bio5: Hormones and the endocrine system (v1.0)

Communication between cells occurs by the production of ligands (proteins, peptides, fatty acids, steroids, gases, and other low molecular compounds). They are either secreted by cells or presented on their surface and act on receptors on or in other cells. Such signaling molecules control important cell processes.

Hormones are signaling molecules that are sent to distant organs to regulate physiology and behavior. Hormone producing cells are found in the endocrine glands (ductless glands) such as thyroid, ovaries, testes, pineal, pituitary, pancreas, parathyroid, hypothalamus, and adrenal. Hormones from the endocrine system are secreted directly into the blood stream. Exocrine glands on the other hand for example secrete various substances through ducts. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucus.

Hormones work slowly, over time, and affect many different processes, including growth and development, metabolism – how your body gets energy from the foods you eat- sexual function, reproduction, and mood. When glands do not produce the right amounts of hormones, diseases develop that can affect many aspects of life. Hormones are vital to your health and well-being. 

Some key hormonal glands are:

1. Adrenal gland secretes steroid hormones such as cortisol and aldosterone. It also makes precursors that can be converted to sex steroids such as androgen, estrogen. 
2. Found deep inside the brain, the hypothalamus produces releasing and inhibiting hormones and controls the “master gland”— the pituitary. Together, the hypothalamus and pituitary tell the other endocrine glands in your body to make the hormones that affect and protect every aspect of your health.
3. The pancreas plays a crucial role in converting food into energy for cells and digestion.
4. Reproductive hormones play a crucial role in sexual development, weight, energy, and fertility. Puberty, menstruation, sperm development and even menopause.
5. The thyroid gland uses iodine from food to make two thyroid hormones that regulate metabolism, whereas the parathyroid glands produce hormones that control calcium. 

Some key hormone signaling molecules are:

1. Estrogen: contributes to the development of female ‘secondary sex characteristics’
2. Testosterone: contributes to the development of male ‘secondary sex characteristics’
3. Cortisol: helps the body in responding to stress
4. DHEA: helps produce other hormones, including testosterone and estrogen.
5. Pregnenolone: helps produce other hormones, including testosterone and estrogen.

Peptide hormones are hormones that are made of small chains of amino acids. The body produces a wide range of peptide hormones, which circulate in the blood and bind to receptors on targeted organs and tissues. In women, for example, the peptide hormones CG and LH are produced by the ovaries or placenta and play a vital role in reproduction. Corticotrophins and growth hormones are also examples of peptide hormones. Corticotrophins cause the release of cortisol, a hormone that helps the body cope with stress, while growth hormone regulates the production of many tissues in the body. 

Insulin, glucagon, thyrocalcitonin, pituitary hormones, and hypothalamic hormones are examples of protein hormones. They are also known as polypeptide hormones.

 

Local hormones are a large group of signaling molecules that do not circulate within the blood. Local hormones are produced by nerve and gland cells and bind to either neighboring cells or the same type of cell that produced them. Local hormones are activated and inactivated quickly. They are released during physical work and exercise. They mainly control smooth and vascular muscle dilation. Strength of response is dependent upon the concentration of receptors of target cell and the amount of ligand (the specific local hormone). Eicosanoids are a primary type of local hormone.

 A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped primarily into two classes: corticosteroids (typically made in the adrenal cortex) and sex steroids (typically made in the gonads or placenta).

Thyroid hormones are any hormones produced and released by the thyroid gland, abbreviated to be named T₃ and T₄. They are hormones that are primarily responsible for the regulation of metabolism. T₃ and T₄ are partially composed of iodine, derived from food. A deficiency of iodine leads to decreased production of T₃ and T₄, enlarges the thyroid tissue and will cause the disease known as simple goitre.

 Hormone secretion in a gland occurs in response to specific biochemical signals and is often subject to negative feedback regulation. For instance, high blood sugar promotes insulin synthesis. Insulin then acts to reduce glucose levels and maintain homeostasis, leading to reduced insulin levels. Upon secretion, water-soluble hormones are readily transported through the circulatory system. Lipid-soluble hormones must bond to carrier plasma glycoproteins to form protein complexes. Some hormones, such as insulin and growth hormones, can be released into the bloodstream already fully active. Other hormones, called prohormones, must be activated in certain cells through a series of steps.

Signal transduction is the process by which a chemical or physical signal is transmitted through a target cell as a series of molecular events. Most signal transduction pathways involve the binding of signaling molecules, to receptors that trigger events inside the cell. The changes elicited by signal sensing (called ligand binding) in a receptor give rise to a biochemical cascade, which is a chain of biochemical events known as a signaling pathway. Most hormones initiate a cellular response by initially binding to either cell surface receptors or intracellular receptors.

Receptors for most peptide as well as many eicosanoid hormones are embedded in the cell membrane as cell surface receptors. Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus.

For steroid or thyroid hormones, their receptors are located inside the cell within the cytoplasm of the target cell. To bind their receptors, these hormones must first cross the cell membrane. They can do so because they are lipid soluble. The combined hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences regulating the expression of certain genes, and thereby increasing the levels of the proteins encoded by these genes. However, it has been shown that not all steroid receptors are located inside the cell.