Endocrine System

Question 1

Endocrine glands

Answer 1

Endocrine glands

• Hormones
  
  • signaling molecules released by secretory cells of endocrine glands into the neighboring vascularized compartment for uptake by capillaries & distribution throughout the body
  • frequently hydrophilic molecules such as proteins, glycoproteins, peptides or modified amino acids with receptors on surface of target cells (just like neurotransmitters)
  • hydrophobic streroid & thyroid hormone circulate on transport proteins but diffuse through the cell membranes & activate cytoplasmic receptoro in target cells
• NO secretory duct
• Endocrine cells
  
  • typically epithelial in origin
  • aggregated as cords or clusters
  • also found in heart, thymus, gut, kidneys, testes, ovaries
• Circulation- allows hormone to act on target cells with receptors for those hormones at a distance from the site of their seretion:
  
  • Paracrine secretion (neighbors :)
    
    • secretion on hormones that act on target cells only a short distance away
    • with localized dispersal in interstitial fluid or throughout loops of blood vessels
    • example: gastrin made by pyloric G cells reaches target cells in fundic glands
  • Juxtacrine secretion (on standby :)
    
    • signaling molecule remains in the secreting cell’s surface on adjacent ECM
    • affect target cells when cells make contact
    • important in embryonic & regenerative tissue interactions
  • Autocrine secretion
    
    • produce molecules that act on themselves or on cells of the same type
    • example: insulin-like growth factor (IGF) produced by several cell types may act on the same cells that produce it
• Target organs for other hormones that can establish a feedback mechanism to control hormone secretion and keep blood hormone levels within strict limits

Question 2

Pituitary gland (Hypophysis)

Answer 2

Pituitary gland (Hypophysis)

• lies below the brain in a small cavity on sphenoid bones (sella turcica)
• Hypophysis-> hypo means under and physis means growth
• formed in embryo partly from the developing brain & partly from the developing oral cavity
  
  • developing brain:
    
    • neural component: neurohypophyseal bud growing down from the floor of the future diencephalon as a stalk or infundibulum that remains attached to the brain
  • developing oral cavity:
    
    • hypophyseal (Rathke) pouch
      
      • outpocketing of ectoderm from the root of the primitive mouth & grows cranially
      • base eventually constricts & separates from the pharynx
      • anterior wall then thickens greatly reducing the pouch’s lumen to a small fissure
• 2 glands united anatomically but with different functions:
  
  1. Posterior neurohypophysis
     
     • retains many histologic features of brain tissue
     • constists of large part (pars nervosa) & the smaller infundibulum stalk attached to the hypothalamus
  2. Anterior adenohypophysis
     
     • derived from the oral ectoderm
     • 3 parts:
       
       • pars distalis (anterior lobe)
       • pars tuberalis (wraps around the infundibulum)
       • pars intermedia (adjacent to the posterior pars nervosa)

Question 3

Hypothalamic-Hypophyseal tract

Answer 3

Hypothalamic-Hypophyseal tract

• Pituitary gland-> connected to the hypothalamus of the base of the brain
• Hypothalamic-Hypophyseal tract-> a bundle of axons that courses into the neurohypophysis from 2 important hypothalamic nuclei
• Hormones undergo axonal transport and accumulate temporarily in the axons of hypothalamic-hypophyseal tract before their release and uptake by capillaries branching from the inferior arteries:
  
  • ADH (Antidiuretic hormone)- synthesized by large neurons in the supraoptic nuclei
  • Oxytocin- synthesized by large neurons in the paraventricular nuclei
• Blood supply derives from 2 groups of vessels coming off the internal carotid artery and drained by the hypophyseal vein:
  
  • Superior hypophyseal arteries- supply the median eminence and the infundibular stalk
    
    • divide into a primary plexus of fenestrated capillaries that irrigate the stalk and median eminence
    • Hypophyseal Portal veins connects primary and secondary plexus
    • Capillaries rejoin to form venules that branch again as larger secondary capillary plexus in the adenohypophysis
  • Inferior hypophyseal arteries
    
    • provide blood mainly for the neurohypophysis

HYPOPHYSEAL VEINS FROM BOTH ADENOHYPOPHYSIS AND NEUROHYPOPHYSIS JOINS TOGETHER TO DISTRIBUTE THE HORMONES TO THE BODY

Question 4

Adenohypophysis (Anterior Pituitary)

Answer 4

Adenohypophysis (Anterior Pituitary)

• 3 parts derived embryonically from the hypophyseal pouch:
  
  • Pars distalis
    
    • accounts for 75% of adenohypophysis
    • has thin fibrous capsule
    • main components: cords of well-stained endocrine cells interspersed with fenestrated capillaries & supporting reticular CT
    • Chromophils
      
      • secretory cells in which hormone is stored in cytoplasmic granules
      • Cytoplasm: well-developed golgi, euchromatic nuclei filled with secretory granules
      • also called basophils and acidophils (based on affinities to stain)
    • Chromophobes
      
      • stain weakly with few or no secretory granules
      • represent a heterogeneous group (stem, undifferentiated progenitor cells, & degranulated cells)
  • Pars tuberalis
    
    • smaller funnel-shaped region surrounding the infundibulum of the neurohypophysis
    • most cells are gonadotrophs
  • Pars intermedia
    
    • narrow zone between Pars distalis & Pars nervosa
    • contains corticotrophs, chromophobes, & small colloid-filled cysts dervie from the lumen of the embryonic hypophyseal pouch
      
      • Corticotrophs are developed and active during fetal life
      • Corticotrophs of Pars intermedia express POMC-> cleave differently from cells in Pars distalis-> produce 2 forms of Melanocyte-stimulating hormone (MSH), y-LPH, & B-endorphin

Question 5

Cells types in Pars distalis of anterior pituitary & their major functions

Answer 5

Cells types in Pars distalis of anterior pituitary & their major functions:

Remember two types of chromophils and their hormone products :
GPA and My FLAT Butt->

Growth hormone, Prolactin are Acidophils

MSH, FSH, LH, ACTH are Basophils

Acidophils:

1. Somatotrophs (somatotropic cells)

• produce Somatotropin (_G_rowth hormone)
• constitute half the cells of Pars distalis
• Acidophils
• stimulate growth in epiphyseal plates of long bones via insulin-like growth factor (IGF) produced in liver

2. Lactotrophs or mammotrophs (lactotropic cells)

• produce _P_rolactin
• Acidophils
• promote milk secretion

Basophils:

3. Gonadotrophs

• target cell: Gonads
• produce _F_SH (Follicle stimulating hormone) & _L_H (Luteinizing hormone)- Pars tuberalis in Pars distalis
• Basophils
• FSH promotes ovarian follicle development & estrogen secretion n women & spermatogenesis in men
• LH promotes ovarian follicle maturation & progesterone secretion in women & interstitial cell androgen secretion (interstitial cell stimulating hormone ICSH) in men

4. Thyrotrophs

• target cell: Thyroid gland
• produce _T_hyrotropin- Pars distalis
• least abundant
• Basophils
• stimulated thyroid hormone synthesis, storage, & liberation

5. Corticotrophs

• target cell: Adrenal cortex
• best developed and active during fetal life
• main protein synthesized is pro-opiomelanocortin (POMC)-> cleaved posttranslationally into the polypeptide hormones:
  
  • _A_denocotricotropic hormones (ACTH)- Pars distalis
  • B-Lipoprotin (B-LPH)
• ACTH stimulates secretion of adrenal cortex hormones
• LPH helps regulate lipid metabolism
• _M_elanocyte-stimulating hormones (MSH)- prominently in Pars intermedia

Question 6

Control of hormone secretion in the Anterior Pituitary

Answer 6

Control of hormone secretion in the Anterior Pituitary:

1. Small neurons near the 3rd ventricle produce peptide-related hypothalamic hormones->

Discharged from axons in the median eminence->

Transported by capillaries of the portal system throughout the Anterior Pituitary->

Control of activities of cells of Anterior Pituitary

• most hormones are releasing hormones that stimulate secretion by specific anterior pituitary cells
• 2 hypothalamic factors are inhibiting hormones that block hormone secretion in specific cells of the adenohypophysis

2. Negative feedbakc by hormones from target organs on secretion of relevant hypothalamic factors & on hormone secretion by the relevant pituitary cells

• Example:
  
  • Stimulus: low body temperature->
  • Hypothalamus secrete TRH (Thyrotropin releasing hormone) to act on Anterior Pituitary->
  • Thyrotropic cells in anterior pituitary release TSH (Thyroid stimulating hormone)->
  • TSH stimulates follicular cells of thyroid glandsto release thyroid hormone->
  • Thyroid hormone stimulates target cells to increase metabolic activities resulting in an increase in basal body temperature->
    
    • Basal body temperature is detected by the hypothalamus->
    • Secretion of TRH by the hypothalamus is inhibited
    • Thyroid hormone also blocks TRH receptors on the thyrotropic cells inhibiting release of TSH

3. Hormone secretion in Anterior Pituitary is affected by other hormones from outside the feedbakc loop

• Example 1:
  
  • Polypeptide Ghrelin (produced in stomach mucosa)->
  • acts as releasing hormone for somatotropin secretion
• Example 2:
  
  • Oxytocin (released in Posterior Pituitary)->
  • increases secretion of Prolactn

Releasing hormone- stimulates activities-> has RH (releasing hormone) like gonadotropin releasing hormone that stimulate gonadotrophs to release hormone

TRH (thryrotropin releasing hormone) stimulates Thyrotrophs

GHRH (growth hormone releasing hormone)

CRH (cortisol releasing hormone

Regulatory hormones’ cell bodies are in the hypothalamus that travels to the median eminence

Inhibiting hormone: Somatostatin (inhibits activity of cells to produce growth hormone and thryotrophs) and Dopamine

Question 7

Neurohypophysis (Posterior Pituitary)

Answer 7

Neurohypophysis (Posterior Pituitary)

• consists of Pars nervosa & infundibular stalk
• composed of neural tissues comtaining unmyelinated axons of large secretory neurons with cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus
  
  • secretory neurons have large diameter axons than typical neurons
  • they have well developed synthetic components related to the production of ADH (or Vasopressin)* & *Oxytocin
• has highly branched glial cells (pituicytes) that resembles astrocytes & are most abundat cell types in Posterior Pituitary

Question 8

ADH (Vasopressin) & Oxytocin

Answer 8

ADH (Vasopressin) & Oxytocin

• transported axonally into the Pars nervosa
• accumulate in axonal dilations (neurosecretory bodies or Herring bodies)
  
  • neurosecretory bodies are faintly eosinophilic structures
  • conta membrane-enclosed granules with oxytocin bound neurophysin I* & *ADH bound neurophysin II

ADH:

• synthesized by large neurons in the supraoptic nuclei
• released in response to increased blood tonicity
• increases permeability of renal collecting ducts to H2O

Oxytocin:

• synthesized by large neurons in the paraventricular nuclei
• stimulates contraction of uterine smooth muscle during childbirth & myoepithelial cells in mammary gland alveoli
• nursing infant induces oxytocin secretion by stimulating sensory tracts that act on the hypothalamus in neurohormonal reflex
• promotes pair-bonding behavior
• together with domapine is the recipe for love :):)

Question 9

Benign Pituitary Adenoma

Answer 9

Benign Pituitary Adenoma

• produce excessive number of functional acidophils and basophils
• Gigantism
  
  • adenomas involving somatotropic cells
  • occurs in children before closure of long bones’ epiphyseal plate
• Acromegaly
  
  • adenoma involving somatotropic cells in adults with musculoskeletal, neurologic, & medical consequences

Question 10

Several conditions that affect Posterior Pituitary function:

Answer 10

Several conditions that affect Posterior Pituitary function:

1. Heritable mutations in the gene for Vasopressin (ADH)-neurophysin
2. Compression from a tumor in adjacent tissues
3. Head trauma

Question 11

Effects of low Vasopressin

Answer 11

Effect of low Vasopressin-

• Diabetes Insipidus- inability to concentrate urine leading to frequent urination (polyuria) & increased thirst (polydipsia)

Question 12

Adrenal glands

Answer 12

Adrenal glands

• also know as Suprarenal glands
• paired organs lying near the superior poles of the kidneys
• embedded in pararenal tissues & fascia
• flattened structures with half-moon shape
• covered by a dense CT capsule that sends thin trabeculae into the glands’ parenchyma
• stroma consists of reticular fibers supporting the secretory cells & microvasculature
• 2 concentric regions:
  
  • Adrenal cortex
    
    • arise from mesoderm
    • yellowish
  • Adrenal medulla
    
    • arise from neural crest
    • reddish brown
    • has dual blood supply: arterial blood form medullary capillaries & venous blood from capillaries of cortex
• lacks a hilum-> superior, middle, & inferior suprarenal arteries arising from large abdominal arteries penetrate the capsule independently & branch immediately to form subcapsular arterial plexus-> arterial plexus form rich network of fenestrated capillaries and sinusoids
• Venous drainage from the glands occur via the Suprarenal veins

Question 13

Adrenal Cortex

Answer 13

Adrenal Cortex

• cells have characteristic features of steroid-secreting cells (acidophilic cytoplasm rich in lipid droplets with central nuclei)
• Cytoplasm: produce smooth ER of interconnected tubules-> contains enzyymes for cholesterol synthesis & conversion of steroid prohormone pregnenolone into specific active steroid hormones
• Mitochondria are spherical & tubular rather than shelflike cristae-> also contain enzymes for converting cholesterol to pregnenolone
• Steroid hormones are NOT stored in granules & diffuse freely between SER & mitochondria
• 3 concentric zones:
  
  1. Zona glomerulosa
     
     • immediately inside the capsule
     • comprising 15% of cortex
     • consists of closely packed arched cords of columnar pyramidal cells with many capillaries
     • makes mineralocorticoids
       
       • steroids that affect uptake of Na, K, & H2O by cells of renal tubules
       • Aldosterone
         
         • principal product
         • major regulation of salt balance
         • stimulates Na reabsorption in the distal convoluted tubule
         • stimulated by Angiotensin II, increase in K concentration, & weakly by ACTH
  2. Zona fasciculata
     
     • occupies 65%-80% of cortex
     • consists of long cords of large polyhedral cells (1-2 cells thick) separated by fenestrated sinusoidal capillaries
       
       • polyhedral cells are filled with lipid droplets & appear vacuolated
       • secrete glucocorticoids especially cortisol
         
         • Cortisol affect carbohydrate metabolism by stimulating gluconeogenesis
         • suppress immune functions
         • induce fat mobilization & muscle proteolysis
       • controlled by ACTH with negative feedback to the concentration of circulating glucocorticoids
       • produce weak androgen
  3. Zona reticularis
     
     • comprises 10% of cortex
     • consists of smaller cells in a network of irregular cords interspersed with wide capillaries-> steroid secreting cells-> produce gonadocorticoids
     • cells are ore heavily stained than other zones-> contain fewer lipid droplets & more lipofuscin pigment
     • also produces cortisol but primarily secrete weak androgens (including DHEA- dehydroepiandosterone-> converted to testosterone in men and women)
     • secretion is also stimulated by ACTH with regulatory feedback

Question 14

Addison disease

Answer 14

Addison disease

• Adrenal cortical insufficiency
• autoimmune disorder which causes degeneration in any layer of adrenal cortex
• leads to loss of glucocorticoids, mineralocorticoids, androgen production

Question 15

Adrenal Medulla

Answer 15

Adrenal Medulla

• composed of large pale-staining polyhedral cells arranged in cords or clumps supported by reticular fiber network
• profuse supply of sinusoidal capillaries intervene between adjacent cords
• few parasympathetic ganglion present
• Chromaffin cells
  
  • medullary parenchymal cells
  • arise from neural crest cells
  • innervated by preganglionic sympathetic neurons
  • considered modified sympathetic postganglionic neurons of sympathetic & parasympathetic ganglia
  • Neuron-LIKE cells
  • lacking axons and dendrites & specialized as secretory cells
  • contain many electron-denses granules for storage and secretion of Catecholamines for fight or flight response:
    
    • Epinephrine (Adrenaline)
      
      • about 80%
      • granules are smaller and less dense than norepinephrine
      • increases heart rate
      • dilates bronchioles
      • dilates arteries or cardiac and skeletal muscles
    • Norepinephrine (Noradrenaline)
      
      • constricts vessels of digestive system & skin increasing blood flow to the heart, musles and brain
      • NE-secreting cells are found in paraganglia (collections of catecholamine-secreting cells adjacent to the autonomic ganglia)
    • conversion of NE to Epinephrine occurs only in chromaffin cells
    • both hormones stimulate glycogen breakdown elevating blood glucose levels
• Chromogranins- proteins that bind both catecholamines together with Ca2+ & ATP in granular storage

Question 16

Benign Pheochromocytomas

Answer 16

Benign Pheochromocytomas

• periodic secretion of high levels of cathecholamines in adrenal medulla that causes swings in blood pressure between hypertension and hypotension

Question 17

Pancreatic Islets

Answer 17

Pancreatic Islets

• Islets of Langerhans
• compact spherical or ovoid masses of endocrine cells embedded within the acinar exocrine tissue of the pancreas (has more than 1 million islets mostly in the gland’s narrow tail region)
• very thin reticular capsule surrounds each islet separating it from the adjacent acinar tissue
• embryonic origin-> epithelial outgrowths from endoderm of developing gut
• cells are polygonal, smaller, & more lightly stained than the surrounding acinar cells arranged in cords separated by fensestrated capillaries
• contain few enterochromaffin cells-> affects digestive system
• Gap junction transfer automatic neural stimulus to other cells
• major islets identified by immunohistochemistry
  
  • alpha or A cells
    
    • secrete primarily glucagon
    • usually located peripherally
    • increase blood glucose
    • make energy stored in glycogen and fat available through glycogenolysis and lipolysis
  • beta or B cells
    
    • produce insulin (insula-> island:)
    • most numerous
    • located centrally
    • cause entry of glucose into cells
    • decrease blood glucose
  • delta or D cells
    
    • secrete somatostatin
    • scattered
    • much less abundant
    • inhibits release of other islet cell hormones through local paracrine action
    • inhibits release of GH & TSH in anterior pituitary & HCL secretion by gastric parietal cells
  • PP cells
    
    • located in head of pancreas
    • secrete pancreatic polypeptide
    • stimulate activity of gastric chief cells
    • inhibit bile secretion, pancreatic enzyme, & bicarbonate secretion, & intestinal motility
  • Sympathetic fibers increase glucagon release & inhibit insulin release
  • Parasympathetic fibers increase both insulin and glucagon

Question 18

Diabetes Mellitus

Answer 18

Diabetes Mellitus

• loss of insulin effect & subsequent failure of cells to take in glucose
• leads to elevated blood sugar or hyperglycemia
• Type 1 DM or Insulin-dependent DM (IDDM)
  
  • caused by loss of B cells from autoimmune destruction
  • treated by regular injection of insulin
• Type 2 DM or Non-insulin-dependent DM (NIDDM)
  
  • B cells are present but fail to produce adequate levels of insulin in response to hyperglycemia (target cells no longer respond to the hormone)
  • commonly occurs with obesity

Question 19

Diffuse Endocrine System

Answer 19

Diffuse Endocrine System

• enterochromaffin cells that are scattered in both the islets & small ducts of the pancreas
• make up the similar cells in the respiratory mucosa
• derived from endodermal cells of embryonic gut or bronchial buds
• produce many of the same polypeptide & neurotransmitter-like molecules such as serotonin (also released by neurosecretory cells in CNS)
• enterochromaffin cells- stained by solutions of chromium salts
• argentaffin cells- stained by silver nitrates
• APUD cells- Amine precursor uptake & decarboxylation
• highly important d/t their role in regulating motility & secretions of all types within the digestive system

Question 20

Thyroid Gland

Answer 20

Thyroid Gland

• located anterior and inferior to the larynx
• consists of 2 lobes united by an isthmus
• originates in early embryonic life from the foregut endoderm near the base of the developing tongue
• synthesizes thyroid hormones Thyroxine (tetra-iodothyronine or T4) & tri-iodothyronine (T3) which help control basal metabolic rate in cells
• also synthesized Calcitonin which is involved in Ca2+ metabolism
• parenchyma is composed of millions of rounded epithelial thyroid follicles
  
  • with simple epithelium & central lumen densely filled with gelatinous acidophilic colloid
• only endrocrine gland in which a large quantity of secretory product is stored
  
  • storage is outside the cells in the colloid of follicle lumen
    
    • colloids contain the largest glycoprotein thyroglobulin (precursor for active thyroid hormones)
    • there is sufficient hormone in follicles to supply the body from up to 2 months with no additional synthesis
• covered by a fibrous capsule from which septa extend into parenchyma dividing it into lobules and carrying blood vessels, nerves, & lymphatics
• follicles are densely packed together separated from one another only by sparse reticular CT
• stroma is well vascularized with fenestrated capillaries for transfer of released hormone to blood
• Thyrocytes
  
  • follicular cells
  • range from squamous (hypoactive) to low columnar (active)
  • controlled by thyroid stimulating hormone (TSH) from the anterior pituitary
  • have apical junctional complexes
  • rest on basal lamina
  • exhibit organelles indicating active protein synthesis & secretion, phagocytosis, & digestion
  • nucleus is round and central
  • Basally, cells are rich in RER & TSH receptor
  • Apically (facing the follicular lumen), are golgi, secretory granules, numerous phagosomes, lysosomes, & microvilli
• Parafollicular cells (C cells)
  
  • found inside the basal lamina of follicular epithelium or as isolated clusters between follicles
  • derived from neural crest
  • larger than follicular cells & stain less intensely
  • smaller amount of RER, large golgi complexes, & numerous small granules containing Cacitonin
    
    • Calcitonin elevates blood Ca2+ levels
    • inhibits osteoclast activities

Question 21

Goiter

Answer 21

Goiter

• excessive growth of thyroid follicles & enlargement of thryoid gland d/t chronic dietary iodine deficiencies which inhibit thyroid hormone production causing thyrotropic cells of anterior pituitary gland to produce excess TSH

Question 22

Production of Thyroid Hormone & its Control

Answer 22

Production of Thyroid Hormone and its Control:

1. Production of Thyroglobulin

• synthesis in RER & glycosylation in golgi apparatus
• thyroglobulin has no hormonal activity but contains 140 tyrosyl residues critical for thyroid hormone synthesis
• thyroglobulin released as an exocrine product from apical vesicles fo thyrocytes into the follicular lumen

2. Uptake of Iodide from blood

• by Na/I symporters (NIS) in the thyrocytes’ basolateral cell membrane-> allows for 30-fold concentration of dietary iodide in thyroid tissue relative to plasma
• decreased levels of circulating iodide triggers synthesis of NIS increasing iodide uptake & compensating for the lower plasma concentration
• apical Iodide/Chloride transporter (pendrin) pumps I- from thyrocytes into colloid

3. Iodination of tyrosyl residues
   * in thyroglobulin with either 1 or 2 atoms occurs in the colloid after oxidation of iodide to iodine by membrane-bound thyroid peroxidase in microvilli surfaces of thyrocytes
4. Formation of T3 & T4 (Thyroxine)
   * occurs as 2 iodinated tyrosines which are covalently conjugated in coupling reactions
5. Endocytosis of iodinated thyroglobulin

• by the thyrocytes involves both fluid-phase pinocytosis & receptor-mediated endocytosis
• endocytic vesicles fuse with lysosomes & thyroglobulin is degraded by lysosomal proteases freeing active thryoid hormones as T3 & T4

6. Secretion of T3 & T4

• at the basolateral domains of thyrocytes occurs in an endocrine manner
• both molecules are immediately taken up by capillaries

Thyroid hormones are carried in blood tightly bound to thryoxine-binding globulin or albumen

Both T3 nd T4 increase the number of mitochondria to help enhance metabolic activity

• T4- more abundant compund (90%)-> halflife is 1 week
• T3- 2-10-fold more active than T4-> halflife is 1.5 days

TSH (Thyrotropin)- major regulator of anatomic and functional state of thyroid follicles

• Secretion is increased by exposure to cold
• Secretion is decreased by heat & stressful stimuli
• Thyroid hormones inhibit release of TSH maintaining levels of circulating T4 & T3 in normal range

Question 23

Graves disease and Hypothyroidism

Answer 23

Graves disease

• autoimmune disorder in which antibodies produce chronic stimulation of follicular cells & release thyroid hormones (hyperthyroidsim)
• causes hypermetabolic state marked by weight loss, nervousness, sweating, heat intolerance

Hypothyroidism

• reduced thyroid hormone levels
• caused by local inflammation (thyroiditis)
• inadequate secretion of TSH by anterior pituitary gland
• causes tiredness, weight gain, intolerance of cold, decresed ability to concentrate

Question 24

Parathyroid glands

Answer 24

Parathryoid glands

• 4 small ovoid masses
• located in the back of thyroid gland embedded in larger gland’s capsule
• microvasculature of each arises from the inferior thyroid arteries
• each is contained within a thin capsule from which septa extend into the gland
• sparse reticular stroma supports dense elongated clusters of secretory cells
• derived from the embryonic pharyngeal pouches
  
  • superior glands from the 4th pouch
  • inferior glands from the 3rd pouch
• 10% of individual have parathyroid tissue attached to the thymus
• Principal (chief) cells
  
  • endocrine cells of Parathyroid gland
  • small polygonal cells with rounded nuclei & pale staining slightly acidophilic cytoplasm
• Oxyphil cells
  
  • often clustered
  • more commonly in older individuals
  • much larger than principal cells
  • acidophilic with abnormally-shaped mitochondria
  • derivatives of principal cells
• Parathyroid hormone (PTH)
  
  • polypeptide in irregulary-shaped cytoplasmic granules
  • important regulator of blood calcium levels
  • 3 major targets:
    
    1. stimulates osteoblasts to produce osteoclast-stimulating factor (OSF)
       
       • increases number & activity of osteoclasts
       • resulting to resorption of calcified bone matrix & release of Ca2+ increasing concentration of circulating Ca2+
       • suppresess PTH production
    2. stimulates Ca2+ reabsoprtion (& inhibits phosphate reabsorption in proximal tubule) in the distal convoluted tubules of the renal cortex
    3. increases Ca2+ absorption in small intestines by stimulating vitamin D activation

Question 25

Hypoparathyroidism & Hyperparathyroidism

Answer 25

Hypoparathyroidism

• diminished secretion of PTH
• causes bones to become more mineralized & denser & striated muscles to exhibit abnormal contractions d/t inadequate Ca2+ ion concentration

Hyperparathyroidism

• stimulates osteoclast number and activity
• leads to increased level of blood Ca2+ that can be deposited pathologically in arteries, cartilage, or kidneys

Question 26

Pineal gland

Answer 26

Pineal gland

• Epiphysis cerebri
• regulates daily rhythms fo bodily acitivites
• small pine cone-shaped organ
• develops from neuroectoderm in the posterior wall of 3rd ventricle & remains attached to the brain by short stalk
• covered by CT of pia mater from which septa containing blood vessels emerge & subdivide variously sized lobules
• Neuroendocrine transducer-> converts sensory input regarding light & darkness into variations into many hormonal functions
  
  • cycle of light & darkness detected in retina->
  • transmitted to the pineal via the retinohypothalamic tract->
  • suprachiasmatic nuclei
  • tracts of sympathetic fibers entering the pineal
  • unmyelinated sympathetic nerve enter the pineal gland & end among pinealocytes forming synapses with some
• Pinealocytes
  
  • prominent and abundant secretory cells
  • slightly basic cytoplasm & irregular euchromatic nuclei
  • have secretory vesicles, many mitochondria, & long cytoplasmic processes extending to the vascularized septa
  • produce Melatonin
    
    • low molecular weight tryptophan derivative
    • promoted by darkness & inhibited by daylight
    • diurnal fluctuation in blood melatonin levels induces rhythmic change in the activity of the hypothalamus, pituitary gland and endocrine tissues that characterize circadian (24 hours, day/night) rhythm
• has interstitial cells (modified astrocytes)
  
  • stain positively for GFAP (Glial Fibrillary Acidic Protein)-> 5% of cells
  • have elongated nuclei
  • more heavily stained that pinealocytes
  • found in perivascular areas & in between groups of pinealocytes
• Corpora arenacea
  
  • Brain sand
  • characteristic features of pineal gland
  • variously sized secretion of Ca2+ & Mg2+ salts
  • increase in number & size with age