The Birth of Endocrinology
The birth date of endocrinology as a science is usually given
as 1902, the year two English physiologists, W. H. Bayliss
and E. H. Starling (Figure 36-1)
|Figure 36-1 Founders of endocrinology.
A, Sir William H.
Bayliss (1860 to 1924).
B, Ernest H. Starling
(1866 to 1927).
, demonstrated the action of
a hormone in a classic experiment that is still considered a
model of the scientific method. Bayliss and Starling were
interested in determining how the pancreas secreted its
digestive juice into the small intestine at the proper time of
the digestive process. They wanted to test the hypothesis
that acidic food entering the intestine triggered a nervous
reflex that released pancreatic juice. To test this hypothesis,
Bayliss and Starling cut away all nerves serving a tied-off
loop of the small intestine of an anesthetized dog, leaving
the isolated loop connected to the body only by its circulation.
Injecting acid into the nerveless loop, they saw a pronounced
flow of pancreatic juice. Thus, rather than a nervous
reflex, some chemical messenger had circulated from
the intestine to the pancreas, causing the pancreas to secrete.
Yet acid itself could not be the factor because it had
no effect when injected directly into the circulation.
Bayliss and Starling then designed the crucial experiment
that was to usher in the new science of endocrinology.
Suspecting that the chemical messenger originated in the
mucosal lining of the intestine, they next prepared an
extract of scrapings from the mucosa, injected it into the
dog’s circulation, and were rewarded with an abundant flow
of pancreatic juice. They named the messenger present in
the intestinal mucosa secretin.
Later Starling coined the term hormone
to describe all such chemical messengers, since
he correctly surmised that secretin was only the first of many
hormones awaiting discovery. The endocrine system, the second
great integrative system controlling
the body’s activities, communicates
by chemical messengers called hormones
, to excite). Hormones
are chemical compounds
released into the blood in small
amounts and transported by the circulatory
system throughout the body to
distant target cells
where they initiate
Many hormones are secreted by
ductless glands composed
of groups of cells arranged in cords or
plates. Since endocrine glands have no
ducts, their only connection with the
rest of the body is by the bloodstream;
they must capture their raw materials
from the extensive blood supply they
receive and secrete their finished
hormonal products into it. Exocrine glands,
in contrast, are provided with
ducts for discharging their secretions
onto a free surface. Examples of exocrine
glands are sweat glands and sebaceous
glands of skin, salivary glands,
and the various enzyme-secreting
glands lining the walls of the stomach
The classical definitions of hormones
and endocrine glands given
above, like so many other generalizations
in biology, gradually are being
altered as new information appears.
Some hormones, such as certain neurosecretions,
may never enter the general
circulation. Furthermore, evidence
suggests that many hormones, such as
insulin, are synthesized in minute
amounts in a variety of nonendocrine
tissues (nerve cells, for example), and
some, such as cytokines, are secreted
by cells of the immune system).
Such hormones may function as neurotransmitters
in the brain or as local tissue
stimulate cell growth or some biochemical
process. Most hormones, however,
are blood borne and therefore diffuse
into every tissue space in the body.
The first formal experiment in endocrinology
was performed in 1849 by a professor of
physiology at the University of Gottingen,
Professor Arnold Adolph Berthold. He conclusively
demonstrated that a blood-borne
signal was produced by the testes, and that
this chemical was responsible for producing
both physical and behavioral characteristics
that distinguished an adult male rooster
from immature chickens and adult male
chickens that had been castrated (capons).
Berthold castrated male chicks and divided
them into three groups. He left one group
of controls to grow normally without their
testes, and he reimplanted the testes into
the second group. The third group was
implanted with testes from different chicks.
As the chicks grew, he observed that the
castrated group developed into capons,
with no interest in hens, lacking rooster
plumage and male aggressive behavior. The
second and third groups of birds were
indistinguishable from each other,with full
male plumage, normal aggressive behavior
and interest in hens. Berthold then killed
the birds and dissected them. He found that
the transplanted testes had developed their
own blood supply and were functioning
normally. From this classic experiment,
Berthold concluded that testes must produce
a blood-borne signal, since there was
no nerve supply to the testes, which produced
all characteristics of maleness.
Compared with the nervous system,
the endocrine system is slow acting
because of the time required for a
hormone to reach the appropriate tissue,
cross the capillary endothelium,
and diffuse through tissue fluid to, and
sometimes into, cells. The minimum
response time is seconds and may be
much longer. Hormonal responses in
general are long lasting (minutes to
days) whereas those under nervous
control are short term (milliseconds to
minutes). We expect to find endocrine
control where a sustained effect is
required, as in many metabolic,
growth, and reproductive processes.
Despite such differences, the nervous
and endocrine systems function without
sharp separation as a single, interdependent
system. Endocrine glands
often receive directions from the brain.
Conversely, many hormones act on the
nervous system and significantly affect
a wide array of animal behaviors.
All hormones are low-level signals.
Even when an endocrine gland is
secreting maximally, the hormone is so
greatly diluted by the large volume of
blood it enters that its plasma concentration
seldom exceeds 10-9 M (or one
billionth of a 1 M concentration). Some
target cells respond to plasma concentrations
of hormone as low as 10-12 M.
Since hormones have far-reaching and
often powerful influences on cells, it is
evident that their effects are vastly
amplified at the cellular level.