The Endocrine System WSO School of Biomedical Sciences, HKU

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1 The Endocrine System WSO School of Biomedical Sciences, HKU Objectives: 1. Be able to identify the endocrine glands and tissues. 2. Be able to describe their locations in the body and the functions of their secretions. 3. Be able to describe the histological features of the endocrine glands,

2 The Endocrine System: Endocrine Glands and endocrine tissues Endocrine glands are discrete glands without ducts, e.g. Pineal gland (1) at the brain in the roof the third ventricle Pituitary gland (2) - found in the sella turcica of the sphenoid bone Thyroid gland (3) - just below the pharynx Parathyroid glands (4) behind the thyroid gland Thymus (5) - posterior to the sternum Adrenal glands (6, suprarenal glands) on top of each kidney Endocrine tissues dispersed in other organs Islet of Langerhans (7) endocrine cells found in the pancreas Gonads (8a & 8b) Ovaries (8a) found in the pelvis granulosa cells, theca cells and corpus luteal cells Testes (8b) - found in the scrotum, Leydig and Sertoli cells.

3 Fenestrated capillary

4 Pituitary Gland and Hypothalamus Where nervous and endocrine systems interact Hypothalamus regulates secretions of anterior pituitary Posterior pituitary is an extension of the hypothalamus Anterior pituitary produces nine major hormones that Regulate body functions Regulate the secretions of other endocrine glands

5 Pituitary Gland Structure Posterior pituitary (neurohypophysis): extension of the nervous system via the infundibulum Secretes neurohormones Anterior pituitary (adenohypophysis) Consists of three areas with indistinct boundaries: pars distalis, pars intermedia, pars tuberalis

6 Hypothalamus, Anterior Pituitary, and Target Tissues

7 Hypothalamus, Posterior Pituitary and Target Tissues

8 Floor of cranial cavity

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10 Three groups of hormone production in the three sites of the hypothalamo-hypophyseal system 1&2. A group of hormones (1) are peptides produced by neurons of the dorsal medial, ventral medial and infundibular nuclei of hypothalamus. Hormones are carried along axons ending in median eminence where they are stored and secreted. These hormones enter the capillaries of the median eminence and transported to the adenohypophysis via the hypophyseal portal system. These hormones regulate the secretion of the anterior pituitary hormones (2).

11 Three groups of hormone production in the three sites of the hypothalamo-hypophyseal system 3. The third group of hormones consists of proteins and glycoproteins of the pars distalis. They are liberated into the secondary capillary plexus of the portal system and are distributed to the general circulation.

12 Adenohypophysis pars distalis Main components are cords of epithelial cells interspersed with capillaries Chromophobes do not stain intensely. They can be undifferentiated cells or chromophils with all secretory granules emptied. 2 types of chromophils: acidophils basophils

13 H&E stain

14 Periodic Acid Schiff, Orange G, Fast green Chromophobe Acidophils Basophil

15 Secretory cells of the pars distalis Hypothalamic Cell type Staining affinity Hormone produced Releasing hormones Inhibiting hormones Somatotrophic cells Acidophilic Somatotrophin (or Growth hormone, GH) SRH Somatostatin Mammotrophic cells Acidophilic Prolactin (PRL) PRH PIH Gonadotrophic cells Basophils Follicle stimulating hormone (FSH); GnRH Luteinizing hormone (LH) Thyrotrophic cells Basophilic Thyrotrophin (TSH) TRH Corticotrophic cells Basophilic Corticotrophin (ACTH ) CRH

16 Adenohypophysis pars tuberalis A funnel shaped region surrounding infundibulum Most cells of this region secrete gonadotrophins and are arranged in cords alongside with blood vessels.

17 Adenohypophysis pars intermedia

18 Neurohypophysis (or pars nervosa) Composed of unmyelinated neurons. Neurosecretions accumulate at the end of the neurons to form Herring bodies ( nm). Secrete two types of hormone: Vasopressin (or antidiuretic hormone) Oxytoxin Each hormone is joined to a specific binding protein, neurophysin.

19 Herring body A neurosecretory cell in the hypothalamohypophyseal tract. Large axon swellings, Herring bodies, contain neurosecretion. In the axon endings, the neurosecretion may be stored as membrane-bound granules.

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21 Herring body Herring body pituicytes Neurohypophysis

22 Superior thyroid artery and vein Carotid artery Middle thyroid vein Inferior thyroid veins Inferior thyroid artery Cricoid cartilage Pyramidal lobe Right and left lobes Isthmus of thyroid Thyrocervical trunk Rt. recurrent laryngeal nerve Left recurrent laryngeal nerve

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24 Vagus n. Superior thyroid artery Common carotid artery capsule Superior parathyroid Thyroid gland Inferior parathyroid Recurrent laryngeal n. Fibrous capsule of thyroid gland Inferior parathyroid (may be more caudally located) Inferior thyroid artery Recurrent laryngeal n. Thyrocervical trunk Subclavian artery

25 Histology of the thyroid gland Thyroid gland consist of vesicular follicles, which are lined with simple cuboidal to squamous epithelium The lumen filled with homogenous colloid, thyroglobulin. Light staining parafollicular cells or C cells lies between follicles or embedded within the follicle.

26 Parafollicular cells

27 Cellular processes of synthesis, iodination of thyroglobulin and its absorption and digestion

28 Formulas of MIT and DIT. The condensation of two molecules of DIT with the elimination of an alanine residue results in the formation thyroxine, T 4 (tetraiodothyronine). Condensation of one molecule of MIT and one molecule of DIT with the elimination of one alanine residue results in the formation of triiodothyronine (T 3 ).

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30 Histology of the parathyroid The parathyroids contain two kinds of epithelial cells: - Principal cells - the major source of parathyroid hormone (PTH) or parathormone. Main action of PTH is to increase blood Ca 2+, Mg 2+ ; decrease blood phosphate; promotes formation of calcitriol. - Oxyphil cells - eosinophilic cells; function unknown.

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33 Pancreas Located along small intestine and stomach; retroperitoneal Exocrine gland Produces pancreatic digestive juices Endocrine gland Consists of pancreatic islets Composed of Alpha cells; secrete glucagon Beta cells; secrete insulin Delta cells; secrete somatostatin

34 Islet of Langerhans beta cells

35 Adrenal Glands Near superior poles of kidneys; retroperitoneal Inner medulla; outer cortex Medulla: formed from neural crest; sympathetic. Secretes epinephrine and norepinephrine Cortex: three zones from superficial to deep Zona glomerulosa Zona fasciculata Zona reticularis

36 Right and left phrenic arteries Middle suprarenal artery Left phrenic veins Superior suprarenal artery Middle suprarenal vein Inferior suprarenal artery Inferior suprarenal artery Right renal artery and vein Superior mesentery artery

37 Hormones of Adrenal Cortex Mineralocorticoids: Zona glomerulosa Aldosterone produced in greatest amounts. Increases rate of sodium reabsorption by kidneys increasing sodium blood levels Glucocorticoids: Zona fasciculata Cortisol is major hormone. Increases fat and protein breakdown, increases glucose synthesis, decreases inflammatory response Androgens: Zona reticularis Weak androgens secreted then converted to testosterone by peripheral tissues. Stimulate pubic and axillary hair growth and sexual drive in females

38 The Adrenal Cortex A, B, C & D Medulla - D

39 Adrenal Medulla Secretory products are neurohormones: epinephrine and norepinephrine Combine with adrenergic membrane-bound receptors All function through G protein mechanisms Secretion of hormones prepares body for physical activity Effects are short-lived; hormones rapidly metabolized Epinephrine Increases blood levels of glucose Increases fat breakdown in adipose tissue Causes dilation of blood vessels in skeletal muscles and cardiac muscles. Epinephrine and norepinephrine increase heart rate and force of contraction; cause blood vessels to constrict in skin, kidneys, gastrointestinal tract, and other viscera

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41 Chromaffin reaction catecholamines react with potassium dichromate to produce a brown colour

42 Pineal Body In epithalamus; produces melatonin and arginine vasotocin

43 Sulcus of corpus callosum Corpus callosum Thalamus and 3rd ventricle Pineal body Optic chiasma Pituitary gland Cerebral aqueduct (Sylvius)

44 Pineal Gland The hormone secreting cells - pinealocytes A, artery BS, brain sand C, capillary CT, connective tissue G, glial cell F, fibroblast V, vein

45 Reference: Junqueira LC and Carneiro J Basic Histology, Text & Atlas. 10 th edition, Lange Medical Books McGraw-Hill, pp