ENDOCRINE SYSTEM
Pituitary gland
The pituitary gland or hypophysis is a small organ lying attached to the ventral part of the brain stem, just caudal to the optic chiasma. There are two lobes.
The anterior lobe (adenohypophysis) secretes thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), FSH, and LH, which regulate the action of the thyroid, adrenals and gonads (see Reproductive system for discussion of the gonads). It also produces prolactin, melanocyte-stimulating hormone (MSH), and somatotropin. FSH and LH are produced under the action of GnRH from the hypothalamus (Oglesbee et al. 1997).FSH in the male bird increases testicle size and stimulates spermatogenesis; in females it stimulates estrogenesis and development of the ovarian follicles. LH stimulates the Leydig cells to produce testosterone in males and in the female it is known to be involved in ovulation. Somatotropin appears to regulate body growth in young birds while ACTH controls the adrenal corticosteroids. TSH controls the thyroid glands; the exact role of the hormone MSH is unknown (Hodges 1981).
Prolactin is involved both in the reproductive system and carbohydrate metabolism. It stimulates brooding and nesting behavior in both males and females and inhibits gonadal activity by suppressing the secretion of FSH and LH. In Columbiformes it stimulates the production of crop milk from the crop epithelium. It is also thought to be associated with premigratory behavior stimulating hyperglycemia and hepatic lipidogenesis (Hodges 1981; King & McLelland 1984).
CLINICAL NOTE
The pituitary should be carefully evaluated in post mortems on budgies as pituitary adenomas and carcinomas are common. A normal pituitary gland measures 2 mm in diameter, but if neoplastic it can be increased 10-fold. Clinical signs caused by hormonal imbalance include persistent hyperglycemia and polydipsia or polyuria, while blindness, exopthalmos, and convulsions can be caused by pressure of the tumor (Rae 2000).
Posterior pituitary
Vasotocin and oxytocin are produced in the hypothalamus and stored in the neurohypophysis or posterior pituitary gland. They act on the uterus and the kidney. Vasotocin plays a major role in body water balance (antidiuretic hormone) and is transported via the bloodstream to the kidney where it causes water resorption. Both hormones play a role in uterine (shell gland) contraction and oviposition, although vasotocin is much more potent.
Pineal gland
This is a conical, pink structure located between the cerebral hemispheres and the cerebellum. This gland is believed to be involved with circadian rhythms, reproductive control, and photoreception (Hodges 1981).
Thyroid gland
These paired, oval glands lie just cranial to the thoracic inlet, lateral to the trachea and medial to the jugular veins. Unlike mammals, birds do not have C cells that produce calcitonin. This is produced instead by the ultimobranchial bodies. Avian thyroglobulin has a higher percentage of iodine than mammals, which is why many birds easily develop iodine deficiency (Oglesbee et al. 1997).
The two hormones thyroxine (T4) and triiodothyronine (T3) play many roles in avian species. These hormones regulate molting by stimulating the growth of new feathers. They also control metabolism, regulation of heat and growth, the reproductive organs, and increasing egg production (Hodges 1981; King & McLelland 1984; Rae 2000).
CLINICAL NOTE
Many birds like the budgie easily become deficient in iodine. This can cause swelling of the thyroid glands called goiter, which can get so large it blocks the esophagus causing regurgitation. In budgies the normal thyroid is pale and should be 2 mm in diameter; any enlargement seen on post-mortem could be a sign of an iodine-deficient diet (Evans 1996;
Rae 2000).
Parathyroid glands
These lie caudal to the thyroid and consist of two pairs of small, yellow glands, which are often fused together. These glands secrete PTH which, as in mammals, controls calcium and phosphorous metabolism.
It raises serum calcium by both increasing calcium reabsorption from the kidney tubule and releasing it from bone via osteoclastic activity. It decreases phosphorous levels by decreasing tubular reabsorption (Hodges 1981; Rae 2000).As some birds, like the domestic, can lay almost one egg daily they have a much higher demand for calcium than mammals. Estrogen stimulates the deposition of calcium in the medullary cavity of bones about 10 days prior to laying (Fig. 6.62). Under the action of PTH this calcium deposit is then shifted from the medullary cavity into ionized plasma calcium and used to calcify the eggshell (King & McLelland 1984; Oglesbee et al. 1997; Rae 2000).
Ultimobranchial bodies
These are small, pink, flattened glands lying just caudal to the parathryoids and consist of C cells, which secrete calcitonin. The role of calcitonin in birds is unknown as it does not appear to lower serum calcium. It may play a role in limiting extensive reabsorption of bone by PTH (Hodges 1981; King & McLelland 1984; Oglesbee et al. 1997).
Adrenal glands
These are small, ovoid glands lying just cranial to the kidneys and gonads on either side of the aorta and caudal vena cava. In birds the cortex is not well differentiated from the medulla (Hodges 1981; Rae 2000). In some male birds, like the domestic fowl, they are firmly attached to the appendix of the epididymis by connective tissue (King & McLelland 1984).
Adrenaline and noradrenaline (norepinephrine) are secreted by the medullary part of the gland. The cortical parts secrete corticosterone and aldosterone. In birds corticosterone has both glucocorticoid and mineralocorticoid activity and so plays a bigger role than aldosterone in electrolyte balance.
Pancreas
This lies in the mesentery of the duodenum and is often divided into three lobes. The endocrine portion has three types of islets: alpha, beta and delta. The alpha cells secrete glucagon, which regulates carbohydrate metabolism, increasing serum glucose levels by gluconeogenesis, lipolysis, and glycogenolysis.
The beta cells produce insulin, which lowers the level of serum glucose by stimulating tissue uptake and storage. Delta cells produce somatostatin, which regulates the levels of glucagon and insulin. Birds have much higher glucagon levels and lower insulin levels than mammals (Rae 2000). It is thought that glucagon plays a far higher role in carbohydrate metabolism than insulin (Hazelwood 2000; Hodges 1981; Oglesbee et al. 1997). The role of insulin is still poorly understood (Rae 2000).Other F cells (sometimes called PP cells) situated in the exocrine tissue of the pancreas secrete avian pancreatic polypeptide. This inhibits gastrointestinal motility and gallbladder and pancreatic secretion. It also induces a sense of satiety via the central nervous system (Hazelwood 2000).
Gastrointestinal endocrine cells
Endocrine cells are scattered along the intestinal epithelium but the majority of endocrine cells are situated in the pylorus. Hormones secreted by the gastrointestinal tract include somatostatin, secretin, and avian pancreatic polypeptide.
KEY POINTS
• Prolactin stimulates broodiness, production of crop milk, and build up of fat stores prior to migration.
• Laying hens deposit calcium in the medullary cavity of bone prior to laying, under the influence
of parathormone.
• Birds have a higher iodine requirement than mammals, which is why goiter is common in some birds.
• In birds corticosterone has both a mineralocorticoid and glucocorticoid effect.
• Glucagon plays a bigger role in avian carbohydrate metabolism than insulin.