2. adrenal medulla Inner portion of adrenal gland; secretes epinephrine, norepinephrine.
3. biological clock Internal time-measuring mechanism; adjusts activities seasonally, daily, or both in response to environmental cues.
4. ecdysone Hormone of many insect life cycles; roles in metamorphosis, molting.
5. endocrine system Integrative system of cells, tissues, and organs functionally linked to the nervous system; exerts control through its secretion of hormones and other chemicals
6. gonad Primary reproductive organ in which animal gametes are produced.
7. hormone Signaling molecule secreted by one cell that stimulates or inhibits activities of any cell with receptors for it. Animal hormones are picked up and transported by the bloodstream.
8. hypothalamic inhibitor Hypothalamic signaling molecule that suppresses a particular secretion by the anterior lobe of the pituitary gland.
9. hypothalamus Forebrain center of homeostatic control over the internal environment (e.g., salt-water balance, core temperature); influences hunger, thirst, sex, other viscera-related behaviors, and emotions.
10. local signaling molecule A secretion by one cell type that alters chemical conditions only in localized tissue regions (e.g., prostaglandin).
11. molting Periodic shedding of worn-out or too small body structures. Permits some animals to grow in size or renew parts.
12. negative feedback mechanism A homeostatic mechanism by which a condition that changed as a result of some activity triggers a response that reverses the change.
13. neurotransmitter Any of a class of signaling molecules secreted by neurons. It acts on cells next to it, then is rapidly degraded or recycled.
14. pancreatic islet Any of the 2 million or so clusters of endocrine cells of the pancreas.
15. parathyroid gland One of four small glands embedded in the thyroid; their secretions raise blood calcium levels.
16. peptide hormone A hormone that binds to a membrane receptor, thus activating enzyme systems that alter target cell activity. A second messenger in the cell often relays the hormone's message.
17. pheromone Hormone-like, nearly odorless exocrine gland secretion. A signaling molecule between individuals of the same species that integrates social behavior.
18. pineal gland Light-sensitive endocrine gland; its melatonin secretions affect biological clocks, overall activity, and reproductive cycles.
19. pituitary gland Endocrine gland that, with the hypothalamus, controls many physiological functions, including activity of many other endocrine glands. Its posterior lobe stores and secretes hypothalamic hormones. Its anterior lobe produces and secretes its own hormones.
20. positive feedback mechanism Homeostatic control; it initiates a chain of events that intensify change from an original condition, then intensification reverses the change.
21. puberty Post-embryonic stage; gametes start to mature and secondary sexual traits emerge.
22. releasing hormone (hypothalamic) Hypothalamic signaling molecule that enhances or slows secretions from target cells in anterior lobe of pituitary gland.
23. second messenger Molecule within a cell that mediates a hormonal signal.
24. steroid hormone Lipid-soluble hormone made from cholesterol that acts on target cell DNA.
25. thyroid gland Endocrine gland; its hormones influence overall growth, development, and metabolic rates of warm-blooded animals.