Chapter 37pages 713-732

Chemical Control of the Animal Body: The Endocrine

System

How Do Animal Cells Communicate?

Individual cells communicate with one another to ensure the proper functioning of the whole organism

Methods of communication between cells fall into four categories:

Direct Synaptic Paracrine Endocrine

Direct

Tissues such as heart muscle have gap junctions that directly link the insides of adjacent cells, allowing ions and electrical signals to flow between them

This type of communication is very fast, but also has a very short range

Non-direct Communication

In the other three types of communication, “sending” cells release messenger chemicals through their plasma membranes

chemicals move to “receiving” cells and alter their physiology by binding to receptors, specialized proteins on the surface or inside the receiving cells

When the messenger binds to a receptor,

the recipient cell responds in a way that is determined by the messenger, receptor, and type of cell

Target Cells

Every cell has dozens of receptors, each capable of binding a specific messenger and stimulating a particular response

Cells with receptors that bind a messenger molecule and respond to it are target cells for that message

Cells without the correct receptors cannot respond to the messenger and are not target cells

Therefore, a given cell can be a target cell for some messenger molecules but not others, depending it’s receptors

Synaptic

Synaptic communication is used in the nervous system Electrical signals within individual nerve cells send

information to the farthest reaches of the body in a fraction of a second

Then, the nerve cell communicates with a small number of other cells at junctions called synapses

At a synapse, a nerve cell elicits responses from a target cell by releasing neurotransmitters across a space between the nerve cell and its target

Paracrine

Cells release chemicals that diffuse through the extracellular fluid to other cells in the immediate vicinity

They influence only a small group of cells, but do so quickly because the distances are very short

Endocrine

Endocrine hormones are released into the bloodstream and move throughout the body in a few seconds.

They trigger responses that may last from a few seconds to a lifetime

Local Hormones

Local hormones diffuse to nearby target cells Many cells engage in paracrine

communication, secreting local hormones into the extracellular fluid

Histamine, released as part of the allergic and inflammatory responses

Cytokines, by which cells of the immune system communicate with one another

Local hormones have only short range actions because they are either degraded rapidly or taken up by nearby cells and cannot get far from the cells that secrete them

Prostaglandins

Prostaglandins - modified fatty acids - important local hormones secreted by cells throughout the body

They have diverse roles During childbirth they cause the cervix to dilate and

help stimulate the muscles of the uterus to contract Prostaglandins contribute to inflammation and pain

sensations

Drugs such as aspirin, acetaminaphen, and ibuprofen provide relief from these symptoms by blocking the enzymes that synthesize prostaglandins

Endocrine Hormones

Endocrine hormones are messenger molecules produced by the endocrine glands

The secretory cells of an endocrine gland are embedded within a network of capillaries, and the cells secrete their hormones into the extracellular fluid surrounding the capillaries

The hormones diffuse into the capillaries and are carried throughout the body by the bloodstream

Oxytocin

Stimulates the contraction of uterine muscles during childbirth because the muscle cells have receptors that bind oxytocin

Oxytocin does not cause other muscles of the body to contract because their cells do not have the necessary receptors

Uterine muscles contain target cells for oxytocin, whereas other muscles do not

Endocrine cellsrelease hormone

The hormone enters theblood and is carriedthroughout the body

The hormone leavesthe capillaries and diffusesto all tissues throughthe extracellular fluid

capillary

(extracellularfluid)

2

1

3

5 The hormone cannot affectcells that only bear receptors towhich the hormone cannot bind

The hormone affectscells bearing receptorsto which the hormonecan bind

uterus

biceps

4

Hormone Release, Distribution, and Reception

Results

The changes induced by hormones may be prolonged and irreversible The onset of puberty or the transformation

of a caterpillar into a butterfly

The changes are temporary and reversible, and help to regulate the physiological systems of the animal body within a course of time of seconds to hours

Vertebrate Endocrine Hormones

Are evolutionarily ancient

Insulin is found not only in vertebrates, but also in protists, fungi, and bacteria, although the function of insulin in these organisms is unknown

Thyroid hormones have been found in invertebrates such as worms, insects, and mollusks, which do not have thyroid glands

Hormones appear to work similarly in the cells of invertebrates

Three Classes of Vertebrate Endocrine Hormones

Peptide hormones - chains of amino acids

Amino acid–derived hormones - composed of one or two modified amino acids

Steroid hormones - synthesized from

cholesterol

How Do Animal Hormones Work?

Hormones act by binding to receptors on or in target cells

Receptors for hormones are found in two locations on target cells:

On the plasma membrane

Inside the cell, within the cytoplasm or the nucleus

Peptide and Amino Acid Hormones

Peptide and amino acid hormones bind to receptors on the surfaces of target cells Cannot diffuse through the phospholipid

bilayer of the plasma membrane and must bind to receptors on the surface of the target cell’s plasma membrane

Hormone–receptor binding activates an enzyme that synthesizes a molecule, called a second messenger, inside the cell

An example is cyclic adenosine monophosphate (cyclic AMP), which regulates many cellular activities

Second Messengers Transfers the signal from the first messenger—

the hormone—to other molecules within the cell, often activating specific intracellular enzymes

These activated enzymes initiate a chain of biochemical reactions that vary depending on the hormone, the second messenger, and the target cell Epinephrine stimulates the synthesis of

cyclic AMP in both heart muscle and liver cells, but the result is different in the two cell types

Cyclic AMP causes heart muscle cells to contract more strongly.

In liver cells, it activates enzymes that breakdown glycogen to glucose

Animation: The Action of Nonsteroid Hormones

Actions of Peptide and Amino Acid-Derived Hormones

(cytoplasm)

(nucleus)

peptide or aminoacid-derivedhormone(first messenger)

(extracellularfluid)

cyclic AMP-synthesizingenzyme

cyclic AMP

ATP

inactiveenzyme

(second messenger)

activeenzyme

reactant

product

plasma membrane

nuclearenvelope

receptor

The hormone binds toa receptor on the plasmamembrane of a target cell

1

The activated enzymescatalyze specific reactions4

The secondmessenger activatesother enzymes

3

Hormone–receptor bindingactivates an enzyme that catalyzesthe synthesis of a second messenger,such as cyclic AMP

2

Steroid hormones bind to receptors inside target cells

Steroid hormones are lipid soluble and diffuse through the plasma membrane of target cells

Bind to receptors inside target cells which are in the nucleus or move into the nucleus after hormone binding

The hormone–receptor complex then binds to the DNA of the promoter region of specific genes and stimulates transcription of messenger RNA

The mRNA travels to the cytoplasm and directs protein synthesis

Steroid Hormone Action on Target Cells

gene

plasmamembrane

ribosome

hormone receptor

steroid hormone

mRNA

(nucleus)

RNA polymerase

DNA

(cytoplasm)

new protein

(extracellularfluid)

A steroid hormonediffuses through theplasma membrane

The hormone binds to areceptor in the nucleus or toa receptor in the cytoplasmthat carries it into the nucleus

The hormone–receptorcomplex binds to DNA andcauses RNA polymerase tobind to a nearby promotersite for a specific gene

RNA polymerase catalyzesthe transcription of DNA intomessenger RNA (mRNA)

The mRNA leaves thenucleus, then attaches to aribosome and directs thesynthesis of a specific proteinproduct

1

2

3

4

5

nuclearenvelope

Animation: The Action of Steroid Hormones

Thyroid Hormone

Although it is not a steroid, thyroid hormone acts intracellularly Actively transported into many cell types Once inside, thyroid hormone binds to

intracellular receptors and activates transcription of specific genes

Hormones that bind to intracellular receptors may take several minutes or even days to exert their full effects

Negative Feedback

Hormone release is regulated by negative feedback mechanisms

A response to a change that counteracts a change and restore the system to its original condition

Example, after jogging on a hot, sunny day, you have lost a quart of water through perspiration

Your pituitary releases antidiuretic hormone (ADH), which causes increased water reabsorption by your kidneys, concentrating your urine

If you drink two quarts of water, your would have excess blood volume

Negative feedback acts to restore the original condition by turning off ADH secretion, and your kidneys eliminates the excess water

Positive Feedback

In a few cases, hormone release is temporarily controlled by positive feedback

In this case, the response to a change enhances the change

Contractions of the uterus early in childbirth push the baby’s head against the cervix, which causes the cervix to stretch

Stretching the cervix sends nervous signals to the mother’s brain, which in turn causes the release of oxytocin

Oxytocin stimulates continued contractions of the uterus, pushing the baby harder against the cervix until delivery is complete

Vertebrate and Invertebrate Endocrine Hormones

Vertebrate and invertebrate endocrine hormones have similar mechanisms of action

Insects molt in order to grow - controlled by the steroid

hormone ecdysone, or molting hormone Ecdysone acts on receptors located within the nucleus and

affects gene transcription, initiating a complex process in which the epithelial cells detach from the old cuticle and secrete a soft new cuticle beneath it

The insect expands its body by pumping itself full of air This splits open the old cuticle and stretches out the new

one to accommodate future growth As the insect emerges, it leaves an insect-shaped cuticle

behind

Insect Molting

Emergingcicada

Old cuticle

Structures and Functions of the Mammalian Endocrine System

The mammalian endocrine system consists of the endocrine hormones and the glands that produce them

The major endocrine glands and organs are: The hypothalamus–pituitary complex The thyroid gland The pancreas The sex organs The adrenal glands

The Major Mammalian Endocrine Glands and Their Hormones

HypothalamusADH, oxytocin, and regulatory hormones for the anterior pituitary Pineal gland

melatonin

Parathyroid glands (on theposterior surface of the thyroidgland) parathyroid hormone

Pancreas islet cellsinsulin, glucagon

Pituitary glandanterior pituitary:ACTH, TSH, GH, PRL, FSH, LHposterior pituitary:oxytocin and ADH

Thyroid glandthyroxine, calcitonin

Thymus glandthymosins

Adrenal glands (one oneach kidney)medulla:epinephrine, norepinephrinecortex:glucocorticoids (cortisol),mineralocorticoids (aldosterone), testosterone

Gonadstestes (male):androgens, especiallytestosteroneovaries (female):estrogens, progesterone

Heartatrial natriuretic peptide

Kidneyserythropoietin

Fatleptin

Digestive tractseveral hormones (seeChapter 34)

testis

ovary

Hypothalamus and Pituitary

Hormones of the hypothalamus and pituitary gland regulate functions throughout the body

The hypothalamus and pituitary gland coordinate the action of many key hormonal systems

The hypothalamus contains clusters of specialized nerve cells called neurosecretory cells

Neurosecretory cells synthesize peptide hormones, store and release when stimulated

Pituitary Gland

Pea-sized gland connected to the hypothalamus by a stalk

Two distinct parts: The anterior pituitary, a true endocrine gland,

composed of several types of hormone-secreting cells in a network of capillaries

The posterior pituitary, mainly a capillary bed and the endings of neurosecretory cells whose cell bodies are in the hypothalamus

The hypothalamus controls the release of hormones from both parts of the pituitary

Pituitary & Hypothalamus Hypothalamic hormones control

hormone release in the anterior pituitary

Neurosecretory cells of the hypothalamus produce +/- seven hormones that regulate release of hormones from anterior pituitary

These hypothalamic hormones are releasing hormones or inhibiting hormones, depending on whether they stimulate or inhibit the release of a particular pituitary hormone

How it works…

1. Neurosecretory cells of the hypothalamus produce releasing and inhibiting hormones (green dots, page 721)

2. Release and inhibiting hormones are secreted into a capillary bed feeding the anterior lobe of the pituitary

3. Endocrine cells of the anterior pituitary secrete hormones in response to releasing hormones, pituitary hormones enter bloodstream

Some hypothalamic hormones (growth hormone-releasing hormone) stimulate the release of pituitary hormones, others inhibit the release of pituitary hormones

hypothalamus

pituitary(anterior lobe)

capillarybed

endocrinecell

blood flow

bloodflow

capillarybed

pituitary(posterior lobe)

Oxytocin and ADH(blue triangles) aresecreted into the bloodvia capillaries in theposterior pituitary

Endocrine cells of theanterior pituitary secretehormones (red squares)in response to releasinghormones; the pituitaryhormones enter thebloodstream

Neurosecretory cells ofthe hypothalamus produceoxytocin and ADH

Releasing or inhibiting hormones(green circles) are secreted intocapillaries feeding the anterior lobeof the pituitary

Neurosecretory cellsof the hypothalamus produce releasing andinhibiting hormones

1

2

1

2

3

The Hypothalamus–Pituitary System

Anterior Pituitary

These regulate hormone production in other endocrine glands:

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the production of sperm and testosterone in males, production of eggs, estrogen, and progesterone in females

Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to release its hormones

Adrenocorticotropic hormone (ACTH) causes the release of the hormone cortisol from the adrenal cortex

More Endocrine Hormones

These hormones of the anterior pituitary do not act on other endocrine glands

Prolactin (with help) stimulates development of milk-producing mammary glands in the breasts during pregnancy

Growth hormone acts on nearly all the body’s cells by increasing protein synthesis, promoting the use of fats for energy, and regulating carbohydrate metabolism

During childhood growth hormone stimulates bone growth, which influences human height; too little growth hormone results in dwarfism, and too much results in gigantism

Anterior Pituitary Malfunctions

Posterior Pituitary

Releases hormones synthesized by cells in the hypothalamus

The hypothalamus contains two types of

neurosecretory cells that send axons into the posterior pituitary

These axons end in a capillary bed into which they release hormones that are then carried by the bloodstream to the rest of the body.

1. Neurosecretory cells synthesize antidiuretic hormone (ADH) or oxytocin. (blue)

2. Secreted into blood via capillaries of posterior pituitary

hypothalamus

pituitary(anterior lobe)

capillarybed

endocrinecell

blood flow

bloodflow

capillarybed

pituitary(posterior lobe)

Oxytocin and ADH(blue triangles) aresecreted into the bloodvia capillaries in theposterior pituitary

Endocrine cells of theanterior pituitary secretehormones (red squares)in response to releasinghormones; the pituitaryhormones enter thebloodstream

Neurosecretory cells ofthe hypothalamus produceoxytocin and ADH

Releasing or inhibiting hormones(green circles) are secreted intocapillaries feeding the anterior lobeof the pituitary

Neurosecretory cellsof the hypothalamus produce releasing andinhibiting hormones

1

2

1

2

3

The Hypothalamus–Pituitary System

ADH and Oxytocin

Antidiuretic hormone (ADH) helps prevent dehydration by causing more water to be reabsorbed from the urine by the kidneys and returned to the bloodstream

Alcohol inhibits the release of ADH and increases urination, resulting in the loss of more water than is consumed with dehydration resulting

Oxytocin causes contractions of uterine muscles during childbirth and triggers “milk letdown” in nursing mothers by causing muscle tissue within the mammary glands to contract in response to the suckling infant

In humans, oxytocin may play a role in emotions, including trust and both romantic and maternal love

Animation: Hypothalamic Control of the Pituitary

Thyroid and Parathyroid Glands

Influence metabolism and calcium levels

Front of the neck below the larynx, the thyroid gland produces two hormones: thyroxine and calcitonin

The parathyroid gland - two pairs of small disks of cells on each side of the thyroid, releases parathyroid hormone

larynx

thyroid glandesophagus

parathyroidglands

trachea

The Thyroid and Parathyroid Glands

Thyroxine

Influences energy metabolism Thyroxine or thyroid hormone, is an iodine-

containing amino acid derivative that works by binding to intracellular receptors that regulate gene activity

By stimulating glucose breakdown and providing the resulting energy from it, thyroid hormone elevates the metabolic rate of many body cells

In juvenile animals, including humans, thyroxine helps regulate growth by stimulating both metabolic rate and nervous system development

Undersecretion of thyroid hormone leads to cretinism, a condition characterized by retardation

Iodine Deficiency

An iodine-deficient diet can reduce the production of thyroxine and trigger a feedback that attempts to restore normal hormone levels by increasing the number of thyroxine-producing cells

The thyroid gland becomes enlarged, forming a goiter

Iodine deficiency in pregnant women and young children is the leading preventable cause of mental retardation

Iodized salt is a simple, and cheap, solution to iodine deficiency

Goiter

Thyroxine

Release is controlled by the hypothalamus and anterior pituitary 1. Thyroid stimulating hormone-releasing

hormone is produced by neurosecretory cells in hypothalamus, travels to anterior pituitary

2. TSH releasing hormone causes anterior pituitary to secrete TSH – thyroid stimulating hormone

3. TSH travels in blood to thyroid and stimulates release of thyroxine

4. Secretion of TSH-releasing hormone and TSH are regulated by negative feedback

Adequate levels of thyroxine inhibit the secretion of both TSH-releasing hormone from the hypothalamus and TSH from the anterior pituitary

releasinghormone

TSH

hormone-producingcells of thethyroid

thyroid gland

endocrinecells of theanteriorpituitary

The releasing hormone causes the anterior pituitary to secrete thyroid-stimulating hormone (TSH)

Neurosecretory cells of the hypothalamus secrete TSH-releasing hormone

Thyroxine inhibitsTSH-releasing hormoneand TSH release bynegative feedback

TSH causes the thyroid tosecrete thyroxine, whichincreases cellular metabolismthroughout the body

2

3

4

thyroxine

1

Negative Feedback in Thyroid Gland Function

Thyroxine Effects

Varied effects in different vertebrates

In amphibians - has the effect of triggering metamorphosis

In 1912, tadpoles were fed minced horse thyroid and metamorphosed prematurely into miniature adult frogs

Thyroxine also regulates the seasonal molting of most vertebrates from snakes to birds to the family dog. Surges of thyroxine stimulate the shedding of skin, feathers, and hair

Parathyroid Hormone and Calcitonin

Regulate calcium metabolism

The proper concentration of calcium is essential to nerve and muscle function, the parathyroid hormone and calcitonin work together to maintain constant calcium levels in the blood

If blood calcium levels drop, parathyroid hormone causes the bones to release calcium and the kidneys to reabsorb more calcium from urine

If blood calcium gets too high, calcitonin inhibits the release of calcium from bone

The Pancreas

Both digestive and endocrine functions

The pancreas produces bicarbonate and enzymes that are released into the small intestine, promoting the digestion of food

The endocrine portion of the pancreas consists of clusters of islet cells that produce one of two peptide hormones: insulin or glucagon

Insulin and Glucagon Control Glucose Levels in the Blood

Insulin and glucagon work in opposition to regulate carbohydrate and fat metabolism Insulin reduces the blood glucose level Glucagon increases it

1. eating raises blood sugar 2. high glucose stimulates insulin release and

inhibits glucagon release 3. Insulin stimulates glucose uptake by body

cells, liver converts glucose glycogen

Maintaining a Glucose Balance

4. glucose uptake into cells and conversion of glucose glycogen reduces blood glucose

5. exercise and fasting also reduce blood glucose

6. low blood glucose stimulates glucagon release and inhibits insulin release

7. glucagon stimulates cells to burn fat instead of glucose, liver converts glycogen to glucose

8. blood glucose is increased

The Pancreas Controls Blood Glucose Levels

Glucagon stimulatescells to burn fat instead ofglucose; the liver convertsglycogen to glucose

7

Bloodglucose isincreased

8 Eatingraises bloodglucose

1

High bloodglucose stimulatesinsulin release andinhibits glucagonrelease

2

Insulin stimulatesglucose uptake bybody cells; the liverconverts glucose toglycogen

3

Glucose uptake intocells and conversion ofglucose to glycogenreduce blood glucose

4 Exerciseand fasting alsoreduce bloodglucose

5

Low blood glucosestimulates glucagonrelease and inhibitsinsulin release

6

high bloodglucose

low bloodglucose

pancreas

liver

insulinglucagon

muscle

Diabetes

Results from malfunctioning insulin control system

Lack of insulin production or failure of target cells to respond to insulin results in diabetes mellitus

In either case, blood glucose levels are high because cells cannot take up glucose unless they are stimulated by insulin, and they rely on fats as an energy source, which leads to high levels of blood lipids

Many diabetics suffer from heart and blood vessel disease caused by fat deposition

Insulin replacement therapy

Sex organs

Produce gametes and sex hormones

Besides producing sperm or eggs, the testes in males and ovaries in females are also important endocrine organs

The testes secrete several steroid hormones, collectively called androgens, the most important being testosterone

The ovaries secrete two types of steroid hormones: estrogen and progesterone

Sex Hormones

Levels increase during puberty

Puberty is the phase of life during which the reproductive systems become mature and functional

It begins when the hypothalamus secretes increasing amounts of releasing hormones, which stimulate the anterior pituitary to secrete more (LH) and (FSH)

LH and FSH stimulate target cells in the testes and ovaries to produce higher levels of sex hormones

Steroid Hormones

Testosterone, secreted by the testes, promotes sperm production and stimulates the development of male secondary sexual characteristics, ie. body and facial hair, broad shoulders, and muscle growth

Estrogen from the ovaries stimulates breast development and the maturation of the female reproductive system, including egg production

Progesterone prepares the reproductive tract to receive and nourish the fertilized egg

Adrenal Glands

Secrete hormones that regulate metabolism and responses to stress The adrenal glands consist of two parts:

The adrenal cortex The adrenal medulla

The adrenal cortexsecretes glucocorticoids,mineralocorticoids,and testosterone

The adrenal medulla secretesepinephrine and norepinephrine

kidney

The Adrenal Glands

Adrenal Cortex

Produces steroid hormones

The outer layer forms the adrenal cortex, which secretes three steroid hormones:

Glucocorticoids, which help control glucose metabolism

Mineralocorticoids, which regulate salt

metabolism Small amounts of testosterone

Cortisol, a Glucocorticoid Release

Glucocorticoid release is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary, which is stimulated by releasing hormones from the hypothalamus Glucocorticoids are released in response to

stimuli such as stress, trauma, or exposure to temperature extremes

Cortisol – the most abundant glucocorticoid Increases blood glucose levels by stimulating

glucose production, inhibiting the uptake of glucose by muscle cells, and promoting the use of fat for energy

Glucose Metabolism

Many different hormones are involved in glucose metabolism: thyroxine, insulin, glucagon, epinephrine, and the glucocorticoids

The reason for so many different hormones in the regulation of glucose can be traced to the metabolic requirements of the brain

Most body cells can produce energy from fats and proteins as well as from carbohydrates

Brain cells only metabolize glucose, so glucose levels in the blood cannot be allowed to fall too far or brain cells rapidly die

Mineralocorticoid Hormones Regulate the mineral (salt) content of the

blood Most important is aldosterone (ADS)

A constant blood sodium concentration is crucial for cellular events, including production of electrical signals by nerve cells

If blood sodium falls, adrenal cortex releases ADS, which causes the kidneys and sweat glands to retain sodium

When blood sodium returns to normal, ADS secretion

is turned off

Testasterone and the Adrenal Cortex

In women and men the adrenal cortex also produces testosterone, although in smaller quantities than produced by the testes

Tumors of the adrenal cortex can lead to excessive testosterone release, causing masculinization of women

Adrenal Medulla

Produces amino acid-derived hormones Located in the center of each adrenal gland Produces two hormones in response to stress

or exercise: epinephrine and norepinephrine

Prepare the body for emergency action by increasing heart and respiratory rates, blood pressure, causing blood glucose levels to rise, and directing blood flow away from the digestive tract and toward the brain and muscles

Hormones also produced in…

Hormones are also produced by the pineal gland, thymus, kidneys, heart, digestive tract, and fat cells

The pineal gland produces the

hormone melatonin Secreted in a daily rhythm, which is

regulated by light entering the eyes

The pineal appears to regulate the seasonal reproductive cycles of many animals

The function of melatonin and the pineal gland in humans is not known

Thymus Gland Located in the chest cavity

behind the breastbone Produces thymosin, stimulates

the development of T-cells Large in infants, but under

the influence of sex hormones, decreases in size after puberty

Elderly produce fewer new T cells than adolescents and are more susceptible to new diseases

Kidneys

Produce erythropoietin, a peptide hormone that is released when oxygen content of the blood is low

Stimulates the bone marrow to increase red blood cell production

The kidneys also produce an enzyme called renin in response to low blood pressure, which catalyzes the production of the hormone angiotensin from proteins in the blood

Angiotensin raises blood pressure by constricting arterioles and stimulating the release of aldosterone by the adrenal cortex, which leads to increased sodium and water reabsorption by the kidney and increased blood volume and pressure

More Hormones

Stomach and small intestines produce a number of peptide hormones that help regulate digestion

Include gastrin, ghrelin, secretin, and cholecytokinin

The heart releases atrial natriuretic peptide (ANP), which inhibits the release of ADH and aldosterone and increases the excretion of sodium

The actions of ANP lead to a drop in blood volume by reducing reabsorption of water and salt by the kidneys

Fat can act as an endocrine organ

In 1995, Jeffrey Friedman at Rockefeller University discovered the peptide hormone leptin, which is released by fat cells

Mice genetically engineered to lack the gene for leptin became obese, leptin injections caused them to lose weight

Researchers hypothesized that by releasing leptin, fat tissue “tells” the body how much fat it has stored and how much to eat

Leptin has other roles that include stimulating the growth of capillaries and speeding wound healing