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