Anatomy-Histology Correlates in Endocrinology PDF

Anatomy-Histology correlates in Endocrinology Nikolai P Pace Department of Anatomy  Objectives Pituitary (hypophysis) – Anterior pituitary – Posterior pituitary Adrenal gland (suprarenal) – Adrenal...

Anatomy-Histology correlates in Endocrinology Nikolai P Pace Department of Anatomy  Objectives Pituitary (hypophysis) – Anterior pituitary – Posterior pituitary Adrenal gland (suprarenal) – Adrenal cortex – Adrenal medulla Thyroid gland – Follicles – Parafollicular cells Parathyroid gland Considered in other lectures: – Endocrine pancreas – Male – Female  Coordination of Body Functions Nervous stimuli – Fast , rapid acting – Conducted along axons of neurons to release chemical mediators called neurotransmitters at synapses – Effects often shorter lived Endocrine – mainly metabolism, growth, differentiation, reproduction – Mediator molecules called hormones – Delivered throughout body via the circulation  Hormone production: Less traditional sources Endothelium: Endothelins NO Cardiocytes: Prostanoids,... ANP Immune system: Cytokines Platelets, mesenchyme: Growth factors Kidney: Erythropoietin RAAS Placenta: All hormones GIT: Gastrin Cholecystokinin Adipocytes: Secretin Leptin Resistin Adiponectin  Chemical characteristics of hormones Amines (from tyrosine) hydroxylation - catecholamines iodination - thyroid hormones Peptides/proteins Steroids (from cholesterol) adrenocorticoids sex hormones active metabolites of vitamin D  Amino acid and derivatives Cathecholamines: derived from phenylalanine and tyrosine Thyroid hormones (thyroxine and triiodothyronine): derived from tyrosine Gonadotropin-releasing hormone (GnRH; decapeptide)  Hormone release Proteins & catecholamines: – secretory granules, exocytosis for incorporation into granules often special sequences cleaved off in granules or after release stimulus →  [Ca2+]i (influx, reticulum) → granules travel along microtubules towards cell membrane (kinesins, myosins) → fusion  Hormone release Thyroid hormones: – made as part of thyroglobulin – stored in folicles – T3 & T4 secreted by enzymatic cleavage Steroid hormones: leave the cell across cell membrane right after synthesis (no storage)  Regulation of hormone release Feedback – Negative  hormone Gland Target tissue inhibition product  Regulation of hormone release Feedback – Negative – Positive (only narrow dose range)  hormone Gland Target tissue product  Regulation of hormone release Feedback – Negative – Positive (only narrow dose range) Nerve regulation – pain, emotions,, injury, stress,... – e.g.  oxytocin with nipple stimulation  Combined feedback Stress etc. CRH secretion in hypothalamus stimulation ACTH secretion in pituitary  plasma ACTH inhibition cortisol secretion in adrenals  plasma cortisol  Regulation of hormone release Rhythms – circadian  Regulation of hormone release Rhythms – circadian light/dark fine/tune endogenous rhythm of cells & suprachiasmatic nucleus of hypothalamus melatonin, cortisol – monthly – seasonal (day length; atavistic) – developmental (puberty, menopause) Pulsations/oscillations gonadotropins  Pulsatility in GnRH & LH release 12:00 14:00 Time of day 16:00  Classes of hormones The hormones fall into two general classes based on their solubility in water. The water soluble hormones are the catecholamines (epinephrine and norepinephrine) and peptide/protein hormones. The lipid soluble hormones include thyroid hormone, steroid hormones and Vitamin D3  Types of receptors Receptors for the water soluble hormones are found on the surface of the target cell, on the plasma membrane. These types of receptors are coupled to various second messenger systems which mediate the action of the hormone in the target cell. Receptors for the lipid soluble hormones reside in the nucleus (and sometimes the cytoplasm) of the target cell. Because these hormones can diffuse through the lipid bilayer of the plasma membrane, their receptors are located on the interior of the target cell  Hormones and their receptors Hormone Class of hormone Location Amine (epinephrine) Water-soluble Cell surface Amine (thyroid Lipid soluble Intracellular hormone) Peptide/protein Water soluble Cell surface Steroids and Vitamin D Lipid Soluble Intracellular  Pituitary I The pituitary is located in a bony fossa in the floor of the cranial cavity – the sella turcica in the sphenoid Connected by a stalk (infunidbulum) to the base of the brain and by a vascular network to the hypothalamus The posterior hypophysis – neurohypophysis – develops from neural ectoderm of the floor of third venticle (infundibulum) The larger anterior hypophysis – adenohypophysis – develops from oral ectoderm as a diverticulum of the buccal epithelium called Rathke’s pouch The hypothalamic-pituitary axis is the master regulator of neuro-endocrine systems Pituitary Development  Anterior Pituitary Anterior lobe of pituitary has 3 parts Derived from Rathke’s pouch  Posterior Pituitary Two parts develop from the embryonic infundibulum – Pars nervosa Neurosecretory nerve endings – Infundibulum Contains neurosecretory axons Form the hypothalamhypo physeal portal tracts  Blood Supply 1.Superior hypophyseal Supply pars tuberalis and infundibulum Branch of ICA & CoW 2. Inferior hypophyseal Supply pars nervosa 3.Hypothalamo-hypophyseal portal tract Branches of SHA form primary capillary plexus Empties into portal veins Form a second fenestrated capillary plexus  Pituitary II The adenohypophysis is composed of clumps and cords of epithelioid cells, separated by large diameter fenestrated capillaries. The neurohypophyisis is a nerve tract that contains neurosecretory bundles whose neurons secrete hormones synthesized by the SON and PVN – oxytocin or ADH. The hypothalamo-hypophyseal portal tract links the hypothalamus to the anterior pituitary – It carries releasing hormones from the hypothalamus to the cells of the anterior pituitary – The adenohypophysis has no direct arterial supply – Fenestrated capillaries that supply the pars tuberalis and median eminence form hypophyseal portal veins – these run along the pars tuberalis to form a second network of fenestrated capillaries that supplies the pars distalis  Pituitary Histology Pars Distalis Pars Intermedia Pars Nervosa  Anterior Pituitary Cells arranged as cords Separated by large-diameter sinusoidal capillaries Cells respond to hypothalmic hormones Four tropic hormones – FSH – LH – ACTH – TSH Two other hormones – GH – Prolactin All glycoproteins of small size  Pituitary Histology The parenchyma of the pars distalis consists of chromophobes and chromophil cells Chromophils are further subdivided into acidophils and basophils according to staining patterns – Basophils stain with basic dyes, hematoxylin – Acidophils cytosol stains with acidic dyes and eosin 5 functional cell types – Somatotropes – produce GH (somatotropin) – acidophil cells – Lactotropes – produce prolactin – acidophils – Corticotropes – produce ACTH - basophils – Gonadotropes – produce FSH and LH – basophils – Thyrotropes – produce TSH - basophils LM Cell type Hormone Releasing (+) or staining inhibiting (-) horm. Acidophil Somatotrope Growth hormone (GH) GHRH (+) = somatotropin Somatostatin (-) Acidophil Mammotrope Prolactin (PRL) [Dopamine (-) = lactotrope estrogen (+)] Basophil Thyrotrope Thyroid stimulating TRH (+) hormone (TSH) = thyrotropin Basophil Gonadotrope Luteinizing hormone GnRH (+) (LH), follicle stimulating hormone (FSH); both = gonadotropin Basophil Corticotrope Adrenocorticotropin CRH (+) (human) (ACTH) = corticotropin  Pituitary Histology These cell types are arranged as cords with intervening capillaries Adenohypophysis-derived hormones are small glycoproteins NOTE – GH is controlled by GHRH (+) and Somatostatin (-) from hypothalamus and gherilin (+) from GIT – Prolactin secretion is inhibited by dopamine from hypothalamus – ACTH is released in response to CRH from hypothalamus – ACTH precursor is POMC, this is cleaved to form ACTH, MSH, endorphin and enkephalin – FSH and LH release are controlled by GnRH – TSH release is controlled by TRH Chromophobe cells stain only weakly These are resting/degranulated chromophils  Acidophils Basophils Capillaries Immunocytochemical localization of growth hormone, LM A.K. Christensen  Immunocytochemical localization of luteinizing hormone in gonadotropes, fluorescence Nucleus Nucleus LH granules A.K. Christensen Neurohypophysis histology Cells called pituicytes and unmyelinated nerve fibers. Pituicytes are analogous to neuroglia ADH and oxytocin are formed in hypothalamic SON and PVN and pass into expanded nerve terminal in the neurohypophyisis, where they are stored Stored neurosecretory material appears as Herring bodies in H&E section  Neurohypophysis Pars nervosa and a stalk (infundibulum) Unmyelinated axons with cell bodies in SON and PVN Axons form the hypothalamohypophyseal tract – Do not terminate on other neurons but next to capillaries – Contain secretory vesicles in all parts of cell  Oxytoxin and ADH 9-aa peptides differing in 2 residues Synthesized as part of a large molecule and cleaved  Thyroid Gland I Located in the anterior neck region, adjacent to the larynx and trachea 2 lateral lobes connected by an isthmus that crosses anterior to the trachea Surrounded by a capsule that sends trabeculae into the parenchyma of the gland In 40%, a pyramidal lobe extends upwards from the isthums Composed of follicles that contain colloid in their lumens The thyroid begins to develop in the fourth week of gestation from a primordium that originates from the endoderm of the floor of the primitive pharynx. This forms the thyroglossal duct that migrates caudally through the tissues of the neck to reach its final destination in front of the trachea The thyroglossal duct atrophies, but leaves the pyramidal lobe – an embryological remnant - in some.  Thyroid Gland II Thyroid follicles are roughly spherical cyst-like structures lined by simple cuboidal follicular epithelium Follicles are richly vascularized Contain colloid – composed of thyroglobulin that that is an inactive storage form of T4 and T3 Follicular epithelium contains two cell types – follicular cells that produce T4 and T3 – parafollicular cells that secrete calcitonin, these are indistinguishable from follicular cells but they do not reach the colloid Calcitonin is physiologically antagonistic to PTH, it lowers serum Ca by suppressing osteoclastic action and promoting bone deposition. Although it is used therapeutically, no clinical disease is associated with its deficiency or complete absence PTH is the primary regulator of calcium metabolism Thyroid Histology Thyroid follicles vary in size and shape and are closely packed Colloid at the center of each follicle Cytoplasm is slightly basophilic and cells have spherical nuclei with prominent nucleoli Ultra structurally, abundant lysosomes and endocytic vesicles are present at the apical cell membrane – these are colloidal resorption vesicles  Parathyroid glands I Small endocrine glands closely associated with the thyroid Ovoid , a few millimeters in diameter, and loosely arranged as two pairs that form the superior and inferior parathyroid glands Usually located in the connective tissue on the posterior surface of the lateral lobes of the thyroid gland The number and location of parathyroid glands varies extensively, sometimes extend to the thymus and mediastinum Structurally each parathyroid is surrounded by a capsule that separates it from the thyroid. Septa from the capsule extend into the gland parenchyma Supplied by the inferior thyroid arteries Develop from endodermal cells derived from the third and fourth branchial pouches  Parathyroid glands II Principal (chief) cells and oxyphil cells constitute the epithelial cells of the parathyroid Principal cells are – More numerous – Produce and secrete PTH – Small polygonal cells with pale staining , slightly acidophilic cytoplasm Oxyphil cells – Constiture a minor portion of the parencyma – Have no secretory role – Larger cells with more distinct acidophilic cytoplasm Adipose cells are present in limited numbers Parathyroid glands Chief cells Oxyphil cells are larger with smaller nuclei and are arranged as clusters  Parathyroid glands III PTH regulates Ca and P levels, it is a peptide hormone that acts via a GPCR PTH release is regulated by serum Ca through a simple negative feedback It acts to – Stimulate osteoclast bone resorption that releases Ca from bone matrix – It stimulates tubular reabsorption of Ca, thereby reducing urinary Ca excretion – It increases urinary P excretion – It regulates conversion of 25-OH D3 to 1,25- (OH)2 D3 by stimulating 1alpha hydroxylase – It increases intestinal absorption of Ca  Adrenal Glands I 2 adrenal glands at the upper pole of each kidney, embedded in peri-renal fat Each composed of structurally and functionally distinct outer cortex and inner medulla The adrenal cortex – produces and secretes steroids – it is sub capsular and constitutes the bulk of gland mass – Develops from mesodermal mesenchyme – Regulated in paryt by pituitary The adrenal medulla – secretes catecholamines – Originates from neural crest cells  Blood Supply Superior, middle and inferior supra renal vessels Branch before entering capsule Three principal systems 1. Capsular vessels 2. Cortical sinusoids, fenestrated 3. Medullary arteriorles that form medullary capillary sinusoids  Adrenal Glands II The adrenal cortex is divided into 3 zones – Zona glomerulosa 15 % superficial – Zona fasciculata 80% middle – Zona reticularis 5% central Zona glomerulosa – Closely packed cells that stain densely – Secrete mineralocorticoid aldosterone that functions in BP control – Aldosterone acts on the distal tubules of the nephron to stimulate Na absorption and K excretion – Under control of the RAAS  Adrenal Glands III Zona Fasciculata – Large polyhedral cells arranged in long straight cords – Separated by sinusoidal capillaries – Secretes glucocorticoids that increase the metabolic availability of glucose and fatty acid for metabolism – Under the control of CRH-ACTH – Cells characterized by lipid droplets (appear vacuolated) – Regulated by ACTH Zona reticularis – Small cells that secrete glucocorticoids and androgens like androstenedione and DHEA – Darker staining than ZF Adrenal Gland Histology Adipose tissue Capsule Cortex Medulla  Adrenal Gland Histology Capsule ZG ZF ZR  Adrenal Gland Histology Zona fasciculata cells are radially arranged and contain intracellular lipid droplets Cells of the zona reticularis are relatively smaller and stain prominently with eosin and contain no lipid droplets  Adrenal Medulla Cells develop from postganglionic cells of sympathetic nervous system They are directly innervated by preganglionic cells and can therefore be regarded as modified secretory postganglionic neurons that lack axonal processes Parenchyma of adrenal medulla is composed of large pale-staining epithelioid cells known as chromaffin cells These are in close proximity to many sinusoidal blood vessels Ultra structurally the chromaffin cells have numerous secretory vesicles that contain catecholamines Exocytosis of secreteory vesicles from chromaffin cells is triggered by release of Ach from presynaptic sympathetic neurons that synapse with each chromaffin cell Adrenal Medulla Histology  Endocrine Pancreas Acinar tissue, adult human pancreas (H&E). Acinar cells stain blue at their base because of the high content of RNA and the presence of nuclei. They are pink at their apex (lumenal aspect) where there is a high content of zymogen proteins (digestive enzymes). The nuclei of centroacinar cells are sometimes seen within an acinus (arrows).  Endocrine Pancreas Islet cells store each hormone in distinct locations (Immunoperoxidase). Serial sections of an islet have been immunostained using antibodies to insulin (image left), glucagon (center) and somatostatin (image right). The presence of the hormones is indicated by the brown stain. The predominance of insulin secreting β-cells is obvious. In the center and image right photos, the location of α-cells and δ-cells is primarily at the border of groups of β-cells.  Second messenger systems Receptors for the water soluble hormones are found on the surface of the target cell, on the plasma membrane. These types of receptors are coupled to various second messenger systems which mediate the action of the hormone in the target cell Second messenger systems include: Adenylate cyclase which catalyzes the conversion of ATP to cyclic AMP; Guanylate cyclase which catalyzes the conversion of GMP to cyclic GMP (cyclic AMP and cyclic GMP are known collectively as cyclic nucleotides); Calcium and calmodulin; phospholipase C which catalyzes phosphoinositide turnover producing inositol phosphates and diacyl glycerol. Types of receptors  Second messenger systems Each of these second messenger systems activates a specific protein kinase enzyme. These include cyclic nucleotide-dependent protein kinases Calcium/calmodulin-dependent protein kinase, and protein kinase C which depends on diacyl glycerol binding for activation. Protein kinase C activity is further increased by calcium which is released by the action of inositol phosphates.  Second messenger systems The generation of second messengers and activation of specific protein kinases results in changes in the activity of the target cell which characterizes the response that the hormone evokes. Changes evoked by the actions of second messengers are usually rapid G-Protein Coupled Receptors Signal Amplification by 2nd messengers  Transmembrane kinase-linked receptors Certain receptors have intrinsic kinase activity. These include receptors for growth factors, insulin etc. Receptors for growth factors usually have intrinsic tyrosine kinase activity Other tyrosine-kinase associated receptor, such as those for Growth Hormone, Prolactin and the cytokines, do not have intrinsic kinase activity, but activate soluble, intracellular kinases such as the Jak kinases. In addition, a newly described class of receptors have intrinsic serine/threonine kinase activity—this class includes receptors for inhibin, activin, TGFb, and Mullerian Inhibitory Factor (MIF). Protein tyrosine kinase receptors  Receptors for lipid-soluble hormones reside within the cell Because these hormones can diffuse through the lipid bilayer of the plasma membrane, their receptors are located on the interior of the target cell. The lipid soluble hormone diffuses into the cell and binds to the receptor which undergoes a conformational change. The receptor-hormone complex is then binds to specific DNA sequences called response elements. These DNA sequences are in the regulatory regions of genes.  Receptors for lipid-soluble hormones reside within the cell The receptor-hormone complex binds to the regulatory region of the gene and changes the expression of that gene. In most cases binding of receptor-hormone complex to the gene stimulating the transcription of messenger RNA. The messenger RNA travels to the cytoplasm where it is translated into protein. The translated proteins that are produced participate in the response that is evoked by the hormone in the target cell Responses evoked by lipid soluble hormones are usually SLOW, requiring transcription/translation to evoke physiological responses. Mechanism of lipid soluble hormone action  Mechanisms of endocrine disease Endocrine disorders result from hormone deficiency, hormone excess or hormone resistance Almost without exception, hormone deficiency causes disease – One notable exception is calcitonin deficiency  Mechanisms of endocrine disease Deficiency usually is due to destructive process occurring at gland in which hormone is produced—infection, infarction, physical compression by tumor growth, autoimmune attack Type I Diabetes  Mechanisms of endocrine disease Deficiency can also arise from genetic defects in hormone production—gene deletion or mutation, failure to cleave precursor, specific enzymatic defect (steroid or thyroid hormones) Congenital Adrenal Hyperplasia  Mechanisms of endocrine disease Hormone excess usually results in disease Hormone may be overproduced by gland that normally secretes it, or by a tissue that is not an endocrine organ. Endocrine gland tumors produce hormone in an unregulated manner. Cushing’s Syndrome Mechanisms of endocrine disease Exogenous ingestion of hormone is the cause of hormone excess—for example, glucocorticoid excess or anabolic steroid abuse  Mechanisms of endocrine disease Activating mutations of cell surface receptors cause aberrant stimulation of hormone production by endocrine gland. – McCune-Albright syndrome usually caused by mosaicism for a mutation in a gene called GNAS1 (Guanine Nucleotide binding protein, Alpha Stimulating activity polypeptide 1). – The activating mutations render the GNAS1 gene functionally constitutive, turning the gene irreversibly on, so it is constantly active. This occurs in a mosaic pattern, in some tissues and not others.  Mechanisms of endocrine disease Malignant transformation of non-endocrine tissue causes dedifferentiation and ectopic production of hormones Anti-receptor antibodies stimulate receptor instead of block it, as in the case of the common form of hyperthyrodism. Grave’s Disease  Mechanisms of endocrine disease Alterations in receptor number and function result in endocrine disorders Most commonly, an aberrant increase in the level of a specific hormone will cause a decrease in available receptors Type II diabetes