Chemical Coordination
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)
, 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 (Gr. hormo-n, 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 physiological responses.
Many hormones are secreted by endocrine glands, small, wellvascularized 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 and intestine.
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 factors (parahormones), which 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.
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). |
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 (Gr. hormo-n, 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 physiological responses.
Many hormones are secreted by endocrine glands, small, wellvascularized 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 and intestine.
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 factors (parahormones), which 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.
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