The endocrine system includes the set of all secretory cells of an organism that produce and secrete into its body chemical messengers, called hormones. They are channeled by exocytosis into the blood and through its circulation, they reach other cells in the body.

• can control of the speed of chemical reactions in the cell
• can control of the movement of substances through the cell membrane
• can stimulate the synthesis and secretion of other substances that are useful to the organism
• can stimulate the proliferation (growth of number) of the target cell
• can stimulate the differentiation (specialization) of the target cell
• have a large range (as opposed to nervous)

Chemical messengers can also be produced and secreted within specific organs, for the purpose of coordinating and regulating the function of cells of the same tissue (autocrine cells) or cells of another tissue, within the organ (paracrine cells). This type of regulatory function is not considered endocrine, because they are not transported through the bloodstream or lymph.

Neurotransmitters are also considered to be chemical messages of a non-endocrine nature, which are secreted from the presynaptic vesicles and diffuse into the synaptic clefts.

Types of Gland (Gland: Tissue with secretory function)

Chemical Composition of Hormones

• Peptides: Peptide chains with ≤50 amino acids.
• Proteins and glycoproteins: Peptides consisting of one or more polypeptide chains that have >50 amino acids (and carbohydrate molecules in the case of glycoproteins).
• Amines: These are derivatives of amino acids (tyrosine and tryptophan).
• Steroids: These are hormones of a lipid nature, derivatives of cholesterol.

Lipid soluble hormones (all steroid hormones and thyroid hormones T3 and thyroxine T4) can easily pass through the phospholipid bilayer of the cell membrane of the target cells and enter them. Water soluble hormones cannot—they must bind to a receptor on the membrane which triggers internal enzymatic reactions.

Steroid Hormones

• Even though these hormones can pass the cell membrane, their action is limited to the receptors that recognize them.
• The protein receptors are located either inside the cytoplasm or inside the nucleus.
• This recognition results in the formation of a hormone-receptor complex.
• This complex moves towards the nucleus and acts on a specific region of the DNA. The result is the activation or inactivation of the transcription of specific genes of the DNA into mRNA molecules, which will lead to the production of specific enzymes or other proteins in the ribosomes.

Example: Thyroxine

• Thyroxine T4 has 4 iodine atoms. Thyroxine T₃ has 3 iodine atoms, but is produced in smaller quantities than T₄.
• Both hormones circulate the blood. When the two forms enter a target cell, all of thyroxine T₄ is converted into thyroxine T₃, because only the T₃ form can be recognized by a protein receptor.
• The T₃ form enters the nucleus and causes activation of the transcription of specific structural genes for protein synthesis in the ribosomes.

Peptide Hormones:

Peptide hormones include: amines, polypeptides, proteins, and glycoproteins. They are water soluble, meaning they require target cells to bear specific protein receptors/channels to recognise them.

The hormone-receptor complex molecule can:

• Activate certain protein transporters or cause the opening of specific protein channels of the cell membrane, affecting the concentration of specific substances in the cytoplasm, and changing the metabolic activities of the target cell. → For example, insulin increases the uptake of glucose of the target cells.
• Cause the formation of a second chemical messenger in the cytoplasm. Thus, a specific metabolic pathway is determined. → For example, the adrenaline-receptor complex causes the activation of the enzyme adenylyl cyclase, which catalyzes the conversion of ATP to cyclic adenosine monophosphate (cAMP). cAMP, as the second chemical messenger, activates the specific enzymes that are necessary for the conversion of glycogen to glucose.

Negative Feedback

• Most endocrine glands secrete small amounts of hormones. Significant amounts of hormones are inactivated by the liver, and excreted by the kidneys.
• This is where the homeostatic mechanism of negative feedback comes in. The gland is continuously fed back with information related to the levels of the hormone it secretes or with information related to the result that the hormone caused and reacts in such a way so that homeostasis is maintained.
• The gland can increase, decrease or stop secreting a hormone.

In the body of every organism, there are receptors, at certain control centers, that perceive the changes that occur inside it. These receptors, (chemoreceptors, baroreceptors, osmoreceptors, etc.) create nerve impulses, which end up in the central nervous system and through the autonomous nervous system, each gland receives the relevant information about its secretory action.

The Pituitary Gland and the Hypothalamus

The pituitary gland is a small endocrine gland located at the base of the hypothalamus and consists of two separate parts. The anterior part is called the anterior lobe or adenohypophysis. The posterior part is called the posterior lobe or neurohypophysis and is an extension of the hypothalamus, with many axons of neurosecretory cells ending here.

The adenohypophysis has secretory cells and therefore produces its own hormones that it then secretes into the blood to regulate the secretory function of a lot of other glands. In contrast, neurohypophysis does not produce hormones. Instead, hormones synthesized in the hypothalamus and secreted by its neurosecretory cell are stored in the neurohypophysis and channeled to the bloodstream through it.

The hypothalamus is part of the brain with an endocrine function. The hypothalamus indirectly regulates the secretory function of glands based on stimuli that it receives from many sensory receptors of the body and from other parts of the brain. The hypothalamus regulates the pituitary by sending releasing factors, which are produced by its neurosecretory cells and are secreted into the hypophyseal portal circulation. The hypophyseal portal circulation consists of a complex of capillary blood vessels of the hypothalamus.

Written by Alexandros Samoutis