Endocrine system - characteristics and functions

Endocrinology is the science dealing with the chemical integration of the physiological functions of an organism.  The science of Endocrinology was born from the experiments of Bayliss and Starling(1902 to 1905). Pende introduced the term ‘Endocrinology’ (Greek, endon=within; krinein= to separate). There are two types of glands in human body: exocrine glands or glands with duct or glands of external secretion e.g. salivary glands and endocrine glands or ductless glands or glands of internal secretion e.g. pituitary gland, thyroid gland.

A hormone is a regulatory chemical that is secreted into the blood by an endocrine gland. The blood distributes the hormones throughout the body. The hormones produce specific physiological effect on certain target organs. The word hormone is derived from the Greek word ‘hormon’ which means ‘to excite’. Bayliss and Starling in 1905 coined the term ‘hormone’.

 Types of chemical messengers

·         Hormones- are products of endocrine glands.

·         Neuro-hormones – are products of nerve cells and released at neurohemal organ.

·         Neurohumors- are products of nerve cells and released at axons.

·         Phytohormones –plant hormones e.g.auxin.

·         Pheromones –ectohormones.

Properties of hormones

 Hormones are low molecular weight chemical messengers. They are secreted in trace amounts in response to specific secretary stimuli. They are soluble in water and act as catalysts.

A single hormone has multiple effects on a single target tissue or on several different tissues. Hormones have high degree of action specificity. They are not species specific and non-antigenic. They are inactivated as soon as their functions are over. Endocrine glands are under the control of nerves.

Chemical classes of hormones

1.     Amines - Secreted by gland cells of nervous origin. e.g.,Adrenaline, ADH.

2.     Steroids - Secreted by glands derived from coelomic mesothelium e.g., corticosteroids, estrogens, androgens.

3.     Proteins - Hormones made up of amino acids and polypeptides e.g., thyroxine, calcitonin, STH.

Steroid hormones are formed from cholesterol. They are lipid-soluble hydrophobic molecules. They are secreted by the gonads, adrenal cortex and placenta e.g., aldosterone, cotisol, estrogen, testosterone and progesterone.

Peptide hormones are short chains of amino acids. They are water-soluble. They are secreted by pituitary and parathyroid glands e.g., oxytocin, vasopressin, calcitonin, insulin

Amine hormones - They are derived from the amino acid tyrosine. They are secreted from the thyroid and the adrenal medulla.

Mechanism of hormone action

 Hormones activate or inhibit enzyme systems e.g., phosporylase. Hormones alter cell permeability. For example Insulin promotes transfer of glucose. Hormones directly activate or suppress particular genes. For example ecdysone causes puffing of certain genes.

Control of hormone secretion
 Humoral signal-secretion depends upon the level of blood ions and nutrients. E.g. insulin production controlled by blood glucose.
Neuronal signal- secretion depends on nerve impulses-stimulation of sympathetic nervous system release catecholamines.
Hormonal signal- some hormones control the release of hormones from another endocrine tissue-hypothalamus hormones.

Hormone secretion can be stimulated / inhibited by other hormones - e.g., trophic hormones, plasma concentrations of ions /nutrients, neurons and mental activity and environmental changes - e.g. light, temperature

Concept of feedback mechanism

The production of a hormone by an endocrine gland is controlled by its circulatory level. A reciprocal relationship exists between the blood level of a hormone and rate of its synthesis and secretion. Feedback mechanisms help in maintaining homeostasis within the endocrine system. This concept is proposed by Moore and Price in 1932. It is a two-way communication between the endocrine gland and the target gland. It is a self-balancing mechanism.
 In a negative feedback system, a gland is sensitive to the concentration of the substance it regulates. When the concentration of the regulated substance reaches a certain threshold level, it inhibits the gland from secreting more hormones until the concentration returns to normal. For example the neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the synthesis and secretion of thyroid hormones. When blood levels of thyroid hormones increase above a certain threshold, TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH.
In a positive feedback, the increased activity of an endocrine gland is followed by stimulation. The positive feedback is not common. For instance oxytocin acts on uterine muscles during child birth. The secretion stops when baby leaves birth canal. The positive feedback is easily observed under experimental conditions e.g. muscular injection of estrogen in female induces ovulation.

Hormone transport 
Hormones are normally present in blood plasma. Steroid and thyroid hormones bind to transport proteins in the plasma. Amines and peptide hormones are hydrophilic and mix easily with blood plasma. Unbound hormones have shorter half life and bound hormones exhibit longer half life. Transport proteins protect circulating hormones.

Hormone receptors
Hormones activate only those cells that have receptors for them. Hormone receptors are located on plasma membrane, in the cytoplasm or in the nucleus. Hormone – receptor interactions exhibit specificity and saturation. Protein hormones react with receptors on the surface of the cell. Steroid hormones react with receptor sites inside a cell.

Functions of endocrine system

1.     Homeostatic function – endocrine system regulates body fluid composition, rate of gaseous exchange and cardiovascular functions.

2.     Integrative function – endocrine system supports the role of nervous system.

3.     Morphogenetic  function – It influences embryonic development

4.     Permissive function - Certain hormones require the presence of another hormone for the expression of their activity.

Endocrine disrupting chemicals (EDCs)

EDCs are both natural and man-made chemicals that may mimic or interfere with the function of hormones in the body. Environmental endocrine disrupting chemicals include persistent organic pollutants (POPs) such as pesticides (DDT), dioxins, polychlorinated biphenyls (PCBs) and plasticizers (biphenol A). Endocrine disruption is an important public health concern. EDCs produce adverse developmental, reproductive, neurological and immune effects in both humans and wildlife. In humans, EDCs may cause male infertility, reproductive abnormalities, reproductive diseases and neuro-developmental disorders.