Introduction to the Endocrine System
Administrative and Study Guidelines
Lab Manual and Materials:
The lab manual has been updated; students are encouraged to print it out.
Incentive: There are bonus points available if the manual is printed and brought to the beginning of the lab on Monday.
Schedule Note: For the endocrine lab, the schedule is different due to the Monday holiday; the lab will occur on Tuesday.
Labs and Prelabs: There are a total of labs throughout the course, but prelabs are scheduled. A "review prelab" will be provided to ensure students can obtain the extra points.
Mastery Assignments: There are two mastery assignments. The first covers the material from the first day of lab, while the second covers both the autonomic and endocrine systems.
Examination Strategies:
Pearson Question Style: The Pearson platform frequently uses "which of the following except" or "which is not" phrasing. It is recommended to rephrase these internally as positive questions.
Visual Content: There will be no pictures or figures on the exam because high-quality images are difficult to reproduce clearly in a testing format. However, students must still know the locations of major organs as they may be tested via text-based questions.
Study Techniques: When studying "compare and contrast" topics, creating a table is highly recommended. The process of figuring out and constructing the table is what facilitates learning, rather than the final product itself.
Overview of Intercellular Communication and Signaling
Defining the Terminology:
The word "crin" is derived from the Greek word for "sieve" (like the verb for sieve). It can be translated as to "discriminate," "distinguish," "separate," or "secrete."
Autocrine Signaling: "Auto" means self. In this mode, a cell releases a signal and possesses its own receptor to respond to that signal.
Example: Neurons often have auto-receptors on presynaptic terminals (e.g., for norepinephrine or adenosine) to prevent positive feedback loops and conserve neurotransmitters.
Paracrine Signaling: "Para" means around, alongside, or near. This signal affects neighboring cells in the immediate vicinity.
Applications: This is critical in blood circulation, blood vessels, the immune system, and the kidneys.
Neurocrine Signaling: This refers to synaptic transmission or neurotransmission. A signal travels a very short distance across a synapse to a postsynaptic cell.
Endocrine Signaling: "Endo" means inside or within. This involves the secretion of a hormone into the bloodstream.
The Nature and Mechanism of Hormones
Definition and Transport:
The word "hormone" comes from a term meaning "to set things in motion."
Hormones are signaling molecules that travel through the blood, meaning all cells in the body are exposed to them.
Selectivity: Only cells that possess specific receptors for a given hormone will respond to it. Cells without receptors remain unaffected.
Capillary Association: Endocrine glands are ductless and must be located near capillary beds. These capillaries are often fenestrated (from the German fenster, meaning window). Fenestrated capillaries have little windows or pores that make them more porous than typical continuous capillaries, facilitating the entry of hormones into the blood.
Comparison of Gland Types:
Both exocrine and endocrine glands derive from epithelial tissue.
Exocrine Glands: Maintain a duct (a pathway) to the surface of the epithelium (e.g., sweat glands or mucus glands).
Endocrine Glands: Are ductless; the connection to the surface is lost, and they are surrounded by an associated capillary bed.
Major Organs of the Endocrine System
Brain Regions:
Pineal Gland: Located in the epithalamus (back of the thalamus, above the superior colliculus). It releases melatonin and is involved in sleep-wake cycle regulation.
Hypothalamus: The integration center for both the autonomic nervous system and the endocrine system. It produces neurohormones.
Pituitary Gland (Hypophysis): Known as the "master gland" because it triggers other organs, though it is strictly controlled by the hypothalamus.
Neck and Torso Regions:
Thyroid Gland: Located in the laryngeal area; produces thyroid hormone and calcitonin.
Parathyroid Glands: Located on the posterior of the thyroid; involved in calcium homeostasis.
Thymus: Primarily involved in the immune system and the hormones that regulate it.
Adrenal Glands: Sit on top of the kidneys. Consists of a cortex (producing steroid hormones from cholesterol) and a medulla (releasing epinephrine and norepinephrine).
Pancreas: A dual-function organ (exocrine and endocrine).
Gonads: Ovaries and testes (secreting sex hormones).
Non-Traditional Endocrine Tissues:
Kidneys: Produce erythropoietin.
Liver: Involved in the synthesis of several hormones and performs approximately functions total.
Comparison: Nervous System vs. Endocrine System
Space and Location:
Nervous System: Acts at specific, direct locations; neurotransmitters act over very short distances (synaptic cleft).
Endocrine System: Acts systemically and is more diffuse; hormones travel long distances throughout the body.
Time and Duration:
Nervous System: Rapid initiation (milliseconds to seconds); short-duration responses.
Endocrine System: Slower initiation (must travel through blood); longer-lasting effects (minutes, hours, days, or even months).
Signal Encoding:
Nervous System: Signal strength is encoded in the frequency of action potentials.
Endocrine System: Signal strength is encoded in the concentration of the hormone in the blood.
Termination:
Nervous System: Specific inactivation mechanisms (enzymes, uptake, or diffusion) limit the signal in time and space.
Endocrine System: Termination depends on the hormone's half-life and degradation by the liver or kidneys.
Stimuli for Hormone Secretion
Humoral Stimulus: Triggered by changes in blood levels of certain ions or nutrients.
Example: Low calcium concentration in the blood triggers the release of parathyroid hormone (PTH) to break down bone and release calcium.
Neural Stimulus: Triggered by nerve fibers.
Example: Preganglionic sympathetic neurons trigger the adrenal medulla to release epinephrine into the blood.
Hormonal Stimulus: Triggered by other hormones (cascades).
Example: Hypothalamic hormones trigger the anterior pituitary to release hormones, which then trigger the thyroid or adrenal cortex.
Chemical Classification of Hormones
Amino Acid-Derived:
Tyrosine derivatives: Includes epinephrine and norepinephrine (catecholamines) and thyroid hormones.
Tryptophan derivatives: Includes melatonin and serotonin.
Peptides and Proteins:
Peptides: Short chains ( to amino acids) without stable tertiary structure (e.g., Angiotensin II, ADH, Oxytocin).
Proteins: Polypeptides with tertiary/quaternary structures (e.g., Growth Hormone, Insulin).
Glycoproteins: Proteins with carbohydrate groups (e.g., TSH).
Lipid-Derived:
Steroids: Derived from cholesterol (e.g., Testosterone, Estradiol, Cortisol, Aldosterone).
Eicosanoids: Derived from arachidonic acid (a -carbon unsaturated fatty acid found in membrane phospholipids).
Special Case - Thyroid Hormone (T3/T4): This is a dimer of two tyrosines linked by an ester bond and iodinated. Despite being amino acid-derived, it is highly hydrophobic.
Hormone Receptors and Regulation
Receptor Locations:
Membrane Receptors: Used by hydrophilic hormones. They trigger intracellular second-messenger cascades (e.g., G-protein coupled receptors or receptors with intrinsic enzyme activity).
Intracellular Receptors: Used by hydrophobic hormones (steroids/thyroid). They bind in the cytoplasm or nucleus and directly affect transcription and translation.
Dynamic Regulation:
Up-regulation: Exposure to a hormone causes the cell to produce and insert more receptors, leading to a larger response in the future.
Down-regulation: Exposure to a hormone leads to the degradation or removal of receptors, resulting in a diminished response (desensitization).
Hormone Transport and Half-Life
Solubility and Transport:
Hydrophilic Hormones: (Most amino acid-derived and peptides) travel free in the plasma.
Hydrophobic Hormones: (Steroids and Thyroid hormones) must travel bound to carrier proteins (e.g., Thyroid Binding Globulin/TBG). A small fraction remains "free" to diffuse into target cells.
Half-Life Concepts:
Definition: The time required for a hormone's concentration in the plasma to reduce by half ().
Role of Carrier Proteins: They make hydrophobic hormones soluble and protect them from rapid degradation or excretion.
Specific Half-Lives:
Epinephrine: Very short; to \,minutes.
Thyroxine (T4): Very long; \,days (regulates metabolism long-term).
Triiodothyronine (T3): \,day.
Cortisol: Roughly \,hours.
Aldosterone: Approximately \,minutes (shorter than other steroids).
Peptides (e.g., Insulin, ADH): Short; approximately \,to \,minutes.
The Hypothalamus-Pituitary Axis
Anatomy:
Connected by the infundibulum (stalk), meaning "funnel."
The pituitary sits in the sella turcica of the sphenoid bone.
Hypophysis is another name for the pituitary; "physis" means growth, and "hypo" means below.
Hypothalamic Nuclei:
Paraventricular Nucleus (PVN): Contains magnocellular and parvocellular neurons.
Supraoptic Nucleus (SON): Contains magnocellular neurons.
Arcuate Nucleus (ARC): Contains neurons that release tropic hormones.
Suprachiasmatic Nucleus (SCN): Involved in circadian rhythms.
The Posterior Pituitary (Neurohypophysis)
Mechanism: Magnocellular neuroendocrine cells in the PVN and SON send axons down the hypothalamic-hypophyseal tract to the posterior pituitary. Hormones are released directly into the capillary bed.
Antidiuretic Hormone (ADH / Vasopressin):
Stimuli for Release: Increased blood solute concentration, low blood pressure, sympathetic activation, and Angiotensin II.
Target: Kidneys (specifically the collecting duct).
Mechanism: Binds G-protein coupled receptors () $\rightarrow$ increases cyclic AMP () $\rightarrow$ activates Protein Kinase A () $\rightarrow$ triggers insertion of Aquaporin channels into the luminal membrane.
Result: Water reabsorption, concentrated urine, and increased blood pressure via vasoconstriction.
Oxytocin:
Stimuli for Release: Stretching of the cervix, suckling, and sexual arousal.
Targets/Effects: Uterus (smooth muscle contraction for birth), mammary glands (milk ejection), and social/behavioral centers in the brain (empathy, bonding).
Feedback: Operates via positive feedback loops during labor and milk ejection.
The Anterior Pituitary (Adenohypophysis)
Mechanism: Uses the hypophyseal portal system. Parvocellular neurons release tropic hormones into a capillary bed at the median eminence. These travel through portal veins to a second capillary bed in the anterior pituitary to stimulate or inhibit endocrine cells.
Anterior Pituitary Hormones:
Growth Hormone (GH): Stimulated by GHRH (Arcuate nucleus), inhibited by somatostatin. Highest release to \,hours after deep sleep onset. Targets bone and muscle. Stimulates liver to release Insulin-like Growth Factors (IGFs).
Anabolic Effects: Protein synthesis, bone mineralization, lipolysis, and glucose sparing (elevating blood glucose).
Disorders: Dwarfism (childhood deficiency), Giantism (childhood excess), Acromegaly (adult excess; hands, feet, face).
Thyroid Stimulating Hormone (TSH): Triggers T3/T4 release.
Adrenocorticotropic Hormone (ACTH): Triggers cortisol release.
Follicle Stimulating Hormone (FSH) / Luteinizing Hormone (LH): Regulate gonads.
Prolactin: Stimulates milk production.
The Thyroid and Parathyroid Glands
Thyroid Anatomy: Follicles filled with colloid (containing thyroglobulin protein).
Follicular Cells: Synthesize thyroglobulin and iodine; process colloid to release T3 and T4.
T3 vs T4: T3 is \times more potent; T4 has a longer half-life. T4 is converted to T3 at target tissues by deiodinase.
Regulation: TRH (Hypothalamus) $\rightarrow$ TSH (Anterior Pituitary) $\rightarrow$ T3/T4 (Thyroid). Negative feedback affects both the hypothalamus and pituitary.
Calcium Regulation:
Parathyroid Hormone (PTH): Peptide hormone released in response to low calcium. Increases osteoclast activity, increases kidney calcium reabsorption, and triggers Calcitriol (Vitamin D) production in kidneys to increase intestinal calcium absorption.
Calcitonin: Peptide hormone from thyroid C-cells (parafollicular) released in response to high calcium. Inhibits osteoclasts and decreases kidney reabsorption.
Vitamin D Synthesis Pathway
Skin: UV light converts cholesterol to an intermediate.
Diet: Absorption of precursors from fortified milk, fish oil, or eggs.
Liver: Conversion to an intermediate product.
Kidney: Under PTH stimulation, converted to the active form, Calcitriol.
Questions & Discussion
Student Question: Does the lab manual need to be colored?
Response: No, it does not need to be printed in color.
Student Question: What is an example of when you would give T3 or T4 to someone? For their metabolism?
Response: Yes, though the instructor notes they do not personally go around injecting people; it is a clinical application for metabolic issues. The instructor focuses on the "big picture," such as which hormone has the longer half-life (T4) and their relative durations compared to others like epinephrine.
Observation: The instructor noted a typo in the slides regarding the classification of T3/T4 as lipid-derived versus amino acid-derived, clarifying that they are tyrosines but functionally hydrophobic.