Biology 202
1999 Second Web Reports
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Pheromones are chemicals employed in intraspecies communication. Pheromones may be proteins, steroids, or any other chemical released by the body. The vomernasal organ (VNO) houses the chemoreceptors involved in detecting pheromones (3). The sensory system that perceives pheromones is similar in many ways to the olfactory system, particularly in that both systems transduce chemicals. Additionally, smell can be "emotionally potent" in humans (4) , and possibly other animals, just as pheromone detection could affect animals' emotional states, at least according to popular culture (5) .

In mammals, the VNO is an elongated tube lined with chemoreceptor cells that extends from the nasal palatine canal (3) . The neurons of the VNO are bipolar cells covered in microvilli or cilia (3) . Chemicals that stimulate the VNO are generally non-volatile and dissolve in the mucus of the nasal organ and diffuse into the duct (3) . In some animals, such as the hamster, large blood vessels near the VNO duct act as pumps, sucking the nasal mucus into the duct to increase the likelihood of contact between the chemicals and the neurons of the VNO (3) . Some volatile molecules may be detected if they reach the chemoreceptors of the VNO, but primarily dissolved substances stimulate the VNO neurons (3) . Either small amounts of specific chemicals or high doses of other chemicals may excite the chemoreceptors (3) . The specificity of the VNO is still ambigious.

The axons of the VNO receptor neurons terminate in the accessory olfactory bulb (AOB), where the input is initially processed (3) . The AOB, which is separate from the main olfactory bulb, connects to the amygdala, which then projects into the hypothalamus and preoptic area (3) . Input from the AOB does not appear to connect directly with the neocortex, which is why animals, such as people, may not be aware of pheromone stimulation (3) . Since the AOB is not distinct in humans, the direct path of the VNO receptors is unclear, if humans have VNO receptors at all (3,7) .

Whether humans have a VNO or not has been debated. During fetal development, a VNO does appear (3,6) . Many doubted the existence of the VNO in adult humans until the 1980's, when revived interest lead to the examination of thousands of human noses in search of the VNO (6) . The majority of nasal passages examined actually did have a small pit that seemed to resemble the VNO of other species (6) . Studies have yet to conclude if the human VNO is functioning or merely vestigial (7) . If the human VNO does function, its behavioral implications are dubious.

In other mammals, such as mice, voles, and hamsters, the VNO appears to play a vital role in several aspects of behavior, particularly mating. Removal of the VNO can be detrimental to the mating habits of rodents. Female mice and voles cannot influence each others' estrus cycles after their VNO's are removed (3) . Additionally, male stimuli cease to quicken puberty in immature female mice and voles without VNO's (3) . Male mice and hamsters do not mate when deprived of both VNO and olfactory input. Sexually experienced males will mate if either the VNO or the olfactory system is intact; sexually inexperienced males have difficulty mating without both systems (3) . Even though the VNO seems to be vital in the mating habits of rodents, the changes in rodent behavior after the removal of the VNO could be caused by something else entirely. A nerve, called the nervus terminalis (NT), is damaged when the VNO is removed (3) . The NT, which follows the vomernasal nerves and ultimately extends to the forebrain after bypassing the AOB, is very rich in LHRH, the hormone responsible for LH release (3) . Changes in levels of LH caused by damage to the NT could explain behavioral changes in rodents after VNO removal. In fact, the suspected human VNO nerves could simply be the NT (3) . However, the role of the NT in rodent mating seems to be marginal, since damaging only the NT has little effect on rodent behavior (3) .

Rodents appear to be affected by the VNO, but are humans? People do communicate chemically. A woman with a very regular menstrual cycle can unconsciously normalize the cycles of other women who live in very close proximity (8) . Vaginal secretions, particularly "volatile aliphatic acids," are also suspected of acting as pheromones (8) . The specific chemicals found in vaginal secretions vary cyclically, and male rhesus monkeys are sexually attracted to the odors of some human vaginal secretions, which contain many of the same chemicals as the secretions of female rhesus monkeys (8) . The biochemical likeness between humans and other primates suggests that humans and rhesus monkeys could react similarly to the same chemical stimulus. Yet, the relationship between these chemical communicators and the human VNO are still not clear.

The human VNO seems to be a possible factor in human behavior. While the role of the VNO in human chemical communication has yet to be established, circumstantial evidence suggests that the VNO could actually influence human activity. The commercial interest of the fragrance industry in establishing the reality of a functioning VNO will speed research, but the resulting data could definitely be distorted to suit the needs of the companies funding the investigations. Consumers should therefore be wary when considering the actual effectiveness of pheromones in perfumes. Also, if humans sense pheromones, they likely produce pheromones. Thus, people might already be benefiting from the wonders of pheromones, without spending any money.

WWW Sources

2) 2) The Pulse: Sex and the Power of Smell, a magazine-style news site

3) The Vomernasal Organ ,

3) 4) Dr. G.H. Gold Lab Homepage: More about Monell.

5) Scent of Eros, another commercial site (OPTIONAL)

4) 6) Making Scents , a fun and informative look at pheromones

7) Making Scents

5) 8) Menstrual Cycles and Odors , ,the demystification of female odor

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