Hormones are the body’s chemical messengers. Released into the bloodstream by the endocrine glands—including the pituitary, pineal, and adrenal glands, as well as the pancreas, thyroid, testes, and ovaries—they travel to other parts of the body, where each triggers a specific effect. Hormones are found in all multicellular organisms—plants, fungi, and animals—and they influence or control many physiological activities, including growth, development, puberty, the regulation of blood sugar levels, and appetite. The endocrine system, alongside the nervous system in animals, is one of the major methods of internal communication in living organisms.
At the beginning of the 19th century, biologists believed that the development of sexual characteristics was controlled primarily by the nervous system. In 1849, German physiologist and curator of the University of Göttingen’s zoological collection, Arnold Berthold, conducted a landmark experiment with chickens. He removed the testes from six male chickens, which resulted in the failure to develop male secondary sexual characteristics. However, when he transplanted testes from another bird into the abdomens of two of his castrated birds, they developed male secondary sexual characteristics as normal. When Berthold dissected the chickens, he discovered that the transplanted testes had formed no nerve connections. He concluded that whatever triggered sexual development had to travel via the bloodstream, rather than through the nervous system.
Despite Berthold’s findings, the belief that communication between organs occurred through electrical signals from the nervous system persisted. In 1902, British physiologist Ernest Starling and his brother-in-law William Bayliss investigated the pancreas and small intestine's nerves in their London laboratories. Testing Ivan Pavlov's theory that pancreatic secretions were controlled by nerve signals, they severed all nerves linked to the vessels supplying the pancreas and small intestine and introduced acid into the small intestine. Remarkably, the pancreas continued to produce secretions. They hypothesized that the acid triggered the small intestine to release a certain substance into the bloodstream. After extracting material from the lining of the small intestine, adding acid, filtering the ensuing fluid, and injecting it into an anesthetized dog, they observed pancreatic secretions within seconds. This demonstrated that the link between the small intestine and pancreas was not transmitted via the nervous system.
The chemical messenger released by the small intestine was named secretin, marking the first substance to be called a hormone. Starling and Bayliss discovered that secretin is released into the bloodstream from the small intestine in response to acidic fluid from the stomach. It then travels through the bloodstream to the pancreas, stimulating the secretion of bicarbonate to neutralize stomach acid. Secretin was found to be a universal stimulant, with the ability to stimulate the pancreas across different species.
Historical Developments in Hormone Research
In 1637, French philosopher René Descartes suggested that animals function like machines, subject to the laws of physics. This concept laid the groundwork for further exploration into biological processes. In 1815, Jean Pierre Flourens, a French physiologist, demonstrated the role of different brain parts in controlling behavior. Later, in 1901, Japanese-American chemist Takamine Jokichi isolated the hormone adrenaline, though the term "hormone" was not coined until four years later. By 1910, Edward Sharpey-Schafer showed the vital role hormones play in regulating body functions.
In 1921, Canadian physician Frederick Banting isolated the hormone insulin and used it to treat diabetes, revolutionizing the treatment of this condition. These discoveries, building on Arnold Berthold's earlier work, underscore the importance of hormones in regulating complex bodily functions.
Arnold Berthold: Pioneer of Endocrinology
Arnold Berthold, born in 1803 in Soest, Germany, was the second youngest of six children. He studied medicine at the University of Göttingen, where he presented his doctoral thesis in 1823. After touring several European universities, he returned to Göttingen as a professor of medicine in 1835 and became curator of its zoological collection five years later. Berthold's research spanned various fields. In addition to his groundbreaking chicken experiments, he identified an antidote for arsenic poisoning and studied myopia, pregnancy, and the formation of fingernails. He passed away in Göttingen in 1861. Since 1980, the German Society for Endocrinology has awarded the Berthold Medal in his honor, recognizing his contributions to the field.
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