Microanatomy of the Endocrine System II

Learning Objectives for Endocrine Microanatomy

• Primary Endocrine Organs: Identify and list the primary organs within the system, specifically the hypophysis (pituitary), pineal gland, thyroid, parathyroid, and adrenal glands.
• Hypothalamus Importance: Explain the critical role of the hypothalamus in regulating the endocrine system.
• Definitions:
  • Neurosecretory Neuron: A neuron that translates neural signals into chemical stimuli (hormones).
  • Conducting Neuron: A neuron primarily involved in transmitting electrical impulses.
• Hypophysis Origin: Describe the embryological origins of the adenohypophysis (epithelial) and neurohypophysis (neural).
• Adenohypophysis Cell Types: Name the specific cells (acidophils, basophils, chromophobes) and the hormones each produces.
• Structural Divisions: Name the main anatomical parts of the adenohypophysis and the neurohypophysis.
• Hypothalamic-Hypophyseal Portal System: Explain how this vascular system functions to transport releasing hormones from the hypothalamus to the anterior pituitary.
• Pineal Gland: Identify its location and physiological function.
• Hormone Classes and Action: List common hormone types and describe their intracellular mechanisms (e.g., cell surface receptors vs. nuclear receptors).
• Hormonal Pathways:
  • Trace releasing hormones from the hypothalamus to acidophils/basophils in the adenohypophysis, then to target organs, including the production and effects of terminal hormones.
  • Trace the production and pathway of oxytocin and antidiuretic hormone ($ADH$) from the hypothalamus to target cells in the mammary gland and kidney.
• Thyroid and Parathyroid Glands: List hormone-producing cells and their actions on systemic physiology.
• Adrenal Gland: List the distinct regions (zones), the specific hormones produced in each, their general actions, and recognize these regions in tissue sections.
• Secondary Endocrine Organs:
  • List hormones from the pancreatic islets and kidneys.
  • Identify hormones produced by enteroendocrine cells in the Gastrointestinal ($GI$) tract.
• Histological Recognition: Be able to identify the following in tissue sections:
  • Hypophysis, adenohypophysis, acidophils, basophils, chromophobes, melanotrophs, and neurohypophysis.
  • Thyroid gland, follicular cells, parafollicular cells, and parathyroid gland.
  • Adrenal gland zones and their products.
  • Endocrine pancreatic cells (Islets of Langerhans).

Comparative Anatomy of Thyroid and Parathyroid Glands

• The physical arrangement of these glands varies significantly across species:
  • Dog: Features two distinct thyroid lobes.
  • Horse: Displays the thyroid lobes clearly.
  • Cattle: Shows a more lobulated thyroid structure.
  • Pig: Displays thyroid tissue consisting of two lobes.

Thyroid Gland: Microanatomy and Hormonal Synthesis

• Regulation: Regulated primarily by Thyroid Stimulating Hormone ($TSH$) secretion from the pituitary.
• Follicles ($F$): The functional units of the thyroid.
  • Follicular Cells: Cuboidal epithelial cells that synthesize and secrete Thyroglobulin.
  • Colloid: Thyroglobulin is stored in the follicle lumen as colloid and combined with iodine.
  • Activation: Cuboidal follicular cells convert stored colloid into active Thyroxine, which increases the basal metabolic rate, thermogenesis, and gluconeogenesis.
• Parafollicular 'C' Cells ($C$):
  • Appearance: Large, pale, and round cells.
  • Function: Secrete Calcitonin in response to increased plasma calcium ($Ca^{2+}$).
  • Effect: Decreases blood calcium levels by inhibiting osteoclast activity (storing calcium in bone) and increasing renal excretion of calcium.
• Thyroid Follicle Activity States:
  • Less Active: Characterized by larger follicles filled with more colloid and flatter follicular cells.
  • More Active: Characterized by smaller follicles with scalloped colloid and taller, more cuboidal follicular cells.

Thyroid Hormone Synthesis and Function

• Synthesis Process:
  1. Thyroglobulin Production: Follicular cells produce thyroglobulin, a protein substrate for $T_3$ and $T_4$.
  2. Stimulation: When triggered by $TSH$, colloid is endocytosed from the follicular lumen back into the follicular epithelial cells.
  3. Cleavage: The colloid is cleaved by proteases to release thyroglobulin from its active components ($T_3$ and $T_4$).
  4. Release: Active thyroid hormones ($T_3$ and $T_4$) are released into circulation, where they may be unbound or attached to plasma proteins.
  5. Recycling: Thyroglobulin is recycled back into the follicular lumen.
• Functions of $T_3$ and $T_4$:
  1. Increases basal metabolic rate, oxygen consumption, and energy use.
  2. Potentiates the effects of growth hormone and somatomedins to promote bone growth, epiphyseal closure, and bone maturation.
  3. Increases the rate of protein synthesis.
  4. Increases the rate of glycogen breakdown and glucose synthesis (gluconeogenesis).
  5. Stimulates cholesterol breakdown and increases the number of $LDL$ receptors, thereby increasing the rate of lipolysis.
  6. Increases heart rate and force of contraction (cardiac output) by increasing $\beta$-adrenergic receptor levels in the myocardium.
  7. Influences lungs and postnatal Central Nervous System ($CNS$) growth; stimulates myelin production, neurotransmitter production, and axon growth. Crucial for linear bone growth.

The Hypothalamic-Pituitary-Thyroid Axis Pathway

• Step 1: Thyrotropin-releasing hormone ($TRH$) is exocytosed from hypothalamic neurons near the third ventricle.
• Step 2: $TRH$ enters the primary plexus of the portal system.
• Step 3: $TRH$ moves to the pars distalis of the pituitary.
• Step 4: $TRH$ reaches thyrotrophs, stimulating the release of Thyroid Stimulating Hormone ($TSH$).
• Step 5: $TSH$ binds to its G-protein coupled receptor ($GPCR$) on thyroid follicular cells, signaling via $cAMP$ to initiate thyroglobulin synthesis and the release of $T_3$ and $T_4$.
• Step 6: $T_3$ and $T_4$ circulate to peripheral target cells (e.g., myocytes, hepatocytes, adipocytes).
• Step 7: Hormones enter target cells by diffusion; $T_4$ is often converted to $T_3$.
• Step 8: $T_3$ enters the nucleus, binds to a nuclear receptor attached to the hormone response element, and initiates gene transcription.

Parathyroid Glands: Structure and Calcium Regulation

• Location: Adjacent to the thyroid gland.
  • Internal Parathyroid: Embedded within the thyroid parenchyma.
  • External Parathyroid: Located away from the thyroid tissue.
• Chief Cells (Principal Cells):
  • These are the primary parenchymal cells of the parathyroid.
  • They possess receptors on their surface that sense lowered levels of interstitial calcium ($Ca^{2+}$).
  • They secrete Parathormone ($PTH$) in response to decreased plasma calcium.
• Functions of $PTH$ (Increases plasma calcium):
  • Bone: Stimulates osteoclasts to increase bone resorption and release calcium.
  • Intestine: Increases intestinal uptake of calcium (often via Vitamin D regulation).
  • Kidney: Increases renal resorption of calcium.
• Regulation Contrast:
  • High Blood Calcium: Triggers Thyroid C cells to release Calcitonin $\rightarrow$ Calcium deposition in bones; reduced uptake in kidneys/intestines.
  • Low Blood Calcium: Triggers Parathyroid Chief cells to release $PTH$ $\rightarrow$ Calcium release from bones; increased uptake in kidneys/intestines.

Adrenal Gland: Gross Anatomy and Embryology

• Vascular Supply: Includes right/left superior, middle, and inferior suprarenal arteries, and left suprarenal vein, branching from the celiac trunk and abdominal aorta.
• Dual Embryonic Origin:
  • Cortex: Derived from the intermediate mesoderm.
  • Medulla: Derived from the neural crest ectoderm.

Microanatomy of the Adrenal Cortex

• Tissue Zonation (Outer to Inner):
  1. Zona Glomerulosa / Zona Arcuata:
     • Structure: Arranged as tufts of epithelial cells (Glomerulosa in ruminants and humans) or as arches of columnar cells (Arcuata in horses, pigs, and carnivores).
     • Product: Mineralocorticoids, primarily Aldosterone and corticosterone.
     • Action: Directs sodium ($Na^+$) conservation in the kidney, salivary glands, sweat glands, and colon. Regulates blood pressure, plasma $Na^+$, and potassium ($K^+$) levels via the renin-angiotensin-aldosterone system ($RAAS$).
  2. Zona Fasciculata:
     • Structure: Composed of radiating columns or cords of spherical cells, separated by sinusoids and bundled as fascicles.
     • Product: Glucocorticoids, primarily Cortisol and cortisone.
     • Histology Note: Often clear or "foamy" due to high cholesterol ester droplet content.
  3. Zona Reticularis:
     • Structure: Formed by polyhedral cells arranged in a network of anastomosing cords and plates with large sinusoids.
     • Product: Small amounts of Androgens (sex steroids).
  4. Zona Intermedia:
     • A narrow band located at the interface of the outer and inner cortices.
     • Contains small, undifferentiated cells acting as blastemic stem cells, which generate replacement cells for the other zones.

The Hypothalamic-Pituitary-Adrenal Axis and Stress Response

• Pathway:
  1. Stress signals perceived by the hypothalamus trigger the exocytosis of Corticotropin-releasing hormone ($CRH$).
  2. $CRH$ enters the primary plexus of the portal system to the pars distalis.
  3. $CRH$ delivered via the secondary plexus binds to $GPCR$ on corticotrophs.
  4. This signals the exocytosis of Adrenocorticotrophic hormone ($ACTH$).
  5. $ACTH$ binds to its $GPCR$ in the adrenal cortex, signaling via $cAMP$ the synthesis of glucocorticoids.
  6. Glucocorticoids diffuse into peripheral target cells, bind to cytoplasmic receptors, enter the nucleus, and affect transcription via hormone response elements.
• Functions of Glucocorticoids (Cortisol):
  • Liver: Gluconeogenesis (increasing blood sugar via glycogen production).
  • Muscle/Fat: Decreases glucose use; causes skeletal muscle breakdown for amino acid supply.
  • Immune System: Immunosuppression.
  • Other Effects: Influence on memory/attention, decreased pain sensitivity, heart disease risk, high blood pressure, and digestive issues.

Adrenal Medulla: The Sympathetic Ganglion

• Nature: Adrenal medullary parenchymal cells are modified sympathetic neurons, essentially a large sympathetic ganglion.
• Chromaffin Cells (Phaeochromocytes):
  • Product: Synthesize and store catecholamines (Epinephrine and Norepinephrine).
  • Staining: Stain boldly brown when exposed to chromium salts.
• Cell Types:
  • Columnar Cells: In horses, ruminants, and pigs; located at the periphery or near sinusoids; produce Epinephrine (formed from norepinephrine via methyltransferase).
  • Rounded Epithelioid Cells: Produce Norepinephrine.
• Physiology: Molecular mediators of the "fight or flight" mechanism. Immediate response to fear/stress results in increased heart rate and glycogen breakdown in skeletal muscle and liver.

Pancreas as an Endocrine Organ

• Composition: The pancreas is a "secondary" endocrine organ. It is $98\%$ exocrine (digestive enzymes) and only $2\%$ endocrine.
• Islets of Langerhans:
  • Alpha Cells ($\alpha$): Produce Glucagon. Stimulated by hypoglycemia. Glucagon signals the release of glucose from hepatocytes, skeletal myocytes, and adipocytes to elevate blood sugar.
  • Beta Cells ($\beta$): Produce Insulin. Sense high plasma glucose concentration. Insulin signals glucose uptake by hepatocytes, myocytes, and adipocytes, lowering blood sugar.
  • Delta Cells ($\delta$): Produce Somatostatin (prevents production of other hormones and certain exocrine secretions).
  • F Cells: Specific functions yet fully detailed in this context.
• Hormone Associations:
  • Glucagon Stimuli: Hypoglycemia, Gastrointestinal Peptide ($GIP$), amino acids.
  • Glucagon Effects: Decreases food intake/appetite; increases satiety, insulin secretion, glucose production, lipid breakdown, and heart rate/contractility.
  • Insulin Effects: Stimulates glycogen synthesis and lipogenesis; inhibits lipolysis and hepatic glucose production.

Other Secondary Endocrine Systems

• Enteroendocrine Cells:
  • Located in the digestive system; also known as Diffuse Neuroendocrine cells, Enterochromaffin cells, or $APUD$ cells (Amine precursor uptake and decarboxylation).
  • Gastrin: Stimulates gastric juice secretion.
  • Cholecystokinin ($CCK$): Mediates digestion/absorption.
  • Ghrelin: Known as the "hunger hormone."
• Adipose Tissue:
  • Leptin: Secreted by unilocular (white) adipocytes. Signals the hypothalamic "lipostat" to regulate energy homeostasis. Binds cytokine receptors to inhibit food intake (inhibits Neuropeptide Y and Agouti-related peptide ($AgRP$)).
• Heart:
  • Atrial Natriuretic Peptide ($ANP$): Secreted by atrial myocardial cells.
  • Target: Distal convoluted tubules of kidneys.
  • Action: Promotes $Na^+$ and water loss to decrease blood pressure.
• Kidney:
  • Renin: Produced by Juxtaglomerular cells; part of the $RAAS$ to control blood pressure.
  • Erythropoietin: Produced by interstitial cells near the proximal tubules during hypoxia; cytokine that controls red blood cell production.

Overview of Endocrine Pathology

• Hyperadrenocorticism / Cushing’s Disease: Excessive adrenal cortex activity.
• Hypoadrenocorticism / Addison’s Disease: Insufficient adrenal cortex activity.
• Hyperthyroidism / Hypothyroidism: Imbalance in thyroid hormone production.
• Hypoinsulinaemia / Type 1 Diabetes Mellitus: Pancreatic insulin deficiency.
• Diabetes Insipidus: Pituitary-related disorder of fluid balance.
• Neoplasia: Endocrine gland tumors resulting in hormone dysregulation.