Endocrine system

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The endocrine system consists of a number of organs (glands) located in different areas of the body which play an important part in the proper functioning of the animal. The organs produce special compounds called hormones, which, in turn, target particular systems or organs, and the way that they function. These glands are called endocrine glands because they do not have an opening to discharge their secretions but discharge them directly into the blood stream. They are then carried to their target systems and organs to carry out their task. In many cases different hormones operating together regulate particular functions. When these get out of balance, the bird’s body cannot function properly and hence performance will suffer, in some cases even to death.

Pituitary gland or hypophysis

The pituitary gland is often called the master gland because many of the compounds it produces target other similar glands to trigger them to produce their compounds that, in turn, influence the functioning of a particular system or organ. Thus, it can be said it is a controlling gland.
The pituitary gland is a pea-sized gland located at the base of the brain and is well protected by the surrounding skull bones. It consists of two parts:

1. Anterior pituitary
2. Posterior pituitary

The anterior pituitary gland is stimulated by special releasing factors from the hypothalamus of the brain to produce and release a number of chemical compounds. These compounds are hormones and include:

1. Thyroid Stimulating Hormone – stimulates the thyroid gland.
2. Adrenocorticotrophic Hormone – stimulates the adrenal cortex.
3. Sex hormones – stimulates the sex glands:
   • Luteinising Hormone
   • Follicle Stimulating Hormone
4. Melanin Stimulating Hormone – function in birds is unknown.
5. Natural Growth Hormone – stimulates growth of the animal.

The amount of these hormones produced by the pituitary gland will influence the level of activity of the target organ or response. The more that is produced the greater will be the response. The posterior pituitary gland produces arginine vasotocin and stores oxytocin that is produced by the hypothalamus. These play a part in the release of the yolk into the oviduct and the actual laying of the egg or oviposition. The secretions produced or stored in the pituitary gland enter the blood stream and are then transported to the part of the body that they target.

Hypothalamus

The hypothalamus is a major part of the brain located at its base and more or less centrally in the skull. As far as its endocrine functions are concerned, they include the production of the releasing factors that act as a control on the anterior pituitary gland, and oxytocin that plays a part in the release of the yolk. The quantity of the releasing factors and oxytocin released is influenced by day length – the longer the day to 18 hours the greater the amount of these compounds released and the greater the effect on the target gland or function.

Adrenal gland

The adrenal glands are small glands approximately 9 mm long located anterior to or in front of the kidneys. There are two such glands, each associated with a particular kidney. Each gland consists of two different types of cells that form two distinct parts of the gland – the adrenal cortex and the adrenal medulla.

The cortex produces three hormones:

1. Corticosterone – carbohydrate and fat metabolism, breakdown of protein and important in the bird’s reaction to stress
2. Aldosterone – retention of sodium
3. 8-hydroxycorticosterone – function unknown

The adrenal medulla produces two compounds:

1. Norepinephrine – fat metabolism
2. Epinephrine – control of blood pressure

Thyroid gland

The thyroid gland consists of two reddish purple glands lying one on each side of the base of the neck. This gland produces two hormones:

1. Thyroxine – helps regulate heat production, carbohydrate metabolism, promotes high blood sugar level, and promotes growth
2. Triiodothyronine – development of skin and feathers, may be involved in the moulting process

Parathyroid glands

These are two small, round, yellowish-white glands located at the base of the thyroid glands at the base of the neck. They produce a hormone called parathormone. Parathormone reacts to low blood calcium levels and works to increase the amount of calcium in the blood.

Ultimobranchial bodies

These are 1-3 mm long and are located just posterior (behind) to the parathyroid glands. They produce a hormone called calcitonin that works to reduce the calcium level in the blood stream. Thus, the hormones parathormone of the parathyroids and calcitonin of the ultimobranchial bodies must be in balance if the calcium levels in the blood are to be in balance to requirements.

Pineal body

The pineal body is a very small gland located above the mid-brain. Using tryptophan (an amino acid) it produces melatonin. Melatonin affects sleep, behaviour and brain electrical activity. Thus the pineal body acts as a biological clock and as such has an effect on the activities of the hypothalamus and its production of releasing factors.

Islets of langerhans

These are small clumps of special cells located in the pancreas, which sits in the duodenal loop of the small intestine. These special cells produce two hormones:

1. Insulin – lowers blood sugar
2. Glucagon – increases blood sugar and affects fatty acid levels

Gonads

The sex organs of males and females are called the gonads. These organs produce hormones called sex hormones and include:

• Oestrogen.
• Testosterone.
• Progesterone.

Both males and females produce and need all three hormones but in different amounts. When a male is castrated, for example, the balance of the sex hormones is affected, leading to the bird taking on female characteristics. This means that a capon or castrated male will, over time, take on much of the appearance and behaviour of a female.

Further information

• Bradley, OC (1960) The Structure of the Fowl, Tom Grahame ed, Oliver and Boyd, Edinburgh, UK.
• Dingle, J.G. (1991) Poultry Husbandry 1: Study Book, DEC, USQ, Toowoomba, Australia.