influence their direction and speed of motion. It is therefore not surprising that 15 odorants have already been identified in vaginal secretion that are recognized by the receptors on the sperm cells.

Concluding his presentation, Hatt mentioned the therapeutic application of scents and gave a brief insight into this interesting field of research. Healthy human skin cells contain around 20 of the 350 scent receptors. Sandalwood scent (sandalore), for instance, enhances the growth and mobility of skin cells and promotes their regeneration and healing. Clinical studies have also shown that synthetic sandalwood scent prolongs the lifetime of human hairs by around 30 percent.

Intensive research is currently being conducted into the therapeutic significance of odorants in cancer cells, in which different scent receptors are to be found than in healthy cells of the same tissue: “It has been shown that administering a certain odorant reduces the division rate of liver carcinoma and leukemia cells (proliferation inhibition). Olfactory receptors can also function as diagnostic markers: The so-called olfactory receptor OR10H1, for example, is specific to bladder carcinoma and is secreted in urine.” Hatt concluded: “The future of olfactory research lies in clinical applications.”