Hormones are informational molecules secreted by the endocrine cells in one part of the body in response to changes in external or internal environment, and carried by blood to another part , where they stimulate or inhibit specific physiological processes for the good of the body as a whole. In other words, the hormones are chemical messengers that regulate the biological processes in the internal organisms.
The first hormone was discovered by two English physiologists : William M. Bayliss and Ernest H. Starling in 1903. It was secretin coming from the duodenal mucous membrane and stimulating the secretion of pancreatic juice. The term hormone was coined by Starling in 1905.
The term “hormone” literally means to stimulate ( Gr. Hormoein = to excite). It signified the role of the hormone discovered first of all. However, we now know that a hormone may stimulate or inhibit the working of the organ it acts on.
Transport of hormones
The endocrine cells release hormones into the extracellular (tissue) fluid. From here, the hormones diffuse into the blood stream. The latter carries them from the site of production to the site of action. They act on specific organs called target organs. The blood contains all the hormones but the cells of a target organ pick up the required hormone and ignore all others. It has been found that the target cell has on its surface or in its cytoplasm a specific protein molecule, called a receptor, which can recognize and pick out the specific hormone capable of acting in that cell. Nontarget cells lack these receptors. Therefore, they do not respond to the circulating hormones. A hormone delivers its ‘message’ to the target cell by changing the shape of the receptor that binds it. The receptor’s new shape sets up certain changes in the cell such as alteration in permeability, enzyme activity, or gene transcription.
Chemical Nature (Classification) of hormones
They may be classified into 4 categories amino acid derivatives, peptides, proteins (polypeptides) and steroids.
1. Amino Acid Derivatives. The hormones epinephrine and norepinephrine from the adrenal. medulla and thyroxine from the thyroid gland are derived from the amino acid tyrosine.
2. Peptides. These hormones are further of 2 types : short peptides and long peptides.
(i) Short Peptides. The hormones oxytocin and vasopressin from the posterior lobe of the pituitary gland and melanocyte-stimulating hormone from the intermediate lobe of the pituitary gland are short peptides.
(ii) Long Peptides. The hormones insulin and glucagon from the pancreas, from the anterior lobe of the pituitary gland, calcitonin from the thyroid gland, and parathormone from the parathyroid glands are long peptides.
3. Proteins (Polypeptides). The gonadotrophic, somatotrophic and thyrotrophic hormones from the anterior lobe of the pituitary gland are proteins.
4. Steroids. The hormones from adrenal cor- tex, testes, ovaries and placenta are steroids. They are synthesized from cholesterol. This shows that some cholesterol is essential for health.
General Characteristics of Hormones
General characteristics of hormones are listed below :
1. The hormones are secreted by living cells to act as chemical messengers. 2. They are carried by blood stream to the target organs. 3. They are organic compounds of diverse nature. 4. They usually have low molecular weight and readily diffuse through the cell membrane. 5. They are effective in very low concentrations. 6. They act away from the site of production. 7. They may accelerate or inhibit specific physiological processes. They neither act as building material nor provide energy. 8. They are often used up in their regulatory action. 9. They do not initiate a reaction but can influence it’s rate . 10. Many hormones, such as peptides, are secreted in an inactive form called prohormone, insulin is produced as proinsulin. 11. The hormones are generally slow in action. 12. The specificity of hormones may vary widely.
For example, the thyroid hormone acts on nearly all cells as it has to control metabolic rate; whereas the thyroid-stimulating hormone of the pituitary gland acts only on the thyroid gland.