Chapter 15

Chapter 15

Endocrine System

Points to Ponder• What is the endocrine system?• Compare and contrast exocrine and endocrine

glands.• What are steroid and peptide hormones?• Name the major glands and their functions in

the endocrine system.• What is diabetes (Type 1 and 2) and how

might you prevent Type 1?• How do the endocrine nervous systems work

with the rest of the systems in the body to maintain homeostasis?

Endocrine system• Mostly comprised of glands• Secretes hormones that move through the bloodstream to target cells

– target cells respond to certain hormones

• Results in a slow but a prolonged response– Takes time to deliver hormones and to take time for cells to respond, but the effect is longer lasting

• Makes use of negative feedback mechanism– Rise in blood glucose level causes the pancreas to release insulin, which promotes glucose until

blood glucose levels fall

What is a target cell?

Exocrine vs. endocrine glands

• Exocrine glands – secrete their products into ducts that carry

these products to other organs or outside the body

• Endocrine glands – secrete their products directly into the

bloodstream

15.1 Endocrine glands

Hormones are Chemical Signals• Hormones

– Means of communication between cells, between body parts, and even between individuals

– Act at a distance between body parts– Travel in the bloodstream from the gland that produces it

to their target cells– Secretions produced by neurosecretory cells in the

hypothalamus• Travel in the capillary network that turns between the

hypothalamus and the pituitary gland• Secretions stimulate/inhibit the pituitary to secrete its hormones

What are hormones?• Hormones:

– Prostaglandins: local hormones• local hormones affect neighboring cells and thus are not carried in the

bloodstream• Sometimes promote pain and inflammation

– Pheromones: • chemical signals that influence the behavior of other individuals• Pheromone released by men

– reduces premenstrual nervousness and tension in women

– Peptide hormones: • Bring about an alteration of the target cell’s structure in some way• Consist of peptides, proteins, glycoproteins, and modified amino acids

– Steroid hormones: • Are all derived from cholesterol• lipids that enter a cell and affect gene activity and thus protein

synthesis

15.1 Endocrine glands

Action of Peptide Hormones• Most hormonal glands secrete peptide hormones• In muscle cells

– Stimulation by epinephrine leads to the breakdown of glycogen to glucose

• Steps of Hormone Action called Enzyme-cascade:1. Bind to a receptor in the plasma membrane

- Hormone does not enter the cell it’s called the first messenger

2. Formation of cAMP (cyclic adenosine monophosphate)- cAMP acts as a second messenger

3. Activation of Protein Kinase

4. Which activates a cascade of enzymes

Action of peptide hormones

Action of Steroid Hormones• Steroid hormones

– produced at the adrenal cortex, the ovaries, and the testes

• Steps of Hormone Action1. Enter the cell because they are lipids2. Binds to a receptor, in the nucleus or cytoplasm3. Inside the nucleus,

• Hormone-receptor complex binds with DNA and activates certain genes

4. Messenger RNA moves to the ribosome in the cytoplasm and protein synthesis follows

– This process• Act more slowly than peptides because it takes more time to

synthesize new proteins than to activate enzymes already present in cells

• But action lasts longer

Action of steroid hormones

Major glands of the endocrine system

1. Hypothalamus2. Posterior pituitary gland3. Anterior pituitary gland4. Thyroid gland5. Parathyroid glands6. Adrenal glands7. Pancreas8. Testes9. Ovaries10.Thymus gland11.Pineal gland

15.2 Hypothalamus and pituitary gland

1. Hypothalamus

• Regulates internal environment through the autonomic nervous system– Helps control heartbeat

– Helps control body temperature

– Helps control water balance

– Controls glandular secretions of the pituitary gland


2. Posterior pituitary gland• Stores antidiuretic hormone (ADH) and oxytocin that are

produced by the neurosecretory cells in the hypothalamus– ADH:

• regulates water balance by reabsorbing water into bloodstream • Action of ADH

– Hypothalamus is sensitive to water-salt balance of the blood – When blood is too concentrated, ADH is released from the posterior

pituitary – Water is absorbed into the bloodstream to regulate water-salt balance

• Negative feedback maintains homeostasis

– Oxytocin: • Causes uterine contractions during childbirth • Allows milk to be released during nursing• Controlled by positive feedback which is not a way to maintain

homeostasis

3. Anterior pituitary gland

• Portal system– Consisting of two capillary systems connected by

a vein– lines between the hypothalamus and anterior

pituitary

• Anterior pituitary gland is controlled by hypothalamic-releasing and hypothalalmic-inhibiting hormones – Hormones pass from the hypothalamus to the

anterior pituitary via the portal system


Hormones Produced by Anterior Pituitary

Hormones that have an effect on other glands:

1.Thyroid-stimulating hormone (TSH): - stimulates the thyroid to produce thyroid hormones

2.Adrenocorticotropic hormone (ACTH): - stimulates the adrenal cortex to produce cortisol

3.Gonadotropic hormones: - stimulate gonads to produce sex cells and hormones

** Blood level of the last hormone in the sequence exerts negative feedback control over the secretion of the first hormone

Hormones Produced by Anterior Pituitary

Hormones that do not affect other endocrine glands:1. Prolactin (PRL):

- stimulates mammary glands to develop and produce milk only after childbirth

- Involved in carbohydrate and fat metabolism

2. Melanocyte-stimulating hormone (MSH): - causes skin cells to produce melanin

3. Growth hormone (GH): - promotes skeletal and muscular growth by stimulating protein

synthesis- Promotes fat metabolism as opposed to glucose metabolism

What happens when the body produces too much or too little GH?

• Pituitary dwarfism – too little GH is produced during childhood that results in

small stature

• Giantism – too much GH is produced during childhood that results in

poor health

• Acromegaly – overproduction of GH as an adult that results in larger

than normal feet, hands, and face


What happens when GH is produced in improper results during childhood?


What happens when GH is produced in high amounts during adulthood?


4. Thyroid gland

• A large gland located below the larynx• Iodine is needed in the diet to allow the thyroid

gland to produce its hormones– Triiodothyronine (T3) and thyroxine (T4)

• It produces:– Thyroid hormone (TH): Regulates metabolism– Calcitonin: Regulate blood Calcium levels

• Calcium plays role in nervous conduction and muscle contraction• Calcitonin helps lower blood Ca2+ levels by stimulating the

deposition of calcium in the bones

• Hormones do not have a target organ instead they stimulate cells of the body to metabolize at a faster rate

Thyroid abnormalities• Simple goiter

– thyroid enlarges due to lack of iodine in the diet• Hypothyroidism – low blood levels of thyroid

hormonesA. Congenital hypothyroidism:

- thyroid does not develop properly and is characterized in a short, stocky person that may be mentally retarded

B. Myxedema: - hypothyroidism in adults characterized by lethargy, weight

gain, loss of hair, cold intolerant and thick, puffy skin

• Hyperthyroidism – excess thyroid hormones in bloodA. Exophthalimic goiter:

- characterized by enlargement of the thyroid gland, protrusion of the eyes, hyperactive and suffers from insomnia

B. Thyroid tumor: can also cause hyperthyroidism

15.3 Thyroid and parathyroid glands

Thyroid abnormalities


5. Parathyroid glands

• Small glands embedded in the surface of the thyroid gland

• Produces parathyroid hormone (PTH): – causes blood Ca2+ level to increase by

promoting osteoclast activity– Promotes reabsorption of Ca2+ by the kidneys– Activates vitamin D at the kidneys to

stimulate the absorption of calcium from the intestines


Regulation of blood calcium

6. Adrenal glands

• Glands that sit on top of the kidneys• 2 parts of each gland

– Adrenal medulla: • controlled by the nervous system

– Adrenal cortex: • portions are controlled by ACTH from the

anterior pituitary

• Stress prompts the hypothalamus to stimulate a portion of the adrenal gland

15.4 Adrenal glands

Adrenal medulla

• Inner portion of the adrenal glands

• Hypothalamus initiates stimulation of hormone secretion via the preganglionic sympathetic nerve fibers that travel to the adrenal medulla

• Produces– hormones that allow a short-term response to

stress (“fight or flight” response)– Epinephrine (adrenaline) – Norepinephrine

15.4 Adrenal glands

Adrenal cortex

• Outer portion of the adrenal glands• Produces hormones that provide a long-term

response to stress• 2 major types of hormones: Mineralocorticoids and

Glucocorticoids– Glucocorticoids:

• Secretion is controlled by ACTH released by the anterior pituitary• regulate carbohydrate, protein (in liver) and fat metabolism

leading to an increase in blood glucose level• Suppress the body’s inflammatory response but suppresses pain

and immunity making a person susceptible to injury/infection– e.g. cortisol and cortisone

• Secrete a small about of female and male sex hormones in both sexes

15.4 Adrenal glands

Adrenal cortex• Mineralocorticoids :

– regulate salt and water balance leading to an increase in blood volume and pressure

– e.g. aldosterone (targets the kidney)• Promotes renal absorption of Na+ and renal excretion of K+

– Secretion is not controlled by anterior pituitary– Renin-angiotensin-aldosterone system:

1. Na+ and blood volume is low2. Kidneys secrete renin3. Renin converts plasma protein angiotensinogen to angiotensin I4. Angiostensin I changes to Angiostensin II via enzyme in the lung capillaries5. Angiotensin II stimulates the adrenal cortex to release Aldosterone6. Blood pressure raises b/c:

1. Angiotensin II constricts the arterioles2. Aldosterone causes the kidneys to reabsorb Na+

– If blood volume is high, atria of the heart will stretch• Cardiac cells release atrial natriuretic hormone (ANH)

– Inhibits the secretion of aldosterone form the adrenal cortex– This causes excretion of Na+ and thus water to decrease blood volume

Summary of the adrenal glands

Adrenal glands can malfunction• Addison’s disease

– Hyposecretion (low) of glucocorticoids by the adrenal cortex – Presence of excessive/ineffective ACTH causes bronzing of skin– Physiological Side Effects:

• Glucose can not be replenished during stress • low levels of aldosterone secretion leads to NA+ and water loss resulting in low blood pressure

Adrenal glands can malfunction

• Cushing syndrome– hypersecretion of glucocorticoids by the adrenal cortex – characterized by weight gain in the trunk of the body

but not arms and legs

15.4 Adrenal glands

7. Pancreas• Not under pituitary control• Composed of 2 tissues:

– Exocrine: produces and secretes digestive juices– Endocrine (islets of Langerhans):

• produce/secrete hormones1. Insulin – Secreted After Eating to lower blood glucose

level- secreted when blood glucose is high and stimulates uptake of glucose by cells (muscle, liver, and adipose cells)

2. Glucagon – Secreted Before Eating to raise blood glucose level- secreted when blood glucose is low and stimulates the

breakdown of glycogen in the liver and adipose1. Stimulates liver break down of glycogen to glucose2. Use of fat (glycerol and fatty acids) and protein (amino acids)

in preference to glucose as energy source

15.5 Pancreas

Regulation of Blood Glucose

Health focus: What is diabetes?• Inability to control blood glucose levels• There are two types: Type 1 and Type 2• 18 million people in the US have diabetes• General symptoms:

– Frequent urination– Unusual hunger and/or thirst– Unexplained change in weight– Blurred vision– Sores that heal slowly or not at all– Excessive fatigue

• Long-term effects are blindness, loss of limbs, nerve deterioration, kidney and cardiovascular disease

Diabetes: Understanding the 2 types

• Type 1:– Usually early-onset – Autoimmune disorder that tends to run in families– Cause and Effect:

• After/during infection cytotoxic T cells attack/destroy pancreatic cells preventing them from producing insulin

• Body turns to metabolism of fat– leads to buildup of ketones in the blood acidosis

– Need insulin injections

Diabetes: Understanding the 2 types

• Type 2:– Usually adult-onset and most common type– Tends to occur in obese, sedentary people– Cause and Effect:

• Cells do not respond to insulin therefore, the number of protein carriers for glucose remains them same Insulin resistance

• Decrease level of glucose absorption

– Usually diet and exercise are important for controlling this and may even prevent this!

Diabetes

• Long term complications– Blindness– Kidney disease– Cardiovascular disorders

• Atherosclerosis• Heart disease• Stroke• Reduced circulation

Testes and Ovaries

• Sex Hormones (estrogen, progesterone, and testosterone) are produced and excreted by the gonads– Hormones feedback to control the

hypothalamus secretion of GnRH (gonadotropic releasing hormone)

– Which in turn controls the pituitary gland secretion of FSH and LH, gonadotropic hormones

8. Testes

• Gonads found in males• Produce androgens (e.g. testosterone)• At time of puberty, gonadotropic hormones

stimulate the release of increased amounts of testosterone– Stimulates growth of the penis and testes– Responsible for 2 male sex characteristics

such as facial, underarm and pubic hair– Prompts the larynx and vocal cords to enlarge

resulting in a lower voice– Promotes muscular strength

15.6 Other endocrine glands

9. Ovaries

• Gonads found in females

• Produce estrogen and progesterone – Stimulates growth of the vagina and uterus– Responsible for secondary sex

characteristics such as female body hair, fat distribution and breast development

– Responsible for egg maturation– Regulates the uterine cycle


Hypothalamic-Pituitary-Ovarian Axis

Hoyer 2005

Ovarian Histology

• Follicular Composition of the Ovary

Primordial Follicle

Primary Follicle

SecondaryFollicle

Early Antral Follicles

1 mg/kg E2100 mg/kg MXC20 µg/kg MXC

Control

Corpora lutea

AntralFollicles

Antralfollicles

E19 – PND 7

500 µm

500 µm500 µm 500 µm

P450scc

Inner MitochrondrialMembrane

pPKAcAMP

HDL

Cholesterol (C27)

StAR protein

Androstenedione in theca cells

Plasma membrane

(+)

Mitochondria

LH

LH-R

Pregnenolone

C27

Pregnenolone

Progesterone

Testosterone in theca cells

Estrogen in Granulosa Cells

Steroidogenic Pathway

FSH

FSH-R

Theca CellsGranulosa Cells

Zachow

Incubation

• Egg enveloped by abdominal fold,

“brood pouch”– Numerous blood vessels

• Body Heat Egg

• Prolactin Stimulate Development

http://www.emperor-penguin.com/emperor-rookery.html

Prolactin in Males and Females

• Prolactin Levels: Male=Female Courtship• Source: Lactotrophic Cells

Anterior Lobe-Pituitary• Induce Parental Behavior

– Care for egg and young

(Vleck, C.M, et al 1999) http://www.coolantarctica.com/gallery/penguins/emperor_penguin_37.htm

Prolactin: Male Gentoo Penguins

• Before and during Incubation– Brood Pouch develops under the influence of an increase

in prolactin and sex steroids

• Chick Phase

• Nest Failure– Decrease

(Williams, T.D. et al 1993)

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Prolactin: Female Gentoo Penguins

(Williams, T.D. et al 1993)

Continuously Elevated Prolactin -Stimulate foraging birds to return to nest, once energy needs are met -Higher Prolactin levels when returning from foraging then after egg laying

Finally: - Prolactin due to seasonal cues

*Ensure switch from:Guarding Foraging Fat Deposition -molting

Pla

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Pro

lact

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(Vleck, C.M et al 2000)

10. Thymus gland

• Lies beneath the sternum

• This gland is largest and most active during childhood

• T lymphocytes mature here

• Secretes hormones called thymosins– aid in differentiation of lymphocytes


11. Pineal gland

• Located in the brain• Secretes melatonin

that regulates the sleep/wake cycle (circadian rhythm)– High levels = sleep– Low levels = wake

• May also regulate sexual development


Photic Stimulation of Hypothalamus

Rods, Cones, &Melanopsin Photoentrainment Regulate Circadian Rhythms

SCN: Suprachiasmatic nucleus - “Master Clock”

PVN:Paraventricular nucleus

Hormonal Control

Hypothalamus

Hormones from other tissues• Erythropoietin:

– secreted by the kidney to increase red blood cell production

• Leptin: – produced by fat cells and acts on the hypothalamus to give

a feeling of being satiated (not hungry)

• Prostaglandins: – A groups of potent chemicals that are not carried in the

bloodstream but work locally on neighboring cells– Some cause smooth muscle contraction– Major impact on reproductive organs

• uterine contractions during menstruation

– Mediate temperature control of the body– Reduce gastric secretion, lower blood pressure, inhibit

platelet clotting– Aspirin and ibuprofen block the synthesis of prostaglandins

Bioethical focus: You decide

• Hormone replacement therapy (HTR) is often used to counteract some of the problems associated with menopause or to prevent an increased risk of osteoporosis

• 2002 a study found that HTR:– May have risks that outweigh the benefits– Does not significantly increase the quality of life– Significantly increased the risk of breast cancer– May increase the risk of cardiovascular disease– Can prevent fractures as a result of osteoporosis


Homeostasis

• The nervous and endocrine systems are important in maintaining homeostasis by:– The hypothalamus bridges regulatory functions

of both the endocrine and nervous systems– The nervous system is able to respond to

changes in the external environment– The endocrine system is able to respond to

changes in the internal environment• Blood pressure and volume explained previously with

ADH (endocrine) and sympathetic nervous system (vessel constriction and increased heart rate)

15.7 Homeostasis

Homeostasis

• Controlling Reproductive System– Hypothalamus controls anterior pituitary

• Controls the release of hormones form the testes and ovaries for sperm and egg development

• Releases oxytocin for stimulation of contractions during birthing

– Nervous system controls muscle contractions of the ducts

• Propels sperm and egg

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Chapter 15