Unit 5 - Endocrine System

� Our body has two major regulatory systems.

� They are the nervous and endocrine systems.

� Together they regulate and co-ordinate the activities of all other body structures.

� The hormones of the endocrine glands regulate and control the functioning of several organs in the body. The hormones of the endocrine glands regulate and control the functioning of several organs in the body.

� Thus, this system in general helps to maintain homeostasis.

� There are several endocrine glands in our body. The major glands are the pituitary, thyroid, parathyroids, pancreas, adrenals, testes and ovaries.

� The endocrine system sends information to the tissues it

controls in the form of chemical signals.

� These signals, in the form of hormones are released into

the circulatory system.

� They are carried to all parts of the body.

� Body cells are able to recognize the chemical signals and

respond to them.

� It is an organ, which secretes eight major hormones.

� These hormones regulate numerous body functions and control the secretory activities of several other endocrine glands. endocrine glands.

� The hypothalamus of the brain is connected to the pituitary.

� The posterior pituitary is an extension of the hypothalamus.

Structure of pituitary gland

� This gland is approximately 1 cm in diameter.

� It weighs 0.5-1g.

� It is placed in a region called the sella turcica of the sphenoid bone in the floor of the skull.

� It is placed inferior to the hypothalamus.

� It is connected to it by a stalk of tissue called the infundibulum.

� Based on origin and function the pituitary is divided into two parts.

� Posterior pituitary or neurohypophysis

� Anterior pituitary or adenohypophysis

Posterior pituitary or Neurohypophysis

� The posterior pituitary is continuous with the brain.

� Hence it is called the neurohypophysis.

� During embryonic development, it is formed as an outgrowth of the inferior part of the brain in the area of the hypothalamus. the inferior part of the brain in the area of the hypothalamus.

� The outgrowth of the brain forms the infundibulum.

� The distal end of the infundibulum enlarges to form the posterior pituitary.

� Since this part of the pituitary is an extension of the nervous system, its secretions are known as neurohormones.

The posterior pituitary, or pars nervosa, stores and releases two

hormones, both of which are produced in the hypothalamus.

� Antidiuretic hormone (ADH)

� ADH promotes the retention of water by the kidneys so that less

water is excreted in the urine and more water is retained in the blood.

� Oxytocin

� In females, oxytocin stimulates contractions of the uterus during labor

and for this reason is needed for parturition (childbirth).

� Oxytocin also stimulates contractions of the mammary gland alveoli

and ducts, which result in the milk-ejection reflex in a lactating

woman.

� In men, a rise in oxytocin secretion at the time of ejaculation has been

measured, but the physiological significance of this hormone in males

remains to be demonstrated.

Anterior Pituitary or Adenohypophysis

� During embryonic development an out pocketing of the roof of the oral cavity arises.

� It is called as the Rathke’s pouch.

� This pouch grows towards the posterior pituitary.

� Later, the pouch loses its connection with the oral cavity and becomes the anterior pituitary.

� The anterior pituitary is subdivided into three areas. � Pars tuberalis

� Pars distalis

� Pars intermedia

The hormones of the anterior pituitary are,

� Growth hormone (GH, or somatotropin)

� GH promotes the movement of amino acids into cells and

the incorporation of these amino acids into proteins, thus

promoting overall tissue and organ growth.

� Thyroid-stimulating hormone (TSH, or thyrotropin)

� TSH stimulates the thyroid gland to produce and secrete

thyroxine (tetraiodothyronine, or T4) and triiodothyronine

(T3).

� Adrenocorticotropic hormone (ACTH, or

corticotropin)

� ACTH stimulates the adrenal cortex to secrete the

glucocorticoids, such as hydrocortisone (cortisol).

� Follicle-stimulating hormone (FSH, or folliculotropin)� Follicle-stimulating hormone (FSH, or folliculotropin)

� FSH stimulates the growth of ovarian follicles in females

and the production of sperm cells in the testes of males.

� Luteinizing hormone (LH, or luteotropin)

� This hormone and FSH are collectively called gonadotropic

hormones.

� In females, LH stimulates ovulation and the conversion of the

ovulated ovarian follicle into an endocrine structure called a

corpus luteum. corpus luteum.

� In males, LH is sometimes called interstitial cell stimulating

hormone or ICSH

� It stimulates the secretion of male sex hormones (mainly testosterone)

from the interstitial cells (Leydig cells) in the testes.

� Prolactin (PRL)

� This hormone is secreted in both males and females.

� Its best known function is the stimulation of milk production by the mammary glands of women after the birth of a baby.

� Prolactin plays a supporting role in the regulation of the male reproductive system by the gonadotropins (FSH and LH)

� It acts on the kidneys to help regulate water and electrolyte balance.

Relationship of the pituitary to the brain

� There is a network of blood vessels on the hypothalamus. It is called the primary capillary network.

� A portal system called the hypothalamohypophyseal portal system extends from a part of the hypothalamus to the anterior pituitary (a portal blood vessel begins and ends as capillaries).

� The portal system in turn opens into the secondary capillary network of the anterior pituitary.

� The neurohormones produced by the hypothalamus are collected by the primary capillary network.

� Through the portal system they enter into the secondary network of the anterior pituitary.

� The thyroid gland is composed of two lobes.

� They are placed on the lateral sides of the upper portion of the trachea.

� These lobes are connected by a narrow band of thyroid tissue called the isthmus.

� The isthmus extends across the anterior aspect of the trachea.

� The thyroid is one of the largest endocrine glands.

� It weighs approximately 20g.

� It is richly supplied with blood capillaries. It is redder than its neighboring tissues.

� The gland is composed of numerous follicles.

� They are small spheres.

� Their walls are made up of cuboidal epithelial cells.

� The central cavity or lumen of each follicle is filled with a � The central cavity or lumen of each follicle is filled with a protein called the thyroglobulin.

� It stores large amount of thyroid hormone.

� The thyroid secretes thyroxine and calcitonin.

� Thyroid hormone (TH) is actually two iodine-containing amine hormones, thyroxine or T4 and triiodothyronineor T3.

� T4 is the major hormone secreted by the thyroid follicles.

� The principal difference between them is that T4 has four � The principal difference between them is that T4 has four bound iodine atoms, and T3 has three (thus, T4 and T3).

� TH affects virtually every cell in the body.

� Like steroids, TH enters a target cell, binds to intracellular receptors within the cell’s nucleus, and initiates transcription of mRNA for protein synthesis.

Effects of thyroid hormone include:

� Increasing basal metabolic rate and body heat production,

by turning on transcription of genes concerned with

glucose oxidation. This is the hormone’s calorigenic

effect (calorigenic = heat producing).

� Regulating tissue growth and development. TH is critical

for normal skeletal and nervous system development and

maturation and for reproductive capabilities.

� Maintaining blood pressure by increasing the number of

adrenergic receptors in blood vessels.

Calcitonin

� Calcitonin is a polypeptide hormone released by the parafollicular, or C, cells of the thyroid gland in response to a rise in blood Ca2+ levels.

� Calcitonin targets the skeleton, where it � Calcitonin targets the skeleton, where it � inhibits osteoclast activity, inhibiting bone resorption and

release of Ca from the bony matrix

� stimulates Ca uptake and incorporation into bone matrix.

� In other animals, calcitonin rapidly reduces blood Ca2+

levels.

� The parathyroid glands are found in association with the thyroid glands.

� They are found embedded in the posterior part of each lobe of the thyroid gland. of the thyroid gland.

� There are four parathyroid glands.

� Inside the glands the cells are organized in densely packed masses.

� The cells of the glands secrete parathyroid hormone.

� Parathyroid hormone (PTH) is the only hormone

secreted by the parathyroid glands.

� PTH, however, is the single most important hormone in

the control of the calcium levels of the blood.

� It promotes a rise in blood calcium levels by acting on the

bones, kidneys, and intestine.

� These glands are found near the superior pole of each kidney.

They are surrounded by adipose tissue.

� The glands are enclosed by a connective tissue capsule.

� The adrenal glands are composed of an inner medulla and

outer cortex.

� These regions are formed from two separate embryonic tissues.

� The medulla consists of closely packed polyhedral cells. They are centrally located in the gland.

� The cortex is composed of smaller cells.

� These cells form three distinct layers, namely� Zona glomerulosa

� Zona fasciculate

� Zona reticularis

� These layers are structurally and functionally specialized.

� The adrenal cortex secretes steroid hormones called

corticosteroids, or corticoids.

� There are three functional categories of corticosteroids

� Mineralocorticoids, which regulate Na+ and K+ balance

Glucocorticoids, which regulate the metabolism of glucose � Glucocorticoids, which regulate the metabolism of glucose

and other organic molecules

� Sex steroids, which are weak androgens (including

dehydroepiandrosterone, or DHEA) that supplement the sex

steroids secreted by the gonads.

� Aldosterone is the most potent mineralocorticoid.

� The mineralocorticoids are produced in the zona

glomerulosa and stimulate the kidneys to retain Na+ and

water while excreting K+ in the urine.

� These actions help to increase the blood volume and

pressure and to regulate blood electrolyte balance.

� The predominant glucocorticoid in humans is cortisol(hydrocortisone), which is secreted by the zona fasciculata and perhaps also by the zona reticularis.

� The secretion of cortisol is stimulated by ACTH from the anterior pituitary.

� Cortisol and other glucocorticoids have many effects on � Cortisol and other glucocorticoids have many effects on metabolism

� they stimulate gluconeogenesis (production of glucose from amino acids and lactic acid) and inhibit glucose utilization, which help to raise the blood glucose level

� they promote lipolysis (breakdown of fat) and the consequent release of free fatty acids into the blood.

� The cells of the adrenal medulla secrete epinephrine and

norepinephrine in an approximate ratio of 4 to 1, respectively.

� The effects of these catecholamine hormones are similar to those

caused by stimulation of the sympathetic nervous system,

except that the hormonal effect lasts about ten times longer.

� increase the cardiac output and heart rate� increase the cardiac output and heart rate

� dilate coronary blood vessels

� increase mental alertness

� increase the respiratory rate

� elevate the metabolic rate

� a rise in blood glucose due to stimulation of hepatic glycogenolysis

(breakdown of glycogen)

� a rise in blood fatty acids due to stimulation of lipolysis (breakdown of

fat).

� The pancreas lies between the greater curvature of the

stomach and the duodenum.

� It is an enlarged structure.

� It is approximately 15 cm long.

� It weighs 85 -100g.

� The pancreas is both an exocrine and an endocrine gland.

� The endocrine part consists of pancreatic islets (islets of Langerhans).

� They are approximalety 500,000 to 1,000,000 in number.

� The islets are distributed in the pancreas.

� The islets are composed of alpha (α) cells (20%) and beta (β) cells (75%).

� While the α cells secrete glucagon the β cells secrete insulin.

� A third type of cells called the delta (δ) cells (5%) has been identified. These cells secrete somatostatin.

� Alpha cells secrete glucagon in response to a fall in blood glucose concentrations.

� Glucagon stimulates the liver to hydrolyze glycogen to glucose (glycogenolysis), which causes the blood glucose level to rise.

� Glucagon also stimulates the hydrolysis of stored fat (lipolysis) and the consequent release of free fatty acids into the blood.

� This effect helps to provide energy substrates for the body during fasting, when blood glucose levels decrease.

� Glucagon, together with other hormones, also stimulates the conversion of fatty acids to ketone bodies, which can be secreted by the liver into the blood and used by other organs as an energy source.

� Beta cells secrete insulin in response to a rise in blood

glucose concentrations.

� Insulin promotes the entry of glucose into tissue cells, and

the conversion of this glucose into energy storage

molecules of glycogen and fat.

� Insulin also aids the entry of amino acids into cells and the

production of cellular protein.

� Thus, insulin promotes the deposition of energy storage

molecules (primarily glycogen and fat) following meals,

when the blood glucose concentration rises.

� This action is antagonistic to that of glucagon, and the

secretion of glucagon is normally decreased when insulin

secretion increases.

� During times of fasting, conversely, the secretion of � During times of fasting, conversely, the secretion of

insulin is decreased while the secretion of glucagon is

increased.

� The tiny, pine cone–shaped pineal gland hangs from the roof of the third ventricle in the diencephalon.

� Its secretory cells, called pinealocytes, are arranged in compact cords and clusters.

� Lying between pinealocytes in adults are dense particles containing calcium salts (“brain-sand” or “pineal sand”).

� These salts are radiopaque, making the pineal gland a handy landmark for determining brain orientation in X rays.

� The endocrine function of the pineal gland is still

somewhat of a mystery.

� Although many peptides and amines have been isolated

from this minute gland, its only major secretory product is

melatonin, an amine hormone derived from serotonin. melatonin, an amine hormone derived from serotonin.

� Melatonin concentrations in the blood rise and fall in a

diurnal (daily) cycle.

� Peak levels occur during the night and make us drowsy,

and lowest levels occur around noon.

� Recent evidence suggests that melatonin also controls the

production of protective antioxidant and detoxification production of protective antioxidant and detoxification

molecules within cells.

� The pineal gland indirectly receives input from the visual

pathways concerning the intensity and duration of

daylight.

� The suprachiasmatic nucleus of the hypothalamus, an

area referred to as our “biological clock,” is richly

supplied with melatonin receptors, and exposure to bright supplied with melatonin receptors, and exposure to bright

light (known to suppress melatonin secretion) can reset

the clock timing.

� As a result, changing melatonin levels may influence

rhythmic variations in physiological processes such as

body temperature, sleep, and appetite.

� Other hormone-producing cells occur in various organs

including the heart, gastrointestinal tract, kidneys, skin,

adipose tissue, skeleton, and thymus

� Adipose cells release leptin, which serves to tell your body how much stored energy (as fat) you have.

� The more fat you have, the more leptin there will be in your blood.

� It also appears to stimulate increased energy expenditure.

� Two other hormones released by adipose cells both affect the sensitivity of cells to insulin. � Resistin is an insulin antagonist

� Adiponectin enhances sensitivity to insulin.

� Enteroendocrine cells are hormone secreting cells sprinkled

in the mucosa of the gastrointestinal (GI) tract.

� These scattered cells release several peptide hormones that

help regulate a wide variety of digestive functions. help regulate a wide variety of digestive functions.

� Gastrin

� Ghrelin

� Secretin

� Cholecystokinin (CCK)

� Incretins [glucose-dependent insulinotropic peptide (GIP) and

glucagon-like peptide 1(GLP-1)]

� Enteroendocrine cells also release amines such as

serotonin, which act as paracrines, diffusing to and

influencing nearby target cells without first entering the

bloodstream.

� The atria contain specialized cardiac muscle cells that

secrete atrial natriuretic peptide (ANP).

� ANP decreases the amount of sodium in the extracellular

fluid, thereby reducing blood volume and blood pressure.fluid, thereby reducing blood volume and blood pressure.

� Interstitial cells in the kidneys secrete erythropoietin, a

glycoprotein hormone that signals the bone marrow to

increase production of red blood cells.

� The kidneys also release renin, which acts as an enzyme � The kidneys also release renin, which acts as an enzyme

to initiate the renin-angiotensin-aldosterone mechanism of

aldosterone release.

� Osteoblasts in bone secrete osteocalcin, a hormone that

prods pancreatic beta cells to divide and secrete more

insulin.

� It also restricts fat storage by adipocytes, and triggers the

release of adiponectin. release of adiponectin.

� This improves glucose handling and reduces body fat.

� Interestingly, insulin promotes the conversion of inactive

osteocalcin to active osteocalcin in bone.

� The skin produces cholecalciferol, an inactive form of

vitamin D3, when modified cholesterol molecules in

epidermal cells are exposed to ultraviolet radiation.

� This compound then enters the blood via the dermal � This compound then enters the blood via the dermal

capillaries, is modified in the liver, and becomes fully

activated in the kidneys.

� The active form of vitamin D3, calcitriol, is an essential

regulator of the carrier system that intestinal cells use to

absorb Ca from food. Without this vitamin, bones become

soft and weak.

� Located deep to the sternum in the thorax is the lobulated

thymus.

� Large and conspicuous in infants and children, the thymus

shrinks throughout adulthood. shrinks throughout adulthood.

� By old age, it is composed largely of adipose and fibrous

connective tissues.

� Thymic epithelial cells secrete several different families of

peptide hormones, including thymulin, thymopoietins,

and thymosins.

� These hormones are thought to be involved in the normal

development of T lymphocytes and the immune response,

but their roles are not well understood.

� Although still called hormones, they mainly act locally as

paracrines.