Endocrine System

Objectives

At the end of this lecture, student will be able to

• Compare control of body functions by the nervous system and endocrine system

• Distinguish between exocrine and endocrine glands

• Describe various endocrine glands

• Explain water soluble and lipid soluble hormones

• Describe the anatomy and histology of thyroid glands

• Explain the steps involved in the synthesis, storage and release of thyroid hormones

• Describe the anatomy, histology and function of parathyroid glands

• List the different hormones secreted from adrenal glands

• Describe the function of adrenal glands

• Describe the anatomy of pancreas

• Explain the role of hormones secreted by pancreas in body functions

• Describe the anatomy and physiological functions of gonads

• Describe the anatomy and physiological functions of pineal gland

• Describe the disorders associated with various endocrine glands

Content

• Nervous system Vs Endocrine system

• Autocrine and endocrine glands

• Hormonal receptors

• Hormones transport in blood

• Anatomy and Histology of

– Thyroid glands

– Parathyroid glands

– Adrenal glands

• Physiology of

– Thyroid hormones

– Parathyroid hormones

– Hormones of adrenal glands

•     Anatomy and Physiology of

- Pancreas

- Gonads

• Role of insulin

• Anatomy and Physiology of

– Pineal gland

• Endocrine disorders

Nervous system Vs Endocrine system

Endocrine System

• The nervous and endocrine systems act together to coordinate functions of all body systems

Exocrine glands

• Secrete their products into ducts

• Ducts carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body

• Sudoriferous (sweat), sebaceous (oil), mucous, and digestive glands

Endocrine glands

• Secrete their products (hormones) into the interstitial fluid surrounding the secretory cells

• From the interstitial fluid, hormones diffuse into blood capillaries

• Blood carries them to target cells throughout the body

• The endocrine glands include:

– Pituitary

– Thyroid

– Parathyroid

– Adrenal

– Pineal glands

• Organs and tissues containing cells that secrete hormones:

– Hypothalamus

– Thymus

– Heart

– Pancreas, ovaries, testes

– Kidneys, stomach, liver, small intestine

– Skin, adipose tissue & placenta

Major Endocrine Glands

The Role of Hormonal Receptors

• Affects specific target cells by binding to specific protein receptors

• Receptors - constantly being synthesized and broken down

• A target cell has 2000 to 100,000 receptors for a particular hormone

• If a hormone is present in excess down - Regulation

• When a hormone is deficient - Upregulation

Circulating and Local Hormones

• Circulating hormones

– Act on distant targets

– Travel in blood

• Local hormones

– Paracrines act on neighboring cells

– Autocrines act on same cell that secreted them

Lipid - Soluble Hormones

• Steroid hormones - Derived from cholesterol. Each

• Thyroid hormones - Iodine + tyrosine

• Nitric oxide (NO) - Hormone and a neurotransmitter

Water - Soluble Hormones

• Amine hormones - Catecholamines, histamine, serotonin and

• Peptide hormones - ADH and oxytocin

• Protein hormones - GH and insulin

• Glycoprotein hormones - TSH

• Eicosanoid hormones - Prostaglandins and leukotrienes

Hormone Transport in Blood

• Water-soluble hormone

– Circulate in the watery blood plasma in a free form

• Lipid-soluble hormone molecules are bound to transport proteins

– The transport proteins - synthesized by liver cells

– Make lipid-soluble hormones temporarily water soluble

– Slow the rate of hormone loss in the urine

– 0.1–10% - free fraction

Thyroid Gland

• Thyroid follicles:

– Microscopic spherical sacs

– Make up most of the thyroid gland

– The wall of each follicle consists primarily of follicular cells

• A basement membrane surrounds each follicle

Inactive: low cuboidal to squamous

Active: Cuboidal to low columnar

• The follicular cells produce two hormones:

– Thyroxine (Tetraiodothyronine or T4)

– Triiodothyronine or T3

• Parafollicular cells or C cells:

– Lie between follicles

– Produce the hormone calcitonin

– Helps regulate calcium homeostasis

Formation, Storage and Release of Thyroid Hormones

• Stores its secretory product in large quantities

• Thyroid hormones are synthesized by attaching iodine atoms to the amino acid tyrosine

Steps involved in the synthesis of Thyroid hormones

• Iodide trapping

• Synthesis of thyroglobulin

• Thyroglobulin (TGB)

• Oxidation of iodide

• Iodination of tyrosine

• Coupling of T1 and T2

• Pinocytosis and digestion of colloid

• Secretion of thyroid hormones

• Transport in the blood

Actions of Thyroid Hormones

• T3 and T4 exert their effects throughout the body

• Increase basal metabolic rate (BMR)

• Stimulate synthesis of additional sodium-potassium pumps

• Calorigenic effect:

– As cells produce and use more ATP, more heat is given off & body temperature rises

• Important role in the maintenance of normal body temperature

• Regulation of metabolism:

– Stimulate protein synthesis

– Increase the use of glucose and fatty acids for ATP production

– Increase lipolysis

– Enhance cholesterol excretion

• Enhance some actions of the catecholamines

• Together with human growth hormone and insulin:

– Thyroid hormones accelerate body growth

– Particularly the growth of the nervous and skeletal systems

Regulation of secretion and actions of Thyroid hormones

Calcitonin

• Produced by the parafollicular cells

• Secretion of CT is controlled by a negative feedback system

• Calcitonin lowers the amount of blood calcium and phosphates by

– Inhibiting bone resorption by osteoclasts

– Accelerating uptake of calcium and phosphates into bone EC matrix

Parathyroid Glands

• Partially embedded in the posterior surface of the lateral lobes of the thyroid gland

• Small, round masses of tissue (0.04 g)

• Contain 2 kinds of epithelial cells

• Chief (principal) cells: Numerous & produce parathyroid hormone

• Oxyphil cell

The roles of calcitonin, parathyroid hormone & calcitriol in calcium homeostasis

Adrenal glands

• Paired suprarenal glands

• Lies superior to each kidney in the retroperitoneal space

• Flattened pyramidal shape

• 3–5 cm in height, 2–3 cm in width

• Mass - 3.5–5 g

• Highly vascularized

• A connective tissue capsule covers the gland

Regions of Adrenal Glands

• Two structurally and functionally distinct regions:

• A large, peripherally located adrenal cortex (80–90%)

• Produces steroid hormones that are essential for life

• A small, centrally located adrenal medulla

• Produces three catecholamine hormones - Norepinephrine, epinephrine & a small amount of dopamine

Adrenal cortex - Zones

Zona glomerulosa (Outer zone)

• Cells are closely packed and arranged in spherical clusters and arched columns

• Secrete hormones called mineralocorticoids  

Zona fasciculate (Middle zone)

• Widest zone

• Cells arranged in long, straight columns

• secrete glucocorticoids

Zona reticularis (Inner zone)

• Cells are arranged in branching cords

• Synthesize small amounts of weak androgens

Mineralocorticoids

Glucocorticoids

• Regulate metabolism and resistance to stress

• Include cortisol (hydrocortisone), corticosterone & cortisone

• Cortisol is the most abundant - 95% of glucocorticoid activity

• Control of glucocorticoid secretion occurs via a typical negative feedback system

Effects of Glucocorticoids

Protein breakdown

• Increase the rate of protein breakdown mainly in muscle fibers

• Thus increases the liberation of AA into the bloodstream

• AA used by body cells for synthesis of new proteins or for ATP production

Glucose formation (gluconeogenesis)

• Liver cells may convert certain AA or lactic acid to glucose

• Neurons and other cells - use for ATP production

Resistance to stress

• Additional glucose supplied by the liver cells provides tissues with a ready source of ATP

• Combat a range of stresses, including:

– Exercise, fasting, fright, temperature extremes

– High altitude, bleeding, infection, surgery, trauma, disease

• Glucocorticoids make blood vessels more sensitive to other hormones - vasoconstriction

Lipolysis

• Glucocorticoids stimulate lipolysis in adipose tissue

Anti-inflammatory effects

• Inhibit WBC to participate in inflammatory responses

• Retard tissue repair - slow wound healing

Depression of immune responses

• Glucocorticoids depress immune responses

• In organ transplant recipients: retard tissue rejection by the immune system

Androgens

In males

• Major androgen from adrenal gland is dehydroepiandrosterone

• After puberty in males, the androgen – testosterone released in much greater quantity by the testes

In females

• Adrenal androgens play important roles (promote libido)

• Converted into estrogens by other body tissues

Adrenal androgens

• Also stimulate growth of axillary and pubic hair in boys and girls

• Contribute to the pre-pubertal growth spurt

Adrenal Medulla

• The hormone-producing cells - Chromaffin cells

• Innervated by sympathetic preganglionic neurons of the ANS

• The two major hormones: Epinephrine (80%) & Nor epinephrine (20%)

• Augment the fight-or-flight response

• Increase HR, FOC, BP & blood flow to the heart, liver, skeletal muscles, and adipose tissue

• Dilate airways to the lungs

• Increase blood levels of glucose and fatty acids

Pancreas

• A flattened organ that measures about 12.5–15 cm in length

• Located in the curve of the duodenum

• Consists of a head, a body, and a tail

• 99% of the cells of the pancreas are arranged in clusters – acini

• Acini produce digestive enzymes

• Pancreatic islets or islets of Langerhans - Scattered among the exocrine acini

Pancreatic Islets

Cell Types in the Pancreatic Islets

Regulation of Insulin & Glucagon secretion

Role of Hormones

Actions of Glucagon

– Glycogenolysis

– Gluconeogenesis

– Release glucose into the blood

Actions of Somatostatin

– Inhibits secretion of insulin and glucagon

– Slows absorption of nutrients from GIT

Actions of Insulin

• Lowers blood glucose level by accelerating transport of glucose into cells

• Glycogenesis

• Decreases glycogenolysis and gluconeogenesis

• Increases lipogenesis

• Stimulates protein synthesis

Actions of pancreatic polypeptide

• Inhibits somatostatin secretion, gall bladder contraction & secretion of pancreatic digestive enzymes

Gonads

• Gonads are the organs that produce gametes

• Sperm in males and oocytes in females

Ovaries

• Paired oval bodies located in the female pelvic cavity, produce several steroid hormones including estrogens and progesterone

Testes

• Oval glands that lie in the scrotum

• Main hormone – testosterone (an androgen or male sex hormone)

Hormones of Ovaries

Estrogens and Progesterone

Together with gonadotropic hormones of the anterior pituitary, regulate the female reproductive cycle, regulate oogenesis, maintain pregnancy, prepare the mammary glands for lactation, and promote development and maintenance of female secondary sex characteristics.

Relaxin

Increases flexibility flexibility of pubic symphysis during pregnancy and helps dilate uterine cervix during labor and delivery.

Inhibin

Inhibits secretion of FSH from anterior pituitary.

Hormones of Testes

Testosterone

Stimulates descent of the testes before birth, regulates spermatogenesis, and promotes development and maintenance of male secondary sex characteristics.

Inhibin

Inhibits secretion of FSH from anterior pituitary.

Pineal Gland

• Small endocrine gland (Mass - 0.1–0.2 g)

• Attached to the roof of the third ventricle of brain at the midline

• Covered by a capsule formed by the pia mater

• Consists of masses of neuroglia and secretory cells (Pinealocytes)

• Secretes melatonin: an amine hormone derived from serotonin

• Contribute to the setting of the body’s biological clock

• Thought to promote sleepiness

Hormones from other tissues

Disorders associated with various endocrine glands

Gigantism

• Hypersecretion of hGH during childhood

• Causes an abnormal increase in the length of long bones

• The person grows to be very tall

• Body proportions – normal

Pituitary dwarfism

• Hyposecretion of hGH during the growth years

• Slows bone growth

• Epiphyseal plates close before normal height is reached

• Other organs of the body also fail to grow

• The body proportions are childlike

Acromegaly

• Hypersecretion of hGH during adulthood

• Cannot produce further lengthening of the long bones

• The bones of the hands, feet, cheeks, jaws – thicken

• Other tissues enlarge

• The eyelids, lips, tongue, nose - enlarge

• The skin thickens and develops furrows

• Especially on the forehead and soles

Diabetes insipidus

• Associated with dysfunction of the posterior pituitary

• Due to defects in ADH receptors or an inability to secrete ADH

• Common symptom - excretion of large volumes of urine

• With resulting dehydration and thirst

• Bed-wetting is common in afflicted children

Diabetes mellitus

• Caused by an inability to produce or use insulin

• Type I – Insulin dependent

• Type II – Non-insulin dependent

Congenital hypothyroidism (Cretinism)

• Hyposecretion of thyroid hormones that is present at birth

• Causes severe mental retardation and stunted bone growth

Myxedema

• Hypothyroidism during the adult years

• Hallmark of this disorder - Edema

• Causes the facial tissues to swell and look puffy

• Slow HR, low BT, sensitivity to cold, dry hair and skin

• Muscular weakness, general lethargy

• Tendency to gain weight easily

Graves’ disease

• Common form of hyperthyroidism

• Autoimmune disorder

• Antibodies mimic the action of TSH

• Stimulate the thyroid gland

• Enlarged thyroid & exophthalmos

Goiter

• Enlarged thyroid gland

• Associated with hyperthyroidism, hypothyroidism, or euthyroidism

Cushing’s syndrome

• Cause - adrenal gland tumor

• Characterized by breakdown of muscle proteins and redistribution of body fat

• Spindly arms and legs

• Rounded moon face

• Buffalo hump

• Hanging abdomen

• Facial skin flush

Addison’s disease

• Hyposecretion of glucocorticoids and aldosterone causes

• Autoimmune disorder - Ab cause adrenal cortex destruction

• Bacteria (tuberculosis) - Trigger adrenal cortex destruction

• Mental lethargy, Muscular weakness

• Anorexia, nausea and vomiting & weight loss

Summary

• Endocrine system consists of endocrine glands (pituitary, thyroid, parathyroid, adrenal, and pineal glands) and other hormone secreting tissues

• Exocrine glands secrete their products through ducts into body cavities or onto body surfaces

• Endocrine glands secrete hormones into interstitial fluid; hormones diffuse into the blood

• Thyroid gland is located inferior to the larynx; secrete thyroxine, triiodothyronine & calcitonin (CT)

• Parathyroid glands are embedded in the posterior surfaces of the lateral lobes of the thyroid gland; secrets parathormone; controls the level of calcium in the blood

• Adrenal glands consists of an outer adrenal cortex and inner adrenal medulla

• Adrenal cortex secretes mineralocorticoids, glucocorticoids, and androgens

• Adrenal medulla secretes epinephrine and norepinephrine; released during stress

• Thyroid gland is located inferior to the larynx; secrete thyroxine, triiodothyronine & calcitonin (CT)

• Parathyroid glands are embedded in the posterior surfaces of the lateral lobes of the thyroid gland; secrets parathormone; controls the level of calcium in the blood

• Adrenal glands consists of an outer adrenal cortex and inner adrenal medulla

• Adrenal cortex secretes mineralocorticoids, glucocorticoids, and androgens

• Adrenal medulla secretes epinephrine and norepinephrine; released during stress

• Pancreas are considered as mixed glands; consists of both exocrine and endocrine functions

• Glucagon increases blood glucose level; insulin decreases blood glucose level.

• Secretion of both hormones is controlled by the level of glucose in the blood

• Ovaries are located in the pelvic cavity and produce estrogens, progesterone, and inhibin; maintains the development of female secondary sex characters and other reproductive structures

• Testes produce testosterone & inhibin; govern the development and maintenance of male secondary sex characteristics and normal male reproductive functions

• Pineal gland secretes melatonin, which contributes to setting the body’s biological clock

• Hypersecreation of hGH hormones produce Acromegaly during adulthood.

• Hypersecreation of hGH hormones produce Gigantism during childhood.

• Hyposecreation of hGH hormones Pituitary dwarfism