Endocrine System

Messenger Molecules Cells must communicate with one another to coordinate cell processes within tissues and to maintain homeostasis. Cell-to-cell communication is carried out via messenger molecules.

Three types of chemical signals are used for cell-to-cell communication.

Four methods of cell-to-cell communication are found in the human body, ranging from direct to remote communication.

Endocrine hormones Produced by endocrine (“ductless”) glands and secreted into the bloodstream. Endocrine hormones may affect a wide array of target cells to produce multiple effects. Two types: peptides (small proteins) and steroids (lipids).

Hormones and Receptors

Peptide Hormones Peptide hormones do not enter the cell directly. These hormones bind to receptor proteins in the cell membrane. When the hormone binds with the receptor protein, a secondary messenger molecule initiates the cell response. Because peptide hormones are water soluble, they often produce fast responses.

Hormone–receptor binding activates an enzyme that catalyzes peptide or amino acid-derived hormone (first messenger) Hormone–receptor binding activates an enzyme that catalyzes the synthesis of a second messenger, such as cyclic AMP 2 The hormone binds to a receptor on the plasma membrane of a target cell 1 cyclic AMP- synthesizing enzyme (cytoplasm) (extracellular fluid) ATP active enzyme receptor product cyclic AMP (second messenger) 4 The activated enzymes catalyze specific reactions plasma membrane inactive enzyme reactant The second messenger activates other enzymes 3 nuclear envelope (nucleus)

Steroid Hormones Steroid hormones enter through the cell membrane and bind to receptors inside of the target cell. These hormones may directly stimulate transcription of genes to make certain proteins. Because steroids work by triggering gene activity, the response is slower than peptide hormones.

receptor in the nucleus or to a receptor in the cytoplasm steroid hormone (extracellular fluid) The hormone binds to a receptor in the nucleus or to a receptor in the cytoplasm that carries it into the nucleus 2 The hormone–receptor complex binds to DNA and causes RNA polymerase to bind to a nearby promoter site for a specific gene 3 A steroid hormone diffuses through the plasma membrane 1 DNA plasma membrane hormone receptor ribosome RNA polymerase 5 The mRNA leaves the nucleus, then attaches to a ribosome and directs the synthesis of a specific protein product RNA polymerase catalyzes the transcription of DNA into messenger RNA (mRNA) 4 mRNA new protein gene nuclear envelope (cytoplasm) (nucleus)

Role of the Hypothalamus The thalamus receives sensory information, relays some to the hypothalamus. Hypothalamus monitors the body for temperature, pH, other conditions. Hypothalamus signals pituitary gland if conditions need to be corrected.

Role of the Pituitary The pituitary is the “master gland” that signals other glands to produce their hormones when needed. The anterior lobe of the pituitary receives signals from the hypothalamus, and responds by sending out the appropriate hormone to other endocrine glands. The posterior pituitary receives oxytocin or antidiuretic hormone (ADH) from the hypothalamus, relays them to the body as necessary.

Neurosecretory cells of the hypothalamus produce oxytocin and ADH 1 produce releasing and inhibiting hormones 2 Releasing or inhibiting hormones (green circles) are secreted into capillaries feeding the anterior lobe of the pituitary 2 Oxytocin and ADH (blue triangles) are secreted into the blood via capillaries in the posterior pituitary blood flow pituitary (anterior lobe) endocrine cell capillary bed pituitary (posterior lobe) 3 Endocrine cells of the anterior pituitary secrete hormones (red squares) in response to releasing hormones; the pituitary hormones enter the bloodstream capillary bed blood flow

Pituitary Hormones Pituitary Hormone Functions Follicle-stimulating hormone Stimulates egg maturation in the ovary and release of sex hormones. Lutenizing hormone Stimulates maturation of egg and of the corpus luteum surrounding the egg, which affects female sex hormones and the menstrual cycle. Thyroid-stimulating hormone Stimulates the thyroid to release thyroxine. Adrenocorticotropic hormone Causes the adrenal gland to release cortisol. Melanocyte-stimulating hormone Stimulates synthesis of skin pigments. Growth hormone Stimulates growth during infancy and puberty. Antidiuretic hormone Signals the kidney to conserve more water. Oxytocin Affects childbirth, lactation, and some behaviors.

Endocrine Hormones Gland Hormones Functions Thyroid Thyroxine Regulates metabolism Calcitonin Inhibits release of calcium from the bones Parathyroids Parathyroid hormone Stimulates the release of calcium from the bones. Islet cells (in the pancreas) Insulin Decreases blood sugar by promoting uptake of glucose by cells. Glucagon Increases blood sugar by stimulating breakdown of glycogen in the liver. Testes Testosterone Regulates sperm cell production and secondary sex characteristics. Ovaries Estrogen Stimulates egg maturation, controls secondary sex characteristics. Progesterone Prepares the uterus to receive a fertilized egg. Adrenal cortex Epinephrine Stimulates “fight or flight” response. Adrenal medulla Glucocorticoids Part of stress response, increase blood glucose levels and decrease immune response. Aldosterone Regulates sodium content in the blood. Testosterone (in both sexes) Adult body form (greater muscle mass), libido. Pineal gland Melatonin Sleep cycles, reproductive cycles in many mammals.

Homeostasis and Hormones Examples: Thyroid and temperature control Thyroid, Parathyroid, and calcium Pancreas and glucose control

Temperature Control

Blood Calcium

Blood Sugar Control

Other hormone roles Controlling sleep cycles (melatonin) Controlling reproductive cycles (melatonin, sex hormones) Growth (growth hormone) Responding to stress or emergencies (epinephrine and other hormones)

Hormones Everywhere! Many other organs besides the endocrine glands produce hormones. Kidneys produce several hormones that regulate blood pressure, which is essential for kidney function. The digestive system produces several hormones that regulate appetite.

The obese mouse on the left does not produce enough leptin, a hormone produced by fat cells. Low body fat stimulates leptin production, which stimulates appetite. The mouse is obese because its low leptin levels give it an enormous appetite. Leptin injections return the mouse’s weight to normal. Humans sometimes have a leptin issue, too, but the problem is a bad leptin receptor on body cells. Alas, leptin injections won’t cure that.