Endocrine Systems Overview

Objectives

The student who successfully completes this course will be able to:

1. List the organs of the endocrine system

• Thymus
• Thyroid
• Parathyroid
• Pituitary
• Pineal
• Adrenal
• Gonads
• Hypothalamus
• Pancreas

2. List the three major chemical classes of hormones and provide examples from each

• Steroids: Cortisol, Aldosterone, Testosterone, Progesterone, Estrogen
• Monoamines: Dopamine, Norepinephrine, Epinephrine, Melatonin, Thyroid hormone
• Peptides: Calcitonin, glucagon, insulin, growth hormone, prolactin, parathyroid

3. Compare and contrast how classes of hormones are produced and stored in the endocrine cell, released from the endocrine cell, and transported in the blood.

• Steroids: Produced in the gonads and adrenal gland
  • Hydrophobic
  • NOT STORED
  • Released through diffusion when produced
  • Bind to receptors through the plasma membrane
• Monoamines: Produced from the amino acid called tyrosine, stored on extracellular space on the cell, released via diffusion
  • Target is ATP dependent protein transport and attach to nuclear receptors
  • Results in responsiveness in cardiac muscles due to using heat from ATP
• Peptides: Transcribed to form mRNA which contains the blueprint for the hormone
  • Stored in secretory vesicles of the endocrine cell until needed
  • Released via exocytosis
  • Target: hydrophilic, bind to cell surface, secondary messenger system
  • Dissolves in blood easily because hydrophilic
  • Fast and short-lived action

4. Compare and contrast the locations of target cell receptors for classes of hormones.

• Steroids - plasma membrane
• Monoamines - Nuclear receptors or ATP dependent proteins
• Peptides- Receptors on the cell surface

5. Describe the locations and the anatomical relationships of the hypothalamus, anterior pituitary, and posterior pituitary, including the hypothalamic-hypophyseal portal system.

• Hypothalamus: Superior to the pituitary gland (right above it)
• Anterior pituitary( ¾) : Closest to the face; in charge of hypothalamic hormones and anterior lobe hormones.
• Posterior pituitary(¼ ): Closest to the back of the head; incharge of antidiuretic and oxytocin release.
• Hypothalamic-hypophyseal portal system: Connects all three glands via blood vessels and veins. Contains primary capillaries, secondary capillaries, and portal veins
  • 1. Hypothalamus releases hormones into the primary capillaries
  • 2. Hormones travel to the secondary capillaries and distribute to the anterior pituitary
  • 3. Portal veins allow the hormones from the hypothalamus to easily influence the anterior pituitary’s hormones
  • * Posterior pituitary is controlled by the hypothalamus neurally

6. Describe major hormones secreted by the anterior pituitary, their primary target(s) and effects.

• Follicle-stimulating hormone (FSH)
  • ovaries and testes
  • F: grows ovarian follicles and secretes estrogen
  • M: sperm production
• Luteinizing hormone (LH)
  • Ovaries and testes
  • F: ovulation
  • M: testosterone secretion
• Thyroid-stimulating hormone (TSH)
  • Thyroid gland
  • Controls growth of thyroid and secretion of thyroid hormone
• Adrenocorticotropic hormone (ACTH)
  • Adrenal cortex
  • Grows adrenal cortex and secretes glucocorticoids
• Prolactin (PRL)
  • Mammary glands
  • Milk synthesis
• Growth hormone (GH)
  • Liver, bone, cartilage, muscle, fat
  • Widespread tissue growth

7. Describe the two hormones stored in the posterior pituitary, their primary targets and effects.

• Antidiuretic hormone (ADH)
  • Kidneys
  • Water retention
• Oxytocin (OT)
  • Uterus and mammary glands
  • Labor contractions, milk release, sexual affection, sperm transport

8. Discuss examples of feedback loops within the endocrine system

POSITIVE: Releasing hormone from the hypothalamus goes to the pituitary gland and then the pituitary releases the hormone that carries out throughout the body.

NEGATIVE: Hormone that was released from the pituitary gland can inhibit the release of more from the hypothalamus.

Terms to know

• Hormone: CHEMICAL messenger that is secreted by an endocrine gland or isolated gland cell, travels through the bloodstream, and triggers a physiological response in distant cells (target cells) with receptors for it
• Endocrine gland: A ductless gland that secretes hormones into the bloodstream: for example, the thyroid and adrenal glands
• Endocrine organ: An organ that secretes hormones: for example, the pancreas and liver
  • Can be used interchangeably, but not all organs are endocrine glands
• Target cell: A cell acted upon by a nerve fiber, hormone, or other chemical messenger

Functions (* = kaap310)

1. Metabolism
2. Reproductive function*
3. Growth and development
4. Stress response
5. Regulation of mood/emotions
6. Immune system regulation*
7. Regulation of the sleep-wake cycle

Nervous vs Endocrine System

Nervous: Communicates by electrical signals and neurotransmitters

Endocrine: Communicates by means of hormones

Nervous: Releases neurotransmitters at synapses at specific target cells

Endocrine: Releases hormones into the bloodstream for general distribution throughout the body

Nervous: Usually has relatively local, specific effects

Endocrine: Sometimes has very general, widespread effects

Nervous: Reacts quickly to stimuli within 1-10ms

Endocrine: Reacts more slowly to stimuli, often taking seconds to days

Nervous: Stops quickly when stimulus stops

Endocrine: May continue responding long after stimulus stops

Nervous: Adapts relatively quickly to continual stimulation

Endocrine: Adapts relatively slowly, may respond for days to weeks

Structures of the endocrine system

• Endocrine glands

All organ systems secrete hormones!

Organs of the Endocrine System

• Hypothalamus
• Pituitary gland
• Pineal gland
• Thyroid gland
• Parathyroid
• Thymus
• Pancreas
• Adrenal glands
• Gonads

Hormones

*3 primary classes: Steroids, Monoamines, Peptides*

Steroids and Steroid Derivatives

• Calcitriol
• Cortisol
• Estrogens
• Progesterone
• Aldosterone
• Androgens

Monoamine

• Dopamine
• Epinephrine
• Melatonin
• Norepinephrine
• Thyroid hormone

Oligopeptides (3-10 Amino Acids)

• Angiotensin II
• Antidiuretic hormone
• Gonadotropin-releasing hormone
• Oxytocin
• Thyrotropin-releasing hormone

Polypeptides (More than 10 Amino acids)

• Adrenocorticotropic hormone
• Calcitonin
• Corticotropin-releasing hormone
• Glucagon
• Growth hormone
• Growth hormone-releasing hormone
• Hepcidin
• Insulin
• Natriuretic peptides
• Pancreatic polypeptide
• Parathyroid hormone
• Prolactin
• Somatostatin

Glycoproteins (protein-carbohydrate complexes)

• Erythropoietin
• Follicle-stimulating hormone
• Inhibin
• Luteinizing hormone
• Thyroid-stimulating hormone

3 main classes of hormones

Steroids

Produced: in the Gonads and Adrenal glands

• Synthesized from cholesterol

Stored: NOT STORED.

• Released as they are produced

Released: via diffusion.

Target: (hydrophobic) passes through the plasma membrane and binds to receptors.

Result: Activated or inhibits transcription for an enzyme or protein.

Monoamines

Produced: from the amino acid tyrosine

• Melatonin: from the amino acid tryptophan

Stored: often in the extracellular space around the cell

Released: diffusion

Target: enters through ATP dependent transport protein, binds to nuclear receptors

Result: liberates heat from ATP, responsiveness of cardiac muscle

Peptides

Produced: gene for the peptide hormone is transcribed to form a molecule of mRNA, which then contains instructions to make the hormone

Stored: Secretory vesicles of the endocrine cell until needed

Released: Exocytosis

Target: Hydrophilic bind to cell surface and lined to a second messenger system

Result: Fast, short and lived.

Anatomy Overview

• Hypothalamus
• Pituitary gland:

¾ anterior (hypothalamus hormones)

¼ posterior

• Hypothalamus connects to the pituitary gland through the hypophyseal portal

Hypophyseal Portal System

• Primary capillaries
• Portal venules
• Secondary capillaries

Feedback Example

POSITIVE

1. The hypothalamus secretes thyrotropin-releasing hormone (TRH)
2. TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH)
3. TSH stimulates the thyroid gland to secrete thyroid hormone (TH)
4. TH stimulates the metabolism of most cells throughout the body

NEGATIVE

1. TH also inhibits the release of TSH from the pituitary
2. TH also slightly inhibits the release of TRH by the hypothalamus

Hypothalamus hormones

• Thyrotropin-releasing hormone (TRH)
• Corticotropin-releasing hormone (CRH)
• Gonadotropin- releasing hormone (GnRH)
• Growth hormone-releasing hormone (GHRH)
• Prolactin-inhibiting hormone (PIH)
• Somatostatin

Pituitary hormones

• Anterior
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Prolactin (PRL)
  • Growth hormone (GH)
• Posterior
  • Antidiuretic hormone (ADH)
  • Oxytocin (OT)

Endocrine System II

Objectives

The student who successfully completes this course will be able to:

1. Describe the anatomy for each endocrine structure including its location, the major hormones secreted, and the hormones’ primary targets and effects.

• Pituitary Gland
  • Location: Underneath the hypothalamus.
  • Major hormones: a lot
  • Primary targets: other organs in the body
  • Effects: many things
• Pineal Gland
  • Location: Roof of 3rd ventricle
  • Major hormones: Melatonin (serotonin derivative)
  • Primary targets: The brain
  • Effects: Allows us to sleep and controls puberty
• Thyroid Gland
  • Location: Beneath the trachea
  • Major hormones: Thyroid hormone (T3 < T4), calcitonin
  • Primary targets: The whole body
  • Effects: Metabolism and skeletal growth
• Parathyroid Gland
  • Location: Behind the thyroid
  • Major hormones: Parathyroid hormone
  • Primary targets: Blood
  • Effects: Maintains blood calcium concentration
• Adrenal Gland
  • Location: Superior to the kidneys (AKA Suprarenal)
  • Major hormones: Medulla: Epinephrine, norepinephrine Cortex: Aldosterone, Cortisol, Androgens in three zones
  • Primary targets: Medulla: Heart, blood vessels, muscles, nervous system Cortex: Kidneys (aldosterone), immune system, muscles, fat tissue (cortisol), skin, bones, muscle development (Androgens)
  • Effects: Fight or flight response, sexual development, regulation of Na/K balance in kidneys
• Pancreas
  • Location: Behind liver and near the gallbladder.
  • Major hormones: Insulin and amylin
  • Primary targets: Glucose
  • Effects: Controls BGL
• Thymus Gland
  • Location: Upper chest , behind the sternum.
  • Major hormones: Thymosin and thymopoietin
  • Primary targets: T cells (white blood cell)
  • Effects: T cell maturation and development
• Gonads
  • Location: Males- Testes Females- Ovaries
  • Major hormones Testosterone, Estrogen, and Progesterone
  • Primary targets:
    • Testosterone- Muscles, bones and reproductive organs
    • Estrogen and progesterone- breasts, bones and reproductive organs
  • Effects:
    • Testosterone- Muscle mass, bone density, facial hair, deeper voice.
    • Estrogen: Regulates menstrual cycle, bone density, breast development.
    • Progesterone: Prepares uterus for pregnancy and regulates menstrual cycle.

2. Provide examples of hormones that are secreted from diffuse endocrine tissues or single endocrine cells.

o Skin, liver, kidneys, heart, skeletal muscle, digestive tract, adipose, bone, and placenta

• Table 17.4

3. Discuss examples of feedback loops within the endocrine system

Anterior Pituitary Hormones

Pineal Gland - back of brain under posterior corpus callosum

• Roof of the 3rd ventricle (posterior)
• Hormone: Melatonin (from serotonin)
• General: maintain circadian rhythm, puberty (?)

Thyroid Gland

• Largest gland that is only endocrine
  • Highly vascularized (4 arteries)
• Hormone: thyroid hormone (T3 [10%] and T4 [90%]), calcitonin
• General: metabolism and skeletal growth

Histology

• Follicular cells (simple cuboidal)
  • Easy diffusion for secretion
  • Release 80 micrograms of thyroid hormone per day
• Parafollicular cells (C Cells)
  • Calcitonin
  • Significantly more in children

Parathyroid Gland

• Typically in 4, 2 on each side. (a person can have more than 4) Posterior thyroid small little circles
• Hormone: parathyroid hormone
• General: regulates blood calcium levels

Parathyroid and thyroid work together to regulate calcium levels throughout the body. Calcitonin helps reduce calcium.

How it works with the thyroid gland (know this!)

Adrenal (Suprarenal) Gland

• Superior pole of each kidney
  • Cortex and the Medulla
• Cortex Hormones: Typically steroid hormones (corticosteroids)
• Medulla Hormones: epinephrine, norepinephrine, dopamine
• General: increase alertness, mobilize high energy fuels
• Sympathetic nervous system (adrenaline)

Zones (superficial to deep)

• Zona glomerulosa (thin, outer layer)
  • Secretes mineralocorticoids
  • Aldosterone
• Zona fasciculata (thick, middle layer)
  • Secretes glucocorticoids and gonadocorticoids
  • Cortisol
• Zona reticularis (narrow, inner layer)
  • Secretes glucocorticoids and gonadocorticoids
  • Androgens (sex hormones)

Aldosterone

• MOST important mineralocorticoid
• Produced only from the zona glomerulosa
• Released in response to falling blood pressure. (causes the release of angiotensin II)
  • Falling blood pressure causes release of angiotensin II, which stimulates aldosterone release from adrenal cortex
  • Aldosterone causes kidney to retain sodium and water, maintaining blood volume and pressure

Cortisol

• Most potent glucocorticoid
• Secreted from zona fasciculata and zona reticularis
  • Released in response to ACTH
    • Stimulate fat and protein catabolism, gluconeogenesis, and release of fatty acids and glucose into blood
      • Helps body adapt to stress, repair damaged tissue
    • Has anti-inflammatory effect
      • Excessive secretion or medical use suppresses the immune system

Androgens

• Primary adrenal sex steroids ( small amounts of estrogen)
• Secreted from zona fasciculata and zona reticularis
  • Released in response to ACTH
    • Dehydroepiandrosterone (DHEA)
      • Major androgen, converted to more potent form in target tissues
      • At puberty, stimulates pubic, axillary hair growth, gland development, and libido
    • Estradiol
      • Major adrenal estrogen
      • Significant source of estrogen in women after menopause

Pancreas

• Endocrine and exocrine functions
• Endocrine cell clusters scattered throughout called pancreatic islets (islets of Langerhans)
• Regulation of glycemia

Pancreatic Islets (islets of Langerhans)

• Alpha cells (20%)
• Beta cells (70%)
• Delta cells (5%)
• Small number of polypeptide cells and others

Alpha cells

• Secretes glucagon (between meals)
• Stimulates gluconeogenesis and glycogenolysis
  • Results in release of glucose into the circulation raising blood glucose levels
• Stimulates fat catabolism and release of free fatty acids
• Also released in response to rising amino acid levels in blood
  • Promotes amino acid absorption

Beta cells

• Secrete insulin and amylin (during and after meals)
• Insulin:
  • Stimulates cells to absorb nutrients and store or metabolize them, lowering blood glucose levels
  • Promotes synthesis of glycogen , fat, and protein
  • Suppresses use of already stored fuels
• Amylin
  • Reduces glucose spikes
  • Slows stomach emptying, modulates gastric secretions, inhibits glucagon secretion and signals satiety

Thymus Gland

Very big in children, smaller in adults

• T-cell maturation site
• Hormones: Thymopoietin, thymosin, thymulin.
• General: stimulate other lymph organs, regulate development and T-cell activity

Gonads

• Ovaries and Testes have both endocrine and exocrine functions
  • Exocrine: Eggs and sperm
  • Endocrine: gonadal hormones(mostly steroids)
• Ovarian hormones:
  • Estradiol, progesterone, inhibin
• Testicular hormones:
  • Testosterone, weaker androgens, estrogen, inhibin

Ovaries

• Hormones: Estradiol, progesterone, inhibin
• Oocyte: egg develops in follicle lines by granulosa cells and surrounded by capsule (theca)
  • Theca cells synthesise androstenedione
  • Granulosa cells convert it to estradiol
• After ovulation, the remains of the follicle become the corpus luteum
  • Secretes progesterone for 12 days following ovulation
    • Shifts to weeks if a pregnancy results
  • Follicle and corpus luteum secrete inhibin

Testes

• Hormones: Testosterone and inhibin
• Seminiferous tubules produce sperm
  • Tubule walls contain nurse cells
  • Interstitial endocrine cells secrete testosterone and other sex steroids.
• General:
  • Testosterone stimulates development of male reproductive system in fetus and adolescent, and sex drive
  • Sustains sperm production
  • Inhibin limits follicle-stimulating hormone secretion to regulate sperm production

“Other” Endocrine structures

Just know the structure and the hormone/function

Table 17.4 REVIEW

Endocrine Pathways

Objectives

The student who successfully completes this course will be able to:

1. Define the terms paracrine and autocrine.

• Paracrine: Short distance signals and only stimulates neary cells.
• Autocrine: Signals that stimulate the same cell

2. List the major types of eicosanoids and describe their functions.

• Prostaglandins: Impacts every system with different roles.
• Prostacyclins: Anti-clotting actions and vasoconstriction
• Thromboxanes: Released from platelets, stimulate vasoconstriction and clotting
• Leukotrienes: Allergic reactions and inflammatory reactions (Leuko = WBC)

3. Describe how prostaglandins impact other body systems

• Table 17.6

4. Describe the three types of hormone stimuli

• How hormones are released
• Neural:
  • Direct stimulation of the exocrine glands that works with the nervous system.
• Humoral (bloodborne)
  • Something circulating in the blood calls for the hormone to be released
• Hormonal
  • Hormones are released by other hormones

5. Explain hormone transport and receptors

• What do hormones travel through? Hydrophilic and hydrophobic hormones travel through the bloodstream
• What are transport proteins? Proteins that are produced in the liver. (TBG and transcortin. Bound and unbound
• Hydrophobic hormone groups: Steroids
• Hydrophilic hormone groups: Peptides and monoamines
• Hormone receptors:
  • Hydrophilic: Peptides and catecholamines. Surface receptors and secondary messenger systems. Quick effects.
  • Hydrophobic: Intracellular receptors. Takes longer because of protein synthesis

6. Describe hormone mode of action

• Intracellular Receptors: Lipid-soluble hormones pass through the cell membrane, bind to intracellular receptors, and directly influence gene expression.
• Cell Surface Receptors: Water-soluble hormones bind to cell surface receptors, triggering signal transduction pathways that lead to various cellular responses.

7. Explain the cascade effect of signal amplification and target cell sensitivity

• “Cascade Effect” - One hormone can be potent and cause a large amount of synthesis of enzyme molecules. The smallest stimulus can cause great effects.
• Target cell sensitivity:
  • Down regulation: Decreasing receptor sensitivity to hormones
  • Up regulation: Increasing receptor sensitivity to hormones

8. Compare and contrast hormone interactions

• Synergistic: Multiple hormones work in synergy to enhance an effect
• Permissive: A specific hormone prepares a target organ for the effect of another hormone
• Antagonistic: Two hormones produce counteracting effects (Insulin vs Glucagon)

9. Discuss examples of feedback loops within the endocrine system (Stress & Adapation)

‘Other’ Messengers

• The body primarily communicates via hormones and neurotransmitters

Paracrine Signals

• Diffuse short distances and stimulate nearby cells.
  • Ex: histamine from mast cells in connective tissue; causes relaxation of blood vessels.

Autocrine Signals

• Stimulate the same cell that secreted them
  • Ex: Hepcidin - stimulates the liver cells that secreted it, regulates their release of stored iron into the blood

Eicosanoids

• Leukotrienes: mediate allergic reactions and inflammatory reactions
• Prostacyclin: inhibits blood clotting and vasoconstriction.
• Thromboxanes (produced by platelets)
  • Override prostacyclin and stimulate vasoconstriction, clotting (opposite of prostacyclin)
• Prostaglandins (PGs)
  • Diverse group with diverse roles
  • Impacts every system in the body

Hormone Stimuli

• Neural
  • Direct stimulation of exocrine glands by the nervous system
• Hormonal
  • Released because another hormone told it to
• Humoral
  • Bloodborne
  • Something circulating through the body causing the trigger for release (ex: insulin)

Hormone Transport

• Travel in bloodstream
  • Hydrophilic (peptides and monanimes)
  • Hydrophobic (steroids)
• Transport proteins (ex: TBG, transcortin) produced by liver
  • Once attached, called bound hormones
  • Only unbound (free) hormones can leave capillary to reach the target cell

Hormone Receptors

• Protein or glycoprotein molecules
  • Plasma membrane, cytoplasm, or in nucleus
• Receptor-hormone interactions
  • Specificity (specific hormones attach to specific receptors)
  • Saturation (has a limit for hormone molecule attachment)

Hydrophilic Hormones

• Peptides and catecholamines
  • Surface receptors
  • Second messenger systems
  • Effects are relatively quick

Hydrophobic Hormones

• Steroids and thyroid hormone
  • Intracellular receptors
  • Take several hours to days to show effects due to lag for protein synthesis (transcription, translation) and accumulation of enough product for significant impact on cell metabolism

Signal Amplification

• “Cascade effect”
  • Hormones are very potent
• One hormone molecule triggers the synthesis of a large number if enzyme molecules
• Very small stimulus can produce a very large effect
• Allows circulating hormone concentrations to remain relatively low

Target Cell Sensitivity to Hormones

• Down-regulation
  • Long-term exposure to high volume of a hormone
    • Ex: high cortisol concentration leads to less receptors on the cells
• Up-regulation
  • Increased number of receptors causes a sensitivity increase

Hormone Interactions

1. Synergistic Effects
   1. Multiple hormones act together for greater effect
      1. Synergism between FSH and testosterone on sperm production
2. Permissive Effects
   1. One hormone enhances the target organ’s response to a different hormone
      1. Estrogen prepares the uterus for action of progesterone
3. Antagonistic Effects
   1. One hormone opposes the action of another
      1. Insulin lowers blood glucose and glucagon raises it

Stress and Adaptation

Stress: Any time homeostasis is off balance

Stressors: Triggers or causes of the change of stress

General Adaptation Syndrome (GAS)

• Elevated levels of epinephrine and cortisol
• Stress response has three stages

1. Alarm reaction
2. State of resistance
3. Stage of exhaustion

Alarm reaction

• Sympathetic nervous system
  • Release of norepinephrine and epinephrine (adrenal medulla)
  • Prepares body for fight or flight
  • Stored glycogen is consumed
  • Increased aldosterone (promotes sodium and water conservation) and angiotensin (increases bp) levels

State of Resistance

• Goal: Provide alternative fuels for metabolism
• After a few hours of increased stressor
• Cortisol
  • Hypothalamus secretes corticotropin releasing hormone
  • Pituitary gland secretes ACTH
  • Adrenal cortex secretes cortisol and other glucocorticoids
  • Promotes breakdown of fat and protein into glycerol, fatty acids, and amino acids for gluconeogenesis (creation of glucose)

Stage of Exhaustion

• When stress continues for several months, fat reserves are gone, body cannot maintain homeostasis
  • Protein breakdown and muscle wasting
• Loss of glucose homeostasis because adrenal cortex stops producing glucocorticoids
• Aldosterone promotes water retention
  • Can lead to hypertension
  • Conserves sodium and hastens elimination of potassium and hydrogen
    • Leads to potassium deficiency and elevated pH, which causes dysfunction of nervous and muscular systems

chippy