Thyroid Gland PDF

Second year Endocrine system Thyroid Gland - It is endodermal in origin lies in the front of the neck & is formed of two lobes connected in the middle by isthmus. - Like any other gland, It is formed of stroma and...

Second year Endocrine system Thyroid Gland - It is endodermal in origin lies in the front of the neck & is formed of two lobes connected in the middle by isthmus. - Like any other gland, It is formed of stroma and parenchyma: (A) Stroma: 1. Capsule: two capsules (highly vascularized): a. Outer layer of dense CT which is continuous with peritracheal fascia. b. Inner layer of loose CT. adherent to the gland 2. Trabeculae (septa): very thin descend from the capsule extends into the mass of the gland dividing it into incomplete lobules. The connective tissue extends inward along with blood vessels, lymphatics, and nerves. 3. Reticular fibers: They form fine network around the thyroid follicles supporting the thyroid parenchyma. (B) Parenchyma: (thyroid follicles) - They are the structural unit of the thyroid gland formed of follicles. - they contain colloid with the follicles are lined with simple cuboidal epithelium resting on BM. - Each follicle is rounded structure filled with a gel-like mass called colloid stains dark pink on H&E. Has strong PAS + ve. 1 Second year Endocrine system - The colloid is used to store inactive thyroid hormones. Actually, the thyroid gland is the only endocrine gland that stores its inactive hormones extracellularly in follicles. - The follicles are surrounded by network of fenestrated blood capillaries. - Each follicle is lined by 2 types of cuboidal cells: principle follicular cells & parafollicular cells. 2 Second year Endocrine system 1. Principle Follicular Cells LM Shape: cuboidal cells. Nucleus: central round. Cytoplasm: basophilic. ✓ The shape of cells differs according the activity of the gland; columnar in hyperactive gland & squamous in inactive gland. E.M: (protein synthesizing cell) 1- apical part of cell present Well developed supranuclear Golgi complex Elongated mitochondria many lysosomes & endocytotic vesicles. Apical secretary vesicles containing thyroglobulin. Many microvilli & Tight junctions between adjacent cells 2- basal part of cell present Abundant basal RER & Free ribosomes. Function -> (Thyroid hormones) 1. Regulate the basal metabolic rate. 2. Increase the absorption of carbohydrates from the intestine 3. Regulate lipid metabolism. 2. Parafollicular Cells LM - They are larger than the follicular cells & present single or in small cluster that rest on the same basement membrane & do not reach the lumen. - They are more numerous in the central region of the thyroid lobes. - They are belonged to the diffuse neuroendocrine system (DNES) - Represent 0.1% of the follicular epithelium. EM Polypeptides secreting cells with moderate number of organelles contain: RER, mitochondria, Large infranuclear Golgi apparatus Numerous basal secretory granules contain hormone. Function They secrete calcitonin hormone which is an antiparathyroid hormone, decrease blood calcium levels & acts to inhibit bone resorption. NB. Interfollicular cells are tangentially cut of the follicular cells. 3 Second year Endocrine system Thyroid gland. C cells (also known as thyroid medullary cells) form small clusters interspersed between thyroid follicles. Thyroid gland. Immunohistochemistry for calcitonin. Thyroid C cells have strong labeling for calcitonin, indicating a high concentration of this protein. 4 Second year Endocrine system Synthesis, Storage & Secretion of Thyroid Gland: 1- Synthesis of thyroglobulin (glycoprotein): A- The protein part of the thyroglobulin is synthesized from the amino acids tyrosine by the rough endoplasmic reticulum. B- The Golgi apparatus adds carbohydrate to the protein to form the thyroglobulin & packed into secretory vesicles. C- The secretory vesicles are discharged into the lumen of the follicle by exocytosis where thyroglobulin contributes in the formation of the colloid. 2- Uptake of iodide: The follicular cells trap the iodide from the capillaries oxidation into iodine released into the follicular lumen. 3- Iodination of thyroglobulin: takes place in the colloid to form monoiodotyrosine& diiodotyrosine. 4- Formation of T3 & T4: by coupling reaction in the colloid, where they are stored in the lumen of the follicle. 5- Secretion of T3 & T4 hormones Upon stimulation of cell receptor (In response to TSH) colloid is taken again by endocytosis. Lysosomes hydrolyze the iodinated thyroglobulin into thyroxin (T3) & (T4). T3 & T4 are released from the basal surfaces of the follicular cell & enter the blood stream. Mechanism of synthesis and release of thyroid hormones 5 Second year Endocrine system NB. Hypofunction of thyroid in children gives a disease called cretinism & in adults leads to myxedema. Hyperfunction of thyroid (hyperthyroidism) causes thyrotoxicosis. Enlargement of the gland is a disease called goiter. Parathyroid Gland o They are 4 small glands embedded in the posterior surface of the thyroid gland. o Like any other gland... It is formed of -» (A) Stroma: 1. Capsule: thin CT capsule. 2. Trabeculae: thin septa that divide the gland into incomplete lobules 3. Reticular fibers: They form reticular network. (B) Parenchyma: The cells of the parathyroid gland are arranged in cords & clumps separated by blood vessels. There are 2 types of cells: chief & oxyphil cells 6 Second year Endocrine system 1. Chief Cell LM Shape: polygonal cells & they are the most numerous cells in the gland. Nucleus: vesicular nuclei. Cytoplasm: pale stained cytoplasm due to large amount of glycogen. With increasing age, they are replaced by adipocytes. EM Well-developed Golgi apparatus, RER, mitochondria & glycogen granules. Irregular shape electron dense secretory granules contain (PTH). Function They secrete the parathyroid hormone (PTH) which release in response to low blood calcium to increase blood calcium level by: 1. Increase bone resorption 2. Increases calcium absorption from intestine 3. Increases calcium reabsorption from distal convoluted NB. Unlike most other endocrine glands, no specific pituitary trophic hormone is involved in its control. Removal of parathyroid glands leads to tetany within 2-3 days & death if left untreated due to low blood calcium. 2. Oxyphil cells Appear at about the 7th year & increase in number with age. LM Shape: polygonal (They are larger than chief cells). Nucleus: small dark stain nucleus. Cytoplasm: acidophilic cytoplasm (due to numerous mitochondria). EM Numerous rounded mitochondria, Golgi complex & few RER. Function Not yet known (it may be stem cell for degenerated chief cells). 7 Second year Endocrine system 8 Second year Endocrine system Suprarenal Gland Site: above the superior poles of the kidneys; embedded in adipose tissue. Parts: the gland consists of 2 regions in fresh section: 1. Cortex - Thick, peripheral & yellow. - It constitutes 80- 90% of the gland. 2. Medulla: - Thin, central & reddish-brown. - It is considered as a modified sympathetic ganglion, where the neurons have acquired an endocrine activity and secrete catecholamines. These 2 regions can be considered as 2 different organs with different origin, function & histological characteristics. Emberyologically -> the cortex is mesodermal in origin. -> the medulla is ectodermal in origin. Like any other gland It is formed of -> (A) Stroma: 1. Capsule: thick capsule covers the gland. 2. Trabeculae: thin septa descend from capsule dividing the cortex & carry blood vessels & nerves. 3. Reticular Fibers: support the parenchymal cells of both cortex & medulla. (B) Parenchyma: The secretory cells are arranged in cortex & medulla. (A) Adrenal Cortex o The adrenal secretions are not stored in the secretory cells but only synthesized & released in need. o It includes 3 layers: Zona Glomerulosa, Zona Fasciculata & Zona Reticularis. 9  Second year Endocrine system Zona Glomerulosa Zona Fasciculata Zona Reticularis % 15% 80% 5% Arrangement In arch or glomeruli In cords or fascicles In network Columnar cells Polyhedral cells Polyhedral cells with basal oval dense nuclei with rounded nuclei with rounded nuclei LM acidophilic cytoplasm acidophilic cytoplasm acidophilic cytoplasm Dark cells: are seen Well developed SER. The same ------- + near the medulla. Mitochondria with tubular The cytoplasm has They have dark cristea. numerous lipid droplets cytoplasm with Few lipid droplets. & is highly vacuolated pyknotic nuclei, Golgi, RER & free ribosomes. like sponge due to suggesting that they EM Electron dense nuclei with dissolved lipid droplets are degenerating steroid prominent nucleoli. during histological cells. secreting cell preparation so they are Lipofuscin called: (Spongiocytes) pigments: are present in this layer. EM The same ------ few lipid droplets It secretes aldosterone It secretes It secretes sex (mineralocorticoids) that glucocorticoids that hormones mainly increase Na reabsorption regulate metabolism of androgen and small & increase K excretion fat, Protein & CHO amount of It is regulated by also it decreases glucocorticoids Function angiotensin II. inflammatory & Allergic development of reactions. axillary & pubic hair Some of these cells in females. secrete androgen hormone NB; all these layers contain many fenestrated bloody capillaries NB; Glucocorticoid secretion is under the control of ACTH hormone. 10 Second year Endocrine system (B) Adrenal Medulla It is formed of: 1- endocrine cells called chromaffin cells. 2- nerve cells called ganglionic cells. 1- Chromaffin Cells LM Shape: large polyhedral cells arranged in groups or cords enclosed by fenestrated blood capillaries. Nucleus: large vesicular. Cytoplasm: - pale basophilic - contains fine granules (They are called chromaffin cells due to the affinity of their granules to react with chromium salts (chromaffin reaction) and give brown color). They are innervated by preganglionic sympathetic nerve fibers. EM polypeptide secreting cells - Moderate amount of rER, prominent Golgi & many free ribosomes. 11 Second year Endocrine system - Abundant membrane limited electron dense secretory granules. Two types 2 (A) Adrenaline Cells: - which contain scattered less electron dense epinephrine granules. - They secrete epinephrine hormone (adrenaline hormone). (B) Noradrenaline Cells: - which contain very dark electron dense norepinephrine granules. - They secrete noradrenaline (norepinephrine) hormone. Function: the granules are rich in catecholamines (adrenaline & noradrenaline) hormones which acts as sympathomimetic. NB Adrenal medulla is considered as a modified sympathetic ganglia so it contains ganglion cells and chromaffin cells are modified postganglionic neurons 2- Ganglion cells - The ganglion cells which are branched cells with large vesicular nuclei, present in-between the chromaffin cells. Functions: - Modulate the secretory activity of the cortex. - Innervate the blood vessels of the cortex. NB. Paraganglia: - These are groups of chromaffin cells & nerve cells similar to those of the suprarenal medulla, but they are present beside the sympathetic ganglia. - These paraganglion cells and the cells of suprarenal medulla may give rise to tumors termed pheochromocytoma. Blood supply of Suprarenal gland -> Adrenal arteries (superior, middle & inferior) penetrate capsule at multiple sites & form Subcapsular plexus which give rise to 1. Capsular branches to supply capsule. 2. Long cortical branches supply medulla (medullary arteriole). 3. Short cortical branches supply the cortex (medullary arteriole). o Short cortical branches drain into the fenestrated medullary sinusoids. So both cortical branches (short & long) empty into medulla forming medullary capillary plexus. 12 Second year Endocrine system o Medulla receive dual blood supply; - arterial (long cortical branches) - venous from cortical capillary network (venous portal circulation), as glucocorticoids reaching the medulla are essential for production of epinephrine by chromaffin cells. In conclusion Cortical arteries and arterioles branch into capillary beds within the cortex to supply that area, then coalesce into veins at the corticomedullary junction that open at the medullary capillary plexus. Medullary arteries and arterioles penetrate the cortex without branching, then form capillary beds in the medulla. Blood from both cortical and medullary veins empties through a single large central vein, which leaves the adrenal and anastomoses with either the vena cava or renal vein. 13 Second year Endocrine system Pancreas o Pancreas is one of the largest glands of the digestive tube. It lies in C- shaped cavity of the duodenum. It is divided into head, body & tail. o It is a mixed gland: composed of both endocrine and exocrine parts. The endocrine pancreas (The Islets of Langerhans) o The islets of Langerhans are more numerous in the tail than in the head & the body of the pancreas. o They appear in histological sections as lightly stained areas. 14  Second year Endocrine system Histological structure of islets of Langerhans A- Stroma: o They are noncapsulated. o Supported by network of reticular fibers. B- Parenchyma: Structure of cells of Islets LM: - Arrangement of the cells: in short cords and separated by network of fenestrated blood capillaries. - Shape of the cells: polygonal. - Cytoplasm: pale basophilic. EM: Polypeptide secreting cells - Moderate amount of rER, Golgi complex & secretory granules. - 3 main types of cells can be identified: A cells, B cells& D cells. the cells of the islets can be differentiated by: 1- Histochemistry 2- Immunohistochemistry 3- Electron microscope (secretory granules) Islet’s of Langerhans showed α (blue arrows), β (yellow arrows), and blood Capillaries (red arrows). Gomori’s Aldehyde Trichrome, X400 15  Second year Endocrine system A cell (alpha) B cells (Beta) D cells (delta) % 20% 70% 5% large small size at the periphery of centrally located within Mainly at the site the islet. the islet. periphery (variable) Cytoplasm using acidophilic basophilic trichrome stain numerous and have their secretory granules Electron lucent a homogeneous have a rectangular homogenous EM (granules) dense core. crystalline dense core granules surrounded by an electron lucent region halo Function secretion of 1- Secretion of secretion of glucagon which insulin lower blood somatostatin increase blood glucose level. inhibits secretion of glucose level. 2- Secretion of C-peptide: GH, insulin & as a consequence of glucagon. insulin production. It prevents neuropathy and other symptoms of diabetic vascular deterioration. Measuring its level can give an idea about the viable beta cell mass. 3- Secretion of GABA (γ amino butyric acid): suppress glucagon secretion. 16 Second year Endocrine system Pineal Gland (Epiphysis cerebri) o It is pea-sized mass of nerve tissue attached by to the posterior wall of the 3rd ventricle of the brain, deep between the cerebral hemispheres at the back of the skull. o It is a neuro-endocrine gland which regulates the functions of the gonads. o Like any other gland... It is formed of (A) Stroma: 1. Capsule: The gland is enclosed by pia matter 2. Trabeculae: very thin. 3. Reticular fibers: They form fine network. (B) Parenchyma: 2 types of cells are forming the parenchyma of gland: 1. Pinealoeytes LM Shape: large branched cells arranged in cords or clumps with long tortuous processes end as flat dilatations near blood capillaries. Nucleus: spherical or lobulated nucleus. Cytoplasm: pale basophilic cytoplasm. EM Many mitochondria, Golgi complex, RER, SER & secretory vesicles. Function -> Secrete melatonin hormone: 1. Regulates normal development & rhythmic changes in the activity of gonads. 2. Inhibitory to gonadotropins (LH & FSH) so, its deficiency leads to precocious puberty. 3. Its Synthesis is inhibited by light & favored by darkness. 2. Interstitial cells o They are present among Pinealocytes. o They are similar to astrocytes neuroglial cells. o They have long cytoplasmic processes. 17 Second year Endocrine system NB. Brain sands (Corpora arenacea) are extracellular calcified concretions. They increase in size & number with advancing age. Their significance is unknown. 18  Second year Endocrine system Diffuse neuro endocrine system (APUD) o Definition: single or small groups of endocrine cells with diffuse distribution in different tissues and organs of the body. o These cells secrete hormones, usually peptides. o They synthesize monoamines by the uptake of amine precursors and their decarboxylation (APUD cells=Amine Precursor Uptake and Decarboxylation). Modes of action of their hormones 1- Paracrine: act on the neighboring cells. 2- Autocrine: act on the cell that secrete the hormone itself. 3- Endocrine: act on target cells in remote tissue. How to identify the DNES in histological sections 1- H&E: These cells are not well seen. 2- Special stain: - chromium salts (chromaffin cells). - silver salts (argentaffin cells). 3- Immunohistochemical stain: using antibodies against their peptide hormone. chromaffin cells argentaffin cells cells 19 Second year Endocrine system Structure of DNES EM: 1- Shape: pyramidal with narrow apex and a wide base. 2- Cytoplasm: polypeptide secreting cells with moderate number of organelles due to slow rate & small amount of secretion: - rER. - mitochondria. - Golgi apparatus. - Secretory granules. Types of DNES 1-Open type: the cell has long microvilli extending into the lumen of the organ. 2- Closed type: where the cell apex is covered by other epithelial cells of the organs. Distribution of DNES - Gastrointestinal tract (enteroendocrine cells). - Respiratory system. - Supraoptic and paraventricular hypothalamic nuclei. - Parafollicular cells of thyroid gland. - Chief cells of parathyroid gland. - Chromaffin cells of the suprarenal medulla. - Cells of islets of Langerhans. - Pinealocytes. 20