Essay on Hormones: Top 5 Essays | Endocrine Glands | Endocrinology

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Essay on Hormones

Essay Contents:

1. Essay on the Meaning of Hormones
2. Essay on the Properties of Hormones
3. Essay on the Classification of Hormones
4. Essay on the Chemistry of Hormones
5. Essay on the Effect of Hormones on Cells

Essay # 1. Meaning of Hormones:

The term hormone refers to any substance in an organism that carries a signal to generate some sort of alteration at the cellular levels. The hormones secreted by the endocrine glands represent one group of hormones that arise in one tissue, or gland and travel a considerable distance through the circulation to reach the target cell expressing cognate receptors.

Paracrine hormones arise from a cell and travel a relatively short distance to interact with their cognate receptors on other neighbouring cells. Autocrine hormones are produced by the same cell that functions as the target for that particular hormone. Hormones regulate specific target tissues in the body and not all cells. Molecular components of the cells, the receptors provide specificity to the hormone-cell interaction.

The concept of receptors originated in the twentieth century with the work of Langley (1906). Receptors may be components of the plasma membrane or they may by cytosolic or nuclear components. Receptors provide a medium through which the hormone initially interacts with the cell.

If each type of cell did not possess specific type of hormone receptors, all cells respond to the chemical messengers. If all cells respond to a single hormone, it will result in uncoordinated muscle contraction and relaxation, and uncontrolled secretion of numerous cellular products, such as enzymes and hormones.

Therefore, all cells do not possess receptors for all hormones, but only a limited number of receptor types. At normal physiological levels, each hormone interacts with its own specific cellular receptor. Receptors recognize differences in hormone structure and thus provide receptor specificity. Some hormones bind to intracellular receptors while others bind to cell surface receptors.

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Essay # 2. Properties of Hormones:

The amount of hormone produced by an endocrine gland is generally very small. Therefore, its concentration in blood is very low. Target cells are very sensitive to the hormone. Some target cells respond to plasma concentration of hormone as low as 10^-12 M. As the secretion and circulation of hormones takes place relatively slow, endocrine systems are best suited for long term regulatory functions.

i. Action in Low Concentration:

Hormones act in very low concentrations. Intravenous injection of a hormone in microgram quantities often produces measurable change in the physiological parameter or character controlled by the hormone.

ii. Storage and Excretion:

Hormones are stored only in the gland in which they are produced. As they are metabolized and excreted as soon as their function is over, they do not have any cumulative action. Hormones, like epinephrine are very quick in their action and are degraded quickly. Other hormones like thyroxine, growth hormone and sex steroids are slow in their action.

iii. Multiple Secretion:

An endocrine gland can produce more than one hormone. For example, pituitary, adrenal cortex, pancreas, etc. On the other hand, glands like parathyroid produce only one hormone.

iv. Dual Control:

A physiological process may be regulated by more than one hormone. Often, these hormones oppose the actions of each other. For example, insulin decreases blood glucose level while glucagon increases it. Similarly, growth and sex are controlled by more than one hormone. One hormone accelerates a physiological process, while the other inhibits it.

v. Rate Limiting Effect:

Hormones control the rates of reaction of many physiological processes. Thyroxine increases the rate of oxygen consumption and insulin controls utilization of glucose. Even in the absence of these hormones, or removal of the thyroid and pancreas, oxygen consumption and glucose utilization do not stop or come to zero level.

vi. Inter-Relationship of Endocrine Glands:

No endocrine gland is completely independent in its action. Endocrine glands are closely interrelated and interdependent. The inter-relationship between different endocrine glands can be synergistic, complimentary, permissive and antagonistic resulting in hypo or hyperactivity of the endocrine glands. This action can be positive or negative. Hyperpituitarism leads to hyperthyroidism.

On the other hand, a reciprocal relationship is also observed. For example, anterior pituitary produces thyroid stimulating hormone, which in turn stimulates thyroid to secrete thyroxine. Thyroxine inhibits the production of TSH by the pituitary. Thus, a hormone regulates its own secretion indirectly through the other glands. This process is known as negative feedback regulation.

vii. Hormones and Vitamins:

Hormonal functions are either directly or indirectly related to vitamins. Vitamin D (Calciferol) is related to parathormone, produced by parathyroid gland, and vitamin E (Tocopherol) to gonads.

Properties of hormones can be summarized as follows:

(a) Hormones are synthesized and secreted by endocrine cells in trace quantities.

(b) Hormones circulate in the blood and reach tissues of the body.

(c) They react with specific receptor molecules present in certain target cells.

(d) Hormones act in catalytic quantities by activating specific enzymes.

(e) A single hormone produces multiple effects on a single target tissue or on several different target tissues.

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Essay # 3. Classification of Hormones:

i. Structural Classification of Hormones:

Hormones can be placed into three molecular groups:

(i) Amines,

(ii) Steroids, and

(iii) Polypeptides and proteins.

The simplest are the proteins. They are the first to be discovered. Epinephrine has been described and its functions were recognized in the beginning of twentieth century. The largest and most complex are the polypeptides, which also include some small proteins. The peptide hormones are particularly interesting as models of molecular evolution.

The sequence of amino acid residues in a given polypeptide hormone is genetically determined. Within the different animal groups, substitutions of individual amino acids produce different analogues of the polypeptide. Some residues never undergo substitution, and are presumably necessary for function while others, which undergo substitution, are functionally neutral and serve only to place the essential residues in the positions appropriate for the activity of polypeptide hormones.

ii. Functional Classification of Hormones:

The actions of hormones on their target tissues are diverse, and cannot be easily generalized.

Three major classes of endocrine mediated effects can be recognized:

(a) Kinetic effects include pigment migration, muscle contraction and glandular secretion.

(b) Metabolic effects consist mainly of changes in the rate and balance of reactions and concentrations of tissue constituents.

(c) Morphogenic effects are concerned with growth and differentiation.

Hormones often have multiple effects, and some in fact cut across at least two of the classes mentioned above. For example, the thyroid hormones have metabolic effects on cells as well as mophogenic effects on certain tissues. This shows that hormones do not produce their end effects directly, but instead activate intermediate processes, which may differ in different tissues or cell types.

Recent advances in the mechanism of action of hormones lend support to this view. There are two major loci for hormone action. Some hormones readily penetrate the surface membranes of their target cells, and react or combine with internal cell constituents. This is characteristic of the steroids, whose lipid solubility allows them to penetrate across the membranes.

Other hormones either because of their large size or polar properties cannot readily enter the cell, and are known to interact with receptors located in the surface of the cell membranes. These receptor molecules are associated with enzymes whose activity is modulated by the hormone-receptor interaction.

Thus a single hormone can produce multiple effects. By activating different membrane bound enzymes associated with similar receptor molecules, a given hormone can produce entirely different effects in different cells or the same cell. Conversely, hormone-receptor specificities and receptor-enzyme specificities account for different channels of action of different hormones.

Table 1. Classification of hormones on the basis of effects:

Hormones may be classified in many ways according to their action, but a convenient classification is to place them in one of the following three categories:

(i) Hormones which keep physiological parameters, such as blood sugar and calcium constant.

(ii) Hormones which control growth and sexual development.

(iii) Hormones which contribute to physiological adjustment to the changing environment.  

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Essay # 4. Chemistry of Hormones:

Hormones generally fall into one of the three following chemical structures:

(I) Polypeptides and proteins,

(II) Steroids, and

(III) Derivatives of tyrosine.

i. Polypeptide and Protein Hormones:

These hormones show a great range of molecular weights, from tripeptides like thyrotrophin releasing hormone (TRH) to growth hormone (GH) with 190 aminoacid residues and the large glycoprotein hormones such as TSH. They may comprise of a single peptide chain or may be more complex. Peptide hormones are formed as prohormones or pre-hormones. The first identified prohormone is proinsulin.

The c-peptide (connecting) is cleaved from this single-chain peptide, leaving insulin, which is a peptide, comprising of two chains A and B (Fig. 1) connected by two disulfide bonds. Peptide hormones are synthesized on the ribosomes and the refragment removed as the product passes into the cisternal space. Carbohydrate residues may be added and the hormone is transferred to the Golgi complex, where it is packed into vesicles. The hormone may then be released to the exterior by exocytosis.

ii. Steroid Hormones:

Steroid hormones are not stored in the cell like peptide hormones but are synthesized as and when required. Steroid hormones comprise three six- carbon rings and a five-carbon ring fused together (Fig. 2).

Methyl groups may be added at C₁₀, C₁₃ and C₁₇. All the steroids are synthesized from acetate. Further synthesis proceeds through norvalonate and squalene to cholesterol, which is the precursor of all steroid hormones.

Cholesterol is converted to pregnenolone in the mitochondria, catalyzed by the enzyme cholesterol desmolase, and then to progesterone or 17-hydroxy pregnenolone in the smooth endoplasmic reticulum. Cortisol is formed from pregnenolone by 17-hydroxylation, and aldosterone by successive-hydroxylation and dehydrogenation. The side chain of 17-hydroxypregnenolone is cleaved to form androstenedione (Fig. 3).

iii. Tyrosine Derived Hormones:

Tyrosine derivatives are the monoamines adrenaline, noradrenaline and dopamine, and the thyroid hormones thyroxin and triiodothyronine. Dopamine is formed from tyrosine. Subsequently, noradrenaline is formed and adrenaline by the action of phenylethanolamine-N-methyl transferase. Thyroid hormones are formed from two molecules of iodinated tyrosine linked through an oxygen molecule (Fig. 4).

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Essay # 5. Effect of Hormones on Cells:

Hormones exert their influence on almost all types of cells in the body.

The large number of effects produced by hormones can be summarized into a few general processes as follows:

(1) Hormones influence cellular synthesis and secretion of other hormones within the endocrine glands and neurons. Hormones produce their effects on the digestive tract and its products such as, enzymes, hydrochloric acid by gastric glands, and bile salts.

(2) Hormones affect the integument, its derivatives and its products. They affect mucus production, peeling, plumage, pigmentation and color change. Production and secretion of sweat, sebum and milk is also controlled by hormones.

(3) Production of pheromones and other odoriferous substances is also under the control of the hormones.

(4) Hormones regulate the overall metabolism of the body. The catabolism and anabolism of proteins, carbohydrates and lipids is greatly influenced by hormones. The production of heat in the body and calorigenesis is also affected by hormones, so as to provide energy for growth and development of the body.

(5) Hormones control the processes of contraction, relaxation and metabolism of the muscles in the body. The properties of skeletal muscles contractile proteins are controlled by hormones. Smooth muscle properties such as, contraction and relaxation of the cardiac muscle, visceral muscles, muscles of the uterine wall, oviducts, urinary bladder and blood vessels are controlled by hormones.

(6) Growth is controlled by hormones as cell proliferation is stimulated or inhibited by hormones. Early stages of development, such as cell divisions and differentiation of the fertilized egg are under the control of hormones.

(7) Hormones control the processes of excretion and osmoregulation. Excretion and reabsorption of organic and inorganic salts such as sodium, potassium, calcium, chloride and bicarbonate are under hormonal control.

(8) Hormones exert permissive effect on the actions of other hormones. A hormone may enhance the action of another hormone manifold.

(9) Reproductive processes involving the development and maturation of the gonads and gametogenesis are greatly influenced by hormones.

(10) Hormones control animal behaviour. The aggressive behaviour, courtship behaviour, mating, migration, homing, nest building, parental care and sexual display are under the control of hormones. Group behaviour is controlled by pheromones.

Hormonal Effects on Other Hormones:

Hormones affect cellular synthesis and secretion of other hormones within other endocrine glands and neurons. One of the best-known examples of the action of one type of hormone on others is hypothalamic hormones on the pituitary hormones.

This forms the hypothalamo-hypophyseal complex. Extracts of hypothalamus contain a number of peptides, which produce specific effects on anterior pituitary activity. These peptides were originally termed releasing or inhibiting factors but now they have been treated as hormones.

a. Thyrotrophin-Releasing Hormone:

This was the first hypophysiotrophic hormone to be obtained in pure form and characterized from crude hypothalamic extracts. Thyrotrophin releasing hormone is extraordinarily potent in promoting thyroid stimulating hormone release. In addition to its stimulation of TSH secretion, TRH promotes the secretion of prolactin and may stimulate or inhibit the release of growth hormone in some species.

b. Luteinizing Hormone-Releasing Hormone:

Leutenizing hormone releasing hormone or Gonadotropin releasing hormone (GnRH) promotes the release of leutenizing hormone and follicle stimulating hormone from the pituitary gland.

c. Growth Hormone Release Inhibiting Hormone:

Investigation of the hypothalamic factors which influence pituitary growth hormone secretion has revealed the secretion of growth hormone release inhibiting hormone or somatostatin. This hormone inhibits growth hormone production from pituitary somatotrophs. Somatostatin inhibits TSH synthesis and release. It also inhibits the basal secretion of insulin and glucagon. Further, it inhibits the secretion of gastric acid, secretin, pepsin and motilin.

Control of Thyroid Activity:

The secretions of thyroid gland are closely regulated by other hormones. The rate of synthesis and release of the thyroid hormones is controlled by thyroid stimulating hormone secreted by the anterior pituitary. TSH secretion is promoted by TRH from the hypothalamus but inhibited by a negative feedback by tetraiodo thyronin and triiodo thyronin on the pituitary.

In turn, TRH secretion is influenced by neural factors and by feedback actions of thyroid hormones and TSH. TSH is a glycoprotein secreted by the pituitary thyrotrophs that influences all aspects of thyroid function including the promotion of iodide uptake, synthesis and release of thyroid hormones. TRH affects the thyrotrophs after attachment to the cell membrane by activation of adenyl cyclase and promotes both synthesis and secretion of TSH.

Action of Glucocorticoids on Insulin:

Glucocorticoids exert an anti-insulin effect in peripheral tissues, effectively inhibiting glucose uptake, which explains why pancreatic diabetes is exacerbated by administration of these steroids and ameliorated by removal of the adrenals. Glucocorticoids directly oppose the action of insulin in adipose and lymphoid tissues and thereby reduce uptake of glucose.

Glucocorticoids also control fat metabolism by potentiating the lipolytic response to growth hormone or catecholamines.