483
CHAPTER THIRTEEN
Endocrine System
ON THE ORIGIN OF MALE BODY ODOR
the human counterparts of genes that aF
ect mating in rodents. The women used a
nasal spray for two weeks to clear their nasal passages. The men wore the same tee
shirt on two consecutive days without using deodorant or soap. Each woman was
then given three tee shirts from men genetically similar to her and three tee shirts
from men genetically dissimilar to her, not knowing which shirts came from which
men. The women sniF
ed the shirts—preferable to smelling urine, as mice do—
and rated the sweat stains on intensity, pleasantness, and sexiness. Like female
rodents, female humans preferred the sweaty tees from the men least like them.
The tee shirt study provided indirect evidence of the existence of human
pheromones. Analysis of the human genome has taken the study of human
pheromones a step further by revealing a possible basis for the female response
to male sweat. The gene that encodes a particular odorant receptor that aF
ects
social behavior in other animals comes in two variants in humans. Women of
genotype RT/WM either do not associate male sweat with a scent at all, or detect
a faint, pleasant, vanilla-like odor. But for women of genotype RT/RT, male
sweat smells like stale urine! In an experiment on 400 women asked to sniF
sixty-six odors in low or high concentrations, those who responded adversely
to androstenone, a metabolite of testosterone found in sweat, shared the RT/
RT genotype. Therefore, perception of male body odor may lie in the nose of
the beholder!
T
he endocrine system produces hormones, which are bio-
chemicals that spread messages in an individual. Less well
understood are pheromones, which are chemical signals
sent between members of the same species. In insects and
rodents, pheromones stimulate social behavior, including
mating. Experiments suggest that sensing pheromones, or at least certain
odors, may in±
uence human sexual attraction, too.
Mice and rats choose mates dissimilar to themselves in a group of genes
that provide immunity. Their sense of smell helps them discern appropriate
mates. Biologists think this mate choice based on smell may protect oF
spring
in two ways: it prevents close relatives from mating and it may team immune
systems with diF
erent strengths.
A mouse’s mating behavior stems from receptors in the olfactory epithe-
lium, a collection of sensory neurons high in the nasal cavity. These receptors—
called trace amine-associated receptors—are attuned to molecules in mouse
urine that direct social behavior. The genes that encode the receptors are
also found in the human genome.
To test if heterosexual humans use the sense of smell to respond to phero-
mones in mate selection, as rodents do, researchers recruited forty-nine young
women and forty-four young men. Each donated DNA, which was typed for
13.1
INTRODUCTION
Regulating the functions of the human body to maintain
homeostasis is an enormous job. Two organ systems func-
tion coordinately to enable body parts to communicate with
each other and to adjust constantly to changing incoming
signals. The nervous system is one biological communica-
tion system: it uses nerve impulses and chemicals called
neurotransmitters. The other is the endocrine system.
The
endocrine system
is so named because the cells,
tissues, and organs that comprise it, collectively called
endocrine glands, secrete substances into the internal envi-
ronment. (“Endocrine” means “internal secretion.”) The
secreted substances, called
hormones,
diffuse from the inter-
stitial fl uid into the bloodstream and eventually act on cells,
called
target cells,
some distance away.
Other glands secrete substances into the internal envi-
ronment that are not hormones by the traditional deF nition,
but they function in similar fashion as messenger molecules
and are sometimes referred to as “local hormones.” These
include
paracrine
secretions, which enter the interstitial
fl uid but affect only neighboring cells, and
autocrine
secre-
tions, which affect only the secreting cell.
Another category of substances, secreted by
exocrine
glands,
enter tubes or ducts that lead to body surfaces. In con-
trast to endocrine secretions, exocrine secretions are released
externally. Two examples are stomach acid reaching the
lumen of the digestive tract and sweat released at the skin’s
surface
(f g. 13.1)
.
The interrelationships of the glands of the endocrine system are obvi-
ous in families that have an inherited cancer syndrome called
multiple
endocrine neoplasia
(MEN). DiF
erent glands are aF
ected in diF
erent
family members, although the genetic cause is the same. One family
member might have a tumor of the adrenal glands called pheochro-
mocytoma; another might have thyroid cancer; yet a third relative
might have parathyroid hyperplasia, a precancerous condition.
13.2
GENERAL CHARACTERISTICS
OF THE ENDOCRINE SYSTEM
Cells of the endocrine system and the nervous system
communicate using chemical signals that bind to receptor
molecules.
Table 13.1
summarizes some similarities and
differences between the two systems. In contrast to the ner-
vous system, which releases neurotransmitter molecules
into synapses, the endocrine system releases hormones
into the bloodstream, which carries these messenger mol-
ecules everywhere. However, the endocrine system is also
precise, because only target cells can respond to a hormone
(f
g. 13.2)
. A hormone’s target cells have speciF c receptors
that other cells lack. These receptors are proteins or glyco-
proteins with binding sites for a speciF
c hormone. The other
chemical messengers, paracrine and autocrine substances,
also bind to speciF c receptors, and some examples of these
are included in the chapter.
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