The Endocrine System

Chapter 15

Introduction

• Nervous system and endocrine system share responsibility for control and coordination of entire body.• Homeostasis- balance of the body.

• Systems have similarities and differences:• Both systems use chemicals to transmit their

messages.• Endocrine system produces hormones• Nervous system produces neurotransmitters

• Endocrine system reacts more slowly to changes but can sustain response longer.

• Nervous system reacts more quickly to changes but cannot sustain prolonged responses.

Basics

• Endocrine glands- basic units of the endocrine system.• Secrete hormones directly into the

bloodstream.• Circulate throughout body and

produce effects when attach to receptors in or outside of cells.

• Ductless glands.

• Exocrine glands- units that secrete their products onto epithelial surfaces through tiny tubes called ducts.

Endocrine vs. Exocrine Glands

Endocrine Glands

Hormones

• Chemical messengers produced by endocrine glands and secreted directly into blood vessels.

• Produce effects when find their receptors in or on cells.• Each body cell has specific receptors to

certain hormones (target).• If body does not have receptor,

hormone will pass by. • Only certain hormones can fit to

receptors and when it occurs, then it changes the activity of the cell.

Control of Hormone Secretion

• “Negative Feedback System”• Endocrine glands will be stimulated to produce

more hormone when it drops below a certain amount in the body.

• If hormone is of adequate levels, gland will either slow or stop production of the hormone.

• Direct Stimulation of Nervous System

• Secretion of some hormones is stimulated by sympathetic nerve impulses when an animal feels threatened.

• Fight or flight response from sympathetic nervous system

The Hypothalamus

• Part of diencephalon of brain.• Has control of pituitary gland.

• This links the conscious mind with the rest of the body. This is an important bridge between the endocrine and nervous systems

Hypothalamus and anterior pituitary gland

• Portal system of blood vessels links hypothalamus with anterior portion of pituitary gland.

• Modified neurons in hypothalamus secrete releasing and inhibiting hormones into portal blood vessels

• Specific for a particular anterior pituitary hormone

• Either promote or inhibit the production of a specific hormone.

Hypothalamus and Posterior Pituitary

• Hypothalamus produces antidiuretic hormone (ADH) and Oxytocin• Are transported to posterior

pituitary where they are stored. • Released into bloodstream by

nerve impulses from hypothalamus.

The Pituitary Gland

• Also referred to as the “hypophysis”.• Master endocrine gland.

• Many hormones direct activity of other endocrine glands.

• Very small• Housed in pituitary fossa of skull. • Actually two separate glands:

• Anterior- adenohypophysis• Produces seven known hormones

• Posterior-neurohypophysis.• Does not produce any hormones but

stores and releases ADH and Oxytocin.

Anterior Pituitary Gland

• Hormones:• 1. Growth Hormone (GH)• 2. Prolactin• 3. Thyroid-stimulating Hormone (TSH)• 4. Adrenocorticotropic Hormone

(ACTH)• 5. Follicle-stimulating Hormone (FSH)• 6. Luteinizing Hormone (LH)• 7. Melanocyte-stimulating Hormone

(MSH).

Growth Hormone (GH)

• Also known as somatotropin and somatotropic hormone.

• Promotes growth in young animals.• Helps regulate metabolism of proteins,

carbohydrates, and lipids in body’s cells.• GH breaks down lipids for energy while

discouraging use of carbs for energy.

DwarfismDwarfismRare now since GH is commercially available

GiantismGiantismBody proportions normal

AcromegalyAcromegalyExcessive GH after epiphyseal plates close Responsive tissues: bones of hands, feet, face

thickened facial features/tongue

Dairy Cows

• Synthetic GH may be used to increase milk production of dairy cows.

• Bovine Somatotropin (BST)• Enhances milk production by

mammary glands.• Can elevate body temperature.• Reduce conception rates• Increases risk of mammary

gland infections• Increases risk of digestive

disorders.

Prolactin

• Helps to trigger and maintain lactation• Continues as long stimulation of nipple

continues. • Mammary glands will shrink to

normal size once stimulation stops.• Has no known effect in male.

Thyroid-Stimulating Hormone (TSH)

• Also referred to as thyrotropic hormone.• Stimulates growth and development of

thyroid gland and causes it to produce hormones.

• TSH production is regulated by feedback from Thyroid gland.

Adrenocorticotropic Hormone (ACTH)

• Stimulates growth and development of the cortex of adrenal gland.

• Stimulates release of some of adrenal gland’s hormones

• Can be released in two ways:• First by feedback system from adrenal glands• Second during stress, can be released by

stimulation of hypothalamus by other parts of brain.

• This sends releasing factor to anterior pituitary through blood vessels.

Follicle-Stimulating Hormone (FSH).

• Stimulates growth and development of follicles in ovaries to create ovum.

• Helps produce estrogens as well.

• Can be used to produce “superovulation”• Can then be harvested for

Artificial Insemination.

Luteinizing (LH)

• Completes process of follicle development that was started by FSH.

• Surge in LH is what causes ovulation to occur.

• High LH is maintained to produce corpus luteum which produces its own hormones.

• In male, LH stimulates interstitial cells to produce testosterone.

• Sometimes LH and FSH are called gonadotropins.

Melanocyte-Stimulating Hormone (MSH)

• Associated with color changes in the melanocytes (pigment cells) of reptiles, fish, and amphibians.

• Can cause pigment to darken when administered artificially.

• Other effects are unknown.

Posterior Pituitary Gland

• Does not produce, but stores and releases ADH and Oxytocin when stimulated.

• ADH• Where does it act?• What does it do?• Lack of can cause what disease?

• Oxytocin• Two targets are uterus and mammary

glands.• What does it do in uterus?• What does it do in mammary glands?

The Thyroid Gland• Consists of two parts called lobes

located on either side of the larynx.• Connected by narrow band called

isthmus in some species. • Composed of tiny follicles, where

thyroid hormone is produced. • Each follicle consists of globule

surrounding thyroid precursor called a colloid.

• Only endocrine gland that stores large amounts of hormone precursor for use later.

• Produces two hormones:• Thyroid hormone• Calcitonin

Thyroid Hormone• T3(Triiodothyronine)- contains three iodine atoms

per molecule.• T4(Tetriodothyronine, thyroxine)-contains four

iodine atoms per molecule. • These hormones are produced when TSH from

anterior pituitary gland reaches the thyroid gland. • T4 produced in greater abundance than T3 but is

mostly converted to T3 before producing effects on target cells• T3 more potent

• Necessary for normal growth and development in young animals. • Influences development and maturation of the

central nervous system, muscles and bones.

Thyroid Dysfunction:Hypothyroidism

• Too little thyroid hormone produced.• Occurs commonly in dogs.• Symptoms include:

• Alopecia• Dry skin• Lethargy• Reluctance to exercise• Weight gain• Seeking out sources of heat.

• Easily treated with thyroid supplementation• Usually occur in middle age.

Thyroid Dysfunction: Hyperthryoidism• Too much thyroid hormone production.• Commonly seen in cats.• Symptoms include:

• Nervousness• Excitability• Weight loss• Increased appetite, but no weight gain• Tachycardia• Vomiting• Diarrhea• Polyuria• Polydipsia

• Treated surgically by removing the thyroid gland, radioactive treatment, or drugs to suppress thyroid production.

Thyroid Dysfunction: Goiter

• Non-cancerous, non-inflammatory enlargement of the thyroid gland.

• Results from iodine deficiency which is a component of thyroid hormone. • Not enough iodine produced, less

hormone that can be produced.• Causes hyperplasia of the gland• Can be treated and prevented with iodine

supplements.• Iodized salt.

Calcitonin

• Produced by C cells located between the thyroid follicles.

• Involved with maintaining homeostasis of the blood calcium levels. • Will prevent hypercalcemia by

decreasing blood calcium levels if they go too high.

• Encourages excess calcium to deposit in bones

Parathyroid Glands

• Small nodules located near, in or on thyroid glands.• Produce:

• Parathyroid Hormone (PTH)

Parathyroid Hormone (PTH)

• Also called parathormone.• Opposite effect of calcitonin.

• Prevents hypocalcemia by increasing blood calcium if gets too low.

• Causes kidneys to retain calcium and intestines to absorb calcium from food.

• Can take calcium out of storage from the bones.

Parathyroid Dysfunction: Hypocalcemia

• Milk fever- when parathormone is overwhelmed in lactating animals.

• Can cause muscle weakness and tremors.• Can progress to seizures and

spasms if left untreated. • “Downer cows”

• Called Eclampsia in dogs and cats• Treated by supplementing calcium

rapidly into system

Adrenal Glands• Located on cranial aspect of kidneys.• Composed of two glands wrapped around one another.

• Outer Adrenal Cortex• Classic endocrine gland tissue• Produce under direction of ACTH:

• Glucocorticoid hormones• Mineralcorticoid hormones• Sex hormones

• Inner Adrenal Medulla• Resembles nervous tissue• Controlled by sympathetic portion of nervous

system• Produces:

• Epinephrine• Norepinephrine

Glucocorticoid Hormones

• Hormones produced are:• Cortisone• Cortisol• Corticosterone

• General hyperglycemic effect by causing blood glucose levels to rise. • Cause catabolism of proteins and lipids

• Products are ultimately converted back to glucose through gluconeogenesis.

• Also may help to maintain blood pressure and help body to resist stress.

Glucocorticoid-Related Diseases• Too much of cortisol in system-Hyperadrenocorticism=Cushing’s

Disease. (Iatrogenic Cushings)• Symptoms include:

• Polyuria• Polydipsia• Polyphagia• Hair loss• Muscle wasting • Slow wound healing

• Too little of cortison in the system- Hypoadrenocorticism=Addison’s Disease.

• Symptoms include:• Weakness• Lethargy• Vomiting• Diarrhea• Can lead to circulatory and kidney failure.

Cushing’s DiseaseCushing’s Disease

pot bellylong hairlaminitis

Excessive amount of glucocorticoids

Addison’s Disease

Glucocorticoid-Related Drugs• Drugs include:

• Hydrocortisone• Prednisone• Dexamethasone• Triamcinolone

• How they act:• Suppress immune system• Affect WBC count• Slow wound healing• Catabolic effect of proteins• Premature parturition• Hyperglycemia• Suppress adrenal cortex stimulation

Mineralcorticoid Hormones

• Regulate levels of electrolytes in body.

• Principle hormone:• Aldosterone

• Affects Sodium, Potassium, and Hydrogen Ions in the body.

• Targets kidney (why?)

Sex Hormones

• Adrenal cortex produces small amounts of sex hormones and effects are generally minimal.

• Male sex hormones- androgens• Female sex hormones-estrogens.

Adrenal Medulla

• Inner gland of adrenal glands and resembles nervous tissue.

• Concerned with sympathetic nervous system.

• Direct stimulation on target tissues.• Produces:

• Epinephrine• Norepinephrine

The Pancreas• Long and flat, located near the

duodenum.• Has both endocrine and exocrine

functions• Pancreatic islets (islets of langerhans)

are tiny clumps of cells that produce hormones

• Produces:• Insulin- by beta cells• Glucagon- by alpha cells• Somatostatin- by delta cells

Insulin

• Controls metabolism and use of glucose.• Is essential for life• Causes glucose, amino acids, and fatty

acids to be absorbed from bloodstream into body’s cells.

• Lowers level of glucose in the blood.

Glucagon

• Opposite effect of insulin• Raises glucose level in the blood.

• Stimulates liver to convert glycogen to glucose

• Stimulates gluconeogenesis• Other hormones do similar things so

deficiency is not as devastating.

Somatostatin

• Inhibits the secretion of insulin and glucagon and GH and diminishes activity of the GI tract.

The Gonads• Reproductive organs:

• Testes (when LH stimulates testes)• Testosterone

• What all does this do?• Where is it produced within the testes?

• Ovaries (when FSH and LH stimulate)• Estrogens

• Estradiol• Estrone

• What all do these do?• Progestins

• Progesterone• Produced by corpus luteum after ovulation• Drugs can be used to delay estrus and

synchronize estrous periods in a group.

The Kidneys

• Produce Erythropoietin• Stimulates red bone marrow to increase

production of red blood cells.• Production is stimulated by decrease of

oxygen in the blood.• Lack of production may result in

anemia.• Synthetic forms are used in cases of

cancer. • Procrit

The Placenta

• Produces small amounts of estrogen and progesterone

• Produces chorionic gonadotropin• What is tested for in pregnancy tests.

The Thymus

• Very important during early development, will shrink as animal approaches adulthood.

• Important in immune system development• Produces:

• Thymosin• Thymopoietin

• Seem to transform into t-lymphocytes (T-cells) which help to attack foreign invaders in the body.

Pineal BodyPineal Body• Influences body’s biological clock• Produces:

• Melatonin • hormone-like substance that affects moods and wake-

sleep cycles• Also plays a role in timing of seasonal estrous cycles in

some species

Prostaglandins• Derived from unsaturated fatty acids• Sometimes called tissue hormones due to the fact

they travel short distances.• Organized in groups A-I

• Remember PGE and PGI?• Produced in a variety of body tissues including:

• Skin• Intestine• Brain• Kidney• Lungs• Reproductive organs• Eye

Prostaglandin effects

• Influence blood pressure• GI function• Respiratory function• Kidney function• Blood clotting• Inflammation • Reproductive functions• PGE’s initiate inflammation so NSAIDs

inhibit PGE production.