By the following three ways the classification of hormones of the animals is done and it is very easy to go through the matter:
i) According to their origin and site of action
ii) According to their chemical nature
iii) According to their solubility and receptor.
Classification of hormones according to origin and site of action
Animal hormones may be of six major types based on the sites of their origin and action. Those are—
Hormones secreted from the typical endocrine (ductless) glands belong to this group. These hormones put forth their actions at sites remote from their origin. Examples—thyroxine of thyroid gland, adrenal corticoids, androgens of testis, estrogen and progesterone of ovary, etc.
Hormones of anterior pituitary are called trophic hormones or trophins because their chief function is to govern and regulate the activities of other endocrine glands.
The hormone which acts at a site very close to its place of origin is called local hormone; for example—gastrin, secretin etc., that are secreted by the mucous membrane of gastrointestinal tract and regulate the secretion of digestive glands. So, it is very important on our discussion about classification of hormones.
In some invertebrates and all vertebrates’ upto man, there are some specialized nerve cells which act like endocrine organs. Chemical substances produced in these cells are carried by blood to various tissues to regulate their functions. These nerve cells are called neurosecretory cells and their secretory products are called neurosecretions or neurohormones. Examples—ecdysone, diapause, juvenile hormone etc., found in insects ; oxytocin and vasopressin of posterior pituitary, hypothalamic releasing factors, melatonin of pineal body etc., found in mammals. In this circumstance, it can be mentioned that ordinary nerve cells also release certain chemicals that act locally on the target cells and are not carried by blood to distant organs. These chemicals are called neurotransmitters, such as, acetylcholine, noradrenaline etc.
In higher animals, there is some blood borne regulator substances which do not originate from specific cells. These chemical substances cannot be called true hormones and as such are termed parahormones which means, hormone like substances; example-erythropoietin, angiotensin etc. Erythropoietin and angiotensin are produced in blood from the plasma proteins by the action of enzymes secreted by the kidney.
These are chemical agents released by one animal into the environment, which produce behavioral, developmental or reproductive effects in other animals of the same species. The pheromones are not true hormones since they are generally produced by exocrine glands instead of endocrine glands. However, the capacity of an exocrine gland to produce pheromone is often dependent on hormonal stimulation and so it has took place in the classification of hormones. Pheromones are also called exohormones as these are secreted outside the body. Pheromones be at variance from hormones in their actions in two following respects:
(i) Pheromones are transmitted via the external environment to draw out adjustments in the bodies of other individuals, whereas hormones detain their activities to the individual in which they are produced.
(ii) Pheromones are species-specific in action, i.e., they are functionally effective in the animals of the same species only, whereas hormones are effective when administered to the individuals of other species as well.
Classification of Hormones according to chemical nature
It is difficult to isolate the hormone secreted by a gland from the blood leaving the gland. Most of the hormones have been isolated by separation and purification of gland-extracts. Only the active principles of an endocrine gland, i.e., the compounds which are found in the gland and have definite physiologic actions are considered as hormones. Chemically, hormones are divisible into three major types, those are:—
Protein or Peptide hormones
The hormones which are made up of many amino acids linked up by peptide bonds belong to this group. Examples—STH, ACTH and prolactin of anterior pituitary, insulin and glucagon of pancreas, parahormone etc., are simple proteins. TSH, FSH and LH of anterior pituitary are glycoproteins. ADH and oxytocin of posterior pituitary are smaller peptides.
Amino acid or Amine hormones
Some hormones are chemically amino acids e.g., thyroxine and triiodothyronine of thyroid gland. Amines are decarboxylated derivatives of single amino acids; examples of amine hormones are adrenalin, serotonin, melatonin etc.
Hormones derived from cholesterol e.g., andorgens, estrogens, progesterone and adrenal corticoids belong to this group.
Classification of Hormones according to solubility and receptor
Classification of Hormones may be grouped into two classes depending on their solubility and location of the receptors though which they act. Those are as follows:
Water soluble hormones acting through membrane receptors
Some hormones are water soluble and they cannot spread through the cell membrane of the target cells. They act on the cell membrane bound receptors to produce their effects. Examples are—amines (adrenalin, noradrenalin etc.) and peptides (glucagon, insulin, parathormone etc.).
Lipid soluble hormones acting through intracellular receptors
Hormones which are lipid soluble can easily penetrate through the lipid matrix of the cell membrane of the target cells and enter into the cell. They produce their effects by acting through some intracllular receptors. Steroid hormones and thyroid hormones are of this type.