In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are formed by the functional grouping together of multiple tissues; the English word "tissue" is derived from the French "tissu", meaning something, "woven", from the verb tisser, "to weave". The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant anatomy; the classical tools for studying tissues are the paraffin block in which tissue is embedded and sectioned, the histological stain, the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.
Animal tissues are grouped into four basic types: connective, muscle and epithelial. Collections of tissues joined in structural units to serve a common function compose organs. While all eumetazoan animals can be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals; the epithelium in all birds and animals is derived from the ectoderm and endoderm, with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that composes the vasculature. By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier; this tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, the digestive tract.
It serves functions of protection and absorption, is separated from other tissues below by a basal lamina. Connective tissues are fibrous tissues, they are made up of cells separated by non-living material, called an extracellular matrix. This matrix can be rigid. For example, blood contains plasma as its matrix and bone's matrix is rigid. Connective tissue holds them in place. Blood, tendon, ligament and areolar tissues are examples of connective tissues. One method of classifying connective tissues is to divide them into three types: fibrous connective tissue, skeletal connective tissue, fluid connective tissue. Muscle cells form the active contractile tissue of the body known as muscle tissue or muscular tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle tissue is separated into three distinct categories: visceral or smooth muscle, found in the inner linings of organs. Cells comprising the central nervous system and peripheral nervous system are classified as nervous tissue.
In the central nervous system, neural tissues form spinal cord. In the peripheral nervous system, neural tissues form the cranial nerves and spinal nerves, inclusive of the motor neurons; the epithelial tissues are formed by cells that cover the organ surfaces, such as the surface of skin, the airways, the reproductive tract, the inner lining of the digestive tract. The cells comprising an epithelial layer are linked via tight junctions. In addition to this protective function, epithelial tissue may be specialized to function in secretion and absorption. Epithelial tissue helps to protect organs from microorganisms and fluid loss. Functions of epithelial tissue: The cells of the body's surface form the outer layer of skin. Inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs. Epithelial tissues help in absorption of water and nutrients. Epithelial tissues help in the elimination of waste. Epithelial tissues hormones in the form of glands; some epithelial tissue perform secretory functions.
They secrete a variety of substances such as sweat, enzymes, etc. There are many kinds of epithelium, nomenclature is somewhat variable. Most classification schemes combine a description of the cell-shape in the upper layer of the epithelium with a word denoting the number of layers: either simple or stratified. However, other cellular features, such as cilia may be described in the classification system; some common kinds of epithelium are listed below: Simple squamous epithelium Stratified squamous epithelium Simple cuboidal epithelium Transitional epithelium Pseudostratified columnar epithelium Columnar epithelium Glandular epithelium Ciliated columnar epithelium In plant anatomy, tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, the vascular tissue. Epidermis - Cells forming the outer surface of the leaves and of the young plant body. Vascular tissue - The primary components of vascular tissue are the xylem and phloem; these transport nutrients internally.
Ground tissue - Ground tissue is less differentiated than other tissues. Ground tis
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are proteins, but in non-protein coding genes such as transfer RNA or small nuclear RNA genes, the product is a functional RNA; the process of gene expression is used by all known life—eukaryotes and utilized by viruses—to generate the macromolecular machinery for life. Several steps in the gene expression process may be modulated, including the transcription, RNA splicing and post-translational modification of a protein. Gene regulation gives the cell control over structure and function, is the basis for cellular differentiation and the versatility and adaptability of any organism. Gene regulation may serve as a substrate for evolutionary change, since control of the timing and amount of gene expression can have a profound effect on the functions of the gene in a cell or in a multicellular organism. In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype, i.e. observable trait.
The genetic code stored in DNA is "interpreted" by gene expression, the properties of the expression give rise to the organism's phenotype. Such phenotypes are expressed by the synthesis of proteins that control the organism's shape, or that act as enzymes catalysing specific metabolic pathways characterising the organism. Regulation of gene expression is thus critical to an organism's development. A gene is a stretch of DNA. Genomic DNA consists of two antiparallel and reverse complementary strands, each having 5' and 3' ends. With respect to a gene, the two strands may be labeled the "template strand," which serves as a blueprint for the production of an RNA transcript, the "coding strand," which includes the DNA version of the transcript sequence.. The production of the RNA copy of the DNA is called transcription, is performed in the nucleus by RNA polymerase, which adds one RNA nucleotide at a time to a growing RNA strand as per the complementarity law of the bases; this RNA is complementary to the template 3' → 5' DNA strand, itself complementary to the coding 5' → 3' DNA strand.
Therefore, the resulting 5' → 3' RNA strand is identical to the coding DNA strand with the exception that Thymines are replaced with uracils in the RNA. A coding DNA strand reading "ATG" is indirectly transcribed through the “TAC” in the non-coding template strand as "AUG" in the mRNA. In prokaryotes, transcription is carried out by a single type of RNA polymerase, which needs a DNA sequence called a Pribnow box as well as a sigma factor to start transcription. In eukaryotes, transcription is performed by three types of RNA polymerases, each of which needs a special DNA sequence called the promoter and a set of DNA-binding proteins—transcription factors—to initiate the process. RNA polymerase. RNA polymerase II transcribes all protein-coding genes but some non-coding RNAs. Pol II includes a C-terminal domain, rich in serine residues; when these residues are phosphorylated, the CTD binds to various protein factors that promote transcript maturation and modification. RNA polymerase III transcribes 5S rRNA, transfer RNA genes, some small non-coding RNAs.
Transcription ends. While transcription of prokaryotic protein-coding genes creates messenger RNA, ready for translation into protein, transcription of eukaryotic genes leaves a primary transcript of RNA, which first has to undergo a series of modifications to become a mature mRNA; these include 5' capping, set of enzymatic reactions that add 7-methylguanosine to the 5' end of pre-mRNA and thus protect the RNA from degradation by exonucleases. The m7G cap is bound by cap binding complex heterodimer, which aids in mRNA export to cytoplasm and protect the RNA from decapping. Another modification is 3' polyadenylation, they occur if polyadenylation signal sequence is present in pre-mRNA, between protein-coding sequence and terminator. The pre-mRNA is first cleaved and a series of ~200 adenines are added to form poly tail, which protects the RNA from degradation. Poly tail is bound by multiple poly-binding proteins necessary for mRNA export and translation re-initiation. A important modification of eukaryotic pre-mRNA is RNA splicing.
The majority of eukaryotic pre-mRNAs consist of alternating segments called introns. During the process of splicing, an RNA-protein catalytical complex known as spliceosome catalyzes two transesterification reactions, which remove an intron and release it in form of lariat structure, splice neighbouring exons together. In certain cases, some introns or exons can be either removed or retained in mature mRNA; this so-called alternative splicing creates series of different transcripts originating from a single gene. Because these transcripts can be translated into different proteins, splicing extends the complexity of eukaryotic gene expression. Extensive RNA processing may be an evolutionary advantage made possible by the nucleus of eukaryotes. In prokaryotes and translation happen together, whilst in eukaryotes, the nuclear membrane separates the two processes, giving time for RNA processing to
Aromatase called estrogen synthetase or estrogen synthase, is an enzyme responsible for a key step in the biosynthesis of estrogens. It is CYP19A1, a member of the cytochrome P450 superfamily, which are monooxygenases that catalyze many reactions involved in steroidogenesis. In particular, aromatase is responsible for the aromatization of androgens into estrogens; the aromatase enzyme can be found in many tissues including gonads, adipose tissue, blood vessels and bone, as well as in tissue of endometriosis, uterine fibroids, breast cancer, endometrial cancer. It is an important factor in sexual development. Aromatase is localized in the endoplasmic reticulum where it is regulated by tissue-specific promoters that are in turn controlled by hormones and other factors, it catalyzes the last steps of estrogen biosynthesis from androgens. These steps include three successive hydroxylations of the 19-methyl group of androgens, followed by simultaneous elimination of the methyl group as formate and aromatization of the A-ring.
Androstenedione + 3O2 + 3NADPH + 3H+ ⇌ Estrone + Formate + 4H2O + 3NADP+ Testosterone + 3O2 + 3NADPH + 3H+ ⇌ 17β-estradiol + Formate + 4H2O + 3NADP+ Aromatase is expressed in the gonads, brain, adipose tissue and other tissues. It is undetectable in adult human liver; the gene expresses two transcript variants. In humans, the gene CYP19, located on chromosome 15q21.1, encodes the aromatase enzyme. The gene has nine coding exons and a number of alternative non-coding first exons that regulate tissue specific expression. CYP19 is present in an early-diverging chordate, the cephalochordate amphioxus, but not in the earlier diverging tunicate Ciona intestinalis. Thus, the aromatase gene evolved early in chordate evolution and does not appear to be present in nonchordate invertebrates. However, estrogens may be synthesized in some via other unknown pathways. Factors known to increase aromatase activity include age, insulin and alcohol. Aromatase activity is decreased by prolactin, anti-Müllerian hormone and the common herbicide glyphosate.
Aromatase activity appears to be enhanced in certain estrogen-dependent local tissue next to breast tissue, endometrial cancer and uterine fibroids. Aromatase is highly present during the differentiation of ovaries, it is susceptible to environmental influences temperature. In species with temperature-dependent sex determination, aromatase is expressed in higher quantities at temperatures that yield female offspring. Despite the fact that data suggest temperature controls aromatase quantities, other studies have shown that aromatase can overpower the effects of temperature: if exposed to more aromatase at a male-producing temperature, the organism will develop female and conversely, if exposed to less aromatase at female-producing temperatures, the organism will develop male. In organisms that develop through genetic sex determination, temperature does not affect aromatase expression and function, suggesting that aromatase is the target molecule for temperature during TSD, it varies from species to species whether it is the aromatase protein that has different activity at different temperatures or whether the amount of transcription undergone by the aromatase gene is what is temperature-sensitive, but in either case, differential development is observed at different temperatures.
Aromatase in the brain is only expressed in neurons. However, following penetrative brain injury of both mice and zebra finches, it has been shown to be expressed in astrocytes. Furthermore, it has been shown to decrease apoptosis following brain injury in zebra finches; this is thought to be due to the neuroprotective actions including estradiol. Research has found that two pro-inflammatory cytokines, interleukin-1β and interleukin-6, are responsible for the induction of aromatase expression in astrocytes following penetrative brain injury in the zebra finch. A number of investigators have reported on a rather rare syndrome of excess aromatase activity. In boys, it can lead to gynecomastia, in girls to precocious puberty and gigantomastia. In both sexes, early epiphyseal closure leads to short stature; this condition is due to mutations in the CYP19A1 gene. It is inherited in an autosomal dominant fashion, it has been suggested that the pharaoh Akhenaten and other members of his family may have suffered from this disorder, but more recent genetic tests suggest otherwise.
It is one of the causes of familial precocious puberty—a condition first described in 1937. This syndrome is due to a mutation of gene CYP19 and inherited in an autosomal recessive way. Accumulations of androgens during pregnancy may lead to virilization of a female at birth. Females will have primary amenorrhea. Individuals of both sexes will be tall, as lack of estrogen does not bring the epiphyseal lines to closure; the inhibition of aromatase can cause hypoestrogenism. The following natural products have been found to have inhibiting effects on aromatase. Extracts of certain mushrooms have been shown to inhibit aromatase in vitro. Aromatase inhibitors, which stop the production of estrogen in postmenopausal women, have become useful in the management of patien
Aromatase inhibitors are a class of drugs used in the treatment of breast cancer in postmenopausal women and gynecomastia in men. They may be used off-label to reduce estrogen conversion when using external testosterone, they may be used for chemoprevention in high risk women. Aromatase is the enzyme, it converts the enone ring of androgen precursors such as testosterone, to a phenol, completing the synthesis of estrogen. As breast and ovarian cancers require estrogen to grow, AIs are taken to either block the production of estrogen or block the action of estrogen on receptors. In contrast to premenopausal women, in whom most of the estrogen is produced in the ovaries, in postmenopausal women estrogen is produced in peripheral tissues of the body; because some breast cancers respond to estrogen, lowering estrogen production at the site of the cancer with aromatase inhibitors has been proven to be an effective treatment for hormone-sensitive breast cancer in postmenopausal women. Aromatase inhibitors are not used to treat breast cancer in premenopausal women because, prior to menopause, the decrease in estrogen activates the hypothalamus and pituitary axis to increase gonadotropin secretion, which in turn stimulates the ovary to increase androgen production.
The heightened gonadotropin levels upregulate the aromatase promoter, increasing aromatase production in the setting of increased androgen substrate. This would counteract the effect of the aromatase inhibitor in premenopausal women since total estrogen increased. Ongoing areas of clinical research include optimizing adjuvant hormonal therapy in postmenopausal women with breast cancer. Although tamoxifen traditionally was the drug treatment of choice, but the ATAC trial showed that AI gives superior clinical results in postmenopausal women with localized estrogen receptor positive breast cancer. Trials of AIs in the adjuvant setting, when given to prevent relapse after surgery for breast cancer, show that they are associated with a better disease-free survival than tamoxifen, but few conventionally-analyzed clinicals trials have shown that AIs have an overall survival advantage compared with tamoxifen, there is no good evidence they are better tolerated. Aromatase inhibitors such as testolactone have been approved for the treatment of gynecomastia in children and adolescents.
Ovarian stimulation with the aromatase inhibitor letrozole has been proposed for ovulation induction in order to treat unexplained female infertility. In a multi-center study funded by the National Institute of Child Health and Development, ovarian stimulation with letrozole resulted in a lower frequency of multiple gestation but a lower frequency of live birth, as compared with gonadotropin but not with clomiphene. In women, side effects include an increased risk for developing osteoporosis and joint disorders such as arthritis and joint pain. Men do not appear to exhibit the same adverse effects on bone health. Bisphosphonates are sometimes prescribed to prevent the osteoporosis induced by aromatase inhibitors, but have another serious side effect, osteonecrosis of the jaw; as statins have a bone strengthening effect, combining a statin with an aromatase inhibitor could help prevent fractures and suspected cardiovascular risks, without potential of causing osteonecrosis of the jaw. The more common adverse events associated with the use of aromatase inhibitors include decreased rate of bone maturation and growth, aggressive behavior, adrenal insufficiency, kidney failure, hair loss, liver dysfunction.
Patients with liver, kidney or adrenal abnormalities are at a higher risk of developing adverse events. Aromatase inhibitors work by inhibiting the action of the enzyme aromatase, which converts androgens into estrogens by a process called aromatization; as breast tissue is stimulated by estrogens, decreasing their production is a way of suppressing recurrence of the breast tumor tissue. The main source of estrogen is the ovaries in premenopausal women, while in post-menopausal women most of the body's estrogen is produced in peripheral tissues, a few CNS sites in various regions within the brain. Estrogen is produced and acts locally in these tissues, but any circulating estrogen, which exerts systemic estrogenic effects in men and women, is the result of estrogen escaping local metabolism and spreading to the circulatory system. There are two types of aromatase inhibitors approved to treat breast cancer: Irreversible steroidal inhibitors, such as exemestane, forms a permanent and deactivating bond with the aromatase enzyme.
Nonsteroidal inhibitors, such as the triazoles anastrozole and letrozole, inhibit the synthesis of estrogen via reversible competition. Available aromatase inhibitors include: Aminoglutethimide inhibits the enzyme P450scc and so decreases synthesis of all steroid hormones. Testolactone Anastrozole Letrozole Exemestane Vorozole Formestane Fadrozole 1,4,6-Androstatrien-3,17-dione 4-Androstene-3,6,17-trione In addition to pharmaceutical AIs, some natural elements have aromatase inhibiting effects, such as damiana leaves. Investigations and research has been undertaken to study the use of aromatase inhibitors to stimulate ovulation, to suppress estrogen production. Aromatase inhibitors have been shown to reverse age-related declines in testosterone, including primary hypogonadism. Extracts of certain mushrooms have been shown to inhibit aromatase when evaluated by enzyme assays, with white mushroom having shown the greatest ability to inhibit the enzyme
Testosterone is the primary male sex hormone and an anabolic steroid. In male humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, the growth of body hair. In addition, testosterone is involved in health and well-being, the prevention of osteoporosis. Insufficient levels of testosterone in men may lead to abnormalities including frailty and bone loss. Testosterone is a steroid from the androstane class containing a keto and hydroxyl groups at the three and seventeen positions respectively, it is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. It exerts its action through binding to and activation of the androgen receptor. In humans and most other vertebrates, testosterone is secreted by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about 7 to 8 times as great as in adult females.
As the metabolism of testosterone in males is more pronounced, the daily production is about 20 times greater in men. Females are more sensitive to the hormone. In addition to its role as a natural hormone, testosterone is used as a medication, for instance in the treatment of low testosterone levels in men and breast cancer in women. Since testosterone levels decrease as men age, testosterone is sometimes used in older men to counteract this deficiency, it is used illicitly to enhance physique and performance, for instance in athletes. In general, androgens such as testosterone promote protein synthesis and thus growth of tissues with androgen receptors. Testosterone can be described as having anabolic effects. Anabolic effects include growth of muscle mass and strength, increased bone density and strength, stimulation of linear growth and bone maturation. Androgenic effects include maturation of the sex organs the penis and the formation of the scrotum in the fetus, after birth a deepening of the voice, growth of facial hair and axillary hair.
Many of these fall into the category of male secondary sex characteristics. Testosterone effects can be classified by the age of usual occurrence. For postnatal effects in both males and females, these are dependent on the levels and duration of circulating free testosterone. Effects before birth are divided into two categories, classified in relation to the stages of development; the first period occurs between 6 weeks of the gestation. Examples include genital virilisation such as midline fusion, phallic urethra, scrotal thinning and rugation, phallic enlargement. There is development of the prostate gland and seminal vesicles. During the second trimester, androgen level is associated with sex formation; this period affects the femininization or masculinization of the fetus and can be a better predictor of feminine or masculine behaviours such as sex typed behaviour than an adult's own levels. A mother's testosterone level during pregnancy is correlated with her daughter's sex-typical behavior as an adult, the correlation is stronger than with the daughter's own adult testosterone level.
Early infancy androgen effects are the least understood. In the first weeks of life for male infants, testosterone levels rise; the levels remain in a pubertal range for a few months, but reach the detectable levels of childhood by 4–7 months of age. The function of this rise in humans is unknown, it has been theorized that brain masculinization is occurring since no significant changes have been identified in other parts of the body. The male brain is masculinized by the aromatization of testosterone into estrogen, which crosses the blood–brain barrier and enters the male brain, whereas female fetuses have α-fetoprotein, which binds the estrogen so that female brains are not affected. Before puberty effects of rising androgen levels occur in both girls; these include adult-type body odor, increased oiliness of skin and hair, pubarche, axillary hair, growth spurt, accelerated bone maturation, facial hair. Pubertal effects begin to occur when androgen has been higher than normal adult female levels for months or years.
In males, these are usual late pubertal effects, occur in women after prolonged periods of heightened levels of free testosterone in the blood. The effects include:Growth of spermatogenic tissue in testicles, male fertility, penis or clitoris enlargement, increased libido and frequency of erection or clitoral engorgement occurs. Growth of jaw, brow and nose and remodeling of facial bone contours, in conjunction with human growth hormone occurs. Completion of bone maturation and termination of growth; this occurs indirectly via estradiol metabolites and hence more in men than women. Increased muscle strength and mass, shoulders become broader and rib cage expands, deepening of voice, growth of the Adam's apple. Enlargement of sebaceous glands; this might cause subcutaneous fat in face decreases. Pubic hair extends to thighs and up toward umbilicus, development of facial hair, loss of scalp hair, increase in chest hair, periareolar hair, perianal hair, leg hair, armpit hair. Testosterone is necessary for normal sperm development.
It activates genes in Sertoli cells. It regulates acute HPA response
The cell is the basic structural and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the "building blocks of life"; the study of cells is called cellular biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as multicellular; the number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years ago. Cells are of two types: eukaryotic, which contain a nucleus, prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotes include two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling, they are simpler and smaller than eukaryotic cells, lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome, in direct contact with the cytoplasm; the nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: Enclosing the cell is the cell envelope – consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule.
Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma and Thermoplasma which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter; the cell wall consists of peptidoglycan in bacteria, acts as an additional barrier against exterior forces. It prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment; some eukaryotic cells have a cell wall. Inside the cell is the cytoplasmic region that contains the genome and various sorts of inclusions; the genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid.
Plasmids encode additional genes, such as antibiotic resistance genes. On the outside and pili project from the cell's surface; these are structures made of proteins that facilitate communication between cells. Plants, fungi, slime moulds and algae are all eukaryotic; these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles in which specific activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA; this nucleus gives the eukaryote its name, which means "true kernel". Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may not be present; the eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins.
All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation and thermosensation. Cilia may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." Motile eukaryotes can move using motile flagella. Motile cells are absent in flowering plants. Eukaryotic flagella are more complex than those of prokaryotes. All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out, maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells possess DNA, the hereditary material of genes, RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery.
There are other kinds of biomolecules in cells. This article lists these primary cellular components briefly
A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: sex steroids. Within those two classes are five types according to the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens and progestogens. Vitamin D derivatives are a sixth related hormone system with homologous receptors, they have some of the characteristics of true steroids as receptor ligands. Steroid hormones help control metabolism, immune functions and water balance, development of sexual characteristics, the ability to withstand illness and injury; the term steroid describes both hormones produced by the body and artificially produced medications that duplicate the action for the occurring steroids. The natural steroid hormones are synthesized from cholesterol in the gonads and adrenal glands; these forms of hormones are lipids. They can pass through the cell membrane as they are fat-soluble, bind to steroid hormone receptors to bring about changes within the cell.
Steroid hormones are carried in the blood, bound to specific carrier proteins such as sex hormone-binding globulin or corticosteroid-binding globulin. Further conversions and catabolism occurs in the liver, in other "peripheral" tissues, in the target tissues. A variety of synthetic steroids and sterols have been contrived. Most are steroids, but some nonsteroidal molecules can interact with the steroid receptors because of a similarity of shape; some synthetic steroids are weaker or stronger than the natural steroids whose receptors they activate. Some examples of synthetic steroid hormones: Glucocorticoids: alclometasone, dexamethasone, cortisone Mineralocorticoid: fludrocortisone Vitamin D: dihydrotachysterol Androgens: oxandrolone, testosterone, nandrolone Oestrogens: diethylstilbestrol and estradiol Progestins: norethisterone, medroxyprogesterone acetate, hydroxyprogesterone caproate; some steroid antagonists: Androgen: cyproterone acetate Progestins: mifepristone, gestrinone Steroid hormones are transported through the blood by being bound to carrier proteins—serum proteins that bind them and increase the hormones' solubility in water.
Some examples are sex hormone-binding globulin, corticosteroid-binding globulin, albumin. Most studies say. In order to be active, steroid hormones must free themselves from their blood-solubilizing proteins and either bind to extracellular receptors, or passively cross the cell membrane and bind to nuclear receptors; this idea is known as the free hormone hypothesis. This idea is shown in Figure 1 to the right. One study has found that these steroid-carrier complexes are bound by megalin, a membrane receptor, are taken into cells via endocytosis. One possible pathway is that once inside the cell these complexes are taken to the lysosome, where the carrier protein is degraded and the steroid hormone is released into the cytoplasm of the target cell; the hormone follows a genomic pathway of action. This process is shown in Figure 2 to the right; the role of endocytosis in steroid hormone transport is not well understood and is under further investigation. In order for steroid hormones to cross the lipid bilayer of cells they must overcome energetic barriers that would prevent their entering or exiting the membrane.
Gibbs free energy is an important concept here. These hormones, which are all derived from cholesterol, have hydrophilic functional groups at either end and hydrophobic carbon backbones; when steroid hormones are entering membranes free energy barriers exist when the functional groups are entering the hydrophobic interior of membrane, but it is energetically favorable for the hydrophobic core of these hormones to enter lipid bilayers. These energy barriers and wells are reversed for hormones exiting membranes. Steroid hormones enter and exit the membrane at physiologic conditions, they have been shown experimentally to cross membranes near a rate of 20 μm/s, depending on the hormone. Though it is energetically more favorable for hormones to be in the membrane than in the ECF or ICF, they do in fact leave the membrane once they have entered it; this is an important consideration because cholesterol—the precursor to all steroid hormones—does not leave the membrane once it has embedded itself inside.
The difference between cholesterol and these hormones is that cholesterol is in a much larger negative Gibb's free energy well once inside the membrane, as compared to these hormones. This is because the aliphatic tail on cholesterol has a favorable interaction with the interior of lipid bilayers. There are many different mechanisms. All of these different pathways can be classified as having either a genomic effect, or a non-genomic effect. Genomic pathways are slow and result in altering transcription levels of certain proteins in the cell; the first identified mechanisms of steroid hormone action were the genomic effects. In this pathway, the free hormones first pass through the cell membrane because they are fat soluble. In the cytoplasm, the steroid may or may not undergo an enzyme-mediated alteration such as reduction, hydroxylation, or aromatization; the steroid binds to a specific steroid hormone receptor known as a nuclear receptor, a large metalloprotein. Upon steroid binding, many kinds of ster