Endocrine system of Birds and Mammals

Birds:

In birds pituitary gland secretes hormone prolactin. Prolactin stimulates production of “Pigeon’s milk” in pigeon’s crop. Prolactin also stimulates and regulates broodiness and certain other kinds of parental behaviour and along with estrogen stimulates full development of the brood pouch. Brood pouch helps to keep the eggs at temperature between 33 and 37°C. Birds thyroid gland produces hormone thyroxine. It regulates normal development of feathers and the molt cycle and plays a role in the onset of migratory behaviour. 

In male birds the teats produce hormone testosterone. It controls the secondary sexual characters of male such as bright plumage colour, comb and spurs all of which strongly influence sexual behaviour. Below parathyroid glands are ultimobranchial glands which are small paired. They secrete hormone calcitonin involved in regulating blood calcium concentration. Bursa fabricuis is a sac that lies just dorsal to cloaca. Its tissues produce secretions that are responsible for the maturation of white blood cells which play important role in immunological reactions.

Mammals:

Mammalian Endocrine system consists of pituitary gland, thyroid gland, Parathyroid gland, Pancreas, adrenal gland, thymus gland, pineal gland and gonads.

(1)        Hypothalamus: It is important part of fore brain which serves as connecting link between nervous and endocrine system. Its neurosecretory cells produce hormones which regulate the release of hormones anterior pituitary gland. Releasing hormones stimulate the release of other hormones while the inhibiting hormones cause inhibition of other hormones from the target organs. Some of its cells secrete antidiuretic hormone (ADH) and oxytocin which are actually stored in their nerve endings located in posterior pituitary gland.

(2)        Pituitary gland (Hypophysis): It is small pea size structure that hangs from the base of the brain. It is attached by short stalk of nerve fibres to hypothalamus.

Anterior Pituitary lobe: Under the influence of hypothalamus it produces its own hormones which tropic (regulate other endocrine glands) as well as others. Among tropic hormones are thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH) and follicle stimulating hormone (FSH), luteinizing hormone (LH), which regulates thyroid gland, adrenal cortex and gonads. Other hormones are growth hormones and prolactin. STH affects metabolism in many tissues and linear secretions which influence growth of bones and soft tissues.

Posterior Pituitary Lobe: The hormone released from here is actually stored secretion of hypothalamus. Antidiuretic hormone (ADH) increases re-absorption of water into the blood by the kidneys and decreases urine output. Other hormone oxytocin stimulates contraction of uterus during labour and release of milk from mammary glands during breast feeding.

(3)        Thyroid gland: Human thyroid gland is located at the base of neck in front of trachea. It is bilobed and butterfly like structure. It secretes thyroxine (Tetra iodo thyroxine or T4), Tri iodothyronine (T3) and calcitonin hormones.

Both T3 and T4 play important roles in controlling the metabolism of the body. Too much or too little of these hormonal levels in the blood can result in serious metabolic disorder, for instance in case of low levels of these hormones in the blood i.e. hypothyroidism results myxedema in elder which is characterized by over weight, sluggish, dry skinned, hair loss, intolerant of cold, confused and depressed individual, often thyroid in enlarged due to low intake of iodine in diet. This condition is called goiter. If hypothyroidism occurs in early age, it causes cretimism characterized by stunted growth, mental retardation and foarse facial features. In case of higher levels of thyroid hormones in the blood hyperthyroidism is produced. It is characterized by protrusion of eye balls (exopthalmia) increased heart beat, heat intolerance, high blood pressure, profuse sweating and weight loss.

(4)        Parathyroid glands: These of two pairs of pea size glands located adjacent to two lobes of thyroid gland in the neck. They release parathyroid hormone (PTH) in response to low level of calcium in the blood. PTH increases re-abosprtion of calcium in the kidneys as well as it induces demineralization of osteoclasts cells of bone to release calcium in the blood.

(5)        Pancreas: Most of the cells of pancreas are related to its exocrine function i.e. secretion of digestive enzymes. There are patches of pancreatic cells known as islets of langerhans which are endocrine glands. Islets consist of two distinct types of cells, alpha cells and beta cells which secrete hormones glucagox and insulin. Both of these hormones play role in regulating blood glucose levels.

Glucogen is secreted in response to decrease in blood glucose level. Glycogen and amino acids are converted into glucose in the liver to bring its level back the normal.

Insulin is secreted in response to higher level of glucose in the blood. It stimulates liver, muscles and adipose cells for uptake of glucose. Deficiency of insulin may lead to common disease diabetes mellitus in which this is high blood glucose level.

(6)        Adrenal gland: It lies above each kidney. Each adrenal gland consists of two parts, an outer cortex and an inner medulla.

Adrenal cortex: It works under the influence of ACTH of pituitary gland. It produces a number of hormones termed as corticosteroid hormones that have important effects on body metabolism.

Cortisol, are involved in glucose metabolism and are produced during anxiety, fever and disease’s Aldosterone increases the re-absorption of Na⁺ and Cl ions by kidney maintaining blood volume and blood pressure Androgens produces another group of corticosteroid hormones called androgens similar to testosterone (male hormone). It is secreted in both sexes.

Androgens causes development of secondary male characters such as growth of facial hair, deepening of the voice and increase in muscle bulk. Excessive secretion of androgens in female lead to masculization (appearance of beard) in ladies.

Adrenal Medulla: It is under the influence of sympathetic nervous system. It secretes adrenaline and non adrenaline under conditions of stress to bring about fight or flight response in emergency situations. They are also formed as emergency hormones.

Adrenaline (epinephrine) increases heart beat, blood glucose, breathing rate and metabolic rate.

Non adrenaline (nor-epinephrine) functions like epinephrine but its primary function is to sustain blood pressure.

(7)        Thymus gland: It lies in upper part of chest, behind the breast-bone and consists of two lobes that join in front of trachea. It secretes several hormones including thymosin that stimulates the development and differentiation of T-lymphocytes after they leave the thymus. T cells defend the body against viruses and other infectious microorganisms.

(8)        Pineal gland: it is a tiny cone shaped body within the brain. It responds to external conditions of light and darkness as sensed through the eyes. It secretes melatonin at night. The level of melatonin varies from day to night and with the season. The variation influences the growth and development of gonads.

(9)        Gonads: Testes: Male gonads or testes secrete androgens, most important of which is testosterone. It stimulates bones and muscles growth and development of secondary sexual characters such as appearance of beard, mustaches etc.

Ovaries: Ovaries secrete estrogen and progesterone. Estrogen maintains female reproductive system and development of secondary sexual characters in female. Progesterone is primarily involved in preparation and maintenance of uterus which supports the growth and development of an embryo.