Endocrinology: Understanding the Hormones of the Body
The body’s cells require specific conditions and environments to survive, function, and perform optimally. Maintaining the body’s functions and conditions in a stable and steady state is called homeostasis. According to Guyton & Hall, homeostasis is a more dynamic equilibrium than constant and unchanging body conditions. Some of the parameters that a person must keep in a steady state include the body temperature, which shall not exceed 37 degrees Celsius, the body pH, which must be maintained at 7.4, and the arterial blood pressure, which shall be around 120/80mmHg. In particular, the constant temperature ensures the body is always warm to prevent fungal infections and maintain proper metabolism.
Moreover, the constant pH provides alkaline conditions allowing millions of cells to function and stay alive. An acidic climate would cause insufficient intake of oxygen to the cells. The blood pressure must remain at acceptable levels. Otherwise, organ damage, such as the heart and blood arteries, would occur. The failure of homeostasis usually leads to diseases and sometimes death. The endocrine system plays a critical role in the process of homeostasis since it releases different hormones that regulate every activity of the cells, tissues, and organs to ensure normal body functioning.
Introduction to Endocrinology
Endocrinology is an area of human biology and medicine that deals with the endocrine system. The primary body organs that comprise the endocrine system include the pancreas, thyroid gland, testes, ovaries, pituitary gland, and adrenals. These organs secrete hormones (chemical substances) into the blood system, allowing the body to return to regular operations. Mostly, the hormones in the blood play a variety of functions and are usually released into the bloodstream to have different effects on the targeted organ. The endocrine system is often affected by the diseases caused by hormonal imbalances. They occur when the hormone levels fluctuate from extremely high to low or when the body fails to react correspondingly to the chemical agents. The most common endocrine disorders include obesity, diabetes, osteoporosis, hypertension, infertility, short stature, menopause, and thyroid disease.
Consequently, part of endocrinology involves diagnosing and curing diseases of this vital body system. In addition, diagnosing involves a thorough evaluation of various symptoms and requires in-depth knowledge of clinical chemistry and biochemistry. To enhance the accuracy of diagnosis, laboratory tests are conducted using diagnostic imaging, while treating these conditions requires constant observation of changes at the molecular and cellular level as well as long-term treatment of the patient as a whole being. Treatments typically revolve around regulating the number of chemical materials (hormones) the organ yields. In the event of hormone insufficiency, the injection of hormone supplements is approved.
Consequently, endocrinologists must possess sufficient knowledge about the functioning of many systems within the body and glands. Other concerns within the given field include a thorough understanding of developmental mechanisms, such as cell proliferation, growth, and differentiation. Usually, differentiation processes revolve around histogenesis and organogenesis. It also concerns the coordination of numerous processes in the body, including metabolism, movement, excretion, reproduction, respiration, and sensory activities. It is possible to evaluate these structures at a chemical or cellular stage and examine them based on chemical materials and secretions by various body parts.
Types of Hormones
The hormones of the endocrine system are released from the glands. These hormones play a particular role since they demand a specific response(s) from the target groups of cells or organs instead of the body as a whole. The exocrine hormones are released into the blood via a duct and usually cause a response to a distant organ or cell, unlike the endocrine hormones released within the tissue or the cell and enter the blood through the capillaries. Concerning the chemical structure, the hormones of the endocrine systems are either amines, peptides or steroid hormones.
The given type of hormones develops from modifying the amino acids but do not usually share identical characteristics. However, they have features resembling those of the peptide and steroid hormones. In general, the amine hormones are synthesized from the amino acid tyrosine, and stored before their release, although the storage mechanism varies depending on the type of the hormone. It is worth noting that some of them are polar, while others are protein-bound. Therefore, adrenaline acts as the membrane receptors, whereas thyroid hormones act directly on the nuclear receptors.
Regarding the effects, amine-derived hormones function as peptides, while thyroid hormones work similarly to steroids. Typically, the initial composition of the amino hormone changes ensuring that the amino sets remain, whereas the carboxyl group is disposed of. The catecholamines, which include epinephrine, norepinephrine, and dopamine, are formed or synthesized from aromatic amino acids, such as tyrosine and phenylalanine, in the adrenal medulla and the brain.
Typically, the synthesis process of amino-based hormones is controlled by other hormones. The amine hormones, such as catecholamines, are made from aromatic amino acids, namely tyrosine and phenylalanine, which the body obtains from food. Usually, tyrosine can be converted from phenylalanine. The sympathetic nerves and central nervous system play a critical role in the given synthesis. Catecholamines are not single amine hormones; others include thyroid hormones and indoleamines. However, their synthesis follows the process described above.
The peptide hormones are made from chains of amino acids. In addition, they are either short or long chains of amino acids, such as oxytocin or extremely lengthy amino acids, such as insulin. Similar to other proteins, these hormones result from the translation and transcription of genes. The peptide hormones are generally synthesized as prohormones requiring further processing to activate. Moreover, they are stored briefly in the vesicles, mostly polar and water-soluble, allowing them to travel freely in the blood, bind receptors on the cell membrane, and transduce signs using the messenger systems. Concerning the impacts, peptide hormones cause transient changes in protein activity, although gene expression changes can occur.
The peptide hormone synthesis develops in several steps naturally occurring in the nucleus and cytoplasm of the secretory cells and tissues. The main activities in this process include gene transcription into the preceding nuclear RNA, post-transcriptional alterations of the precursor messenger RNA, translation of the ripe messenger RNA, and co-translational and post-translational modifications of the hormonal peptide. This process is regulated due to the above biosynthetic steps to achieve the secretory needs of endocrine glands. Once combined, the peptide hormones are enclosed into secretory cells until an appropriate stimulus trigger their release into the extra-cellular cells. However, this secretion is not consistent; rather, it assumes pulsatile and rhythmic patterns, ensuring sufficient chemical production to prevent the further or excessive release of chemicals. Since peptide hormones are water soluble, this characteristic contributes to rapid hormone erosion by plasma proteases and a shorter half-life than other hormones, such as amines and steroids.
Steroid hormones usually develop from cholesterol. They have a chemical structure similar to the peptide hormones, but unlike the peptide hormones made in the entire body, steroid hormones are produced by a few organs. Generally, steroid organs are synthesized in a series of reactions from the cholesterol and are released immediately following their synthesis. Regarding their solubility, these hormones are typically non-polar and need carrier proteins to move in the blood. They usually bind to the intracellular receptors, directly changing the gene expression. These alterations lead to immediate impacts, such as slower onset, but show longer duration than peptide hormones. The primary examples of steroid hormones include cortisol, testosterone, estrogen, progesterone, and aldosterone.
Cells that ensure the synthesis of steroid hormones have large quantities of smooth endoplasmic reticulum, which is the area that combines the steroids. Steroid hormones are lipophilic and usually move easily across the cell membranes, both out of their nucleus and cytoplasm of secretory cells and into their target tissues and cells. According to this characteristic, steroid-secreting tissues do not store hormones in secretory cells. Instead, they release their hormone on a need basis, particularly when a stimulus triggers the endocrine cell. In such a case, the precursors in the cell cytoplasm are quickly converted to active chemical agents, which then disappear through simple diffusion.
Endocrine Glands and Tissues
The endocrine glands discharge hormones directly into the bloodstream. They have feedback mechanisms that allow proper maintenance of hormones to prevent excess hormone discharge.
The Pituitary Gland
The given type of gland, the master gland, controls other organs in the endocrine system. It consists of two embryologically-distinct tissues, namely the anterior lobe and the posterior lobe. The anterior lobe, called adenohypophysis, comprises three distinct cell tissues: chromophobes, basophils, and acidophils. On the one hand, this section plays a major function in releasing adrenocorticotrophic, human growth, thyroid-stimulating, follicle-stimulating, luteinizing and prolactin hormones. On the other hand, the posterior lobe or the neurohypophysis consists mostly of axons from hypothalamic neuro-secretory nerve cells conjoined with glial cells. Moreover, it stores and releases two main hormones: antidiuretic and oxytocin.
The Thyroid Gland
The thyroid gland is butterfly-shaped, located at the throat’s base. It consists of two main lobes separated by a strip of tissue known as the isthmus. A closer look at the gland reveals one or more light-colored nodules emerging from its surface. These structures are called parathyroid glands. The microscopic structure is distinctive, with cells arranged in spheres known as thyroid follicles filled with colloids. In addition to the epithelial cells, the gland houses parafollicular cells that secrete calcitonin. However, the cell structure of the parathyroid is completely different from the thyroid gland. The thyroid gland secretes three main hormones, calcitonin, triiodothyronine, and thyroxine, essential for body growth and metabolism.
The Adrenal Glands
The adrenal glands are bilateral structures perched at the top of each kidney. They consist of two main parts, namely the cortex and the medulla. The cortex is the outer region, which generally combines more than three-quarters of the adrenal mass, unlike the medulla part, which is the innermost region of the adrenal gland. Adrenal histology includes connective tissues that are adequately vascularized with blood vessels, lymphatics, and nerves. A unique layer is the partitioning between the cortex and medulla. While the medulla is homogenous, the cortex has three concentric zones: glomerulosa, fasciculate, and reticular. Overall, the adrenal medulla secretes two main hormones: adrenalin and noradrenalin. However, the adrenal cortex produces mineralocorticoids, glucocorticoids, and gonadal hormones.
The gonads glands comprise the ovaries and testes. The male testes are egg-shaped and held by the sac-like scrotum. The histology of the testes concludes that they are covered with tunica vaginalis, albuginea, and vascular. On the one hand, these sections are highly vascularized by connective tissues to allow the production of sperm cells and the secretion of testosterone hormones that induce masculinity in males. On the other hand, ovaries in females are almond-shaped on each side of the uterus, attached to the broad ligament in the lateral pelvic wall. Its histology depicts fossa bounded by iliac capillaries. The anterior section is thin and straight, while the posterior segment is unattached, convex and rounded. This gland secretes two hormones, namely estrogen and progesterone.
Neuroendocrine Glands of the Pancreas
The given glands are situated deep in the abdomen and behind the stomach. The endocrine pancreas is a long and thin organ. In addition, its microscopic structure contains pancreatic islets, a cluster of cells scattered throughout the organ. The islets release their hormones into the bloodstream, hence being served or surrounded by numerous capillaries. The organ releases five important hormones. They include insulin, which regulates blood glucose. The glucagon assists insulin in controlling the blood glucose, somatostatin coordinates the release of other hormones, gastrin helps digestion, and the vasoactive intestinal peptide regulates water secretion and uptake from the intestines.
The Action of Hormones with Cell Membranes
Hormones react differently with cell membranes. However, the main determining factors include the plasma membrane, lipids, proteins, and biosynthesis of receptors. The plasma membrane, which covers the outer structure of the cell, is an essential structure in this process as it ensures the hormone-target cell interaction. Moreover, it contains all necessary components, such as trans-membrane signals, receptors, and the adenylate cyclase, which must be present in the initial actions of the peptide and catecholamines hormones. The plasma membrane is also involved in the later events of hormone action, such as a platform for transferring amino acids, ions and membrane proteins during hormonal secretion. However, lipids influence the action of hormones with cell membranes as they must be present to insulate the interior of the cells and tissues from the extra-cellular environment. The major lipid elements include cholesterol and phospholipids, which have partly water-soluble sites. In addition, the proteins play a critical role in this action. In particular, the peripheral proteins in the plasma membrane are covalently linked to the membrane’s internal proteins and cytoplasmic surface. They allow communication between the outer and inner layers of the plasma membrane. The integral proteins include the adenylate cyclase and the transport structures for the ions. Finally, the biosynthesis of receptors is a necessary factor in the action of hormones with the cell membranes. The synthesis of receptors must respond to the concentrations of the hormones.
Moreover, it is the only way the hormones can move to the target cell. The main factors that regulate the receptor’s activity and the receptors’ biosynthesis include ionic composition, pH, and temperature. Precisely, any alteration in the ionic structure makes the fatty acids react to Ca2+, Na+, and K+, while changes in pH influence the activity of ions. Abnormally low temperatures adversely affect the receptor binding activity. To understand the precise action of hormones with the cell membrane, it is necessary to consider the specific types of hormones and their interactions with the cell membrane.
Amino Acid-Based Hormone Action with Cell Membrane
- cAMP Second Messenger
Hormones play a critical role in the endocrine system. They control different metabolic activities and coordinate processes of various body parts, while other hormones regulate the rate and type of body growth. Normally, the target tissues of a particular hormone have specific receptors of the given hormone. The amino-acid-based hormones are highly water-soluble and, therefore, cannot pass through the cell membranes, which consist of lipids or cholesterol. Therefore, their target receptors are fixed on the cell membrane so that the hormone can bind to the specific receptor. In particular, the amino hormones or the first messenger react with the cell membrane while binding to its receptor before linking to a G protein. Following this reaction, the G protein is activated as it attaches to the GTP, displacing GDP. Afterward, the binding of the hormone to the specific receptor on the target cell triggers the enzyme adenyl cyclase in the cell membrane, causing the production of the cyclic AMP (cAMP). This agent acts as the secondary messenger that diffuses through the cell membrane and triggers the protein kinase to affect the biochemical changes. Moreover, the given process does not stop after the protein kinase release and the biochemical changes’ effectuation. After the cells have received the effects, the cAMP is deactivated by a group of phosphodiesterase enzymes, thereby ceasing the further action of the hormones.
- Calcium Second Messenger
In this case, calcium acts as the second messenger. In particular, the amino-based hormone binds to the cell receptor, activating the G protein. Moreover, G proteins are GTP binding since they have an intrinsic GTPase activity, which can be either stimulatory or inhibitory. The G proteins activate the phospholipase enzyme, which hydrolyzes phospholipids PIP2 into IP3 and diacylglycerol. These agents act as the second messengers, allowing the hormones to interact with their target tissues and cells. During the given process, the DAG plays a primary role in activating the protein kinases, which does not affect the changes as in the case of cAMP, while the IP3 activates the release of Ca2+ stores.
Moreover, the calcium ions or the Ca2+ (third messenger) alter cellular responses. It is worth noting that target cell activation and response depends on several factors. However, the main ones are:
- The blood levels of the hormone
- The relative number of receptors on the target cell
- The affinity of these receptors for the hormone
Steroid Action with Cell Membrane
In contrast to the amino hormones, steroid and thyroid hormones (T4 and T3) can diffuse through the cell membrane due to their small size and lipid permeability. In most cases, these hormones have their receptors inside the cell, freely floating in the cytoplasm. In addition, the binding of hormones to the specific receptor initiates the enzymatic activities necessary to realize biochemical changes. Thus, the hormones can influence intracellular metabolism directly. While in the cell, they attach to specific target tissue receptors. Then, the receptor protein-hormone alignment inserts into the nucleus. However, two processes precede this action. Firstly, the hormone may be transported to a nuclear receptor target. Secondly, the hormone-receptor protein may temporarily bind with DNA, ensuring its capacity to perform as a gene activator. Amino materials are responsible for attaching the receptor to a particular sequence of DNA.
Moreover, an appropriate mRNA is developed using transcription, leaving the nucleus before being translated into the ribosomes into a particular enzyme or protein. This process lasts a certain period and causes corresponding effects and actions of the steroid hormones. However, this action may be challenged by actinomycin and puromycin at the transcription and translation stages. In other words, receptor activation describes conformational alterations in the receptor triggered by the binding hormone. The major impact of this initiation is that the receptor becomes qualified to bind DNA. However, the activated receptors adhere to the response materials, which are normally short and specific sequences of DNA situated in the hormone-responsive genes.
In all cases, hormone-receptor complexes bind DNA in pairs, which affects the transcription process from those alleles the receptor bound must be affected. Normally, the receptor binding triggers the transcription process, causing the hormone-receptor complex to function as a transcription factor. As one can forecast, certain variations relate to the issues mentioned above, depending on the particular receptor under study. For example, in the absence of the steroid hormone, some intracellular receivers may not attach their hormone response materials as required and might silence the transcription process.
However, when complex to the steroid hormone, the receptors may be triggered strongly to stimulate the transcription process. Furthermore, some receivers bind DNA with a different intracellular receptor and not with any other of their kind. An example is glucocorticoids, a type of steroid hormone that considerably impacts the physiology of all cells in the body but rarely binds to any intracellular hormone.
Peptide Hormones Action with Cell Membrane
The different impacts of peptide hormones upon target membranes are supported by a set of chemical activities, which connect to the primary region of peptide hormone activity, the plasma membrane, and a set of target cell reactions. They include the development of specific control proteins, regulating lipid and glucose metabolism, changing tissue permeability and ion transfer, and releasing target tissue elements, such as protein hormones. A common characteristic of hormonal materials, which is evident due to amino-acid precursors and protein chemical materials, is their capacity to synthesize with particular receptor regions in the plasma layers of their target tissues. The process by which peptide hormones influence the target cells to evoke typical functional reactions starts from interacting with a highly dedicated region of the plasma membrane called the hormone receptor site. The cholesterol and protein elements of the membrane are structured as a mosaic, whereby some proteins are superficially connected to the upper layer of the phospholipids bi-layer, while others are partially attached to the hydrophobic region of the bi-layer. Others perforate the plasma membrane completely. Usually, the peptide hormones activate adenylate cyclase, ensuring that the earliest biochemical event follows the binding of peptides to cell receptors in the plasma membrane. The purpose of the cyclic AMP as an intracellular mediator of peptide hormone action is to activate the enzyme protein kinase. This enzyme is situated in the cytosol and consists of two parts in the inactive state. The regulatory and the catalytic elements coalesce to form the dormant holoenzyme (RC), which is generally triggered by cAMP. The binding of the cyclic AMP to the structural part leads to the dissociation of the RC matter and stimulation of the proteins in the catalytic region. In dormant tissues, most of the present enzymes take the form of an inactive RC holoenzyme and the addition of an exogenous cAMP, leading to the dissociation of the holoenzyme and stimulation of phosphorylation of protein materials by the catalytic portion. The stimulation of protein kinase appears as the major or single activity, due to which cAMP acts as a second messenger in the transfer of hormonal signals. The location of protein kinase in several intracellular organs is consistent with the primary role of the enzyme to support the major actions of cAMP upon cellular processes. However, the differences in cellular reactions to cAMP and peptide hormones are influenced by the properties of the protein kinase present at loci of cAMP action within the target cell. Thus, the reaction of a given cell to cyclic AMP depends on the activities that control gene expression and cellular distinction, specifically, on the development of relevant enzymes.
Endocrinology is an area of biology and medicine that deals with the endocrine system. The hormones of the endocrine system are released from several glands. However, they are specific in that each hormone demands a particular response from the target groups of cells or organs rather than from the whole body. Amine hormones are synthesized from the alteration of the amino materials. Nevertheless, they do not usually share identical characteristics with peptide and steroid hormones.
On the one hand, the amine hormones, such as catecholamines, consist of the aromatic amino acid tyrosine and phenylalanine, normally obtained from food. On the other hand, the peptide hormones include strands of amino acids. The hormones are either short or long chains of amino acids. Moreover, the main activities in their synthesis involve gene transcription into the preceding nuclear RNA. The given process is followed by post-transcriptional alterations of the precursor messenger RNA, translation of the ripe messenger RNA, and finally, the co-translational and post-translational modifications of the hormonal peptide. However, steroid hormones consist of cholesterol. Cells that ensure the synthesis of steroid hormones have large quantities of smooth endoplasmic reticulum, which is the area that combines the steroids.
Moreover, steroid hormones are lipophilic and usually move easily across the cell membranes, both out of their nucleus and cytoplasm of secretory cells and into their target tissues and cells. The modes of action are different. In particular, the amino hormones react with the cell membrane combining the receptors before binding to a G protein. Following this reaction, the G protein is activated as it attaches to the GTP, displacing GDP. On the one hand, the binding of the hormone on the specific receptor of the target cell triggers the enzyme adenyl cyclase in the cell membrane, causing the production of the cyclic AMP (cAMP).
On the other hand, steroid hormones diffuse through the cell membrane due to their small size and permeability to meet their receptors located inside the cell. These receptors freely float in the cell cytoplasm, while binding of hormones to the specific receptor initiates the enzymatic activities necessary to implement functional changes. Finally, peptide hormones activate adenylate cyclase, ensuring that the earliest biochemical event follows the binding of peptides to cell receptors in the plasma membrane. The purpose of the cyclic AMP as an intracellular mediator of peptide hormone action is to stimulate the enzyme protein kinase. Kinase appears as the major activity, due to which cAMP acts as a second messenger in transferring hormonal instructions. The location of protein kinase in several intracellular organs depends on the primary function of the enzyme, which is to support various actions of cAMP upon cellular processes.