Endocrine Histology Flashcards
Hormonal control mechanisms
Endocrine control
Paracrine control
Autocrine control
Water soluble or hydrophobic hormones
Steroid hormones -hydrophobic, can cross cell membrane receptors inside cell
Protein hormones- hydrophilic - cant cross cell membrane receptors on cell membrane
Types of capillary
Continuous
Fenestrated
Discontinuous
Continuous capillary
Typically found in connective tissue
-cardiac, skeletal, smooth muscle
-skin
-lungs
-CNS
Uninterrupted vascular endothelium that rests on a continuous basal lamina
Fenestrated capillary
Typically found in endocrine glands, sites of fluid or metabolite absorption
Numerous openings (Fenestrations) providing channels across capillary wall
A continuous basal lamina across fenestrations on the basal plasma membrane surfaces
May also have thin, non membranous diaphragm, across opening referred to as filtration pores
Discontinuous capillary or sinusoids
Typically found in liver, spleen, bone marrow
Larger in diameter, more irregular shaped. Large openings in endothelial cytoplasm, separated by wide, irregular, intercellular gaps.
A discontinuous basal lamina
Embryology of the thyroid gland
Originates by cell proliferation between 1st and 2nd pharyngeal pouches near base of tongue
The migrates caudally along midline, division of the thyroid into right and left lobes also occurs during this time
4th week an endodermal proliferation (thyroid primordium) develops at tip of the foramen caecum of developing tongue
Migrates inferiorly to its final site anterior and inferior to larynx
Until the 5th week the thyroid remains connected to the foramen caecum by the thyroglossal duct
The gland reaches its final site in 7th week
What does the thyroid gland consist of
Mainly spherical follicles composed of simple epithelium of thyrocytes surrounding a lumen with thyroglobulin (colloid)
Thyroglobulin is a large glycoprotein in which tyrosine residues are iodinated as precursors of the thyroid hormones T3 and T4
Thyroid follicles also contain a few smaller and paler cells called parafollicular cells or C cells which produce calcitonin
Process of producing thyroid hormones via stored thyroglobulin
Both phases are promoted by TSH & may occur simultaneously in same cell
In exocrine phase: glycoprotein thyroglobulin made and secreted into follicular lumen, iodide pumped across cells into lumen, iodide converted to iodine by membrane bound thyroid peroxidase
Added to tyrosine residues of thyroglobulin to form monoiodotyrosine or diiodotyrosine which are then covalently coupled to form T3 and T4 still within glycoprotein
In endocrine phase: iodinated thyroglobulin endocytosed by thyrocytes and degraded by lysosomes, free active T3 &T4 released to adjacent capillaries in an endocrine matter
Where are the parathyroid glands
On posterior side of thyroid gland
Embryology of parathyroid glands
Pharyngeal pouch derivatives
Third pouch: inferior parathyroid glands, thymus
Fourth pouch: superior parathyroid gland and parafollicular cells (produce calcitonin in thyroid)
The inferior and superior parathyroid glands descend with the thymus and come to rest on the posterior surface of thyroid gland low in the neck
What do parathyroid glands consist of
Primarily of principal cells (chief cells) that secrete PTH (parathyroid hormone), that raises blood Ca2+ levels
What is calcitonin
Hormone thyroid gland secretes to help regulate calcium levels in blood by decreasing it. Produced by C cells in thyroid
Opposes actions of PTH (increases blood calcium levels)
What are the pancreatic islets of Langerhans
Small clusters of pale cells (neuroendocrine) embedded within exocrine tissue
They have primarily: B cells making insulin, a cells making glucagon and delta cells making somatostatin (GH inhibiting hormone)
Each islet consists of 2000 to 3000 cells surrounded by a network of fenestrated capillaries and supported by reticular fibres
Tissues the pancreas is divided into
Exocrine portion- acinar and duct tissue
Exocrine pancreas- formed by protein secretory acini with apically located zymogen granules
Endocrine portion- islets of Langerhans
Difference between exocrine and endocrine tissue
Exocrine glands secrete their substances through ducts onto body’s surfaces
Endocrine glands secrete their substances directly into bloodstream, ductless glands
What are adrenal glands
Also known as suprarenal glands
Located on top of kidneys
Left; semi lunar shape, higher T11
Right; pyramidal shape, lower T12
Secrete many hormones including cortisol, aldosterone and adrenaline
Help to regulate several body functions including metabolism, blood pressure and body’s responses to stress
Composed of a cortex and medulla
What does the adrenal cortex consist of
3 histologically distinct zones; cells producing steroid hormones, drained by same system of capillaries
Most superficial zona glomerulosa with round cell clusters produce mineralcorticoids such as aldosterone, regulates electrolyte levels
Wider middle zone fasciculata with elongated strands of cells produce glucocorticoids such as cortisol, regulates carbohydrate metabolism. Cells known as spongiocytes
Innermost zona reticularis (largest zone) make a weak androgen DHEA converted to testosterone in men& women.
What does the adrenal medulla consist of
Inner region of the glands
Neural crest-derived chromaffin cells synthesising either adrenaline or noradrenaline that regulate the stress response
Capillaries of adrenal cortex
Sinusoidal type
Numerous holes and sieve plates
Basal lamina is normally lacking and is patchy if present. This is in contrast to the capsular and medullary capillaries which are uninterrupted
Pineal gland
Develops from embryonic neuroectoderm, remains attached to the brain and contains modified neurons called pinealocytes that secrete amine melatonin
Pineal concreations called corpora arenacea
Neural connections from the retina to pinealocytes allow diurnal secretion of melatonin and rhythms in physiological activities
Output of pineal melatonin is modulated by light through nervous pathways which input as sympathetic innervation to gland
What are paraganglia
A collection of cells that came from embryonic nervous tissue (neural crest origin) and are found near adrenal glands and some blood vessels and nerves
Associated with autonomic nervous system- parasympathetic and sympathetic
What is the diffuse neuroendocrine system
Composed of neuroendocrine cells
Widespread in GI and respiratory tract
Many different products depending upon location
Local paracrine
Definition of an endocrine gland
A ductless gland that secretes hormones directly into the blood
Structure of thyroid gland
Large unpaired gland in neck formed of two lateral lobes connected by an isthmus
It is palpable but often not with ageing
Also has a pyramidal lobe
All supplied by superior and inferior thyroid arteries
Venous drainage occurs via superior, middle and inferior thyroid veins
Location & relation of thyroid gland
Located on front of neck just under Adam’s apple
Resides within visceral compartment of neck
Anteriorly: lies behind strap muscles of neck- sternohyoid, sternothyroid and omohyoid
Posteriorly: wraps around cricoid cartilage and tracheal rings
Arterial blood supply of thyroid
Supplied by two main arteries- superior and inferior thyroid artery
The superior thyroid artery is the first branch of the external carotid artery, this artery supplies the superior anterior and superior posterior portions of gland
The inferior artery arises from thyrocervical trunk (branch of subclavian artery), artery travels superomedially to reach inferior pole of thyroid, tends to supply posteroinferior aspect of gland
Venous blood supply of thyroid
Venous drainage is carried out by superior, middle and inferior thyroid veins
The superior and middle veins drain into the internal jugular veins whereas the inferior drains into the brachiocephalic vein
Arterial blood supply of the parathyroid
The posterior aspect of the thyroid gland is supplied by the inferior thyroid artery so its branches also supply the parathyroid glands
Venous blood supply parathyroid
Venous drainage is carried out by the parathyroid veins which drain into the superior, middle and inferior thyroid veins
Secretomotor
Refers to the capacity of a structure (often a nerve) to induce a gland to secrete a substance
Vasomotor
Actions upon a blood vessel which alter its diameter
How are the thyroid and parathyroid gland innervated
By the cervical sympathetic ganglion
These nerves are vasomotor and not secretomotor. Endocrine secretion of parathyroid hormone/thyroid hormone is controlled hormonally/depended on calcium levels
Adrenal gland location and relation
The adrenal glands are located in the posterior abdomen, between kidneys and diaphragm
They’re retroperitoneal (located in space behind peritoneum membrane forming the lining of the abdominal cavity) with parietal peritoneum covering their anterior surface only
Perinephric fascia (connective tissue) encloses the adrenal glands and kidneys, this fascia attaches glands to crura of diaphragm
Separated from kidneys by perirenal fat
The adrenal glands sit in close proximity to many other structures in abdomen
What is the left adrenal gland anterior and posterior to
Anterior- stomach, pancreas and spleen
Posterior- left crus of the diaphragm
What is the right adrenal gland anterior and posterior to
Anterior- inferior vena cava, right lobe of liver
Posterior- right crus of the diaphragm
Adrenal gland embryology
The adrenal cortex is derived from the mesoderm of the posterior abdominal wall
The adrenal medulla is of ectodermal origin, arising from neural crest cells
Arterial blood supply of adrenal glands
The adrenal glands have a rich blood supply via 3 main arteries
Superior adrenal artery- arises from the inferior phrenic artery
Middle adrenal artery- arises from the abdominal aorta
Inferior adrenal artery-arises from the renal arteries
Venous blood supply adrenal arteries
On the left side, the left adrenal vein leaves the left adrenal gland and passes inferiorly to enter the left renal vein
On the right side the right adrenal vein leaves the right adrenal gland and almost immediately enters the inferior vena cava
Innervation of the adrenal gland
The adrenal medulla is innervated by the sympathetic nervous system from mostly the greater splanchnic nerve (T8-L1)
Fibres of the greater splanchnic nerve will enter the adrenal gland to synapse on membranes of chromaffin cells
The chromaffin cells release adrenaline and noradrenaline directly into bloodstream when stimulated by sympathetic nervous system
What are the splanchnic nerves
Arise from within the sympathetic thoracic trunk to innervate abdomen
The greater splanchnic nerve helps with the motility of the foregut and provides sympathetic innervation to the adrenal medulla
What is the pancreas
L shaped organ positioned at L1 level
With the exception of the tail of the pancreas it is a retroperitoneal organ, located deep within upper abdomen
The pancreas is a glandular organ with both digestive (exocrine) and hormonal (endocrine) functions
It consists of 2 types of secretory tissue: exocrine (acinar cells, secrete digestive enzymes) and endocrine (islets of Langerhans, releases insulin and glucagon)
Embryology of the pancreas
Pancreatic development begins with the formation of a dorsal and ventral pancreatic bud
Both join with the foregut through a duct
The ventral bud rotates and fuses with the dorsal bud
At this point of fusion, the main ducts of the ventral and dorsal pancreatic buds fuse, forming the main pancreatic duct
The dorsal pancreatic bud forms the neck, body and tail of the developed pancreas and the ventral pancreatic bud forms the head and uncinate process (hook)
Structure of the pancreas
Typically divided into 4 parts
Head: widest part, lies within C shaped curve created by duodenum & is connected to it by connective tissue
Uncinate process: a projection arising from the lower part of the head and extending medially to lie beneath the body of the pancreas, lies posterior to superior mesenteric vessels
Neck: located between head and body, overlies superior mesenteric vessels which form a groove in its posterior aspect
Body: centrally located crossing midline of human body to lie behind stomach and to left of superior mesenteric vessels
Tail: the left end of pancreas that lies within close proximity to hilum of spleen, contained within splenorenal ligament with the splenic vessels, this is only part of pancreas that’s intraperitoneal
Anatomical relations to pancreas
Stomach lies anterior to pancreas, separated from pancreas by lesser sac
Duodenum- the c shaped duodenum curves around and outlines head of pancreas
Spleen-located posteriorly and laterally, the lienorenal ligament connects spleen to tail of pancreas
The aorta and vena cava pass posteriorly to head, the superior mesenteric artery and vein lie behind neck and anterior to uncinate process
Posterior to neck the splenic and superior mesenteric veins unite to form hepatic portal vein
Splenic artery traverses the superior border pancreas
Arterial blood supply of pancreas
The pancreas is supplied by the pancreatic branches of the splenic artery (the celiac trunk). These include the great pancreatic artery which supplies tail and dorsal pancreatic artery which supplies blood
Head is also supplied by superior and inferior pancreaticoduodenal arteries
The Gastroduodenal artery (branch of common hepatic artery from celiac trunk) branches into anterior superior pancreaticoduodenal artery and posterior superior pancreaticoduodenal artery, both supply the anterior and posterior superior region of head
Venous blood supply in pancreas
Pancreatic veins are responsible for draining deoxygenated blood from pancreas
The pancreatic veins draining venous blood from body and tail empty into splenic vein
The anterior superior pancreaticoduodenal vein empties into superior mesenteric vein, while posterior variant empties into hepatic portal vein
Both the anterior and posterior inferior pancreaticoduodenal veins drain into superior mesenteric vein
Innervation of pancreas
The pancreas receives involuntary innervation via ANS
-it’s sympathetic innervation originates from greater and lesser splanchnic nerves
-its parasympathetic innervation originates from vagus nerve
Inside the organ they carry nerve impulses to acinar cells and pancreatic islets
Parasympathetic fibres induce secretion from acinar cells and pancreatic islets ultimately resulting in release of pancreatic juice insulin and glucagon
Sympathetic fibres cause vasoconstriction and inhibition of secretion pancreatic juice but stimulates release of glucagon but inhibits release insulin
Why is vision often affected in patients with large pituitary adenomas
Because optic chiasm lies in close proximity above the pituitary gland
