Endocrine System Quiz

Endocrine Glands

• Endocrine glands are ductless glands that release hormones into the extracellular space through diffusion.
• Hormones have different pathways to exert their action: endocrine (enter blood stream), paracrine (affect nearby cells), and autocrine (act on the same cell that produced it).
• Target cells must have specific membrane or intracellular receptors for hormones to bind.
• Hormones are chemical signals secreted by endocrine cells to regulate metabolic functions of target cells.

Types of Hormones

• Amino acid-based hormones: example Thyroid & adrenomedullary hormones
• Protein & peptide hormones: example all other hormones
• Steroid hormones: derived from cholesterol; example sex hormones (testosterone, progesterone, estrogen) & adrenocortical hormones
• Endocrine glands derived from mesoderm produce steroid hormones; those derived from ectoderm or endoderm produce amines, peptides, or protein hormones.

Mechanism of Hormone Action

• Water-soluble hormones (all amino acid based hormones except thyroid hormone) exert effects by activating plasma membrane receptors and releasing intracellular second messengers.
• Lipid-soluble hormones (steroids and thyroid hormone) diffuse into the cell, bind to intracellular receptors, migrate to the nucleus, and alter gene expression.

Control of Hormone Release

• Most hormone synthesis/release is regulated through negative feedback mechanisms.
• Endocrine gland stimuli may be humoral, neural, or hormonal.

Humoral Stimulus

• Hormone release caused by altered levels of certain critical ions or nutrients.

Neural Stimulus

• Hormone release caused by neural input.

Hormonal Stimulus

• Hormone release caused by another hormone (a tropic hormone).

Endocrine System

• The major endocrine glands include pituitary, pineal, thyroid, parathyroid, adrenal, and endocrine pancreas.
• Other endocrine components are found in the ovaries & testes, pancreas & kidneys, and diffuse neuroendocrine cells in the GIT & kidneys.
• The histological organization of endocrine glands includes a capsule, stroma, septa, reticular fibers, and cells of the gland within the parenchyma.
• Endocrine cells are specialized for secretion, with appearance dependent on secretory product (peptide or steroid).
• Endocrine cells are commonly arranged into cords or groups and are closely associated with fenestrated capillaries.

Hypothalamus

• The hypothalamus is the major integrating link between the nervous and endocrine systems.
• It oversees many internal body conditions.
• It receives nervous stimuli from receptors throughout the body.
• The hypothalamus stimulates cellular activity in various parts of the body by directing and regulating the release of hormones from the pituitary gland.

Pituitary Gland

• The pituitary gland (hypophysis) is located in the sella turcica of the sphenoid bone in the base of the skull.
• It has two subdivisions that arise from different embryonic sources: adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary).
• The adenohypophysis develops from an evagination of the oral ectoderm (Rathke's pouch) and the neurohypophysis develops from neural ectoderm as a down growth of the floor of the diencephalon.
• Blood supply to the pituitary gland involves superior and inferior hypophyseal arteries and the hypothalamo-hypophyseal portal system.

Subdivisions of Hypophysis

• Anterior Pituitary (adenohypophysis): Pars distalis, Pars tuberalis, and Pars intermedia.
• Posterior Pituitary (neurohypophysis): Pars nervosa and infundibulum.

Control of Secretion in the Adenohypophysis

• Primarily controlled by hypothalamic hormones via the hypophyseal portal system (releasing and inhibiting hormones).

Negative Feedback

• A stimulus (e.g., low body temperature) causes the hypothalamus to secrete thyrotropin-releasing hormone (TRH), which acts on the anterior pituitary.
• Thyrotropic cells in the anterior pituitary release thyroid-stimulating hormone (TSH).
• TSH stimulates follicular cells of the thyroid gland to release thyroid hormone (TH).
• TH stimulates target cells to increase metabolic activities resulting in an increase in basal body temperature.
• Increased body temperature inhibits TRH secretion, inhibiting TSH synthesis and release, thereby dampening TH production.

Pars Tuberalis

• A funnel-shaped region surrounding the infundibulum.
• Most cells are basophilic gonadotropic cells secreting follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Highly vascularized by arteries and the hypophyseal portal system.

Pars Intermedia

• Lies between the pars distalis and the pars nervosa.
• Contains colloid-filled cysts (remnants of Rathke's pouch).
• In the fetus, basophils produce melanocyte-stimulating hormones (MSH) important for melanocyte activity.

Neurohypophysis (Posterior Pituitary)

• Consists of pars nervosa and infundibular stalk.
• Unlike the pars distalis, it does not contain secretory cells.
• Composed of unmyelinated axons, Herring bodies, pituicytes, and fenestrated capillaries.
• Contains neurosecretory bodies (Herring bodies) that store and release oxytocin and ADH.

Hormones of Pars Nervosa

• Vasopressin (ADH) and oxytocin.
• Released in response to respective stimuli (plasma osmolarity or uterine/mammary gland stimuli).
• ADH increases water reabsorption in the kidney and oxytocin stimulates contraction of uterine muscle and myoepithelial cells around mammary glands during childbirth or nursing.

Pineal Gland

• Location: posterior of the third ventricle, attached to the brain by a short stalk
• Origin: neuroectoderm in the roof of the diencephalon
• Shape: cone or pine-shaped and covered by pia mater with septa of connective tissue
• Pinealocytes: secretory cells, having slightly basophilic cytoplasm, secretory vesicles, mitochondria & long cytoplasmic processes
• Produce melatonin (derived from serotonin), a low molecular-weight tryptophan derivative.

Unmyelinated Sympathetic Nerve Fibers

• Enter the pineal gland along its septa; associated indirectly with photoreceptive neurons in the retinas and running to the pinealocytes to stimulate melatonin release in darkness.
• The pineal is involved in circadian rhythms.

Interstitial Cells

• Most closely resemble astrocytes.
• Have elongated nuclei and cytoplasmic processes.
• Found in perivascular areas and between groups of pinealocytes.
• About 5% of cells in the gland.

Corpora Arenacea

• Characteristic feature of the pineal gland.
• Variously sized concretions of calcium and magnesium salts.
• Form by precipitation around extracellular protein deposits.
• Increase in number and size with age and have no known function.

Adrenal Gland

• Paired organ near the superior poles of the kidneys; embedded in adipose tissue.
• Shape: almond/triangular/flattened structures.
• The right is pyramid-shaped and sits directly on top of the right kidney.
• The left is more crescent-shaped and lies along the medial border of the left kidney.
• Histological structure: covered by a dense capsule that sends thinner septa into the interior.
• Two functionally different regions: cortex (yellowish peripheral layer) and medulla (reddish-brown central layer).

Origin of Adrenal Gland

• Adrenal cortex from mesoderm and adrenal medulla from neural crest, with sympathetic ganglion cells having the same origin.
• The origin of the layers is important because they have different functions.

Suprarenal Cortex

• Accounts for 80-90% of the gland.
• Contains cells that synthesize and secrete several steroid hormones, but do not store them.
• Cells have characteristic features of steroid-secreting cells: central nuclei, acidophilic cytoplasm, and rich in lipid droplets, smooth ER with enzymes for cholesterol synthesis, mitochondria with tubular cristae .

Adrenocortical Hormones

• Mineralocorticoids, glucocorticoids, and androgens are synthesized from cholesterol and a component of low-density lipoproteins. -Hormone synthesis and release are by enzymes of SER & mitochondria.

Organization of Adrenal Cortex

• The cords of cortical cells are arranged differently and are specialized to produce different classes of steroid hormones.
• Three concentric zones (glomerulosa, fasciculata, and reticularis) from the capsule inward

Zona Glomerulosa

• The outer small zone immediately inside the connective tissue capsule.
• Forms 15% of the cortex
• Cells are columnar, arranged in spherical or arched cords, and surrounded by fenestrated capillaries ( important for fluid and electrolyte homeostasis).

Function of Zona Glomerulosa

• Secrete mineralocorticoids, primarily aldosterone, to regulate salt and water balance.
• Aldosterone stimulates Na+ reabsorption and water, in the distal convoluted tubules;
• It also increases Na+ absorption by the intestine.
• Decrease Na+ & increase K+ content of sweat & saliva.
• Aldosterone secretion is stimulated by angiotensin II and also by an increase in plasma K+ and decrease Na+ concentrations; weakly by ACTH

Renin-Angiotensin Mechanism

• A mechanism related to blood pressure regulation.
• Decreased blood pressure triggers renin release from the kidney, which initiates a cascade that results in aldosterone secretion.

Zona Fasciculata

• The intermediate layer of cells in the suprarenal cortex
• The largest layer, accounting for up to 80% of the cortex.

Zona Fasciculata Consists of:

• Polyhedral cells larger than zona glomerulosa cells.
• Arranged in radial columns (one to two layers thick).
• Stain lightly acidophilic.
• Sinusoidal capillaries between the columns of cells.
• Many lipid droplets (appear vacuolated called spongiocytes)

Function of Zona Fasciculata

• Secrete glucocorticoids, especially cortisol and a small amount of androgens, regulating carbohydrate, fat, and protein metabolism.
• Cortisol has an anti-inflammatory effect and suppresses the immune response.
• Secretion is controlled by ACTH from the anterior pituitary and negative feedback.
• Stress and catecholamines stimulate secretion.

Zona Reticularis

• The innermost layer of the suprarenal cortex, constituting about 7% of the cortex
• Darkly staining acidophilic cells.
• Arranged in anastomosing cords.
• Similar to spongiocytes but smaller
• Fewer lipid droplets and more lipofuscin pigment.
• Several cells near the suprarenal medulla are dark with electron-dense cytoplasm and pyknotic nuclei, which suggests that this zone contains degenerating parenchymal cells.

Function of Zona Reticularis

• Produces glucocorticoids (mainly cortisol) but primarily secretes the weak androgen dehydroepiandrosterone (DHEA).
• DHEA gets converted to testosterone in other tissues.
• ACTH stimulates secretion and negative feedback system regulated with the pituitary and hypothalamus.

Fetal Adrenal Cortex

• A layer comprising 80% of the total gland, thick, containing cells secreting sulfated DHEA, controlled by fetal pituitary
• DHEA gets converted to active estrogens and androgens, primarily entering maternal circulation..
• After birth, the provisional cortex involutes and the permanent cortex develops in three layers (zones).

Clinical Applications of Adrenal Gland

• Hyperfunction: Cushing's syndrome (excess glucocorticoids), Conn's syndrome (excess aldosterone), and excessive adrenal androgens which may lead to precocious puberty (boys) and hirsutism/virilization in girls.
• Hypofunction: Addison's disease (destruction of the adrenal cortex).

Blood Supply of Adrenal Gland

• Capsular arteries and cortical arterioles (supplying the cortex).
• Medullary arterioles (branching into medullary capillary network).
• Adrenal medulla has a more vascular supply with dual blood supply, arterial blood from medullary capillaries, and blood of cortical capillaries, which carries cortical hormones.

Adrenal Medulla

• Composed of chromaffin cells (pheochromes) – large, pale-staining polyhedral cells arranged in cords or clumps.
• Few parasympathetic ganglion cells, alongside a profuse supply of sinusoidal capillaries.
• Secretion through chromaffin reaction.

Origin of Medullary Cells

• Chromaffin cells arise from neural crest cells, the same origin as postganglionic neurons in sympathetic and parasympathetic ganglia.
• They are modified sympathetic postganglionic neurons without axons/dendrites, specialized as secretory cells.

Medullary Hormones

• Secrete catecholamines like epinephrine and norepinephrine, derived from tyrosine.
• Epinephrine, the major product (80%).
• Granules in chromaffin cells (epinephrine granules, less electron dense & smaller), contain catecholamines, Ca2+, ATP & proteins called chromogranins.

Medullary Hormones - Function

• Medullary hormones deal with physical/emotional stress (fight-or-flight).
• Increase blood pressure, blood glucose, vasoconstriction, and heart rate, as well as metabolic effects
• Disorders: Pheochromocytoma is tumor of chromaffin cells, leading to hyperglycemia and transient elevation of blood pressure.

Endocrine Pancreas

• Composed of spherical aggregates, called islets of Langerhans, scattered among the exocrine acini.
• More than a million islets in the human pancreas, concentrated at the tail.
• Very thin capsule of reticular fibers surrounds each islet.
• Origin: Endodermal (epithelial tissues).

Islets of Langerhans Cells

• Each islet consists of polygonal/rounded cells that are smaller and lightly stained than the surrounding acinar cells.
• Arranged in cords that are separated by capillaries
• Autonomic nerve fibers contact some of the endocrine cells and capillaries.

Pancreatic Islet Cells

• Four cell types:
  • Alpha cells (20%): Secrete glucagon; near periphery, stained pink with Gomori; granules with homogenous electron-dense core and lower density peripheral zone
  • Beta cells (70%): Secrete insulin; centrally located; smaller; numerous; stains blue with Gomori; granules with one or more dense crystals in a low density matrix.
  • Delta cells (5–10%): Secrete somatostatin; scattered and less abundant; larger granules, less electron dense
  • F/PP cells (<5%): Secrete pancreatic polypeptide; more common in islets of the head of the pancreas; granules with smaller sizes and variable density.

Pancreatic Hormones

• Insulin: anabolic hormone, increases the storage of glucose, fatty acids, and amino acids in cells and tissues, lowering blood sugar.
• Glucagon: catabolic hormone that mobilizes glucose, fatty acids, and amino acids from stores into blood, raising blood sugar

Neuronal Control of Glucagon & Insulin

• Sympathetic fibers during stressful situations, emergencies, and exercise increase glucagon release and inhibit insulin release.
• Parasympathetic fibers during restful times and digestion stimulate insulin release..

Cell Types & Function

• Alpha cells (glucagon): Increase blood glucose.
• Beta cells (insulin): Decrease blood glucose.
• Delta cells (somatostatin): Paracrine; inhibits hormone release; inhibits GH & TSH.
• F cells (pancreatic polypeptide): Stimulates gastric chief cells; inhibits bile/pancreatic enzyme/bicarbonate secretion & intestinal motility

Medical Applications

• Insulin-dependent (type 1) diabetes: autoimmune destruction of beta cells, resulting in insulin insufficiency.
• Insulin-independent (type 2) diabetes: failure of cells to respond to insulin (insulin resistance), often associated with obesity.

Thyroid Gland

• Location: cervical region, anterior to the larynx, two lobes united by an isthmus.
• Surrounded by dense, irregular collagenous CT capsule; lobules.
• Embedded in capsule: parathyroid glands.

Thyroid Gland Origin

• Main bulk from the primitive pharynx (endodermal lining).
• Parafollicular cells (C-cells) from neural crest.
• Develops between the first and second pharyngeal pouches.

Cellular Organization

• Thyroid follicle is the structural/functional unit (20-30 million).
• Follicular cells (98%): cuboidal basophilic; central nucleus. Parafollicular cells (2%): larger, less intensely stained.
• Central lumen is colloid (acidophilic, gelatinous substance).

Follicular Cells

• Single layer of cuboidal basophilic cells.
• Protein secreting cells.
• Contain apical secretory granules and abundant lysosomes & surface microvilli.
• Shape (squamous to low columnar) varies with functional activity; active glands have low columnar epithelium & glands with squamous follicles are considered hypoactive.
• Produce thyroid hormones (T3 & T4).

Parafollicular (C) Cells

• Found inside the basal lamina or clusters between follicles.
• Derived from neural crest cells.
• Larger and less intensely stained than follicular cells.
• Contain fewer rough ER, larger Golgi complexes, numerous small granules with polypeptide hormone (calcitonin)

Thyroid Hormone (T3 & T4) Synthesis & Secretion

• Regulated by: iodide levels in follicular cells; binding of TSH to TSH receptors.
• Steps:
• Thyroglobulin synthesized/released into lumen.
• Iodide uptake/oxidation.
• Iodinated tyrosines (DIT, MIT) form T3 & T4 in colloid.
• Endocytosis & hydrolysis of thyroglobulin by lysosomal enzymes.
• T3 & T4 released into blood.

Thyroid Hormone (T3 & T4)

• T4 is more abundant (90%) and a prohormone/reservoir for active T3.
• T3 is more active form (2-10x stronger), with a shorter half-life (1.5 days vs. week).
• Most circulating thyroid hormone is bound to transport proteins.

Thyroid Hormone Function

• Important for growth, cell differentiation, basal metabolic rate, and oxygen consumption in cells.
• Affects protein, lipid, and carbohydrate metabolism.

Thyroid Hormone Secretion Control

• TSH from anterior pituitary stimulates synthesis/release.
• TSH increases epithelium activity/height.
• TSH receptors at follicular cell bases.
• Negative feedback.
• Iodine levels.

Medical Applications

• Iodine deficiency goiter: thyroid gland enlargement due to iodine deficiency, resulting in accumulation of non-iodinated thyroglobulin Hypothyroidism: leads to cretinism in infants; autoimmune disorders; fatigue, weight gain, pale or puffy face; cold intolerance, joint/muscle pain
• Hyperthyroidism: Grave's disease; autoimmune; TSH receptors stimulated; continuous thyroid hormone release; bulging eyes (exophthalmos)

Parathyroid Gland

• Usually 4 in number.
• Located on posterior surface of the thyroid gland.
• Superior & inferior glands..
• Usually embedded in thyroid capsule
• Derived from pharyngeal pouches.

Parathyroid Gland Structure

• Capsule, septa, and clusters of secretory cells.
• As age increases, Chief Cells are replaced by adipocytes..
• Two cell types: Chief cells and oxyphil cells

Chief Cells

• Polygonal cells with round nuclei and pale-staining, slightly acidophilic cytoplasm.
• Cytoplasm contains secretory granules with parathyroid hormone (PTH).
• Main target cell of PTH are osteoblasts which release osteoclast-stimulating factor, causing increase in the number & activity of osteoclasts.

PTH Action

• Increase blood calcium by increasing the number and activity of osteoclasts; increases Ca2+ & Mg2+ reabsorption from the distal tubules;
• Inhibits HPO4⁻² reabsorption so more is in Urine;
• Promotes activation of vitamin D, increasing Ca2+ and Mg2+ absorption from the gastrointestinal tract

Oxyphil Cells

• Less numerous, larger than chief cells.
• Stain deeply with eosin.
• Appear in groups/isolated cells, have more mitochondria, little RER & glycogen in cytoplasm.
• Possibly inactive phase of chief cells.
• Appear around puberty and increase in number; unknown function.

Medical Applications

• Hyperparathyroidism: decreased blood phosphate, increased blood calcium.
• Osteitis fibrosa cystica: bones are less stress-resistant; prone to fractures
• Hypoparathyroidism: increased blood phosphate, decreased blood calcium.
• Generalized convulsions (spasam/tetany)
• Treated with Calcium salts & Vit D to promote Ca2+ uptake in gut

Test your knowledge on the endocrine system, its functions, and the various hormones involved. This quiz will cover hormone pathways, gland classifications, and the integration between the nervous and endocrine systems. Challenge yourself and see how much you really know about this essential biological system!