Exotic endocrinology

Question 1

Describe the hormonal control of larval metamorphosis in amphibians

Answer 1

• Relies on thyroid hormones
• Hypothalamus secretion TRH, stimualtes TSH from pituitary, T3 and T4 produced
• Controlled by T3 and T4

Question 2

Descibe the inter-renals in amphibians

Answer 2

• Adrenal tissue in amphibians (and fish)

- Adrenal cells intermingled with renal tissue

Question 3

Where are the islet cells in amphibians?

Answer 3

Singly or in clusters in teh pancreas

Question 4

Describe the APUD cells in amphibians

Answer 4

• Amine precursor uptake and decarboxylation cells
• Scattered in many organs esp epithelium of GI tract
• Secrete variety of polypeptide hormones

Question 5

Describe calcitonin in exotic species

Answer 5

• Decrease serum caliciu
• Stimulates osteobalsts to increase bone formation
• In birds, no active decrease of serum Ca2+, just reduction in Ca2+ resorption from bone
• Produced by ultimobranchial bodies in brids

Question 6

Describe the ultimobranchial body in birds

Answer 6

• Paired, near paratyroid glands or thyroid dependign on species
• Small: 1.0-3.0mm diameter
• Cords/islands of C cells, parathyroid nodules in loose connective tissue stroma
• C-cells make calcitonin

Question 7

Describe PTH in exotics

Answer 7

• Same roles as in mammals
• Calcitriol activation, phosphate excretion, retention of calcium in kidney
• Absorption of Ca2+ in gut
• Activates osteoclasts to resorb bone and release Ca2+
• But birds more sensitive to PTH than mammals

Question 8

What are the 3 forms of serum calcium in birds?

Answer 8

• Same as mammals
• Ionised, protein bound and complexed
• Ionised active
• Ionised calcium is measured as total cahnges based on protein bound amount and pH (pH affects protein binding)

Question 9

What is the role of vitamin D?

Answer 9

• Regulates Ca2+ absorption across intestinal wall
• Induces formation of carrier protein calbindin
• Stimulates bone formation by osteoblasts

Question 10

Describe vitmain D in birds and reptiles

Answer 10

• D3 form required, cannot use D2 easily (unlike mammals)
• Sources include diet, supplements and conversion using UV-B
• Plants have D2, poor source for reptiles and birds
• Supplement is contain D3, if for mammals may be D2 and unsiotable
• Can convert D2 to D3 with sufficient UV-B light exposure

Question 11

Describe calcium homeostasis in amphibians

Answer 11

• Across skin or GI tract
• Excretion via urine, faeces and skin
• Storage in bone, endolymphatic sacs, skin

Question 12

Describe calcium homeostasis in rabbits

Answer 12

• Plasma Ca2+ 30-50% higher than other animals
• Passive absorption across gut wall, vit D not required but more efficient if present
• No feedback, absorb as much as is in diet
• Excretion via kidneys (can excrete up to 60%)
• Facilitated by high plasma calcium
• High tolerance for high levls of dietary Ca+

Question 13

Compare glucose homeostatic mechanisms in birds to mammals

Answer 13

• Higher glucagon, lower insulin
• Glucagon more important in carbohydrate metabolism
• Blood glucose poor indicator of energy balance

Question 14

Why is blood glucose a poor indicator of energy balance in birds?

Answer 14

• Many species hae small declines in glucose during polonged fasts
• Generally maintain glucose levels well
• Limited fluctuation

Question 15

Outline hyperglycaemia in reptiles

Answer 15

• Unlikely to be due to diabetes mellitus
• Can be affected by physiological and environmental variables
• Housing, lighting, heating, diet, vitamin and mineral supplementation
• Changes to environmenta, previous medical history, treatment with drugs
• Seasonal cycles: hibernation, reproduction (high glucose in breeding season, low in sutum, high after emerge from hibernation)
• Stress-related hyperglycaemia

Question 16

Describe the process of ecdysis

Answer 16

• Shedding of skin
• Cells in intermediate layer of epidermis replicate, form new 3 layer epidermis
• Lymph infiltrates between top and new layers, enzymes released, shed upper layer
• Controlled by thyroid hormones

Question 17

Describe thyroid hormones in birds

Answer 17

• T3 lower than in mammals
• Bird specific assay to detect T4, specific processing
• T4 levels fluctuate over 24 hours (higher at night when fasting)
• Linked to reproduction
• Avian thyroglobulin higher percentage iodine than mammals so easily develop goitre
• To assess T4, use TSH-stimulation test

Question 18

Outline the relationship between thyroid hormonnes and reproduction

Answer 18

Increased day length leads to increased T4 which increases reproductive activity

Question 19

Outline the control of song in birds

Answer 19

• Song nuclei affected by photoperiod and testosterone
• Mostly males that sing
• Seasonal stimulation of song nuclei by photoperiod and testosterone
• Photoperiod has direct stimulation on photoreceptors in brain
• Testosterone (and eostrogenic and androgenic metabolites) increases in spring, stimulates singing
• Melatonin attenuates increases in song volume induced by long days

Question 20

Describe the circadian system in birds

Answer 20

• Pineal gland (melatonin)
• Retinae of eyes (photic signal back to pineal gland)
• Self-sustained oscillatory in hypothalamus (generates own internal rhythm, affected by melatonin and light from pineal retina)
• Importance varies across species

Question 21

What is the dominant glucocorticoid in reptiles, birds, amphibians and rodents vs most mammals?

Answer 21

Corticosterone vs cortisol in most mammals

Question 22

Outline glucocorticoid release

Answer 22

• Stress leads to increased stimulation of corticotropin from PVN of hypothalamus
• Stimulates release of ACTH from pituitary
• Stimulates adrenal cortex (fasciculata region) to secrete glucocorticoids (corticosterone dominant in reptiles, birds, amphibians and rodents)