There are many ways that cells can use to communicate with each other. The method used often depends on the proximity of the cell sending the signal to the one which will receive it. Examples include autocrine, where the cell signals to itself, and paracrine, where a cell signals to a nearby neighbour.
However, when cells need to communicate a message over a long distance, they can use the endocrine system. Endocrine signalling uses chemicals called hormones to send messages throughout the body. The hormones are released from the cell into the bloodstream and can travel around the entire body. In contrast, the exocrine system secretes its products into the extracellular environment.
Ever wonder why you're feeling a bit stressed? Why you grow during puberty? What causes that lovely womanly gift once a month? All of these things and many others are controlled by the hormones of the endocrine system.
Hormones have a very diverse array of functions. Their secretion is a tightly controlled process and varies throughout your life, sometimes daily, monthly or yearly. Different hormones can have effects on your growth, both as a child and in puberty (the imaginatively named growth hormone), sexual development and libido (oestrogen and testosterone), and menstruation (follicle-stimulating hormone (FSH), luteinising hormone (LH), oestrogen and progesterone). Hormones are also released in response to stress (adrenocorticotropic hormone (ACTH) and cortisol). They have other functions too, including in metabolism and homeostasis. In short, they are very important and very complex. This is why so many diseases and illnesses can be caused by hormonal imbalances.
Pregnancy is associated with fluctuations in hormone levels, including oestrogen, progesterone, prolactin and oxytocin. This is both for the mother (prolactin is involved in lactation) as well as hormones to help the foetus develop, such as placental growth hormone. These changes in hormonal levels explain why pregnant women can sometimes be a bit ... uh ... changeable in mood!
The key organs in endocrine signalling are glands. Glands are the target for signals, often coming from the brain. In response to these signals they will release the appropriate hormone. A gland involved in endocrine signalling specifically releases hormones into the bloodstream. Examples of glands include the islets of Langerhans in the pancreas, and the thyroid gland.
The endocrine system mostly starts in the area of the brain known as the hypothalamus. When a particular response is required, the hypothalamus releases a signal. This signal will usually be targeted to another area of the brain, the pituitary gland. The pituitary gland interprets the signal and then itself releases the appropriate chemical signal, usually a hormone. This signal will then travel through the body via the bloodstream to reach its required destination, which will be a gland. The gland then responds to the signal from the pituitary to secrete a particular hormone into the bloodstream. This will produce a response in the body.
There are many types of hormones - the chemicals which are responsible for endocrine signalling. Like most signalling pathways, hormones will bind to receptors on specific cells. This will produce the desired response within the cell.
The different types of hormones include:
Peptide hormones: these are made from strings of amino acids. They are often made in the cell in advance and packaged into secretory vesicles until the signal is received for them to be released via exocytosis. Peptide hormones are often made in a longer form (called pre-hormone or pre-pro-hormone) which are then cleaved to create the mature form. When released, they will bind to cell-surface receptors and trigger a response inside the target cell. Peptide hormones usually bind to G-protein coupled receptors (GPCR) or tyrosine kinase receptors. Examples of peptide hormones include insulin, ACTH and thyroid stimulating hormone (TSH).
Steroid hormones: these are lipids formed from cholesterol. They travel around the body bound to carrier proteins as they are water-insoluble. Once they have entered the target cell, they bind directly to nuclear receptors, which then travel into the nucleus. These hormones directly regulate gene expression of target genes. Examples include cortisol and oestrogen.
Each separate endocrine pathway is regulated by a specific set of hormones released from certain glands. The hormones will be released in response to a change in the body - e.g. insulin will be released when sugar has been eaten and ACTH is released in response to stress and in turn triggers the release of cortisol.
Many of the endocrine pathways also operate on a negative feedback loop - one of the target organs of many of the final-stage hormones is the pituitary gland, which then prevents the release of more hormone - for example the thyroid hormones T3 and T4 feedback to the pituitary gland and prevent the release of TSH.
Some examples of hormone pathways and glands are detailed below:
Function: metabolic control (conversion of food to energy). The thyroid hormones T4 and T3 require iodine to function. A lack of iodine can lead to thyroid-related diseases (see below).
Function: release of steroid hormones. Found just above the kidney, and is divided into three layers:
Function: Stress response, immune/inflammatory response, carbohydrate and protein metabolism.
The pancreas is divided into two sections - exocrine and endocrine. The exocrine pancreas is responsible for the release of enzymes and other products to aid digestion. The endocrine cells in the pancreas are responsible for a range of hormones.
The endocrine cells in the pancreas are called the islets of Langerhans. Specific endocrine cells include the alpha-cells which release glucagon,beta-cells, which are responsible for the release of insulin, and C-cells, which release somastatin.
The most famous hormone released by the pancreas is insulin. The release of insulin is triggered by the ingestion of glucose (sugar).
Glucagon, released from the alpha-cells, has the opposite effect to insulin and is released when glucose levels are low. Glucagon therefore triggers the production of glucose from stores such as glycogen.
Pineal: found buried deep in the brain. Reponsible for the production of melatonin, an important hormone for circadian (time-dependent) rhythms. It is responsive to changes in light.
Ovaries/testes: responsible for the release of the sex steroids (also called gonadotrophins), oestrogen and testosterone (small amounts of these are also secreted from the zona reticularis of the adrenal cortex). These are steroid hormones and are responsible for sexual development as well as secondary sexual characteristics such as facial and body hair. They are released in response to gonadotrophin releasing hormone (GnRH) from the hypothalamus and follicle stimulating hormone (FSH) and luteinising hormone (LH) from the pituitary.
Parathyroid: found adjacent to the thyroid gland. Responsible for the release of parathormone, whcih helps to maintain calcium homeostasis throughout the body.
Thymus: found in the chest. Responsible for the release of the hormone thymosin, which is important for development of the immune T cells. The thymus is only functional as an endocrine gland until puberty.
Other hormone systems
Growth hormone: released from the pituitary in response to growth-hormone releasing hormone (GHRH) from the hypothalamus. It is a peptide hormone which signals through the janus-kinase-signal transducer and activation of transcription (JAK-STAT) pathway (tyrosine kinase receptors). Targets the liver to release insulin-like growth factor-1 (IGF-1). Responsible for growth in adolescence as well as bone mass, protein and carbohydrate metabolism.
Prolactin: similar to growth hormone in structure. Binds to a cytokine receptor. Responsible for lactation during pregnancy.
Due to the complex nature of endocrine signalling, many disorders and illnesses are associated with endocrine systems. More detail about the specific endocrine disorders can be found elsewhere on Fastbleep, but this is a brief list of some medical conditions caused by defective endocrine signalling:
Pituitary tumours: overgrowth of cells or over-secretion of hormones from this gland can lead to several disorders, including;
In both cases of diabetes, diagnosis can be made by testing blood sugar levels - it is usually elevated in diabetes sufferers. If left untreated, diabetes can lead to blindness, chronic kidney disease and neuropathy (damage to the nerves), especially in hands and feet. Severe neuropathy can lead to the hand or foot needing to be amputated.
University of Manchester FLS Lecture Courses - Endocrinology and Reproduction (2nd Year, 2006-2007, co-ordinator Dr. Steve Bidey) and Clinical Endocrinology (3rd Year, 2008-2009, co-ordinator Dr. Donald Ward).
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