Steroid hormone

Steroid hormones are synthesized in the adrenal cortex, gonads, and placenta; all are derived from cholesterol and many are of clinical importance. Steroid hormones are synthesized in the mitochondria and the smooth endoplasmic reticulum. Because they are lipophilic, they cannot be stored in vesicles from which they would readily diffuse, and are therefore synthesized when needed as precursors. Upon stimulation of the stem cell, steroid hormone precursors are converted to active hormones and diffuse out of the stem cell by simple diffusion as their intracellular concentration increases.

Because all steroid hormones are derived from cholesterol, they are not soluble in plasma or other body fluids. As a result, the steroids bind to transport proteins that increase their half-life and ensure ubiquitous distribution. Protein-bound steroids are in equilibrium with a small fraction of free steroids, which are “active.” Steroids can act rapidly, by binding to cell surface receptors, or slowly, by binding to cytoplasmic or nucleic receptors, ultimately activating gene transcription.

The adrenal glands are made up of the adrenal medulla and the adrenal cortex. The adrenal cortex is divided into three main anatomic zones: the zona glomerulosa, which produces aldosterone; and the zona fasciculata and reticularis, which together produce cortisol and adrenal androgens. The medulla synthesizes catecholamines. More than 30 steroids are produced in the adrenal cortex; they can be divided into three functional categories: mineralocorticoids, glucocorticoids, and androgens. Steroids that are produced almost exclusively by the adrenal glands are cortisol, 11-deoxycortisol, aldosterone, corticosterone, and 11-deoxycorticosterone. Most other steroid hormones, including estrogens, are produced by the adrenal glands and gonads.

Mineralocorticoids

Mineralocorticoids are formed in the zona glomerulosa. The main function of mineralocorticoids is to promote tubular reabsorption of sodium and secretion of potassium and hydrogen ions in the collecting tubule, distal tubule, and collecting ducts. When sodium is reabsorbed, water is simultaneously absorbed. Sodium and water absorption increase fluid volume and blood pressure. Aldosterone is the most potent mineralocorticoid, accounting for about 90% of the total mineralocorticoid activity. Mineralocorticoid potency in descending order is aldosterone, 11-deoxycorticosterone, 18-oxocortisol, corticosterone, and cortisol.

Although cortisol has primarily glucocorticoid activity, it also has some mineralocorticoid activity. Cortisol has 1/400 the potency of aldosterone, but its concentration is about 80 times that of aldosterone. Adrenal production of cortisol is approximately 25 mg/day and that of aldosterone is 100 μg/day. Corticosterone has mainly glucocorticoid activity and some mineralocorticoid activity. Aldosterone secretion is regulated primarily by the renin-angiotensin system; it is also stimulated by increased serum potassium concentrations. Hyperkalemia and angiotensin II cause an increase in aldosterone. To a lesser extent, elevated sodium concentration suppresses aldosterone secretion, and corticotropin allows aldosterone secretion.

Glucocorticoids

Glucocorticoids are produced primarily in the zona fasciculata. Glucocorticoids affect metabolism in several ways. Glucocorticoids stimulate gluconeogenesis and decrease the use of glucose by cells. Cortisol reduces protein stores in all cells of the body except the liver and increases protein synthesis in the liver. Cortisol also increases amino acids in the blood, decreases amino acid transport to extrahepatic cells, and increases amino acid transport to liver cells. Cortisol mobilizes fatty acids from adipose tissue, increases plasma free fatty acids, and increases the use of free fatty acids for energy. Cortisol, the most clinically important glucocorticoid, accounts for approximately 95% of all glucocorticoid activity.

Corticosterone accounts for a small but significant amount of the total glucocorticoid activity. Cortisol secretion is regulated almost entirely by corticotropin, which is secreted by the anterior pituitary gland in response to corticotropin-releasing hormone (CRH) from the hypothalamus. Serum cortisol inhibits CRH and corticotropin secretion, thus preventing excessive cortisol secretion from the adrenal glands. Corticotropin stimulates cortisol secretion and promotes the growth of the adrenal cortex in conjunction with growth factors such as insulin-like growth factor (IGF)-1 and IGF-2. There is a circadian rhythm in cortisol secretion; the highest cortisol levels occur about 1 hour before waking up. Stress, pain, and inflammation cause increased cortisol production.

Androgens

The term “androgen” refers to any steroid hormone that has masculinizing effects. In men, androgens are responsible for the development of secondary sexual characteristics. Androgens play a less important role in women; however, adrenal androgens are responsible for much of the growth of pubic and axillary hair. Testosterone is the main androgen. Androgens are produced in the adrenal glands and the gonads. In men, the adrenal glands produce about 100 μg/day of testosterone and the testes produce about 7,000 μg/day. In women, 50% to 60% of testosterone is derived from androstenedione conversion in peripheral tissues, 30% is produced directly by the adrenal glands and 20% by the ovary.

Adrenal androgens are formed primarily in the zona reticularis. Dehydroepiandrosterone (DHEA) is the main steroid produced by the adrenal glands. The sulfation of DHEA produces DHEA sulfate (DHEA-S). Adrenal androgens are moderately active male sex hormones. Some of the adrenal androgens are converted to testosterone. The mechanism of stimulation of androgen secretion from the adrenal glands is not well understood. Adrenarche is the maturation of the adrenals, which causes an increase in these androgens and occurs between the ages of 5 and 20.

Adrenarche, therefore, begins well before puberty. Adrenal androgen secretion is partially regulated by corticotropin but also by other unknown factors. The testes secrete testosterone, dihydrotestosterone (DHT), and androstenedione. Gonadal androgen production is controlled by the hypothalamic secretion of GnRH, which causes the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Testosterone is secreted by the Leydig cells of the testes in response to LH stimulation. Most of the testosterone is converted to the more active DHT in target tissues.

Estrogens and progestins

In women, the main function of estrogen is to promote the proliferation and growth of specific cells in the body that is responsible for the development of most secondary sexual characteristics. Progestins are responsible for preparing the uterus for pregnancy and the breasts for lactation. In men, estrogens and progestins generally do not play a clinically significant role in the development of sexual characteristics. In women, estrogens and progestins are derived from the adrenal gland or the gonads. In women with intact ovaries, the adrenal contribution to circulating estrogen is negligible. Estrogens and progestins are secreted at different rates during different parts of the female menstrual cycle.

Estradiol is the prominent ovarian estrogen; estrone and estriol are two other estrogens. Estradiol is 12 times more potent than estrone and 80 times more potent than estriol. The ovaries produce estrone in small amounts, but most of it is formed by peripheral conversion from androgens. Estriol is primarily a metabolite of estrone and estradiol in non-pregnant women. In pregnancy, however, estriol is the main placental estrogen. DHEA-S from the fetal adrenal glands is converted to estriol by the placenta.

The main progestin is progesterone; a minor progestin is 17-hydroxy-progesterone. In the first half of the menstrual cycle, small amounts of progesterone are produced, about half by the ovaries and half by the adrenal cortex. Larger amounts of progesterone are secreted in the last half of the menstrual cycle by the corpus luteum. Men produce a small amount of estrogen (about 1/5 of the production of a non-pregnant woman). Sertoli cells convert a small amount of testosterone to estrogen. Also, estrogens are formed from testosterone and androstenediol in the periphery of the liver.

Animals

Discover different types of animals today

A-Z Animals is the world’s most trusted online animal encyclopedia. Whether you want cute animals, types of wild animals, exotic pets, strange animals, or are looking for complete pet guides for dogs and cats, you’ll find it here! We believe that if people know more about the world’s creatures, they will take better care of them. That’s why we add new animals for you to discover, every day!

The 7 main different types of animals

1. Mammals

The official class of mammals is Mammalia. Animals that are considered mammals include warm-blooded vertebrates that have hair or fur and whose young drink milk. Unlike other types of animals such as birds and insects, all mammalian babies drink milk that comes from their mother’s body. This is one of the key ways to tell if an animal is a mammal.

2. Reptiles

Lizards, dinosaurs, crocodiles, turtles, and snakes all belong to that ancient and robust class of animals known as reptiles. This is a diverse group with more than 10,000 different species and a large representation in the fossil record. Once the dominant terrestrial vertebrates on the planet, reptiles still occupy nearly every ecosystem outside of the far north and south.

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3. Fish

Fish are aquatic vertebrates. They typically have gills, paired fins, a long, scale-covered body, and tend to be cold-blooded. “Fishes” is a term used to refer to lampreys, sharks, coelacanths, and ray-finned fishes, but it is not a taxonomic group, which is a clade or group containing a common ancestor and all its descendants.

4. Birds

Birds, members of the class Aves, include more than 10,400 living species. Their feathers distinguish them from all other classes of animals; no other animal on earth has them. If you see an animal with feathers, it is definitely a bird. Like mammals, birds are warm-blooded vertebrates with four-chambered hearts. However, they are more closely related to reptiles and are thought to have evolved from dinosaurs.

5. Amphibians

The official class of amphibians is Amphibia. To qualify as an amphibian, an animal must be a vertebrate, require water to survive, be cold-blooded, and spend time both on land and in water. Although other animals only live on land or in water, amphibians have the unique ability to thrive equally in both. Amphibians cover more than 6,000 different species worldwide, but around 90% of them are frogs.

6. Invertebrates

The definition of an invertebrate is any animal that does not have a backbone or backbone. The most prolific and easily recognizable members of the invertebrate family are insects. It is estimated that there may be more than 30 million individual species of invertebrates that represent between 90 and 95 per cent of all organisms on the planet.

7. Insects

All insects are part of the taxonomic phylum Arthropoda and are collectively known as arthropods. It is common to see this name misspelt as “anthropoid”, but this is not the correct term. They can be found in almost every environment on the planet and currently account for more than half of all known living organisms in the world. They have gone through many cycles of evolution depending on the resources available to them.

Vaccine

Vaccine, suspension of weakened, killed, or fragmented microorganisms or toxins, or other biological preparation, such as those consisting of antibodies, lymphocytes, or messenger RNA (mRNA), that is given primarily to prevent disease. A vaccine can confer active immunity against a specific harmful agent by stimulating the immune system to attack the agent. Once stimulated by a vaccine, antibody-producing cells called B cells (or B lymphocytes) remain sensitized and ready to respond to the agent should it ever enter the body.

A vaccine can also confer passive immunity by providing antibodies or lymphocytes already produced by an animal or human donor. Vaccines are usually given by injection (parenteral administration), but some are given orally or even through the nose (in the case of the flu vaccine). Vaccines applied to mucosal surfaces, such as those lining the gut or nasal passages, appear to stimulate a greater antibody response and maybe the most effective route of administration.

The first vaccinations

The first vaccine was introduced by British physician Edward Jenner, who in 1796 used the cowpox virus (vaccinia) to confer protection against smallpox, a related virus, in humans. However, before such use, Asian doctors applied the principle of vaccination and gave children dried scabs from the lesions of people suffering from smallpox to protect against the disease.

While some developed immunity, others developed the disease. Jenner’s contribution was to use a substance similar to, but safer than, smallpox to confer immunity. Thus, he took advantage of the relatively rare situation in which immunity to one virus confers protection against another viral disease. In 1881, French microbiologist Louis Pasteur demonstrated immunization against anthrax by injecting sheep with a preparation containing attenuated forms of the bacillus that causes the disease. Four years later he developed a protective suspension against rabies.

Vaccine effectiveness

After the time of Pasteur, an intensive and widespread search for new vaccines was carried out, and vaccines against bacteria and viruses, as well as vaccines against poisons and other toxins, were produced. Through vaccination, smallpox was eradicated worldwide in 1980 and polio cases were reduced by 99 per cent. Other examples of diseases for which vaccines have been developed include mumps, measles, typhoid, cholera, plague, tuberculosis, tularemia, pneumococcal infection, tetanus, influenza, yellow fever, hepatitis A, hepatitis B, some types of encephalitis, and typhus. although some of those vaccines are less than 100 per cent effective or are used only in high-risk populations. Vaccines against viruses provide particularly important immune protection because, unlike bacterial infections, viral infections do not respond to antibiotics.

Types of vaccines

The challenge in vaccine development is to design a vaccine strong enough to prevent infection without making the individual seriously ill. To that end, researchers have devised different types of vaccines. Weakened or attenuated vaccines consist of microorganisms that have lost the ability to cause serious disease but retain the ability to stimulate immunity. They can produce a mild or subclinical form of the disease. Attenuated vaccines include measles, mumps, polio (the Sabin vaccine), rubella, and tuberculosis. Inactivated vaccines are those that contain organisms that have been killed or inactivated with heat or chemicals.

Inactivated vaccines elicit an immune response, but the response is usually less complete than with live vaccines. Because inactivated vaccines are not as effective in fighting infections as those made with attenuated microorganisms, larger quantities of inactivated vaccines are administered. Vaccines against rabies, polio (the Salk vaccine), some forms of influenza, and cholera are made from inactivated microorganisms. Another type of vaccine is a subunit vaccine, which is made from proteins found on the surface of infectious agents. Influenza and hepatitis B vaccines are of that type. When toxins, the metabolic byproducts of infectious organisms, are inactivated to form toxoids, they can be used to boost immunity against tetanus, diphtheria, and whooping cough (whooping cough).

In the late 20th century, advances in laboratory techniques allowed for the refinement of approaches to vaccine development. Medical researchers could identify the genes of a pathogen (disease-causing microorganism) that code for the protein or proteins that stimulate the immune response to that organism. That allowed immune-stimulating proteins (called antigens) to be mass-produced and used in vaccines. It also made it possible to genetically alter pathogens and produce weakened strains of viruses. In this way, the harmful proteins of pathogens can be removed or modified, thus providing a safer and more effective method of manufacturing attenuated vaccines.

Recombinant DNA technology has also proven useful in developing vaccines against viruses that cannot be cultured successfully or are inherently dangerous. The genetic material that codes for the desired antigen is inserted into the attenuated form of a large virus, such as the vaccinia virus, which “piggybacks” the foreign genes. The altered virus is injected into an individual to stimulate the production of antibodies against foreign proteins and thus confer immunity. The approach potentially allows the vaccinia virus to function as a live vaccine against various diseases, once it has received genes derived from the relevant disease-causing microorganisms.

A similar procedure can be followed using a modified bacterium, such as Salmonella typhimurium, as the carrier of a foreign gene. Human papillomavirus (HPV) vaccines are made from virus-like particles (VLPs), which are prepared using recombinant technology. The vaccines do not contain live biological or genetic material from HPV and therefore cannot cause infection. Two types of HPV vaccines have been developed, including a bivalent HPV vaccine, made with VLPs of HPV types 16 and 18, and a quadrivalent vaccine, made with VLPs of HPV types 6, 11, 16 and 18.