Endocrine Signaling

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.

When discussing endocrine signaling, this information, these directions, and these signals come in the form of certain small molecules called hormones.

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.

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:

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Parathyroid gland

• Adrenal gland

• Pancreas

• Ovaries (specific to females)

• Testes (specific to males)

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 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.

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.

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.

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.

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.

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.

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