25-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 25: Hormonal control

25-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 25: Hormonal control in animals


Animal hormones

• Hormones are molecules that regulate an organism’s internal environment

• They are secreted by glands and nerve cells in response to stimuli

• Transported via blood system or diffuse through tissue to target organs and cells

• Modify action of target organ


Fig. 25.1: Hormonal control of a physiological function


Sites of action

• Autocrine hormones affect the cell that secreted them

• Paracrine hormones affect local tissues as they diffuse through extracellular fluid

• Endocrine hormones affect distant organs and tissues as they circulate in blood

• Pheromones affect other organisms, often over long distances, as they are released externally


Mechanisms of hormone action

• Target organs and cells possess receptors that are specific for a hormone

• Water-soluble hormones– catecholamines, peptides, proteins, eicosinoids– interact with surface receptors, trigger a secondary

response inside cell

• Lipid-soluble hormones– adrenocortical and gonadal steroids, ecdysones, thyroid

hormones– interact with intracellular receptors


Hormone control systems

• Hormone secretion controlled by changes in local environment

• Information from environment may cause release by

– innervation of hormone-secreting cells by neurons– direct release of hormones from neurosecretory cells– direct release of hormones from hormone-secreting cells


Neurosecretion

• Hormones are released from nerve endings of specialised neurohaemal organs

– bundles of neurosecretory cells– provided with a rich blood supply

• Cell bodies contain secretory granules– when stimulated, they release the granules into the blood

supply


Neurohaemal organs

• In vertebrates, neurohaemal organs are concentrated in the brain

• Hypothalamus– pituitary gland (hypophysis)

posterior pituitary gland and median eminence (neurohypophysis)

– pineal gland

• Adrenal medulla


Neurohypophysis

• Posterior pituitary secretes hormones that affect water balance and reproduction

– vasopressin (antidiuretic hormone) affects blood pressure

– oxytocin milk let-down birth

• Median eminence secretes hormones that stimulate or inhibit specific anterior pituitary gland hormones


Pineal gland

• Photosensory cells in fish, amphibians and some reptiles

• Secretory pinealocytes in birds, mammals and snakes

– produce melatonin derived from serotonin (a neurotransmitter)

– influence diurnal and seasonal rhythms


Adrenal medulla

• Part of adrenal glands– surrounded by adrenal cortex

• Adrenal medulla secretes catecholamines – adrenaline, noradrenaline and dopamine– neurotransmitters

• In blood stream, these neurotransmitters– mobilise energy reserves– influence cardiac contractility– affect blood pressure


Non-neural endocrine glands

• Associated with digestive and excretory systems• Grouped according to whether they respond to

changes in– external environment via neurosecretory hormones– internal environment

(cont.)


Non-neural endocrine glands (cont.)• Glands that respond to external environment

– anterior pituitary gland– thyroid gland– adrenal glands– gonads

• Glands that respond to external environment– parathyroid and ultimobranchial glands– islets of Langerhans– gastrointestinal mucosa


Fig. 25.13: Origins of some non-neural endocrine glands


Anterior pituitary gland

• Secretes hormones that act on other organs or on cells of the body

• Secretion controlled by neurosecretions of hypothalamus

• Most anterior pituitary hormones are subject to hormonal feedback control

– presence of hormone and/or target organ hormone in blood inhibits release of anterior pituitary hormone or releasing hormone from hypothalamus

(cont.)


Anterior pituitary gland (cont.)

• Double-chain glycoprotein hormones– TSH (thyrotropic hormone) acts on thyroid gland– FSH (follicle-stimulating hormone) and LH (luteinising

hormone) act on ovaries and testes

• Peptide hormones– ACTH (adrenocorticotrophic hormone) acts on adrenal

cortex– MSH (melanophore-stimulating hormone) acts on

melanophores

(cont.)


Anterior pituitary gland (cont.)

• Single-chain protein hormones– GH (growth hormone) acts on the liver and many other

cells in the body– PRL (prolactin) acts on mammary glands, gonads,

secretory epithelia and other structures


Thyroid glands• Hormones released by thyroid glands stimulate

– growth and development in immature vertebrates– metabolic rate in mature vertebrates

• Release under control of TSH• Secrete thyroxine (T4) and triiodothyronine (T3)

– synthesis depends on adequate intake of iodine– low iodine intake results in decrease in thyroid hormone

secretion– may lead to serious developmental and metabolic

problems


Adrenal cortex

• Outer layer of adrenal gland produces two classes of corticosteroids

• Glucocorticoids– influence carbohydrate and protein metabolism– e.g. cortisol, corticosterone

• Mineralocorticoids– influence Na+ and K+ balance– e.g. aldosterone


Corticosteroids

• Glucocorticoid secretion increased by ACTH in response to stressful conditions

– induce enzymes that synthesise glucose from non-carbohydrate sources (gluconeogenesis)

– prolonged action of glucocorticoids affect muscle and lymphoid tissues

• Mineralocorticoid secretion controlled by plasma K+ and angiotensin II levels


Gonads

• Ovarian follicles secrete steroidal oestrogens• Interstitial cells in testes secrete steroidal

androgens• Oestrogens and androgens influence

– growth– sexual characteristics– gamete development


Oestrogens

• FSH stimulates ovarian follicles to produce oestrogens

• LH + FSH cause ovulation• LH stimulates follicle cells to form corpus luteum• Corpus luteum secretes progesterone

– progesterone prepares reproductive tract for coating eggs (oviparous animals) or for embryo (viviparous animals)


Androgens

• FSH stimulates sperm production• LH stimulates secretion of androgens

– most common androgen is testosterone

• In mammals, gonads produce inhibin which inhibits secretion of FSH

– allows differential simulation of gonadotrophins by single hypothalamic-releasing hormone (GnRH)


Parathyroid glands• Parathyroid hormone (PTH) increases Ca2+

concentration– mobilises bone Ca2+

– promotes Ca2+ reabsorption by kidneys– stimulates synthesis of vitamin D, enhancing Ca2+

absorption from gut

• Responds to drop in blood Ca2+ levels• Excessive PTH secretion leads to demineralisation

of bone– osteoporosis


Ultimobranchial glands

• Ultimobranchial gland (thyroidal C-cells)– calcitonin depresses plasma Ca2+ – promotes uptake by bone– promotes excretion by kidneys

• Responds to increase in blood Ca2+ levels


Islets of Langerhans

• Hormone-secreting cells in pancreas• Composed of three cell types

– α-cells secrete glucagon– β-cells secrete insulin– γ-cells secrete somatostatin

• Insulin– increases permeability of cells to glucose and amino

acids– increases concentration of cytoplasmic glucose

transporters– leads to induction of metabolic enzymes


Effects of insulin

• Release of insulin from β-cells, results in– increased glucose utilisation– increased storage of glucose as glycogen

• As a consequence– increased fat production– increased uptake of amino acids– increased rate of protein synthesis– decreased blood glucose concentration

(cont.)


Effects of insulin (cont.)

• Insulin produces hypoglycaemic and anabolic effects

– hypoglycaemic = lowering blood glucose– anabolic = promoting tissue synthesis

• Glucagon produces hyperglycaemic and catabolic effects

– secreted when blood glucose levels fall– decreases glucose oxidation in cells– increases glycogen breakdown– promotes glucose synthesis from amino acids


Other hormones

• Gastrointestinal mucosa– hormones secreted by cells lining gut– cells detect substances in gut and secrete hormones into

blood vessels– secretin, gastrin, pancreozymin/cholecystokinin (CCK)

• Polypeptide growth factors – secreted by a variety of cells– mitogens stimulate mitotic division of cells– epidermal GF, nerve GF, transforming GF– regulate growth and differentiation


Hormones in insects

• Neurosecretory cells of brain and ganglia secrete brain hormone

• Corpora cardiaca (sing. corpus cardiacum) secrete bursicon

• Corpora allata (sing. corpus allatum) secrete juvenile hormone

• Prothoracic glands secrete ecdysone


Control of moulting• Moulting (ecdysis) in arthropods is controlled by

hormones• Prothoracicotropic hormone (PTTH) is

manufactured in brain and released by corpora cardiaca

• PTTH stimulates prothoracic gland to release ecdysone

• Other hormones associated with ecdysis include– ecdysis-triggering hormone, eclosion hormone, bursicon

Documents

25-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 25: Hormonal control