Pancreas and other EndocrineGlands Pancreatic Islets (Islets of Langerhans) Pineal Gland Thymus Gastrointestinal Tract Gonads and Placenta
Pancreatic Islets These 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
Glucagon Secreted by: Alpha cells Produced in response to a fall in blood glucose concentrations. ◦ Glucagon Stimulates the liver to hydrolyze glycogen to glucose Stimulates 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
Insulin Secreted by: Beta cells Produced 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 Gland Is 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 It 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 Rhythms The primary center for circadian rhythms in the body is the SCN These 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 Secretion Excessive 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
Thymus A 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 The 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 Tract The 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.
Gonads The gonads(testis and ovary) secrete sex steroids. ◦ Males-androgens ◦ Females- estradiol-17β and progestogens
Testis Consists 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]
Ovary Estrogen 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]
Placenta It is the organ responsible for nutrient and waste exchange between the fetus and the mother
Placenta It 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 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 ParacrineRegulation Nitric 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 factor Endothelins-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 AutocrineRegulation Cytokines- 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
Prostaglandins The most diverse group of autocrine regulators Members 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 pathways 1. Arachidonic acid is converted by the enzyme cyclo-oxygenase into other prostaglandins 2. Arachidonic acid is converted by the enzyme lipoxygenase into leukotrienes
Prostaglandins Prostaglandins 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 PGE Thromboxane A₂- promotes clotting by stimulating platelet aggregation and vasoconstriction Prostacylin 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 ProstaglandinActions1. 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 labor3. Digestive system-the stomach and intestines produce prostaglandins,which are believed to inhibit gastric secretions and influence intestinal motility and fluid absorption4. 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 ProstaglandinActions5. 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 sythesis6. 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 platelets2. Type II isoform(COX2)-is induced in a number of cells in response to cytokines involved in inflammation ◦ Produces prostaglandins that promote
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