Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
White blood cell
White blood cells are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells. Leukocytes are found throughout the body, including lymphatic system. All white blood cells have nuclei, which distinguishes them from the other blood cells, the anucleated red blood cells and platelets. Types of white blood cells can be classified in standard ways. Two pairs of broadest categories classify them either by cell lineage; these broadest categories can be further divided into the five main types: neutrophils, basophils and monocytes. These types are distinguished by their physical and functional characteristics. Monocytes and neutrophils are phagocytic. Further subtypes can be classified; the number of leukocytes in the blood is an indicator of disease, thus the white blood cell count is an important subset of the complete blood count.
The normal white cell count is between 4 × 109/L and 1.1 × 1010/L. In the US, this is expressed as 4,000 to 11,000 white blood cells per microliter of blood. White blood cells make up 1% of the total blood volume in a healthy adult, making them less numerous than the red blood cells at 40% to 45%. However, this 1 % of the blood makes a large difference to health. An increase in the number of leukocytes over the upper limits is called leukocytosis, it is normal. It is abnormal, when it is neoplastic or autoimmune in origin. A decrease below the lower limit is called leukopenia; this indicates a weakened immune system. The name "white blood cell" derives from the physical appearance of a blood sample after centrifugation. White cells are found in the buffy coat, a thin white layer of nucleated cells between the sedimented red blood cells and the blood plasma; the scientific term leukocyte directly reflects its description. It is derived from the Greek roots leuk- meaning "white" and cyt- meaning "cell".
The buffy coat may sometimes be green if there are large amounts of neutrophils in the sample, due to the heme-containing enzyme myeloperoxidase that they produce. All white blood cells are nucleated, which distinguishes them from the anucleated red blood cells and platelets. Types of leukocytes can be classified in standard ways. Two pairs of broadest categories classify them either by cell lineage; these broadest categories can be further divided into the five main types: neutrophils, basophils and monocytes. These types are distinguished by their physical and functional characteristics. Monocytes and neutrophils are phagocytic. Further subtypes can be classified. Granulocytes are distinguished from agranulocytes by their nucleus shape and by their cytoplasm granules; the other dichotomy is by lineage: Myeloid cells are distinguished from lymphoid cells by hematopoietic lineage. Lymphocytes can be further classified as T cells, B cells, natural killer cells. Neutrophils are the most abundant white blood cell, constituting 60-70% of the circulating leukocytes, including two functionally unequal subpopulations: neutrophil-killers and neutrophil-cagers.
They defend against fungal infection. They are first responders to microbial infection, they are referred to as polymorphonuclear leukocytes, although, in the technical sense, PMN refers to all granulocytes. They have a multi-lobed nucleus; this gives the neutrophils the appearance of having multiple nuclei, hence the name polymorphonuclear leukocyte. The cytoplasm may look transparent because of fine granules. Neutrophils are active in phagocytosing bacteria and are present in large amount in the pus of wounds; these cells are not able to die after having phagocytosed a few pathogens. Neutrophils are the most common cell type seen in the early stages of acute inflammation; the life span of a circulating human neutrophil is about 5.4 days. Eosinophils compose about 2-4% of the WBC total; this count fluctuates throughout the day and during menstruation. It rises in response to allergies, parasitic infections, collagen diseases, disease of the spleen and central nervous system, they are rare in the blood, but numerous in the mucous membranes of the respiratory and lower urinary tracts.
They deal with parasitic infections. Eosinophils are the predominant inflammatory cells in allergic reactions; the most important causes of eosinophilia include allergies such as asthma, hay fever, hives. They secrete chemicals that destroy these large parasites, such as hook worms and tapeworms, that are too big for any one WBC to phagocytize. In general, their nucleus is bi-lobed; the lobes are connected by a thin strand. The cytoplasm is full of granules that assume a characteristic pink-orange color with eosin stain
Connective tissue is one of the four basic types of animal tissue, along with epithelial tissue, muscle tissue, nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes that envelop the brain and spinal cord are composed of connective tissue, they protect the body. All connective tissue consists of three main components: ground substance and cells. Not all authorities include blood or lymph as connective tissue because they lack the fiber component. All are immersed in the body water; the cells of connective tissue include fibroblasts, macrophages, mast cells and leucocytes. The term "connective tissue" was introduced in 1830 by Johannes Peter Müller; the tissue was recognized as a distinct class in the 18th century. Connective tissue can be broadly subdivided into connective tissue proper, special connective tissue. Connective tissue proper consists of loose connective tissue and dense connective tissue Loose and dense connective tissue are distinguished by the ratio of ground substance to fibrous tissue.
Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue. Dense regular connective tissue, found in structures such as tendons and ligaments, is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions. Special connective tissue consists of reticular connective tissue, adipose tissue, cartilage and blood. Other kinds of connective tissues include fibrous and lymphoid connective tissues. Fibroareolar tissue is a mix of fibrous and areolar tissue. New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue. Fibroblasts are the cells responsible for the production of some CT. Type I collagen is present in many forms of connective tissue, makes up about 25% of the total protein content of the mammalian body.
Characteristics of CT: Cells are spread through an extracellular fluid. Ground substance - A clear and viscous fluid containing glycosaminoglycans and proteoglycans to fix the body water and the collagen fibers in the intercellular spaces. Ground substance slows the spread of pathogens. Fibers. Not all types of CT are fibrous. Examples of non-fibrous CT include adipose blood. Adipose tissue gives "mechanical cushioning" to the body, among other functions. Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place; the matrix of blood is plasma. Both the ground substance and proteins create the matrix for CT. Connective tissues are derived from the mesenchyme. Types of fibers: Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved. Loose and dense irregular connective tissue, formed by fibroblasts and collagen fibers, have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, carbon dioxide and waste substances to diffuse from cells back into circulation.
They allow organs to resist stretching and tearing forces. Dense regular connective tissue, which forms organized structures, is a major functional component of tendons and aponeuroses, is found in specialized organs such as the cornea. Elastic fibers, made from elastin and fibrillin provide resistance to stretch forces, they are found in the walls of large blood vessels and in certain ligaments in the ligamenta flava. In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma—or structural support—for the parenchyma—or functional part—of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos, capable of differentiation into all types of mature connective tissue. Another type of undifferentiated connective tissue is mucous connective tissue, found inside the umbilical cord. Various types of specialized tissues and cells are classified under the spectrum of connective tissue, are as diverse as brown and white adipose tissue, blood and bone.
Cells of the immune system, such as macrophages, mast cells, plasma cells and eosinophils are found scattered in loose connective tissue, providing the ground for starting inflammatory and immune responses upon the detection of antigens. There are many types of connective tissue disorders, such as: Connective tissue neoplasms including sarcomas such as hemangiopericytoma and malignant peripheral nerve sheath tumor in nervous tissue. Congenital diseases include Ehlers-Danlos Syndrome. Myxomatous degeneration – a pathological weakening of connective tissue. Mixed connective tissue disease – a disease of the autoimmune system undifferentiated connective tissue disease. Systemic lupus erythematosus – a major autoimmune disease of connective tissue Scurvy, caused by a deficiency of vitamin C, necessary for the synthesis of collagen. For microscopic viewing, most of the connective tissue staining-techniques, colour tissue fibers in contrasting shades. Collagen may be differentially stained by any of the following: Van Gieson's stain Masson's trichrome stain Mallory's t
Granulocytes are a category of white blood cells characterized by the presence of granules in their cytoplasm. They are called polymorphonuclear leukocytes or polymorphonuclear neutrophils because of the varying shapes of the nucleus, lobed into three segments; this distinguishes them from the mononuclear agranulocytes. In common parlance, the term polymorphonuclear leukocyte refers to "neutrophil granulocytes", the most abundant of the granulocytes. Granulocytes are produced via granulopoiesis in the bone marrow. There are four types of granulocytes: Basophils Eosinophils Neutrophils Mast cellsExcept for the mast cells, their names are derived from their staining characteristics. Neutrophils are found in the bloodstream and are the most abundant type of phagocyte, constituting 60% to 65% of the total circulating white blood cells, consisting of two subpopulations: neutrophil-killers and neutrophil-cagers. One litre of human blood contains about five billion neutrophils, which are about 12–15 micrometers in diameter.
Once neutrophils have received the appropriate signals, it takes them about thirty minutes to leave the blood and reach the site of an infection. Neutrophils do not return to the blood. Mature neutrophils are smaller than monocytes, have a segmented nucleus with several sections. Neutrophils do not exit the bone marrow until maturity, but during an infection neutrophil precursors called myelocytes and promyelocytes are released. Neutrophils have three strategies for directly attacking micro-organisms: phagocytosis, release of soluble anti-microbials, generation of neutrophil extracellular traps. Neutrophils are professional phagocytes: they are ferocious eaters and engulf invaders coated with antibodies and complement, as well as damaged cells or cellular debris; the intracellular granules of the human neutrophil have long been recognized for their protein-destroying and bactericidal properties. Neutrophils can secrete products that stimulate macrophages. Neutrophils have two types of granules.
Primary granules contain cationic proteins and defensens that are used to kill bacteria, proteolytic enzymes and cathepsin G to break down proteins, lysozyme to break down bacterial cell walls, myeloperoxidase. In addition, secretions from the primary granules of neutrophils stimulate the phagocytosis of IgG antibody-coated bacteria; the secondary granules contain compounds that are involved in the formation of toxic oxygen compounds and lactoferrin. Neutrophil extracellular traps comprise a web of fibers composed of chromatin and serine proteases that trap and kill microbes extracellularly. Trapping of bacteria is a important role for NETs in sepsis, where NET are formed within blood vessels. Eosinophils have kidney-shaped lobed nuclei; the number of granules in an eosinophil can vary because they have a tendency to degranulate while in the blood stream. Eosinophils play a crucial part in the killing of parasites because their granules contain a unique, toxic basic protein and cationic protein.
These cells have a limited ability to participate in phagocytosis, they are professional antigen-presenting cells, they regulate other immune cell functions, they are involved in the destruction of tumor cells, they promote the repair of damaged tissue. A polypeptide called interleukin-5 interacts with eosinophils and causes them to grow and differentiate. Basophils are one of the least abundant cells in bone blood. Like neutrophils and eosinophils, they have lobed nuclei. Basophils have receptors that can bind to IgE, IgG, histamine; the cytoplasm of basophils contains a varied amount of granules. Granule contents of basophils are abundant with histamine, chondroitin sulfate, platelet-activating factor, other substances; when an infection occurs, mature basophils will be released from the bone marrow and travel to the site of infection. When basophils are injured, they will release histamine, which contributes to the inflammatory response that helps fight invading organisms. Histamine causes increased permeability of capillaries close to the basophil.
Injured basophils and other leukocytes will release another substance called prostaglandins that contributes to an increased blood flow to the site of infection. Both of these mechanisms allow blood-clotting elements to be delivered to the infected area. Increased permeability of the inflamed tissue allows f
A microscope is an instrument used to see objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small structures using such an instrument. Microscopic means invisible to the eye. There are many types of microscopes, they may be grouped in different ways. One way is to describe the way the instruments interact with a sample to create images, either by sending a beam of light or electrons to a sample in its optical path, or by scanning across, a short distance from the surface of a sample using a probe; the most common microscope is the optical microscope, which uses light to pass through a sample to produce an image. Other major types of microscopes are the fluorescence microscope, the electron microscope and the various types of scanning probe microscopes. Although objects resembling lenses date back 4000 years and there are Greek accounts of the optical properties of water-filled spheres followed by many centuries of writings on optics, the earliest known use of simple microscopes dates back to the widespread use of lenses in eyeglasses in the 13th century.
The earliest known examples of compound microscopes, which combine an objective lens near the specimen with an eyepiece to view a real image, appeared in Europe around 1620. The inventor is unknown. Several revolve around the spectacle-making centers in the Netherlands including claims it was invented in 1590 by Zacharias Janssen and/or Zacharias' father, Hans Martens, claims it was invented by their neighbor and rival spectacle maker, Hans Lippershey, claims it was invented by expatriate Cornelis Drebbel, noted to have a version in London in 1619. Galileo Galilei seems to have found after 1610 that he could close focus his telescope to view small objects and, after seeing a compound microscope built by Drebbel exhibited in Rome in 1624, built his own improved version. Giovanni Faber coined the name microscope for the compound microscope Galileo submitted to the Accademia dei Lincei in 1625; the first detailed account of the microscopic anatomy of organic tissue based on the use of a microscope did not appear until 1644, in Giambattista Odierna's L'occhio della mosca, or The Fly's Eye.
The microscope was still a novelty until the 1660s and 1670s when naturalists in Italy, the Netherlands and England began using them to study biology. Italian scientist Marcello Malpighi, called the father of histology by some historians of biology, began his analysis of biological structures with the lungs. Robert Hooke's Micrographia had a huge impact because of its impressive illustrations. A significant contribution came from Antonie van Leeuwenhoek who achieved up to 300 times magnification using a simple single lens microscope, he sandwiched a small glass ball lens between the holes in two metal plates riveted together, with an adjustable-by-screws needle attached to mount the specimen. Van Leeuwenhoek re-discovered red blood cells and spermatozoa, helped popularise the use of microscopes to view biological ultrastructure. On 9 October 1676, van Leeuwenhoek reported the discovery of micro-organisms; the performance of a light microscope depends on the quality and correct use of the condensor lens system to focus light on the specimen and the objective lens to capture the light from the specimen and form an image.
Early instruments were limited until this principle was appreciated and developed from the late 19th to early 20th century, until electric lamps were available as light sources. In 1893 August Köhler developed a key principle of sample illumination, Köhler illumination, central to achieving the theoretical limits of resolution for the light microscope; this method of sample illumination produces lighting and overcomes the limited contrast and resolution imposed by early techniques of sample illumination. Further developments in sample illumination came from the discovery of phase contrast by Frits Zernike in 1953, differential interference contrast illumination by Georges Nomarski in 1955. In the early 20th century a significant alternative to the light microscope was developed, an instrument that uses a beam of electrons rather than light to generate an image; the German physicist, Ernst Ruska, working with electrical engineer Max Knoll, developed the first prototype electron microscope in 1931, a transmission electron microscope.
The transmission electron microscope works on similar principles to an optical microscope but uses electrons in the place of light and electromagnets in the place of glass lenses. Use of electrons, instead of light, allows for much higher resolution. Development of the transmission electron microscope was followed in 1935 by the development of the scanning electron microscope by Max Knoll. Although TEMs were being used for research before WWII, became popular afterwards, the SEM was not commercially available until 1965. Transmission electron microscopes became popular following the Second World War. Ernst Ruska, working at Siemens, developed the first commercial transmission electron microscope and, in the 1950s, major scientific conferences on electron microscopy started being held. In 1965, the first commercial scanning electron microscope was developed by Profess
In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm; the main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, various cytoplasmic inclusions. The cytoplasm is about 80% water and colorless; the submicroscopic ground cell substance, or cytoplasmatic matrix which remains after exclusion the cell organelles and particles is groundplasm. It is the hyaloplasm of light microscopy, high complex, polyphasic system in which all of resolvable cytoplasmic elements of are suspended, including the larger organelles such as the ribosomes, the plant plastids, lipid droplets, vacuoles. Most cellular activities take place within the cytoplasm, such as many metabolic pathways including glycolysis, processes such as cell division; the concentrated inner area is called the endoplasm and the outer layer is called the cell cortex or the ectoplasm.
Movement of calcium ions in and out of the cytoplasm is a signaling activity for metabolic processes. In plants, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming; the term was introduced by Rudolf von Kölliker in 1863 as a synonym for protoplasm, but it has come to mean the cell substance and organelles outside the nucleus. There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles the vacuoles and sometimes the plastids; the physical properties of the cytoplasm have been contested in recent years. It remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelles while maintaining the cell’s structure; the flow of cytoplasmic components plays an important role in many cellular functions which are dependent on the permeability of the cytoplasm. An example of such function is cell signalling, a process, dependent on the manner in which signaling molecules are allowed to diffuse across the cell.
While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures require aid in moving through the cytoplasm. The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm. There has long been evidence, it is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal solution and at other times like an integrated network, forming a solid mass. This theory thus proposes that the cytoplasm exists in distinct fluid and solid phases depending on the level of interaction between cytoplasmic components, which may explain the differential dynamics of different particles observed moving through the cytoplasm, it has been proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. In this theory, the greater the concentration of cytoplasmic components, the less the cytoplasm behaves like a liquid and the more it behaves as a solid glass, freezing larger cytoplasmic components in place.
A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defence strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of small proteins and metabolites, helping to kickstart growth upon the cell's revival from dormancy. There has been research examining the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an alternative approach, the aggregate random forces within the cell caused by motor proteins explain the non-Brownian motion of cytoplasmic constituents; the three major elements of the cytoplasm are the cytosol and inclusions. The cytosol is the portion of the cytoplasm not contained within membrane-bound organelles. Cytosol makes up about 70% of the cell volume and is a complex mixture of cytoskeleton filaments, dissolved molecules, water; the cytosol's filaments include the protein filaments such as actin filaments and microtubules that make up the cytoskeleton, as well as soluble proteins and small structures such as ribosomes and the mysterious vault complexes.
The inner and more fluid portion of the cytoplasm is referred to as endoplasm. Due to this network of fibres and high concentrations of dissolved macromolecules, such as proteins, an effect called macromolecular crowding occurs and the cytosol does not act as an ideal solution; this crowding effect alters. Organelles, are membrane-bound structures inside the cell that have specific functions; some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, in plant cells, chloroplasts. The inclusions are small particles of insoluble substances suspended in the cytosol. A huge range of inclusions exist in different cell types, range from crystals of calcium oxalate or silicon dioxide in plants, to granules of energy-storage materials such as starch, glycogen, or polyhydroxybutyrate. A widespread example are lipid droplets, which are spherical droplets composed of lipids and proteins that are used in both prokaryotes and eukaryotes as a way of storing lipids such as fatty acids and sterols.
Lipid droplets make up much of the volume of adipocytes, which are specialized lipid-st
Ticks are small arachnids 3 to 5 mm long, part of the order Parasitiformes. Along with mites, they constitute the subclass Acari. Ticks are ectoparasites, living by feeding on the blood of mammals and sometimes reptiles and amphibians. Ticks had evolved by the Cretaceous period, the most common form of fossilisation being immersed in amber. Ticks are distributed around the world in warm, humid climates. All ticks belong to one of two major families, the Ixodidae or hard ticks, the Argasidae or soft ticks. Adults have ovoid or pear-shaped bodies which become engorged with blood when they feed, eight legs; as well as having a hard shield on their dorsal surfaces, hard ticks have a beak-like structure at the front containing the mouthparts whereas soft ticks have their mouthparts on the underside of the body. Both families locate a potential host by odour or from changes in the environment. Ticks have four stages to their lifecycle, namely egg, larva and adult. Ixodid ticks have three hosts, taking at least a year to complete their lifecycle.
Argasid ticks have up to seven nymphal stages, each one requiring a blood meal. Because of their habit of ingesting blood, ticks are vectors of at least twelve diseases that affect humans and other animals. Fossilized ticks are known from the Cretaceous onwards, most in amber, they most originated in the Cretaceous, with most of the evolution and dispersal occurring during the Tertiary. The oldest example is an argasid bird tick from Cretaceous New Jersey amber; the younger Baltic and Dominican ambers have yielded examples which can be placed in living genera. The tick Deinocroton draculi has been found with dinosaur feathers preserved in Cretaceous Burmese amber from 99 million years ago. There are three families of ticks; the two large ones are the sister families of Ixodidae and Argasidae. The third is Nuttalliellidae, named for the bacteriologist George Nuttall, it comprises a single species, Nuttalliella namaqua, is the most basal lineage. Ticks are related to the mites, within the subclass Acarina.
RDNA analysis suggests that the Ixodidae are a clade, but that the Argasidae may be paraphyletic. The Ixodidae contains over 700 species of hard ticks with a scutum or hard shield, which the Argasidae lack; the Argasidae contains about 200 species. They have no scutum, the capitulum is concealed beneath the body; the family Nuttalliellidae contains only a single species, Nuttalliella namaqua, a tick found in southern Africa from Tanzania to Namibia and South Africa. The phylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014 maximum parsimony study of amino acid sequences of twelve mitochondrial proteins; the Argasidae appear monophyletic in this study. Tick species are distributed around the world, but they tend to flourish more in countries with warm, humid climates, because they require a certain amount of moisture in the air to undergo metamorphosis, because low temperatures inhibit their development from egg to larva. Ticks are widely distributed among host taxa, which include marsupial and placental mammals, reptiles such as snakes and lizards, amphibians.
Ticks of domestic animals cause considerable harm to livestock by transmission of many species of pathogen, as well as causing anaemia and damaging wool and hides. Some of the most debilitating species occur in tropical countries. Tropical bont ticks affect most domestic animals and occur in Africa and the Caribbean; the spinose ear tick has a worldwide distribution, the young feeding inside the ears of cattle and wild animals. In general, ticks are to be found wherever their host species occur. Migrating birds carry ticks with them on their journeys; the species of tick differed between the autumn and spring migrations because of the seasonal periodicities of the different species. For an ecosystem to support ticks, it must satisfy two requirements: the population density of host species in the area must be high enough, humidity must be high enough for ticks to remain hydrated. Due to their role in transmitting Lyme disease, ixodid ticks the North American I. scapularis, have been studied using geographic information systems to develop predictive models for ideal tick habitats.
According to these studies, certain features of a given microclimate – such as sandy soil, hardwood trees and the presence of deer – were determined to be good predictors of dense tick populations. Ticks, like mites, are arthropods that have lost the segmentation of the abdomen that their ancestors had, there has subsequently been a fusion of the abdomen with the cephalothorax; the tagmata typical of other Chelicera have been replaced by two new body sections, the anterior capitulum, retractable and contains the mouthparts, the posterior idiosoma which contains the legs, digestive tract, reproductive organs. The capitulum is a feeding structure with mouthparts adapted for piercing skin and sucking blood. Features of the capitulum include the basis capitulum and hypostome; the basis capitulum supports the rest of the feeding structures. Palps have a sensory role and are composed of three sections; the hypostome is used for blood extraction and is a hollow, tube-like structure. The ventral side of the idiosoma bears sclerites, the gonopore is located between