By Thomas L. Fuller, New York
I hope the illustrated chicken models of Hank and Henrietta have informed you of what makes our feathered friends “tick,” as the endocrine system will be the last part of my series on the biology of the chicken.”
I feel it only appropriate that the system of hormones, or the endocrine system, ties this series together. For hormones, along with the nervous system, actually tell the chicken what to do.
The endrocrine system is made up of a number of organs and major glands located throughout the bird. These glands and organs produce a special chemical messenger called a “hormone.” Hormones are very specific compounds that target particular systems and influence the way they function. Therefore, endocrine glands secrete their “messenger” hormone directly into the blood stream. Through the transport system of the chicken, the compound travels to a specific area to perform its task.
When discussing this system, what better place to start than with the pituitary gland. Often known as the “master gland,” this organ produces hormones that stimulate other glands, thereby influencing the functions of other systems or glands. Located at the base of the brain and well protected by the skull, the pituitary gland consists of two definitive parts, the anterior and the posterior lobes.
The anterior pituitary lobe is stimulated by the hypothalamus of the brain to produce and release a number of hormones. Among these is TSH, or thyroid stimulating hormone. Targeting the thyroid glands that are located on each side of the bird’s throat, it produces the hormone thyroxin. Thyroxin is the compound involved in carbohydrate metabolism, growth rate and temperature control. An additional hormone created by this gland, triiodothyronine, influences production of the skin and feathers.
The anterior lobe of the pituitary also produces and releases ACTH, or adenocorticotrophic hormone. This chemical messenger stimulates the adrenal glands. These glands are located on our chickens’ kidneys. The word adrenal comes from the Greek prefix “ad,” meaning “on” or “above,” while the suffix “renal” refers to the kidney.
These powerful glands are also broken down into two distinct parts, the adrenal cortex and adrenal medulla. The cortex produces three hormones, one of which the function is still unknown. The other two, corticosterone and aldosterone, influence carbohydrate and fat metabolism. They are also involved in the critical breakdown of proteins and sodium retention. The adrenal medulla produces two hormones, epinephrine and norepinephrine. You may know them better as adrenaline and noradrenaline, respectively. These compounds are active in the “fight or flight” response to stress. By affecting blood pressure and fat metabolism, these chemical messengers protect our Hank and Henrietta when faced with a threat that requires taking action.
Additional influence by the anterior pituitary lobe triggers the release of the sex hormones. Two hormones, LH, or luteinizing hormone, and FSH, or follicle stimulating hormone, stimulate the sex glands or gonads. Both Hank and Henrietta are affected by these gonadotropin compounds. For Hank, LH acts to stimulate cells that produce testosterone, while FSH promotes the production of sperm. Henrietta is influenced by LH to induce ovulation. For her, the combination of LH and FSH trigger steroid type hormones directing traits of the female.
The anterior lobe of the pituitary deserves one more recognition as the initial stimulation for GH, or growth hormone. The secretion of this chemical messenger stimulates growth and affects a wide range of physiological functions.
As mentioned, the other definitive part of the pituitary is the posterior lobe. This section of the “master gland” has less influence but is still critical to the overall health of our chicken. Here vasotocin is produced, which influences the water balance within our birds’ systems. This hormone also controls the social and sexual behavior for our Hank and Henrietta. Though not made here, oxytocin is stored here and plays a part in the release of the yolk and the actual laying of the egg, or oviposition.
Not all the glands in Hank and Henrietta’s endocrine system are influenced by the pituitary, or the “master gland.” The hypothalamus has influence over the pituitary gland and directly over the release of oxytocin. The environmental factor of light hours influences this gland to release its hormone. There is a direct effect on egg production when oxytocin induces the release of the yolk. It is this reasoning that suggests we give Henrietta 14 to 18 hours of light to secure optimum egg production.
Parathyroid glands located at the base of the thyroid gland react to low blood calcium levels in the blood. Once this deficiency has been detected, a hormone (parathormone) will work to release more calcium from areas it is stored. This is a critical balancing process consider-]ing how much calcium Henrietta needs in the formation of her eggshell.
The pineal body is a very small gland that acts as a “biological clock” for our birds. It uses an amino acid (protein) to produce melatonin. Melatonin affects sleep, behavior and brain electrical activities. In controlling these factors, it affects the hypothalamus, thereby influencing releasing factors to other glands and systems.
The pancreas is both an exocrine and endocrine gland. Through tiny tubes, the pancreas influences digestion in the small intestine. Because of the use of ducts (tubes) to transfer its compound, it is referred to as an exocrine gland. Here “exo” means out or outside of, referring to outside of the blood. As an endocrine gland, it secrets its chemical messengers of insulin and glucagon “within” the blood. The area of the pancreas where these two hormones are located is called “islets of langerhans.”
Blood sugar stays in balance by a negative feedback system, much like the way a furnace works in your house. The thermostat will call for heat when needed and turn off when the right temperature is reached. Glucagon releases sugar into the blood. When the correct amount is reached then the hormone insulin is called for to lower the amount of sugar in the blood.
In concluding my series on “The Biology of the Chicken,” I find the endocrine system to be critically important to our birds’ health. As with all the systems, we have had many similarities to our own mammalian systems. However, the chicken, being a bird and the only vertebrate capable of true flight, has undergone some unique variations in physiology. I hope Hank and Henrietta have given you some help with understanding the biology of our “feathered friends.”
Thomas Fuller is a retired biology teacher and lifelong poultry owner. You can read his entire series by purchasing past issues of our magazine at countryside network.com. Part 1 began with the digestive system in our August/September 2015 issue.
Originally published in the October/November 2016 issue of Backyard Poultry magazine.