The Endocrine system consists of various glands and nodes which secrete hormones. Hormones are chemical messengers which travel in the blood to activate target cells. These target cells have special receptors, into which only certain hormones can fit. For example, testosterone act on the male gonads, but not the adrenal glands.
Hormones tell the body what to do. The endocrine system is closely linked to the nervous system, because both deal with communications and co-ordination, but there are a few distinct differences.
Hormones Act slowly, and for a long period of time. They cause gradual changes.
Nerve Impulses Act very quickly, and only last for an extremely short period of time. The changes they cause are instant.
Below are some of the main avian endocrine glands:
Pituitary: The pituitary Gland is sometimes called the master gland. It sits in the brain base, and releases various hormones which trigger other endocrine glands to release hormones. It has two parts, the anterior and posterior lobe. It releases:
- Natural Growth Hormone
- Thyroid Stimulating Hormone
- Adrenocorticotrophic Hormone (ACTH) – Stimulates the adrenal cortex
- Sex hormones (LuteinisingHormone (LH) and Follicle Stimulating Hormone (FSH)) – To stimulate the sex glands
- Melanin Stimulating Hormone – Unknown function (in birds)
The posterior pituitary also produces arginine vasotocin and stores oxytocin both of which play a role in egg yolk release.
The hypothalamus is located centrally in the brain, at the base. It produces oxytocin as mentioned above, and plays a role in controlling the anterior pituitary gland.
Pineal Body: This is a small gland in the centre of the brain. It produces melatnin through the ue of tryptophan (a type of amino acid). Melatonin affects electrical goings-on in the brain, and behaviour, as well as sleep.The Pineal body has been described as a ‘biological clock’ and part of it function is to alert the hypothalamus as to when to release certain hormones. An example of this is laying. In the wild, birds only lay in spring, because the pineal body ‘tells’ the hypothalamus that spring has come, and the hypothalamus begins producing LH and FSH.
Adrenal Glands: These are a pair of small glands, some 9mm long, located in front of the kidneys. They have two regions, the adrenal cortex and the adrenal medulla. The cortex produces 3 hormones…
- 8-hydroxycorticosterone – A hormone with an unknown function
- Corticosterone – Stress reaction, protein breakdown, fat and carbohydrate metabolism
- Aldosterone – Regulates sodium retention
The medulla produces two compounds:
Epinephrine – This plays a role in controlling blood pressure
Norepinephrine – Fat metabolism
These two glands are found at the base of the neck, each one lying at a different side. They produce:
- Triiodothyronine – Involved in feather (and beak and skin) development. Scientists think it may be involved in the moulting process
- Thyroxine Regulates carbohydrate metabolism, heat production and promotes high blood sugar and growth
Parathyroid Glands: are two assisting bodies located behind the thyroid glands. They produce Parathormone which regulates and controls the calcium level in the blood.
Ultimobranchial bodies: These are located behind the parathyroid glands. They produce calcitonin and lower the blood calcium, thus calcitonin and Parathormone must be in balance.
Islets of Langerhans: This group of specialized cells is located in the pancreas. It produces two hormones:
- Glucagon – Raises blood sugar, and affects fatty acids
- Insuli – Lowers blood sugar
Gonads or reproductive organs produces these hormones:
All of the hormones are needed in both genders, but the amounts vary. Hens need a lot more of the latter two, while cockerels produce much more of the former. When a cockerel is casrated, he becomes a capon and stops producing testosterone. Overtime, he takes on the behaviour, and to some degree, the appearance of a hen!