You may have heard of the endocrine system, and how it controls the hormones in your body. You may have heard of endocrinology (the study of the endocrine system) or endocrinologists (the doctors that treat the endocrine system). But what about endocrine signaling, what could this be? How do endocrine hormones signal the body to behave or change in certain ways and how does endocrine signaling differ from other forms of cell signaling?
Because of the importance of these pathways and mechanisms in our bodies, as well as the bodies of many other organisms, we will endeavor to answer all of these questions.
Let's Define Endocrine Signaling
Before we define endocrine signaling, we should define the larger group that it is a member of: cell signaling.
Cell signaling is a form of cellular communication, in which a (or multiple) cell(s) transmits and receives information, signals or directions between itself and the environment.
When discussing endocrine signaling, this information, these directions, and these signals come in the form of certain small molecules called hormones.
Hormones are molecules, produced by endocrine cells, that are transmitted through the blood (the circulatory system). From the blood, hormones reach their effector cells in effector organs, which are the cells that their signal is supposed to affect.
When hormones reach effector organs, they can cause a number of changes and alterations in the cell, depending on the specific hormone and the specific cell.
Now, let's define endocrine signaling. The process of endocrine signaling involves a cell targeting a distant cell through the bloodstream. The signaling molecule is released by one cell, then travels through the bloodstream to bind to receptors on distant target cells.
It's important to outline the endocrine organs that take part in this form of cell signaling and the effector organs that alter themselves or their behavior as a response to it.
Organs and Hormones of Endocrine Cell Signaling
Organs that release hormones are part of the endocrine system. The glands and organs within this system include:
Endocrine Organs & Glands, from proximal to distal:
What's the difference between an organ and a gland?
Glands are distinguished from organs in that their only function is to secrete substances, while organs are usually multifunctional structures. Organs can be involved in secretion, but also have other functions including activities like absorption, mechanical digestion, protection, and so on.
Now that we know that we recognize the members of the endocrine system, let's examine the hormones secreted by select organs, and the target organs they affect.
The pineal gland
Secretes: melatonin
Effector organs: (primarily) the brain; also, the ovaries, blood vessels, GI tract
Overall effect: to control the sleep-wake cycle
The hormones produced by the hypothalamus - vasopressin and oxytocin- are actually stored in, and then secreted from the posterior pituitary!
The pituitary gland
Secretes: From the anterior pituitary - FSH, LH, ACTH, TSH, Prolactin, Growth Hormone. Remember them with the acronym- FLAT PiG (the "i" stands for irrelevant, or nothing there).
Effector organs: ovaries and testes, adrenal glands, thyroid, breasts, bones, muscles, and more!
Overall effect: FSH and LH are reproductive hormones that cause sexual maturity and activity in the ovaries, testes, and other secondary sex cells. ACTH causes cortisol release. TSH causes thyroid hormone release. Prolactin causes breast milk accumulation. Growth hormone causes growth!
The thyroid gland
Secretes: Thyroid hormones -T3 and T4.
Effector organs: bones, brain, muscles, liver, kidneys, eyes, heart, and more.
Overall effect: Controls your body's metabolism. Too little thyroid hormone is called hypothyroidism; and makes a person more likely to be cold, gain weight, and have a slow heartbeat. Too much thyroid hormone is called hyperthyroidism; and makes a person more likely to be too warm, lose weight, and have a fast; erratic heartbeat.
The pancreas is a special case as an endocrine organ. This is because it releases endocrine hormones with endocrine signaling, but it also releases some exocrine hormones with exocrine signaling.
Exocrine vs Endocrine Signaling Pathway - the Pancreas
First, let's examine the pancreas' endocrine hormones. These include insulin, glucagon, and somatostatin. Insulin, which you have likely heard of before, is the hormone that's either dysfunctional or completely absent in diabetes. The purpose of insulin is blood sugar and hence, metabolism, and regulation.
When sugar is high in the blood, usually after a carbohydrate-rich meal, insulin gets released from the pancreas, into the blood, as all endocrine hormones do.
Once in the circulatory system, insulin can travel to the cells of the body; and inform the relevant target cells in the effector organs that blood sugar is high. This allows the body to use the blood sugar to make energy.
Without this message, or signal, from the pancreas via the endocrine hormone insulin, the effector cells wouldn't know that sugar is high, and wouldn't be able to utilize it for energy as effectively. This leads to problems from both poor energy utilization, and hyperglycemia - high blood sugar - like damage to the eyes and the nerves, as well as lots of peeing and dehydration.
Importantly, insulin is released into the blood. This is what defines it as an endocrine hormone, the fact that it travels through the bloodstream to act on distant effector organs like the liver or the muscles. This pathway; being released into the bloodstream, is also true for glucagon, and somatostatin, the other endocrine hormones of the pancreas. It is also true for the endocrine hormones listed above, like vasopressin, thyroid hormones, and growth hormone.
But the pancreas has both exocrine and endocrine functions. How exactly do those exocrine functions work?
Well, the exocrine hormones of the pancreas are released, not into the bloodstream, but into tubes and ducts. These tubes and ducts lead directly into nearby organs. The two important ducts in this scenario are called the pancreatic duct and the common bile duct. The exocrine hormones released into them include things like pancreatic lipase, pancreatic amylase, and trypsinogen.
While the primary function of the endocrine pancreas is to regulate blood sugar, the primary function of the exocrine pancreas is to aid digestion of the particles of food that travel from the stomach to the duodenum (of the small intestine). Once squirted into the duodenum via the pancreatic and common bile ducts, the exocrine hormones work to help break down macromolecules of food into smaller molecules, that can be more easily absorbed. Trypsinogen helps break down proteins, amylase helps break down carbohydrates, and lipase helps break down fats.
We can see then, that the important distinctions between the endocrine and the exocrine pancreas are how hormones are released, and the distance the hormones travel.
Endocrine pancreas hormones are released into the blood, to act on target organs that may be very far away (anatomically) from the pancreas. Exocrine pancreas hormones are released into ducts, to act within organs that are quite near to the pancreas.
Endocrine Signaling Example
Above, we looked at an endocrine vs exocrine signaling comparison with the pancreatic hormones as our example. Now, to have a deeper understanding of the endocrine signaling pathway, we should go through yet another example.
The parathyroid gland is actually a set of four small glands located on either side of the thalamus. It releases a parathyroid hormone, whose function is to control calcium levels in the body.
Parathyroid hormone causes increased levels of calcium in three ways: 1) by increasing calcium release from the bones, 2) by increasing calcium absorption from the GI tract, and 3) by decreasing calcium excretion in the urine.
The parathyroid gland is not located next to the kidneys or the GI tract, or most bones. In order to reach its target organs, it must travel long distances, through the circulatory system (the bloodstream). Thus, parathyroid hormone is a classic example of an endocrine hormone.
Compare and Contrast Endocrine and Paracrine Signaling
Paracrine signaling is yet another form of cell signaling. In paracrine signaling, the molecules that affect change are released from a secretory cell and travel only short distances, diffusing onto their effector cells nearby (Table 1). The greatest distinctions between endocrine and paracrine signaling are their differences in distance traveled, and the fact that paracrine signaling does not occur through the bloodstream, but merely by signaling molecules being released in close proximity to their effector cells.
Table 1: Endocrine Vs. Paracrine signaling
| Endocrine Signaling | Paracrine Signaling |
| Distance | Can be long distances | Short distances, close proximity |
| What medium does it travel through | Through the bloodstream | Direct diffusion |
| When is this used in humans | Metabolism of glucose | Wound healing |
| Examples | Insulin release in blood due to high blood sugar (glucose) | Small molecules like FGF (fibroblast growth fiber) and PDGF (platelet-derived growth factor) lead to fibroblast proliferation in tissues that were damaged (wounded) and are now healing |
Endocrine Signaling - Key takeaways
- Endocrine signaling is the form of cell signaling in which the signals are hormones that are released into the blood and act on distant target cells.
- Organs and glands of the endocrine system in humans include the pancreas, the thyroid, the pituitary gland, the adrenal glands, the ovaries, the testes, and more.
- The pancreas is an example of an organ that has both endocrine and exocrine functions.
- Exocrine hormones of the pancreas are released directly to the GI system through ducts (i.e. trypsinogen) while endocrine hormones of the pancreas are released into the blood (i.e. insulin).
- Paracrine signaling differs from endocrine signaling in that its molecules diffuse directly onto the target cells, they are not released into the blood.
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