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The Endocrine System
An Overview of the Endocrine System
Fig 19.1
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Introduction
• Maintenance of homeostasis involves coordinating activities of organs and systems throughout the body
• The nervous system and endocrine system work together to monitor and adjust physiologic activities
• In general, the nervous system performs short-term very specific responses to environmental stimuli
- chemical messengers are neurotransmitters (NTs)
• The endocrine system regulates longer-term, ongoing metabolic processes throughout the body
- chemical messengers are hormones (‘to excite’)
Endocrine System Overview
• Includes all of the endocrine cells and tissues of the body
• Endocrine cells are glandular secretory cells that release hormones directly into the interstitial fluids, lymphoid system, or blood
• Hormones alter the metabolic activities of many different tissues and organs
• Organized into 4 groups based on chemical structure
- amino acid derivatives; peptide hormones; steroid hormones; and eicosanoids
Hormones• Amino acid derivatives – small molecules similar to
amino acids - derivatives of tyrosines such as the thyroid hormones and the catecholamines
• Peptide hormones, chains of amino acids and are the largest group- all pituitary gland hormones
• Steroid hormones derived from cholesterol- reproductive organs and suprarenal glands
• Eicosanoids, small molecules with a 5-carbon ring at one end; and are released by most body cells- coordinate cellular activities & affect enzymatic activity in ECF
Endocrine Stimuli
• Enzymes control all cellular activity- hormones influence cellular operations by changing the types, activities, or quantities of key enzymes
- a hormone can regulate the metabolic operations of its target cells
• Endocrine activity is controlled by endocrine reflexes triggered by:1) Humoral stiumli – changes in the composition of the ECF
2) Hormonal stimuli – arrival or removal of a specific hormone
3) Neural stimuli – arrival of NTs at neuroglandular junctions
Endocrine System
‘Pure’ endocrine glands: pituitary, pineal, thyroid, parathyroid, and adrenal
Organs containing endocrine cells: pancreas, thymus, gonads, and hypothalamus
Feedback Loops
• Endocrine reflexes are regulated by some form of feedback
• In direct negative feedback control1) endocrine cell responds to a disturbance in homeostasis by releasing its hormone into the circulatory system
2) released hormone stimulates a target cell
3) the target cell response restores homeostasis
• Example: circulating calcium levels decline- PTH is released and responses of target cells (osteoclasts) elevate blood calcium levels
Feedback Loops
• More complex endocrine reflexes involve one or more intermediary steps and 2 or more hormones- controlled by complex negative feedback loops
- or rarely, by positive feedback loops
• Complex negative feedback loops - most common- secretion of 1 hormone triggers secretion of a 2nd hormone
• Positive feedback loops are rare- oxytocin causes smooth muscle contractions in the uterus, uterine contractions further stimulate oxytocine release
The Hypothalamus and Endocrine Regulation
• Coordinating centers in the hypothalamus (master gland) regulate the NS and endocrine system activities by 3 different mechanisms1) by secreting regulatory hormones that control endocrine cells in the adenohypophysis (anterior lobe) of the pituitary gland:
Releasing hormones (RH) stimulate production of one or more hormones
Inhibiting hormones (IH) prevent the synthesis and secretion of specific pituitary hormones
The Master Gland
2) The hypothalamus acts as an endocrine organ, releasing the hormones ADH and oxytocin into the circulation at the neurohypophysis (posterior lobe)
3) The hypothalamus contains autonomic centers that have direct neural control over the endocrine cells of the suprarenal medullae
- when the sympathetic division is activated, the suprarenal medullae release hormones into the bloodstream
Hypothalamic Control over Endocrine Organs
The Pituitary Gland
• Or hypophysis lies in the sella turcica- resembles a golf club with a stalk or infundibulum that extends from the hypothalamus and the head of the club as the gland
• 2 lobes: the adenohypophysis (anterior lobe) and the neurohypophysis (posterior lobe)- 9 important peptide hormones are released by the pituitary
The Pituitary Gland
• Neurohypophysis
ADH and Oxytocin• Adenohyposphysis
ACTH – adrenocorticotropic hormone
TSH – thyroid-stimulating hormone
GH – growth hormone
PRL – prolactin
FSH – follicle-stimulating hormone
LH – luteinizing hormone
The Pituitary Gland
Gross Anatomy -Histological Organization of the Pituitary Gland and Its Subdivisions
Pituitary Hormones and Their Targets
Hypophyseal Portal System
• Adenohypophysis hormones under control of the hypothalamus - secretes specific regulatory factors:- hypothalamic neurons release regulatory factors into the surrounding interstitial fluids through fenestrated (‘window’) capillaries
- primary capillary plexus receives blood from the superior hypophyseal artery
- before leaving the hypothalamus, the capillary network unites to form a series of larger vessels, that form a secondary capillary plexus, which branches among the endocrine cells
- BVs that link the 2 capillary networks are called portal vessels
The Pituitary Gland and the Hypophyseal Portal System
Low magnification – the entire Pituitary Gland
Posterior Pituitary Pars Intermedia Anterior Pituitary
Pars TuberalisPituitary Stalk
High mag of the Anterior Pituitary – note eosinophilic and basophilic cells
Growth Hormone ACTH
Immunohistochemictry of the Anterior Pituitary
TEM of the Anterior Pituitary – note different granule morphology
C – corticotrophs (ACTH); G – gonadotrophs (FSH,LH) S – somatotrophs (growth hormone); T Thyrotrophs (TSH) ; F – fenestrated capillary
High mag of the Posterior Pituitary – neurons, capillaries, and pituicytes
High mag of the Posterior Pituitary – note Herring Bodies (arrow)
TEM of the Posterior Pituitary UN -unmyelinated neuron; asterisks – Herring Body
The Thyroid Gland
• Sits on thyroid cartilage of the larynx
- butterfly-like appearance
• Consists of 2 main lobes connected by the isthmus
• Controls metabolism- Thyroxine (T4) and Triiodothyronine (T3)
• Involved in calcium homeostasis- calcitonin
Anatomy and Histological Organization of the Thyroid Gland
The Thyroid Gland
• Largest pure endocrine gland
• Internally, composed of hollow follicles- separated by areolar CT rich in capillaries- walls are formed of cuboidal or squamous epithelial cells (follicular cells)- lying within the epithelium are parafollicular (C) cells- central lumen filled with colloid (‘gluelike’) consisting of thyroglobulin (protein precursor to thyroid hormone)
• Amino-based TH and protein based Calcitonin
Anatomy and Histological Organization of the Thyroid Gland
Anatomy and Histological Organization of the Thyroid Gland
The Regulation of Thyroid Secretion
Negative Feedback Loop
TRH: thyrotropin-releasing hormone
TSH: thyroid-stimulating hormone
The Parathyroid Glands
• Lie on the posterior surface of the thyroid gland surrounded by CT capsules (number varies)
• Contains thick branching cords composed of 2 types of endocrine cells- small abundant parathyroid or chief cells – glandular cells that produce PTH- Oxyphil cells and transitional cells – likely immature of inactive principal cells
• Regulates calcium homeostasis- PTH increases calcium levels and is essential to life:1) stimulates osteoclasts to release calcium from bones2) decreases secretion of calcium by the kidney3) activates vit D, which stimulates uptake of Ca by the intestine
Anatomy and Histological Organization of the Parathyroid Glands
High mag of thyroid follicles – note Parafollicular or C-Cells (arrows)
Low Magnification of the Parathyroid Gland
Parathyroid Gland – note small dark staining chief cells and larger, eosinophilic oxyphil cells
The Thymus
• Located on top of the heart
• Secretes many chemicals that help T cells of the immune system develop (thymosins)
• Atrophies as one ages
The Endocrine System – The Thymus
• Paired pyramidal organs on the superior surface of the kidneys – highly vascularized
• 3 groups of 60 small suprarenal arteries supply each gland - the superior suprarenal arteries from the inferior phrenic artery- middle suprarenal arteries from the aorta; - inferior suprarenal arteries from the renal artery
• Veins- left suprarenal vein drains into the renal vein and the right suprarenal vein drains into the inferior vena cava
The Suprarenal (Adrenal) Glands
The Suprarenal Glands
• Divided into 2 regions:- Suprarenal cortexThe Zona Glomerulosa – mineralocorticoidsThe Zona Fasciculata – glucocorticoidsThe Zona Reticularis – androgens- Suprarenal medullaChromaffin cells produce epinephrine and norephinephrine- modified ganglionic sympathetic neurons- secrete catecholamines: the amine hormones epinephrine and norepinephrine- active in the ‘fight, flight, and fright’ (fight or flight) response- hormones stored in secretory vesicles
Anatomy and Histological Organization of the Suprarenal Gland
Low magnification of the Adrenal Gland
Medulla
Cortex
Cortex
Adrenal Cortex
Zona Glomerulosa
Zona Fasciculata
Zona Reticularis
Adrenal Cortex
Zona Glomerulosa
(clumps, cords, and follicle like structures
Zona Fasciculata
(cords of spongiocytes)
Adrenal Gland
Zona Fasciculata
cords of spongiocytes
Zona Reticularis
darker staining cells
Adrenal Medulla
The Pancreas
• Located under the stomach
• Contains endocrine and exocrine cells
• Exocrine acinar cells, form most of the gland- secrete digestive enzymes into the small intestine
• Endocrine cells are contained in spherical bodies- pancreatic islets or islets of Langerhans- about 1 million scattered among the exocrine cells
The Pancreas
• Each islet contains 4 major cells• Alpha cells – glucagon• Beta cells – insulin• Delta cells – somatostatin (growth-hormone
inhibiting hormone)• F cells – pancreatic polypeptide (PP)• Other digestive hormones discussed in digestive
chapter
Anatomy and Histological Organization of the Pancreas
Endocrine Tissue of the Reproductive System
• Testes:- Interstitial cells produce androgens (testosterone)
promotes production of functional sperm, maintains secretory glands, influences 2nd sexual characteristics, and stimulates muscle growth
- Nurse cells (or sustentabular cells) secrete inhibin
• Ovaries- Follicular cells produce estrogens and secrete inhibin
- Corpus luteum releases progestins and relaxin
Low magnification of the Pancreas with Islets of Langerhans (arrows)
High mag of an Islet – note Beta cells and more eosinophilic Alpha2 cells
Acini
Alpha Cells
Glucagon Insulin
Immunohistochemistry of a pancrearic Islet of Langerhans
The Pineal Gland
• Small pine-cone shaped gland (or epiphysis cerebri) - part of the epithalamus
• Contains neurons, glial cells, and special secretory cells called pinealocytes- synthesize the hormone, melatonin
- melatonin slows the maturation of sperm, oocytes, and reproductive organs
- production rates rises at night and declines during the day
Pineal Gland
N – neuroglia
P –pinealocytes
S – Brain Sand
The Endocrine System – Pineal Gland
Hormones and Aging
• Exhibits relatively few changes with advancing age
• One can expect- changes in reproduction hormone levels at puberty
- decline in the concentration of reproductive hormones at menopause in women
Clinical Note - Endocrine Abnormalities
Clinical Note - Joan’s MRI
Auto-immune disease: Hashimoto thyroiditis, slow destructuion of thyroid cells results in inadquate TH synthesis and release