Intercalated discs are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells connected by intercalated discs to work as a single functional organ or syncytium. By contrast, skeletal muscle consists of multinucleated muscle fibers and exhibit no intercalated discs. Intercalated discs support synchronized contraction of cardiac tissue, they occur at the Z line of the sarcomere and can be visualized when observing a longitudinal section of the tissue. Three types of cell junction make up an intercalated disc — fascia adherens and gap junctions. Fascia adherens are anchoring sites for actin, connect to the closest sarcomere. Desmosomes stop separation during contraction by binding filaments. Desmosomes are known as macula adherens. Gap junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. Molecular and comprehensive studies have shown that intercalated discs consist for the most part of mixed type adherens junctions, termed composite junctions or areae compositae.
These represent an amalgamation of typical fascia adherens proteins. Thus cardiomyocyte adherens junctions desmosomes. Histology image: 22502loa – Histology Learning System at Boston University — "Ultrastructure of the Cell: cardiac muscle, intercalated disk "
In biochemistry, intercalation is the insertion of molecules between the planar bases of deoxyribonucleic acid. This process is used as a method for analyzing DNA and it is the basis of certain kinds of poisoning. There are several ways molecules can interact with DNA. Ligands may interact with DNA by electrostatically binding, or intercalating. Intercalation occurs when ligands of an appropriate size and chemical nature fit themselves in between base pairs of DNA; these ligands are polycyclic and planar, therefore make good nucleic acid stains. Intensively studied DNA intercalators include berberine, ethidium bromide, daunomycin and thalidomide. DNA intercalators are used in chemotherapeutic treatment to inhibit DNA replication in growing cancer cells. Examples include doxorubicin and daunorubicin, dactinomycin. Metallointercalators are complexes of a metal cation with polycyclic aromatic ligands; the most used metal ion is ruthenium, because its complexes are slow to decompose in the biological environment.
Other metallic cations that have been used include iridium. Typical ligands attached to the metal ion are dipyridine and terpyridine whose planar structure is ideal for intercalation. In order for an intercalator to fit between base pairs, the DNA must dynamically open a space between its base pairs by unwinding; the degree of unwinding varies depending on the intercalator. This unwinding causes creating an opening of about 0.34 nm. This unwinding induces local structural changes to the DNA strand, such as lengthening of the DNA strand or twisting of the base pairs; these structural modifications can lead to functional changes to the inhibition of transcription and replication and DNA repair processes, which makes intercalators potent mutagens. For this reason, DNA intercalators are carcinogenic, such as the exo 8,9 epoxide of aflatoxin B1 and acridines such as proflavine or quinacrine. Intercalation as a mechanism of interaction between cationic, polycyclic aromatic systems of the correct size was first proposed by Leonard Lerman in 1961.
One proposed mechanism of intercalation is as follows: In aqueous isotonic solution, the cationic intercalator is attracted electrostatically to the surface of the polyanionic DNA. The ligand displaces a sodium and/or magnesium cation present in the "condensation cloud" of such cations that surrounds DNA, thus forming a weak electrostatic association with the outer surface of DNA. From this position, the ligand diffuses along the surface of the DNA and may slide into the hydrophobic environment found between two base pairs that may transiently "open" to form an intercalation site, allowing the ethidium to move away from the hydrophilic environment surrounding the DNA and into the intercalation site; the base pairs transiently form such openings due to energy absorbed during collisions with solvent molecules. Molecular tweezers Intercalation
Arthropod head problem
The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, how they are evolutionarily related to each other. While the dispute has centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897, have been published, including several in the 2000s; the arthropod head problem is popularly known as the endless dispute, the title of a famous paper on the subject by Jacob G. Rempel in 1975, referring to its intractable nature. Although some progress has been made since that time, the precise nature of the labrum and the pre-oral region of arthropods remain controversial. According to recent research, it has been suggested that some key events in the evolution of the arthropod body resulted from changes in certain Hox genes' DNA sequences.
The trunks of arthropods are built out of repeated segments, which are associated with various structures such as a pair of appendages, apodemes for muscle attachment, ganglia and, at least embryologically, coelomic cavities. While many arthropod segments are modified to a greater or lesser extent, it is assumed that the ancestral state was for all of the segments to be nearly identical. However, while the segmental organisation of the trunks of adult arthropods can be readily seen, that of the head is much less obvious. Arthropod heads are fused capsules that bear a variety of complex structures such as the eyes and mouth parts; the challenge that the arthropod head problem has to address is to what extent the various structures of the arthropod head can be resolved into a set of hypothetical ancestral segments. Given the high compaction and complexity of adult arthropod heads, much attention has been directed towards understanding the developmental processes that give rise to them, in the hope that they will reveal their segmental organisation more clearly.
A typical insect head possesses a pair of antennae. Lying above the oesophagus is the brain or supraesophageal ganglion, divided into three pairs of ganglia: the protocerebrum and tritocerebrum from front to back. Nerves from the protocerebrum lead to the large compound eyes. Circum-oesophageal connectives lead from the tritocerebrum around the gut to connect the brain to the ventral ganglionated nerve cord: nerves from the first three pairs of ganglia lead to the mandibles and labium, respectively; the position of the mouth and the circum-oesophageal connectives allows a distinction to be made between pre- and post-oral structures. The myriapod head is similar to that of the insects; the crustacean head is broadly similar to that of the insects, but possesses, in addition, a second pair of antennae that are innervated from the tritocerebrum. In place of the labium, crustaceans possess a second pair of maxillae. Chelicerate head structures differ from those of mandibulates. Behind the mouth lies another pair of mouthparts, the pedipalps, behind them lie the series of walking limbs.
In chelicerates, the leg-bearing segments are fused with the anterior segments to form a prosoma, so that in living arthropods a distinct head only exists in mandibulates. The arthropod head problem has until been predicated on the Articulata theory, i.e. that the arthropods and annelids are close relatives. Although arthropods are direct developers that do not possess a trochophore-like larva, the annelids do. During annelid metamorphosis, segments are added close to the posterior of the body, behind the mouth. Recognition of this led to the concept of a primary, non-segmental component of the body in annelids known as the acron being developed, from which the brain is derived; because the arthropod and annelid heads, in the light of the Articulata theory, were assumed to be structurally homologous in some way, the arthropod head was often considered to incorporate a non-segmental acronal component. Taking the homology between annelid and arthropod heads at face value, Swedish workers such as Hanström and Holmgren assumed that a large part of the arthropod head must correspond to the acron, a view followed by several prominent American insect workers such as Butt and Snodgrass.
They proposed that all pre-oral structures in insects were non-segmental, although such a view is at odds with the preoral position of bona fide appendages such as the antennae. A less extreme set of theories propose that only the protocerebrum and associated structures should be considered to be acronal; the view that the arthropod head must contai
In chemistry, intercalation is the reversible inclusion or insertion of a molecule into materials with layered structures. Examples are found in transition metal dichalcogenides. One famous intercalation host is graphite. Intercalation expands the van der Waals gap between sheets; this energy is supplied by charge transfer between the guest and the host solid, i.e. redox. Two potassium graphite compounds are KC8 and KC24. Carbon fluorides are prepared by reaction of fluorine with graphitic carbon; the color is white, or yellow. The bond between the carbon and fluorine atoms is covalent, thus fluorine is not intercalated; such materials have been considered as a cathode in various lithium batteries. Treating graphite with strong acids in the presence of oxidizing agents causes the graphite to oxidise. Graphite bisulfate, +−, is prepared by this approach using sulfuric acid and a little nitric acid or chromic acid; the analogous graphite perchlorate can be made by reaction with perchloric acid. Another well-known family of intercalation hosts are the layered metal dichalcogenides such as titanium disulfide.
In characteristic manner, intercalation is analyzed by X-ray diffraction, since the spacing between sheets increases, by electrical conductivity, since charge transfer alters the number of charge carriers. A structurally related species is iron oxychloride. An extreme case of intercalation is the complete separation of the layers of the material; this process is called exfoliation. Aggressive conditions are required involving polar solvents and aggressive reagents. In biochemistry, intercalation is the insertion of molecules between the bases of DNA; this process is used as a method for analyzing DNA and it is the basis of certain kinds of poisoning. Clathrates are chemical substances consisting of a lattice that contains molecules. Clathrate compounds are polymeric and envelop the guest molecule. Inclusion compounds are molecules, whereas clathrates are polymeric. Intercalation compounds are not 3-dimensional, unlike clathrate compounds. According to IUPAC, clathrates are "Inclusion compounds in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."
Clathrate compound: where a molecule is included into a lattice Graphite intercalation compound Intercalation Stacking
The intercalated duct called intercalary duct, is the portion of an exocrine gland leading directly from the acinus to a striated duct. The intercalated duct forms part of the intralobular duct; this duct has the thinnest epithelium of any part of the duct system, the epithelium is classified as "low" simple cuboidal. They are found in both the pancreas and in salivary glands