Endocrine SystemIntroduction:

• Animal hormones are usually secreted by organs specialized for their production and transported through the circulatory system to a very specific target tissue

• Overall Function – guide growth and development, regulate metabolism and maintain general homeostasis

Hormones in Invertebrates• Insects – V.B. Wigglesworth (1930’s) did

work on insect growth hormones by studying the metamorphosis of insects

• Insects show a pattern of growth different to that of vertebrates because of exoskeleton (limits size)

• Growth exerts pressure against inner surface of exoskeleton until it is shed

Wigglesworth performed experiments on bloodsucking insect – Rhodnius

• Goes through 5 nymphal stages, each separated by a molt

• must obtain a blood meal during each nymphal stage – stimulates release of hormones that cause molting at the end of a definite time interval following meal

• last molt occurs approx. 28 days after blood meal• if decapitated during days 1 – 7 after meal – no molting• if decapitated 8 days or after – molts in 28 days and

produces a headless moth• if decapitated 8 days after and joined to a another

Rhodnius (decapitated during days 1 – 7) – both insects molt into adults

• therefore, some stimulus passes via the blood from one insect to another and induces molting

• Stimulus must be a hormone whose secretion by the head begins about 8 days after the blood meal

• Brain Hormone (BH) – stimulates glands in the prothorax

• Prothoracic Gland – secretes a 2nd hormone, Ecdysone, which induces molting

• Wigglesworth then experimented to find out whether a molt will result in an adult or another nymphal stage

• found a 3rd hormone – Juvenile Hormone (JH) – produced by a pair of glands (corpus allatum) located behind brain

• when JH is present in high concentrations at time of molting – another nymphal stage follows molt

Hormones in VertebratesHormones are divided into four chemical

groups:

1. Peptide hormones – composed of chains of amino acids (peptide refers to comparatively short amino acid chains – longer chains are proteins)

– both peptides and proteins can function as hormones – all are referred to as peptide hormones

– largest group of hormones, water soluble, ex. Oxytocin and antidiuretic hormone (ADH)

2. Amino-acid derivatives – simplest hormones, simple molecules made from single amino acids ex. tyrosine is used to make epinephrine (adrenaline)

3. Steroid hormones – made from cholesterol, structure resembles cholesterol

– secreted by ovaries, placenta, testes, and adrenal cortex

4. fatty acid derivatives – also called prostaglandins

– produced by nearly every type of cell in body, and affect variety of body processes

– referred to as “local hormones” because act on cells in their immediate environment

• modify cyclic AMP levels, interact with other hormones to regulate various metabolic activities

Hormones function by binding to specific receptors on and in target cells

• some hormones bind to surface receptors (peptide and amino acid-based hormones are water soluble and cannot pass through cell membrane)

– react with protein receptors on target cell’s plasma membrane – may cause the following affect:

• surface receptor may be linked directly to channels in membrane that open when hormone binds

• ex. binding of epinephrine to heart muscle cells opens calcium channels increasing strength of contraction

Second messenger system• most common• hormone is the first messenger (binds to

receptor)• triggers release inside the cell of a

chemical (second messenger) – this initiates a series of biochemical reactions

• usually, cyclic AMP (cAMP)is the second messenger

• binding of hormone activates an enzyme that converts ATP to cAMP

• cAMP then initiates a chain of reactions

Second Messenger system

Some hormones bind to intracellular receptors

• Steroids are lipid soluble and pass through the cell membrane

• hormones then bind to receptors inside the cell (usually protein receptors in nucleus)

• receptor-hormone complex binds to DNA and stimulates certain genes to transcribe mRNA (resulting in protein synthesis)

• steroid and thyroid hormones work by stimulating the expression of genes

Steroid Hormones

Hormones are regulated by feedback mechanisms • negative feedback usually controls hormones in

animals – the secretion of the hormone stimulates a response in target cell that inhibits further secretion of the hormone

• few cases are controlled by positive feedback – end result stimulates release of more hormone

– ex. oxytocin stimulates contractions of uterus in childbirth – contractions stimulates further release of oxytocin

The Endocrine SystemMammals have types

of glands:• exocrine glands –

have ducts and produce secretions that are released outside the body or into digestive system

• endocrine glands – no ducts, release hormones into bloodstream

Hypothalamus • part of the brain that contains clusters of special

nerves cells called neurosecretory cells• neurosecretory cells form peptide hormones,

store them, and release them when stimulated• hypothalamus controls the pituitary gland• hypothalamus and pituitary gland together

coordinate action of many key hormonal messaging systems

• hypothalamus produces at least 9 different peptide hormones that control the anterior pituitary - releasing hormones or inhibiting hormones – stimulate or inhibit release of pituitary hormones

Pituitary Glandconsists of two lobes: anterior and posterior pituitary

(both parts controlled by hypothalamus)

1. anterior pituitary – produces several peptide hormones (1st four of these control hormone production in other endocrine glands):

– Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) – stimulate production of sperm and testosterone in males and of eggs, estrogen, and progesterone in females

– Thyroid-Stimulating Hormone (TSH) – stimulates thyroid gland to release its hormones

– Adrenocorticotropic Hormone (ACTH) – causes release of hormones from adrenal cortex

remaining hormones do not act on other endocrine glands

– prolactin – stimulates development of mammary glands (exocrine glands) during pregnancy

– endorphins – inhibit the perception of pain by binding to receptors in brain

– melanocyte-stimulating hormone (MSH) – stimulates synthesis of skin pigment, melanin

– human growth hormone – regulates body’s growth

2. posterior pituitary – releases two hormones actually produced in hypothalamus:

– Antidiuretic hormone (ADH)• prevent dehydration• increases permeability to water of collecting

ducts of nephrons• causes water to be reabsorbed from urine

and retained in body– Oxytocin – causes contraction of uterus

during childbirth and triggers “milk letdown reflex” in nursing mothers

Thyroid Gland• lies at the front of the neck around larynx

Produces:– thyroxine – iodine-containing modified amino

acid that raises metabolic rate of most body cells – levels are finely tuned by negative feedback

– release of thyroxine is stimulated by thyroid-stimulating hormone (TSH) from anterior pituitary (which is stimulated by a releasing hormone from hypothalamus)

– calcitonin – peptide important in calcium metabolism

Negative Feedback in Thyroid Gland

Parathyroid glands

• four small disks embedded in the back of the thyroid gland

• secrete parathyroid hormone (PTH) – controls concentration of calcium in blood and body fluids along with calcitonin (together they regulate calcium absorption and release by the bones)

• PTH stimulates release of calcium

• calcitonin inhibits release of calcium from bones

Pancreas • acts as both an exocrine and endocrine gland• exocrine portion releases digestive secretions to digestive

system through pancreatic duct• endocrine portion is made up of clusters of cells called islets of

Langerhans cells – produce peptide hormones• beta cells produce insulin• alpha cells produce glucagon• work antagonistically to each other to control carbohydrate and

fat metabolism, and control blood glucose levels• insulin promotes storage of fuel molecules• stimulates tissues (including liver, muscle, and fat cells) to take

up glucose from blood by facilitated diffusion• glucose is either immediately used for fuel or stored as glycogen• also inhibits liver cells from releasing glucose• overall effect is to lower glucose level in blood• effect of glucagon is opposite• raises blood glucose level by stimulating liver cells to convert

glycogen to glucose• also mobilizes fatty acids and amino acids

Gonads (testes and ovaries)

• testes produce androgens (steroid hormones including testosterone)

• ovaries produces estrogen and progesterone (both steroids)

• sex hormones play key role in puberty and development of secondary sex characteristics

• FSH and LH stimulate the gonads to produce sex hormones

Adrenal glands made up of two glands: 1) adrenal medulla and 2)

adrenal cortex

1. adrenal medulla is located in the center of each gland and is derived during development from nervous tissue

– hormone production is controlled directly by nervous system (sympathetic NS)

– produces epinephrine (adrenaline) and norepinephrine (noradrenaline) in response to stress (prepare body for emergency)

– increase heart and respiratory rates, cause blood glucose levels to rise, and directs blood flow to brain and muscles

2.adrenal cortex is the outer layer of glands• secretes three types of steroid hormones called

glucocorticoids made from cholesterol– glucocorticoid release is stimulated by ACTH (from

pituitary gland which is stimulated by hypothalamus releasing hormones)

– act similar to glucagon – raise blood glucose levels and promotes use of fats for energy instead of glucose

• secretes aldosterone – regulates sodium content of the blood (stimulates kidneys and sweat glands to retain sodium)

• also produces testosterone in low amounts (both males and females)

Other body tissues act as endocrine glands

1. pineal gland – located between two hemispheres of brain – secretes melatonin which affects daily rhythms – poorly understood

2. thymus – located behind breastbone – secretes thymosin which affects immune system

Also where T lymphocytes of immune system mature

3. stomach and small intestine – digestion is controlled by both nerves and hormones

– endocrine cells in certain regions of digestive tract produce at least 4 hormones to regulate digestion• gastrin (stimulates production of acid in

stomach)• secretin (stimulates pancreas to release

sodium bicarbonate)• cholecystokinin (stimulates pancreas to

release enzymes)• gastric inhibitory peptide (inhibits acid

production and peristalsis to slow down rate of passage of chyme into small intestine to allow more time for digestion and absorption