Pancreas and Other Endocrine Glands

Pancreas and other Endocrine GlandsPancreatic Islets (Islets of

Langerhans)Pineal GlandThymusGastrointestinal TractGonads and Placenta

Pancreatic IsletsThese structures are most

common in the body and tail of the pancreas

On a microscopic level, the most conspicuous cells in the islets are the alpha and beta cells

Hormones secreted by the pancreatic islets:

Glucagon Insulin

Pancreatic Islets

GlucagonSecreted by: Alpha cellsProduced in response to a fall in blood

glucose concentrations.◦Glucagon Stimulates the liver to hydrolyze

glycogen to glucoseStimulates the hydrolysis of stored fat

(lipolysis) and the consequent release of free fatty acids into the blood.◦This effect helps provide energy

substrates during fasting when blood glucose levels decrease

InsulinSecreted by: Beta cellsProduced in response to a rise in

blood glucose concentrations◦Promotes the entry of glucose into

tissue cells and the conversion of this glucose into glycogen and fat

Insulin aids the entry of amino acids into cells and the production of cellular protein

The actions of Insulin and Glucagon are antagonistic

After a meal◦Insulin> Glucagon

During times of Fasting◦Insulin<Glucagon

Pineal GlandIs found on the roof of the third

ventricle near the corpora quadrigema, where it is encapsulated by the meninges covering the brain

The pineal gland of a child weighs about 0.2 g and is 5-8 mm long and 9mm wide◦This gland begins to regress at age 7

and in the adult appears as a thickened strand of fibrous tissue

Pineal Gland

Pineal GlandIt lacks direct nervous connection

to the rest of the brain but it is highly innervated by the sympathetic nervous system from the superior cervical ganglion

Secretes melatonin◦Production and secretion is

stimulated by activity of the Suprachiasmatic Nucleus (SCN) in the hypothalamus of the brain

Circadian RhythmsThe primary center for circadian

rhythms in the body is the SCNThese are Rhythms of

Physiological activity that follow a 24-hour pattern

The circadian activity of the SCN is automatic

Circadian Rhythms

◦environmental light/dark changes are required to synchronize this activity to a day/night cycle Secretion of melatonin increases with

darkness and peaks at the middle of the night

During the day, the neural pathways from the retina of the eyes to the hypothalamus act to depress the activity of the SCN (decreasing melatonin secretion)

Melatonin SecretionExcessive melatonin secretion in

humans is associated with a delay in the onset of puberty.

Melatonin secretion is highest in children between the ages of 1 and 5 and decreases thereafter, reaching its lowest levels at the end of puberty

ThymusA bilobed organ positioned in

front of the aorta and behind the manubrium of the sternum

It is relatively large in newborns and children and sharply regresses in size after puberty.◦The thymus in adults becomes

infiltrated with strands of fibrous and fatty connective tissue.

Thymus

ThymusThe Thymus secretes hormones

that help to regulate the immune system

Site of production of T cells (thymus dependent cells)

In addition to providing T cells, the thymus secretes a number of hormones that are believed to be stimulate T cells after they leave the thymus

Gastrointestinal TractThe stomach and intestine

secrete a number of hormones that act on the gastrointestinal tract itself and on the pancreas and gallbladder.

The effects of these hormones coordinate the activities of different regions of the digestive tract and the secretions of pancreatic juice and bile.

GonadsThe gonads(testis and ovary)

secrete sex steroids.◦Males-androgens◦Females- estradiol-17β and

progestogens

TestisConsists of two compartments:

1. Seminiferous tubules-which produce sperm cells

2. Interstitial tissues-contains the Leydig cells which secrete Testosterone

◦ Testosterone- is needed for the development and maintenance of the male genitalia [penis and scrotum] and the male accessory organs [prostate, seminal vesicles ,epididymis and vas deferens]

Testis

OvaryEstrogen is secreted by small structures

within the ovary called ovarian follicles◦The ovarian follicles contain the egg cell (or

ovum) and granulosa cells that secrete estrogen

By about the midcycle,one of these follicles grows very large and ,in the process of ovulation,extrudes its ovum.◦The empty follicle under the influence of the

Luteinizing hormone becomes the corpus luteum [which secretes progesterone and estradiol-17β]

Ovary

Placenta

It is the organ responsible for nutrient and waste exchange between the fetus and the mother

PlacentaIt secretes:

◦Large amounts of estrogen and progesterone

◦A number of polypeptide and protein hormones Human Chorionic Gonadotropin (hCG)-

similar to LH Somatomammotropin- similar in action to

both growth hormone and prolactin

Autocrine and Paracrine Regulation

Autocrine regulators-if they are produced and act within the same organ

Paracrine regulators-if they are produced within one tissue and regulate a different tissue of the same organ

Examples of Paracrine RegulationNitric Oxide-can function as a neurotransmitter

in memory processes and in other processes can be produced by the endothelium of blood vessels◦ Functions as the regulator previously known as

endothelium-derived relaxation factorEndothelins-directly promote vasoconstriction

◦ Endothelin-1 –is produced by the epithelium of the airways

Bradykinin-promotes vasodilation*endothelins and bradykinin are very important in the control of blood pressure. These are also involved in artherosclerosis

Examples of Autocrine RegulationCytokines- molecules that regulate

different cells of the immune system◦Lymphokines- cytokines produced by

lymphocytes (interleukins are the specific molecules involved)

Neutrophins-including nerve growth factor,guide regenerating peripheral neurons that have been injured

Growth factors- promote growth and cell division in any organ

*cytokines may also function as growth factors

Growth factors

◦Platelet-derived growth factor◦Epidermal growth factor◦Insulin-like growth factor

*stimulate cell division and proliferation of their target cells

Prostaglandins

ProstaglandinsThe most diverse group of autocrine

regulatorsMembers of a family called the

eicosanoids[molecules derived from the precursor arachidonic acid]◦Upon stimulation arachidonic acid is released

from phospholipids in the cell membrane and may enter one of the two possible metabolic pathways1. Arachidonic acid is converted by the enzyme

cyclo-oxygenase into other prostaglandins2. Arachidonic acid is converted by the enzyme

lipoxygenase into leukotrienes

ProstaglandinsProstaglandins of the E series (PGE)-can cause

smooth muscle to relax in the bladder,bronchioles intestine and unterus and the same can cause the vascular smooth muscle to contract

PGF₂α -has exactly the opposite effects as PGEThromboxane A₂- promotes clotting by stimulating

platelet aggregation and vasoconstrictionProstacylin or PGI₂-has the opposite effects as

Thromboxane A₂

*these antagonistic effects ensure that, while clotting is promoted, the clots will not normally form on the walls of the intact blood vessels

Examples of Prostaglandin Actions1. Immune system-prostaglandins promote the

development of pain and fever.2. Reproductive system- plays a role in ovulation and

corpus luteum function in the ovaries and in contraction of the uterus.

◦ Excessive prostaglandin production leads to dysmenorrhoea,endometriosis and premature labor

3. Digestive system-the stomach and intestines produce prostaglandins,which are believed to inhibit gastric secretions and influence intestinal motility and fluid absorption

4. Respiratory system-The leukotrienes are potent bronchoconstrictors and these compounds together with some prostaglandins may cause respiratory distress and contribute to bronchoconstriction in asthma

Examples of Prostaglandin Actions5. Circulatory system- Prostacylin,a

vasodilator ,and Thromboxane A₂ ,a vasoconstrictor play a role in blood clotting.

◦ In fetus PGE₂ is believed to promote ductus arteriosus –a short vessel that connects the pulmonary artery to the aorta fails to close after birth. It can be closed by the administration of drugs that inhibit prostaglandin sythesis

6. Urinary system- Prostaglandins produced in the renal medulla cause vasodilation resulting in increased blood flow and increased excretion of water and electrolytes in the urine

Inhibitors of Prostaglandin Synthesis

Aspirin-most widely used member of a class of drugs known as nonsteroidal anti-inflammatory drugs(NSAIDs)◦Other members of this class are

indomethacin and ibuprofen◦These drugs specifically inhibit the cyclo-

oxygenase enzyme that is needed for prostaglandin sythesis

◦ Inhibit inflammation but with side effects like: Gastric bleeding Possible kidney problems Prolonged clotting time

Inhibitors of Prostaglandin Synthesis

2 isoenzyme forms of cyclo-oxygenase:

1. Type I isoform(COX1)-produced in a constant fashion by cells of the stomach and kidneys and by blood platelets

2. Type II isoform(COX2)-is induced in a number of cells in response to cytokines involved in inflammation

◦ Produces prostaglandins that promote the inflammatory condition