When we worry about the health of our family and our friends, discussion often gravitates towards our hormones, the most talked about being insulin and the thyroid hormone. Hormones are signaling molecules found in most multicellular organisms. They facilitate communication between organs and tissues in our body that are far apart. The signals regulate a wide range of physiological and behavioral processes, such as growth and maturation, sleep, digestive functions, and stress responses.
Our school education has provided us with an understanding of the role of hormones in human physiology, with the respective endocrine glands that secrete hormones being described by their size and location in our bodies.
What is noticeable about these descriptions is how small these glands are. The adrenal glands, found atop each kidney, weigh 5-10 grams in adults. The pineal gland in the midline of the brain is the size and shape of a grain of rice and weighs 50-150 mg. The thyroid gland in the neck resembles a butterfly in shape and weighs about 25 grams.
Size versus function
One puzzle that has remained unaddressed is the huge differences in the size of the endocrine glands themselves. The thyroid gland may weigh no bigger than a chapstick, but a pineal gland is a mere grain of rice. This question has been recently answered, for many of our body’s hormones, in an analysis by the Alon group at the Weizmann Institute in Rehovot, Israel; the study was published in the journal iScience. They started with the number of cells in an endocrine gland that secrete hormones. Thus, the parathyroid gland, four of which are found in the neck, each the size of a lentil seed (in Hindi- masur; Tamil, avarai) weighs 120 mg and has about 10 million cells that secrete the parathyroid hormone. On the other extreme, the adrenal cortex, at over 5 grams, is much larger and has 4.5 billion cells that secrete cortisol.
All cells that are the target of a hormone molecule have a receptor for that molecule on their surface. Cells bearing these receptors can be tagged and their numbers estimated in slices of tissue using microscopes. This study shows that the number of hormone-secreting cells are in proportion to the number of cells that are targeted. Every hormone producing cell has about 2,000 target cells.
The adrenal cortex is relatively large, as is the thyroid. Adrenaline binds to all cells in the body that have a nucleus. The thyroid hormones keep up metabolic balance throughout the body. The parathyroid hormone comes from a diminutive gland, and its targets include the kidney, the pancreas and some parts of the central nervous system.
Some hormones are secreted by organs that have other functions too. The pancreas, weighing 80-100 grams, has a major role in secreting digestive enzymes. Only 1-2% of cells in the pancreas produce insulin, which targets the liver and muscle cells.
Adjustments to hormonal levels by dietary and other health measures can have a large impact on our well-being. For example, intermittent fasting reduces circulating insulin concentrations because the absence of food intake diminishes the requirement for insulin secretion. Lowered insulin levels also result from high fibre-diets, regular exercise, sufficient sleep and low stress levels. Reduced insulin levels make the cells of our bodies become more efficient at taking up glucose from the blood. This helps prevent insulin resistance. The small size of these organs belies the influence that they have on our well-being.
(The article was written in collaboration with Sushil Chandani, who works in molecular modelling. sushilchandani@gmail.com)
Published – December 28, 2024 09:25 pm IST
