Endocrinology Quiz: Hormones and Glands

Anatomy-Histology Correlates in Endocrinology

•  Objectives include the pituitary (hypophysis), anterior pituitary, posterior pituitary, adrenal gland (suprarenal) - adrenal cortex, adrenal medulla, thyroid gland - follicles, parafollicular cells, parathyroid gland, and aspects of the endocrine pancreas (male and female).

Coordination of Body Functions

• Nervous stimuli act quickly, conducting along neuron axons to release neurotransmitters at synapses. Neuron effects are generally short-lived.
• Endocrine stimuli primarily affect metabolism, growth, differentiation, and reproduction. Hormone mediators are distributed throughout the body via the circulatory system.

Nervous vs. Endocrine Systems

Hormone Production: Less Traditional Sources

• Hormones are produced by a variety of tissues, not just glands. Examples include endothelium (endothelins, NO, prostanoids), immune system (cytokines), platelets/mesenchyme (growth factors), cardiocytes (ANP), placenta (all hormones), adipocytes (leptin, resistin, adiponectin), kidney (erythropoietin, RAAS), and the gastrointestinal tract (GIT - gastrin, cholecystokinin, secretin). The hypothalamus and pituitary also produce hormones.

Chemical Characteristics of Hormones

• Amines (from tyrosine): Catecholamines (hydroxylation) and thyroid hormones (iodination).
• Peptides/Proteins: Chains of amino acids.
• Steroids (from cholesterol): Adrenocorticoids, sex hormones, and active vitamin D metabolites.

Chemical Characteristics of Hormones (Examples)

• Amines: Norepinephrine is an example of an amine hormone.

• Peptides/Proteins: Oxytocin is an example of a peptide hormone, and Growth hormone is an example of a protein hormone.

• Steroids: Cortisol is an example of a steroid hormone.

Amino Acid and Derivatives (Catecholamines)

• Catecholamines are derived from phenylalanine and tyrosine.
• Tyrosine is an amino acid.
• Dopamine, norepinephrine, and epinephrine are examples of catecholamines.
• These are connected by enzymatic steps.

Thyroid Hormones

• Thyroid hormones (thyroxine and triiodothyronine) are derived from tyrosine.

Hormone Release

• Proteins and catecholamines are released by exocytosis from secretory granules. Sometimes these hormones undergo cleavage to become active after release.
• Release is often triggered by changes in calcium concentration.
• Thyroid hormones are made as part of thyroglobulin and stored in follicles. T3 and T4 are secreted by enzymatic cleavage.
• Steroid hormones are released directly across the cell membrane after synthesis (no storage).

Regulation of Hormone Release

• Feedback (Negative & in some cases Positive, limited)
  • Gland releases hormone which triggers a response in a target tissue. The response product inhibits the release of the initial hormone.
• Nerve regulation: Pain, emotions, etc.. influence hormone release. Ex: oxytocin during nipple stimulation.

Combined Feedback (Example) - Stress Response

• Stress triggers CRH release in the hypothalamus.
• CRH stimulates ACTH release from the pituitary.
• ACTH stimulates cortisol release from the adrenals.
• Cortisol levels increase, and they provide feedback inhibition on the secretion of CRH and ACTH (hypothalamus and pituitary).

Regulation of Hormone Release - Rhythms

• Circadian rhythms: Light/dark fine-tune endogenous rhythm of cells (suprachiasmatic nucleus of hypothalamus). Examples: melatonin and cortisol.
• Monthly, seasonal, developmental rhythms (puberty and menopause).
• Pulsatility/oscillations are seen in gonadotropins.

Pulsatility in GnRH & LH Release

• GnRH and LH release are often described in pulsatile patterns.

Classes of Hormones

• Water-soluble hormones include catecholamines (epinephrine & norepinephrine) and peptide/protein hormones.
• Lipid-soluble hormones include thyroid hormones, steroid hormones, and vitamin D3.

Types of Receptors

• Water-soluble hormone receptors are on the plasma membrane, often coupled to second messenger systems.
• Lipid-soluble hormone receptors are intracellular, often in the nucleus or cytoplasm, because these hormones can diffuse across membranes.

Hormones and Their Receptors

Pituitary I

• The pituitary gland is in the sella turcica of the sphenoid bone.
• Connected to the hypothalamus via a stalk and vascular system.
• The posterior pituitary (neurohypophysis) develops from neural ectoderm. The anterior pituitary (adenohypophysis) develops from the floor of the mouth.
• The hypothalamic-pituitary axis controls neuroendocrine systems.

Pituitary II

• Adenohypophysis is composed of cells and cords with fenestrated capillaries.
• Neuronal tract containing neurosecretory bundles synthesized in the SON & PVN forms the neurohypophysis, storing oxytocin and ADH.
• The hypothalamo-hypophyseal portal tract links the anterior pituitary to the hypothalamus by releasing hormones.
• Adenohypophysis has no direct arterial supply, but instead receives fenestrated capillaries producing a portal vein network.

Pituitary Development

• The pituitary gland develops sequentially from oral ectoderm (Rathke's pouch for anterior lobe) and neural ectoderm (posterior lobe).
• The posterior lobe develops earlier than the anterior lobe.

Anterior Pituitary

• Anterior lobe has pars tuberalis, pars intermedia, and pars distalis.
• Derived from Rathke's pouch.

Posterior Pituitary

• Posterior lobe has pars nervosa and infundibulum.
• Neurosecretory nerve endings, storing oxytocin and ADH.
• Connecting to the hypothalamus via hypothalamo-hypophyseal tract.

Blood Supply (Pituitary)

• Superior hypophyseal artery supplies the pars tuberalis and infundibulum.
• Inferior hypophyseal artery supplies the pars nervosa.
• Hypothalamo-hypophyseal portal tract links hypothalamus and anterior pituitary.

Pituitary Histology

• Anterior pituitary is made of chromophobes and chromophils (acidophils and basophils).
• Five functional cell types include somatotropes (GH), lactotropes (prolactin), corticotropes (ACTH), gonadotropes (FSH & LH), and thyrotropes (TSH).
• All anterior pituitary hormones are glycoproteins.

Endocrine Pancreas

• Acinar tissue (exocrine) and islet cells (endocrine)
• Islet cell groups including Alpha, Beta, Delta, and F cells secrete hormones in specific locations. 

Second Messenger Systems

• Receptors for water-soluble hormones are found on the plasma membrane.
• These receptors couple to secondary messengers (systems including adenylate cyclase, guanylate cyclase, calcium/calmodulin, and phospholipase C) to trigger cellular response.

Types of Receptors (Membrane Receptors)

• Receptor Kinase: Contains ligand activation and phosphorylation (tyrosine kinase activity), activation of intracellular kinases (e.g., JAK).

• G-Protein-Coupled Receptors: Hormone-receptor binding activates related G-proteins and intracellular effector molecules (e.g., adenylate cyclase).

• Ligand-Gated Ion Channels: Hormone binding opens channels, which changes ion movement.

Second Messenger Systems (Continued)

• Specific protein kinases are activated by various second messengers. This leads to changes in target cell activity. Calcium/calmodulin-dependent protein kinase (CAMK) and protein kinase C (PKC) are examples.

• Second messenger systems lead to rapid cellular responses.

Receptors for Lipid-Soluble Hormones (Intracellular receptors)

• These hormones pass through the cell membrane and interact with intracellular receptors (often in the nucleus).
• The hormone-receptor complex binds to specific DNA sequences, influencing gene expression and altering cellular responses.
• These responses are slower than those with membrane receptors because they require protein synthesis.

Mechanisms of Endocrine Disease

• Deficiency can result from destructive processes (infection, infarction, tumor compression, autoimmune attack) or genetic defects (gene deletion, mutation, or enzymatic defects).
• Excess can result from overproduction by the gland, ectopic production, or tumors.
• Exogenous ingestion of hormones can also cause problems.
• Receptor number/function abnormalities or activating receptor mutations can cause disease.

Thyroid Gland I

• Two lateral lobes connected by an isthmus, located in the anterior neck.
• Surrounded by a capsule.
• Follicles contain colloid, storage of thyroid hormones.
• Pyramidal lobe is sometimes present.
• Thyroid develops from endoderm of the primitive pharynx.
• The thyroglossal duct disappears, except for its remnant -- the pyramidal lobe.

Thyroid Gland II

• Thyroid follicles are lined with simple cuboidal epithelium and contain colloid.
• Follicular cells make T3 and T4. 
• Parafollicular cells make calcitonin.
• Calcitonin is a regulator of calcium levels.
• Thyroid follicle structure has prominent colloid, cytoplasm, and spherical nuclei.

Thyroid Histology

• Thyroid follicles are roughly spherical and densely packed.
• Colloid occupies the center of each follicle.
• Cytoplasm is basophilic with spherical nuclei and prominent nucleoli.
• Abundant lysosomes and endocytic vesicles are present at the apical cell membrane (resorption vesicles).

Thyroid Follicular Epithelial Cell and Function

• This describes the process of iodine uptake and incorporation into thyroid hormones.

Congenital Hypothyroidism

• Deficiency of thyroid hormone during development. 
• Identified by physical characteristics (thick tongue, pale skin, slow growth and developmental delays, prolonged jaundice etc.) and a general decreased activity.

Hypothyroidism/Hyperthyroidism

• Hypothyroidism (low thyroid hormone): Symptoms include fatigue, depression, difficulty concentrating, slow heart rate, muscle cramps, weight gain, dry skin, cold intolerance, and constipation.
• Hyperthyroidism (high thyroid hormone): Symptoms include nervousness, irritability, restlessness, increased perspiration, racing heart, palpitations, weight loss, thin skin, hair loss, intolerance to heat, irregular menstrual cycles, and bulging eyes. The types of thyroid disorders each have characteristic symptoms.

Parathyroid Glands I

• Small endocrine glands adjacent to the thyroid gland. 
• Usually located on the posterior surface of the lateral lobes of the thyroid gland.
• The number and location are variable, sometimes extending to the thymus and mediastinum.

Parathyroid Glands II

• Principal (chief) cells and oxyphil cells make up the epithelial cells of the gland.
• Principal cells are more numerous, produce PTH, and have pale-staining slightly acidophilic cytoplasm.
• Oxyphil cells are larger, have more distinct acidophilic cytoplasm, and have no secretory role.
• Adipose cells are present in limited numbers.

PTH Effects on Bone/Kidney

• PTH stimulates bone resorption, releasing calcium (Ca2+) into the extracellular fluid (ECF).
• PTH affects the kidney by reducing calcium loss in the urine while also inhibiting phosphate reabsorption. This maintains extracellular Ca2+ levels and homeostasis.

Parathyroid Glands III

• PTH regulates extracellular calcium and phosphate. 
• PTH release is regulated by calcium in a negative feedback system.
• PTH affects bone (stimulating osteoclasts, releasing Ca2+ from the matrix) and kidney (increasing Ca2+ reabsorption, reducing Ca2+ excretion; increasing phosphate excretion). 

Adrenal Glands I

• Paired glands located superior to the kidneys.
• Composed of two parts: cortex and medulla.
• Adrenal cortex produces steroid hormones. 
• Adrenal medulla produces catecholamine hormones..

Adrenal Glands II

• The adrenal cortex is divided into three zones (glomerulosa, fasciculata, reticularis, each producing different hormones.
• Zona glomerulosa cells produce mineralocorticoids (aldosterone). 
• Aldosterone regulates sodium and potassium balance in the kidneys. 
• The RAAS (Renin-Angiotension-Aldosterone) system controls aldosterone secretion. (This system includes multiple steps and enzymes). The regulation of aldosterone and minerals is important for overall homeostasis.

Adrenal Glands III

• Zona fasciculata produces glucocorticoids (cortisol). These hormones affect a variety of functions including carbohydrate, protein, and lipid metabolism and immune response.

• Zona reticularis cells produce adrenal androgens. They have important roles in sexual function and development.

Histology of the Adrenal Gland

• The cortex is highly vascular (richly supplied with sinusoidal capillaries).
• The adrenal medulla has numerous chromaffin cells closely associated with sinusoidal blood vessels.

Adrenal Gland Histology (Specific Images)

• Images show the location of the zones in the adrenal cortex and their differing structure.  For example, the zona glomerulosa, fasciculata, and reticularis may be visible.

Regulation of Aldosterone Secretion (Renin-Angiotensin-Aldosterone System: RAAS)

• This describes the complex hormonal pathway affecting blood pressure and volume that includes renin, angiotensin, and aldosterone and their interactions in the kidneys, lungs, and adrenal cortex.

Effects of Glucocorticoids

• Effects of cortisol on the body systems are extensive, affecting carbohydrates, lipids, protein and nucleic acids, the fluid and electrolytes, bones, secretory actions, connective tissues, and the immune system.

Physiological Actions of Cortisol

• Cortisol has many wide-ranging effects, including promotion of gluconeogenesis, increasing fat breakdown, reducing inflammation response, suppressing the immune system, and impacting bone metabolism.

Regulation of Cortisol Release

• ACTH from the pituitary regulates the release of cortisol, and cortisol negatively regulates ACTH and CRF secretion. Circadian rhythms also affect cortisol release.

Adrenal Medulla (Histological Detail)

• Chromaffin cells are large pale-staining epithelial cells within close proximity to sinusoidal blood vessels.
• They have numerous secretory vesicles filled with catecholamines (epinephrine and norepinephrine). Catecholamines are formed within the chromaffin cells and these cells are responsive to sympathetic nervous stimulation.

Sympatho-Adrenomedullary System (SAS)

• The SAS describes the function of sympathetic nervous stimulation leading to hormone release associated with stress and fight-or-flight response.

Endocrine Pancreas

•  Sections are provided that describe the appearance of exocrine and endocrine tissue as well as a description of hormone production (cells) and their location in the endocrine pancreas.

Test your knowledge on key concepts in endocrinology, focusing on the thyroid and parathyroid glands, hormone release patterns, and the adrenal cortex. This quiz covers the structural and functional aspects of various hormones, including their secretion and regulation.