The endocrine system is responsible for sending "messages" to the various organs and tissues of the body. These signals are provided by chemicals of different nature, called hormones, a term coined in 1905 starting from the Greek verb ormao ("substance that stimulates or awakens").
Until recently, it was believed that hormones were produced exclusively by the endocrine glands. Today we know that this function also belongs to single cells or groups of cells, such as neurons or certain cells of the immune system. The heart, for example, despite being a muscle, produces a hormone called atrial natriuretic peptide (PAN), which is secreted into the blood and increases sodium excretion in the kidney. Stomach, adipose tissue, liver, skin and intestine also have the ability to produce hormones.
As a whole, the endocrine system is therefore made up of glands and cells responsible for the production of particular substances, called hormones.
The activity of the endocrine system is strongly correlated to that of the nervous system. Between the two there is an "important anatomical and functional connection, represented by the" hypothalamus. Through the pituitary peduncle this anatomical formation regulates the activity of the pituitary, the most important human endocrine gland.
Located at the base of the brain and the size of a bean, the pituitary or pituitary gland, in turn, controls the functioning of many cells, organs and tissues.
In addition to the pituitary, the main endocrine glands are:
the thyroid
the parathyroid glands
the endocrine portion of the pancreas
the adrenal glands or capsules
the gonads
the thyme
the epineal gland (epiphysis)
According to the traditional theory, hormones, after being produced by glands or cells, are secreted into the blood (endocrine mechanism of action). From here they are transported to target tissues, where they perform their function by influencing cellular activity. Today it has been widely demonstrated that some hormones can influence the functionality of the same structures that produced them (autocrine mechanism of action) or of the adjacent ones (paracrine mechanism of action).
It should be remembered that hormones:
they act in infinitesimal concentrations
to perform their function they need to bind to a specific receptor
Furthermore, a hormone can have different effects depending on the tissue in which it is captured.
Steroid hormones (androgens, cortisol, estrogen, progesterone, etc.) are lipophilic and as such can easily cross the cell membrane, both to enter and to exit the target cell. This lipophilicity turns into a big disadvantage when the steroid hormones have to be transported in the bloodstream. As they are not soluble, they must in fact bind to specific carrier proteins, called carriers, such as albumin or SHBG (sex hormon binding proteins). This bond prolongs the half-life of the hormone, protecting it from enzymatic degradation. In proximity to the target cell, the complex carrier protein + hormone must dissolve, since the hydrophobicity of these carriers would prevent them from entering the intracellular environment.
The target of any steroid hormone is the nucleus, which it can reach directly or indirectly, for example by binding to a cytoplasmic receptor. Once here, it regulates gene transcription to direct the synthesis of new proteins.
Peptide hormones (growth hormone, LH, FSH, parathyroid hormone, insulin, glucagon, erythropoietin etc.) are hydrophobic and as such cannot enter the target cells directly. To do this, they rely on specific receptors on the cell surface. The receptor hormone complex triggers a series of events mediated by a complex of second messengers.
While steroid hormones directly regulate protein synthesis, the second messengers triggered by peptide hormones modify the functions of already existing proteins.
Cortisol, for example, increases the number of lipases (enzymes responsible for the degradation of triglycerides present in adipose tissue), while adrenaline, with a faster action, activates the already existing lipases. For this reason the cell's response to the hormones of protein nature is generally faster.
With the recent advances in science, all the general discourse made up to this point has been questioned. In fact, some peptide hormones have been discovered capable of activating second messengers which, similarly to steroid hormones, activate gene transcription, driving the synthesis of new proteins. Thanks to other studies, the existence of membrane receptors for steroid hormones has also emerged, capable of activating second messenger systems and stimulating rapid cellular responses.
