Human Anatomy: Endocrine Glands (Lecture Notes)

HUMAN ANATOMY

ENDOCRINE GLANDS

(Lecture Notes)

Necdet Ersoz
Gazi University Medical School

Endocrine glands are pituitary gland, thyroid gland, parathyroid gland, and suprarenal glands.

Pituitary Gland

ü  Pituitary gland is a reddish-grey, ovoid body, about 12 mm in transverse and 8 mm in anteroposterior diameter.

ü  An average adult weight of 500 mg.

ü  Is continuous with the infundibulum (a hollow, conical, inferior process from the tuber cinereum of the hypothalamus)

ü  Lies within the pituitary fossa.

ü  Is covered superiorly by a circular diaphragm sellae of dura mater.

ü  Diaphragm sellae is pierced centrally by an aperture for the infundibulum, and separates the anterior superior aspect of the pituitary from the optic chiasma.

ü  Inferiorly the pituitary is separated from the floor of the fossa by a venous sinus that communicates with the circular sinus.

ü  The meninges blend with the pituitary capsule and are not separate layers.

ü  Pituitary has two major parts, the neurohypophysis and adenohypophysis, which differ in their origin, structure, and function.

ü  The neurohypophysis is a diencephalic downgrowth connected with the hypothalamus.

ü  The adenohypophysis is an ectodermal derivative of the stomatodeum.

ü  Both include parts of the infundibulum.

ü  The infundibulum has a central infundibular stem.

ü  The term neurohypophysis includes the median eminence, infundibular stem and neural lobe or pars posterior.

ü  Surrounding the infundibular stem is the pars tuberalis, a component of the adenohypophysis.

ü  The main mass of the adenohypophysis may be divided into the pars anterior (pars distalis) and the pars intermedia, which are separated in fetal and early postnatal life by the hypophysial cleft, a vestige of Rathke’s pouch, from which it develops.

ü  Neurohypophysis includes the pars posterior (pars nervosa, posterior or neural lobe), infundibular stem, and the median eminence.

ü  Adenohypophysis includes the pars anterior (pars distalis or glandularis), pars intermedia, and pars tuberalis.

Neurohypophysis

ü  In early fetal life the neurohypophysis contains a cavity continuous with the third ventricle.

ü  Axons arising from groups of hypothalamic neurons (e.g. the magnocellular neurons of the supraoptic and paraventricular nuclei) terminate in the neurohypophysis.

ü  The long magnocellular axons pass to the main mass of the neurohypophysis. They form the neurosecretory hypothalamohypophysial tract and terminate near the sinusoids of the posterior lobe.

ü  Some smaller parvocellular neurones in the periventricular zone have shorter axons, and end in the median eminence and infundibular stem among the superior capillary beds of the venous portal circulation. These small neurons produce releasing and inhibitory hormones which control the secretory activities of the adenohypophysis via its portal blood supply.

ü  The neurohormones stored in the main part of the neurohypophysis are vasopressin (antidiuretic hormone) and oxytocin.

ü  Near the posterior lobe, astrocytes are replaced by pituicytes, which constitute most of the non-excitable tissue in the neurohypophysis. Pituicytes are dendritic neuroglial cells of variable appearance, often with long processes running parallel to adjacent axons.

ü  Typically their cytoplasmic processes end on the walls of capillaries and sinusoids between nerve terminals fibrils.

ü  Axons also end in perivascular spaces; they lie close to the walls of sinusoids, but remain separated from them by two basal laminae, one around the nerve endings, and the other underlying the fenestrated endothelial cells.

ü  The spaces between the basal laminae are occupied by fine collagen fibrils.

Adenohypophysis

ü  The adenohypophysis is highly vascular.

ü  It consists of epithelial cells of varying size and shape arranged in cords or irregular follicles, between which lie thin-walled vascular sinusoids supported by a delicate reticular connective tissue.

ü  Most of the hormones synthesized by the adenohypophysis are trophic. They include the peptides growth hormone (GH), involved in the control of body growth, and prolactin (PRL) which stimulates breast growth.

ü  Glycoprotein trophic hormones are the large pro-opiomelanocortin precursor of adrenocorcitotrophin (ACTH), which controls the secretion of certain suprarenal cortical hormones.

ü  Thyroid stimulating hormone (TSH); follicle stimulating hormone (FSH), which stimulates growth and secretion of oestrogens in ovarian follicles and spermatogenesis (acting on testicular Sertoli cells)

ü  Luteinizing hormone (LH), which induces progesterone secretion by the corpus luteum and testosterone synthesis by Leydig cells in the testis.

ü  Pro-opiomelanocortin is cleaved into a number of different molecules including ACTH

ü  β-Lipotropin is released from the pituitary, but its lipolytic function in humans is uncertain.

ü  The epithelial endocrine cells, which secrete the different adenohypophysial hormones, are distinguished in part by their differing affinities for acidic and basic dyes.

Vessels of Pituitary Gland

ü  The arteries of the pituitary arise from the internal carotid arteries via a single inferior and several superior hypophysial arteries on each side.

ü  The former come from the cavernous part of the internal carotid artery, the latter from its supraclinoid part and from the anterior and posterior cerebral arteries.

ü  The inferior hypophysial arteries divide into medial and lateral branches, which anastomose across the midline and form an arterial ring around the infundibulum. Fine branches from this circular anastomosis enter the neurohypophysis.

ü  The superior hypohysial arteries supply the median eminence, upper infundibulum, and, via arteries of the trabeculae, the lower infundibulum.

ü  A confluent capillary net, extending through the neurohypophysis, is supplied by both sets of hypophysial vessels.

ü  Reversal of flow can occur in cerebral capillary beds lying between the two supplies.

ü  Short portal vessels run from the lower infundibulum to the pars anterior. There is no direct arterial supply.

ü  The portal system carries hormone-releasing factors, probably elaborated in parvocellular groups of hypothalamic neurons, and these control the secretory cycles of cells in the pars anterior.

ü  The pars intermedia appear to be avascular.

ü  The route for venous drainage of the neurohypophysis and adenohypophysis is via long and short portal vessels into the dural venous drainage.

ü  The venous drainage carries hypophysial hormones from the gland to their targets and also facilitates feedback control of secretion.

Thyroid Gland

ü  The thyroid gland is the body’s largest endocrine gland.

ü  It produces thyroid hormone, which control the rate of metabolism, and calcitonin, a hormone controlling calcium metabolism.

ü  The thyroid gland affects all areas of the body except itself, spleen, testes, and the uterus.

ü  Lies deep to the sternothyroid and sternohyoid muscles, located anteriorly in the neck at the level of the C5-T1 vertebrae.

ü  It consists primarily of right and left lobes, anterolateral to the larynx and trachea.

ü  A relatively thin isthmus unites the lobes over the trachea, usually anterior to the second and third tracheal rings.

ü  Is surrounded by a thin fibrous capsule, which sends septa deeply into the gland.

ü  Dense connective tissue attaches to the cricoid cartilage and superior tracheal rings.

ü  External to the capsule is a loose sheath formed by the visceral portion of the pretracheal layer of deep cervical fascia.

Surfaces and Relations

ü  The convex lateral (superficial) surface is covered by sternothyroid, whose attachment to the oblique thyroid line prevents the upper pole of the gland from extending on to thyrohyoid.

ü  The medial surface of the gland is adapted to the larynx and trachea; its superior pole contacts the inferior pharyngeal constrictor and the posterior part of cricothyroid, which separate it from the posterior part of the thyroid lamina and the side of the cricoid cartilage.

ü  The external laryngeal nerve is medial to this part of the gland as it passes to supply cricothyroid.

ü  Inferiorly the trachea and, more posteriorly, the recurrent laryngeal nerve and oesophagus, are medial relations.

ü  The posterolateral surface of the thyroid gland is close to the carotid sheath, and overlaps the common carotid artery.

ü  The anterior border of the gland is thin, and near the anterior branch of the superior thyroid artery it slants down medially.

ü  The posterior border is rounded and related inferiorly to the inferior thyroid artery and its anastomosis with the posterior branch of the superior thyroid artery.

ü  On the left side, the lower end of the posterior border lies near the thoracic duct.

ü  The parathyroid glands are usually related to the posterior border.

ü  The isthmus is covered by sternothyroid, from which it is separated by pretracheal fascia.

ü  More superficially it is covered by sternohyoid, the anterior jugular veins, fascia, and skin.

ü  The superior thyroid arteries anastomose along its upper border and the inferior thyroid veins leave the gland at its lower border.

Arteries

ü  The thyroid gland is supplied by the superior and inferior thyroid arteries and sometimes by an arteria thyroidea ima from the brachiocephalic trunk or aortic arch.

ü  The arteries are large and their branches anastomose frequently both on and in the gland, ipsilaterally and contralaterally.

ü  The superior thyroid artery, which is closely related to the external laryngeal nerve, pierces the thyroid fascia and then divides into anterior and posterior branches.

ü  The anterior branch supplies the anterior surface of the gland.

ü  The inferior thyroid artery approaches the base of the thyroid gland and divides into superior (ascending) and inferior thyroid branches to supply the inferior thyroid branches to supply the inferior and posterior surfaces of the gland.

ü  The superior branch also supplies the parathyroid glands.

ü  The relationship between the inferior thyroid artery and the recurrent laryngeal nerve is highly variable and of considerable clinical importance.

ü  Iatrogenic injury to the nerves that supply the larynx represents a major complication of thyroid surgery.

ü  The recurrent laryngeal nerve is usually related to the posterior branch of the inferior thyroid artery, which may be replaced by a vascular network.

Veins

ü  The venous drainage of the thyroid gland is usually via superior, middle, and inferior thyroid veins.

ü  The superior thyroid vein emerges from the upper part of the gland and runs with the superior thyroid artery towards the carotid sheath: it drains into the internal jugular vein.

ü  The middle thyroid vein collects blood from the lower part of the gland: it emerges from the lateral surface of the gland and drains into the internal jugular vein.

ü  The inferior thyroid veins arise in a glandular venous plexus, which also connects with the middle and superior thyroid veins.

ü   These veins form a pretracheal plexus, from which the left inferior vein descends to join the left brachiocephalic vein and the right descends obliquely across the brachiocephalic artery to join the right brachiocephalic vein at its junction with the superior vena cava.

ü  The inferior thyroid veins often open via a common trunk into the superior vena cava or left brachiocephalic vein.

Lymphatics

ü  Thyroid lymphatic vessels communicate with the tracheal plexus and pass to the prelaryngeal nodes just above the thyroid isthmus and to the pretracheal and paratracheal nodes; some may also drain into the brachiocephalic nodes related to the thymus in the superior mediastinum.

ü  Laterally the gland is drained by vessels lying along the superior thyroid veins to the deep cervical nodes.

ü  Thyroid lymphatics may drain directly, with no intervening node, to the thoracic duct.

Innervation

ü  The thyroid gland receives its innervation from the superior, middle, and inferior cervical sympathetic ganglia.

ü  Postganglionic fibres from the inferior cervical ganglion form a plexus on the inferior thyroid artery which accompanies the artery to the thyroid gland, and communicates with the recurrent and external laryngeal nerves, with the superior cardiac nerve, and with the plexus on the common carotid artery.

Microstructure

ü  The thyroid gland has a thin capsule of connective tissue, extends into the glandular parenchyma and divides each lobe into irregularly shaped and sized lobules.

ü  The functional units of the thyroid are follicles, which are spherical and cyst-like, between 0.02 – 0.9 mm in diameter.

ü  Follicles consist of a central colloid core surrounded by a single-layered epithelium resting on a basal lamina.

Parathyroid Glands

ü  The hormone produced by the parathyroid glands, parathormone (PTH), controls the metabolism of blood phosphorus and calcium in the blood.

ü  The parathyroid glands target the skeleton, kidneys, and intestine.

ü  They are small, yellowish-brown, ovoid or lentiform structures, usually lying between the posterior lobar borders of the thyroid gland and its capsule.

ü  They are commonly 6 mm long, 3-4 mm across, and 1-2 mm from back to front, each weighing about 50 mg.

ü  Typically there are two on each side, superior and inferior, but there may be only three or many minute parathyroid islands scattered in connective tissue near the usual sites.

ü  The superior parathyroid glands are more constant in location than the inferior and are usually to be found midway along the posterior borders of the thyroid gland, although they may be higher.

ü  The inferior pairs are more variably situated and may be within the fascial thyroid sheath, below the inferior thyroid arteries and near the inferior lobar poles; or outside the sheath, immediately above an inferior thyroid artery, or in the thyroid gland near its inferior pole.

ü  The superior parathyroids are usually dorsal, and the inferior parathyroids ventral, to the recurrent laryngeal nerve.

Vessels and Lymphatics

ü  Both superior and inferior parathyroid glands are supplied by the inferior thyroid arteries, the superior thyroid may be supplied by the superior thyroid artery or from anastomoses between the superior and inferior thyroid arteries in 10-15% cases.

ü  The glands drain into the plexus of veins on the anterior surface of the thyroid.

ü  Lymph vessels are numerous and associated with those of the thyroid and thymus gland.

Innervation

ü  The nerve supply is sympathetic, either direct from the superior or middle cervical ganglia or via a plexus in the fascia on the posterior lobar aspects.

ü  Parathyroid activity is controlled by variations in blood calcium level, it is inhibited by a rise and stimulated by a fall.

ü  The nerves are believed to be vasomotor but not secretomotor.

Microstructure

ü  Each parathyroid gland has a thin connective tissue capsule with intraglandular septa but lacks distinct lobules.

ü  These glands synthesize and secrete parathyroid hormone (PTH) concerned with the control of the level and distribution of calcium and phosphorus.

ü  In childhood, the gland consists of wide, irregular, interconnecting columns of chief or principal cells separated by a dense plexus of fenestrated sinusoidal capillaries.

ü  After puberty adipose tissue accumulated in the stroma and typically accounts for about one third of the adult tissue mass, increasing further with age.

Suprarenal (Adrenal) Glands

ü  Lie immediately superior and slightly anterior to the upper pole of either kidney.

ü  Golden yellow in colour, each gland possesses two functionally and structurally distinct areas, an outer cortex and an inner medulla.

ü  The glands are surrounded by connective tissue containing perinephric fat, enclosed within the renal fascia, and separated from the kidneys by a small amount of fibrous tissue.

ü  The mean transverse dimensions of the body of the suprarenal glands are 61 mm (right) and 79 mm (left) and the mean transverse dimensions of suprarenal limbs are 28 mm (right) and 33 mm (left).

ü  Each weigh approximately 5 g (the medulla contributes about one-tenth of the total weight).

ü  The renal gland is pyramidal in shape.

ü  The left gland has a more semilunar form and is flattened in the anteroposterior plane. The left gland is marginally larger than the right.

ü  The bulk of the right suprarenal sits on the apex of the right kidney and usually lies slightly higher than the left gland, which sits on the anterolateral aspect of the upper pole of the left kidney.

Right Suprarenal Gland

ü  Lies posterior to the inferior vena cava.

ü  Lies posterior to the right lobe of the liver and anterior to the right crus of the diaphragm and superior pole of the right kidney.

ü  It often overlaps the apex of the upper pole of the right kidney as the two lower projections (limbs) straddle the renal tissue.

ü  The anterior surface faces slightly laterally and possesses two distinct facets.

ü  The medial facet is somewhat narrow, runs vertically and lies posterior to the inferior vena cava.

ü  The lateral facet is triangular and lies in contact with the bare area of the liver.

ü  The lowest part of the anterior surface may be covered by peritoneum, reflected onto it from the inferior layer of the coronary ligament. At this point it may lie posterior to the lateral border of the second part of the duodenum.

ü  Below the apex, near the anterior border of the gland, the hilum lies in a short sulcus from which the right suprarenal vein emerges to join the inferior vena cava. This vein is particularly short, which makes surgical resection of the gland potentially hazardous, because ligation may be difficult.

ü  The posterior surface is divided into upper and lower areas by a curved transverse ridge.

ü  The largest upper area is slightly convex and rests on the diaphragm.

ü  The small lower area is concave and lies in contact with the superior aspect of the upper pole of the right kidney.

ü  The medial border of the gland is thin and lies lateral to the right coeliac ganglion and the right inferior phrenic artery as the artery runs over the right crus of the diaphragm.

Left Suprarenal Gland

ü  Lies on closely applied to the left crus of the diaphragm and is separated from it only by a thin layer of fascia and connective tissue.

ü  The medial aspect is convex whilst the lateral aspect is concave since it is shaped by the medial side of the superior pole of the left kidney.

ü  The superior border is sharply defined while the inferior surface is more rounded.

ü  The anterior surface has a large superior area covered by peritoneum on the posterior wall of the lesser sac, which separates it from the cardia of the stomach and sometimes from the posterior aspect of the spleen.

ü  The smaller inferior area is not covered by peritoneum and lies in contact with the pancreas and splenic artery.

ü  The hilum faces inferiorly from the medial aspect and is near the lower part of the anterior surface.

ü  The left suprarenal vein emerges from the hilum and runs inferomedially to join the left renal vein.

ü  The posterior surface is divided by a ridge into a lateral area adjoining the kidney and a smaller medial area which lies in contact with the left crus of the diaphragm.

Arteries

ü  Suprarenal glands have an extensive vascular supply in relation to their size.

ü  Each gland is supplied by superior, middle and inferior suprarenal arteries, whose main branches may be duplicated or even multiple.

ü  They ramify over the capsule before entering the gland to form a subcapsular plexus, from which fenestrated sinusoids pass around clustered glomerulosal cells and between columns in the zona fasciculate to a deep plexus in the zona reticularis.

ü  Venules from this plexus pass between medullary chromaffin cells to medullary veins, which they enter between prominent bundles of smooth muscle fibres.

ü  Some relatively large arteries bypass this indirect route and pass directly to the medulla.

ü  The superior suprarenal artery arises from the inferior phrenic artery, a branch of the abdominal aorta: it is often small and may be absent.

ü  The middle suprarenal artery arises from the lateral aspect of the abdominal aorta at the level of the superior mesenteric artery. It ascends slightly and runs over the crura of the diaphragm to the suprarenal glands, where it anastomoses with the suprarenal branches of the inferior phrenic and renal arteries. The right middle suprarenal artery passes behind the inferior vena cava and near the right coeliac ganglion and is frequently multiple. The left middle suprarenal artery passes close to the left coeliac ganglion, splenic artery, and the superior border of the pancreas.

ü  The inferior suprarenal artery arises from the renal arteries, usually from the main renal artery but occasionally from its upper pole branches.

Veins

ü  Medullary veins emerge from the hilum to form a suprarenal vein, which is usually single.

ü  The right vein is very short, passing directly and horizontally into the posterior aspect of the inferior vena cava.

ü  An accessory vein is occasionally present and runs from the hilum superomedially to join the inferior vena cava above the right suprarenal vein.

ü  Their short course renders both right vessels liable to injury or even avulsion from the inferior vena cava during surgery if undue traction is applied, producing a side hole in.

ü  The left suprarenal vein descends medially, anterior and lateral to the left coeliac ganglion. It passes posterior to the pancreatic body and drains into the left renal vein.

ü  Since the venous drainage from each gland is usually via a single vein, infarction of that gland is more likely to be caused by damage to a suprarenal vein than to one of the suprarenal arteries.

Lymphatic Drainage

ü  Small lymphatic channels from both cortex and medulla drain to the hilum, from where larger calibre lymphatics emerge to        drain directly into the lateral groups of para-aortic nodes.

Innervation: Suprarenal Plexus

ü  On each side, the suprarenal plexus lies between the medial aspect of the gland and the coeliac and aorticorenal ganglia.

ü  It contains mostly preganglionic sympathetic fibres, which originate in the lower thoracic spinal segments and which reach the plexus via branches from the coeliac ganglion and plexus, and via the greater splanchnic nerve.

ü  These fibres synapse, often in deep invaginations, on large medullary chromaffin cells, which may therefore be considered to be homologous with postganglionic sympathetic neurons.

ü  A preponderance of non-myelinated axons has been described in the human suprarenal plexus.

ü  Both cortex and medulla also contain acetylcholinesterase ( AChE)-positive axons that presumably reach the gland from the coeliac plexus: some synapse with ganglion cells in the zona fasciculate and reticularis.

Microstructure

ü  The suprarenal gland has an outer cortex, which is yellowish in colour and forms the main mass, and a thin medulla, forming about one-tenth of the gland, which is dark red or greyish, depending on its content of blood.

ü  The medulla is completely enclosed by cortex, except at the hilum.

ü  The gland has a thick collagenous capsule from which trabeculae extend deep into the cortex.

ü  The capsule contains a rich arterial plexus which supplies branches to the gland.

ü  Suprarenal Cortex: consists of the zona glomerulosa, zona fasciculate, and zona reticularis. The outer subcapsular zona glomerulosa consists of a narrow region of small polyhedral cells in rounded clusters. Cortical cells produce several hormones and the cells of the zona fasciculate and reticularis are also rich in ascorbic acid. Cells in the zona glomerulosa produce mineralocorticoids, e.g. aldosterone, which regulates electrolyte and water balance. Cells in the zona fasciculate produce hormones maintaining carbohydrate balance (glucocorticoids) e.g. cortisol (hydrocortisone). Cells in the zona reticularis produce sex hormones (progesterone, oestrogens, and androgens)

ü  Suprarenal Medulla: composed of groups and columns of chromaffin cells (phaeochromocytes) separated by wide venous sinusoids and supported by a network of reticular fibres. Chromaffin cells, so-called from their colour reaction to dichromate fixatives, form part of the neuroendocrine system and are functionally equivalent to postganglionic sympathetic neurons. They are neural crest derivatives and synthesize, store (as granules), and release the catecholamines noradrenaline and adrenaline into the venous sinusoids. Release is under preganglionic sympathetic control, mediated by the sympathetic neurons that occur either singly or in small group in the medulla. The sinusoids are lined by fenestrated endothelium and drain to the central medullary vein and hilar suprarenal vein. Under normal circumstances, little adrenaline or noradrenaline is released; secretion is increased in response to fear, anger, or stress. Unlike the cortex, the suprarenal medulla is not essential to life.

Note: These notes are taken from Gazi University Faculy of Medicine Prof. Dr. Meltem BAHCELIOGLU's Anatomy Lectures.