International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the
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
Stromatolites or stromatoliths are layered mounds and sheet-like sedimentary rocks that were formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe. Fossilized stromatolites provide records of ancient life on Earth. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens. Stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping and cementation of sedimentary grains by biofilms of microorganisms cyanobacteria, they exhibit a variety of forms and structures, or morphologies, including conical, branching and columnar types. Stromatolites occur in the fossil record of the Precambrian, but are rare today. Few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic precipitation.
Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology. Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. Biddanda et al. found that cyanobacteria exposed to localized beams of light moved towards the light, or expressed phototaxis, increased their photosynthetic yield, necessary for survival. In a novel experiment, the scientists projected a school logo onto a petri dish containing the organisms, which accreted beneath the lighted region, forming the logo in bacteria; the authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony. In both light and dark conditions, the cyanobacteria form clumps that expand outwards, with individual members remaining connected to the colony via long tendrils; this may be a protective mechanism that affords evolutionary benefit to the colony in harsh environments where mechanical forces act to tear apart the microbial mats.
Thus these sometimes elaborate structures, constructed by microscopic organisms working somewhat in unison, are a means of providing shelter and protection from a harsh environment. Some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life, namely stromatolites. However, others regard these patterns as being due to natural material deposition or some other abiogenic mechanism. Scientists have argued for a biological origin of stromatolites due to the presence of organic globule clusters within the thin layers of the stromatolites, of aragonite nanocrystals, because of the persistence of an inferred biological signal through changing environmental circumstances. Stromatolites are a major constituent of the fossil record of the first forms of life on earth, they peaked about 1.25 billion years ago and subsequently declined in abundance and diversity, so that by the start of the Cambrian they had fallen to 20% of their peak.
The most supported explanation is that stromatolite builders fell victim to grazing creatures. Another hypothesis is. Proterozoic stromatolite microfossils include cyanobacteria and some forms of the eukaryote chlorophytes. One genus of stromatolite common in the geologic record is Collenia; the connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation. Fluctuations in metazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes. While prokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis. Cyanobacteria use water, carbon dioxide, sunlight to create their food.
A layer of mucus forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone; these laminations can accrete over time. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis. Layered spherical growth structures termed oncolites are similar to stromatolites and are known from the fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria, common in the fossil record and in modern sediments; the Zebra River Canyon area of the Kubis platform in the dissected Zaris Mountains of south
Biomineralization is the process by which living organisms produce minerals to harden or stiffen existing tissues. Such tissues are called mineralized tissues, it is an widespread phenomenon. Examples include silicates in algae and diatoms, carbonates in invertebrates, calcium phosphates and carbonates in vertebrates; these minerals form structural features such as sea shells and the bone in mammals and birds. Organisms have been producing mineralised skeletons for the past 550 million years. Ca carbonates and Ca phosphates are crystalline, but silica organisms are always non crystalline minerals. Other examples include copper and gold deposits involving bacteria. Biologically-formed minerals have special uses such as magnetic sensors in magnetotactic bacteria, gravity sensing devices and iron storage and mobilization. In terms of taxonomic distribution, the most common biominerals are the phosphate and carbonate salts of calcium that are used in conjunction with organic polymers such as collagen and chitin to give structural support to bones and shells.
The structures of these biocomposite materials are controlled from the nanometer to the macroscopic level, resulting in complex architectures that provide multifunctional properties. Because this range of control over mineral growth is desirable for materials engineering applications, there is significant interest in understanding and elucidating the mechanisms of biologically controlled biomineralization. Among metazoans, biominerals composed of calcium carbonate, calcium phosphate or silica perform a variety of roles such as support and feeding, it is less clear. One hypothesis is. Iron oxide particles may enhance their metabolism. If present on a super-cellular scale, biominerals are deposited by a dedicated organ, defined early in the embryological development; this organ will contain an organic matrix that directs the deposition of crystals. The matrix may be collagen, as in deuterostomes, or based on chitin or other polysaccharides, as in molluscs; the mollusc shell is a biogenic composite material, the subject of much interest in materials science because of its unusual properties and its model character for biomineralization.
Molluscan shells consist of 95–99% calcium carbonate by weight, while an organic component makes up the remaining 1–5%. The resulting composite has a fracture toughness ≈3000 times greater than that of the crystals themselves. In the biomineralization of the mollusc shell, specialized proteins are responsible for directing crystal nucleation, phase and growths dynamics and give the shell its remarkable mechanical strength; the application of biomimetic principles elucidated from mollusc shell assembly and structure may help in fabricating new composite materials with enhanced optical, electronic, or structural properties. The most described arrangement in mollusc shells is the nacre - prismatic shells, known in large shells as Pinna or the pearl oyster. Not only the structure of the layers differ, but their mineralogy and chemical composition differ. Both contain organic components and the organic components are characteristic of the layer, of the species; the structures and arrangements of mollusc shells are diverse, but they share some features: the main part of the shell is a crystalline Ca carbonate, despite some amorphous Ca carbonate occurs.
The examination of the inner structure of the prismatic units, nacreous tablets, foliated laths… shows irregular rounded granules. Fungi are a diverse group of organisms. Studies of their significant roles in geological processes, "geomycology", has shown that fungi are involved with biomineralization and metal-fungal interactions. In studying fungi's roles in biomineralization, it has been found that fungi deposit minerals with the help of an organic matrix, such as a protein, that provides a nucleation site for the growth of biominerals. Fungal growth may produce a copper-containing mineral precipitate, such as copper carbonate produced from a mixture of 2CO3 and CuCl2; the production of the copper carbonate is produced in the presence of proteins made and secreted by the fungi. These fungal proteins that are found extracellularly aid in the size and morphology of the carbonate minerals precipitated by the fungi. In addition to precipitating carbonate minerals, fungi can precipitate uranium-containing phosphate biominerals in the presence of organic phosphorus that acts a substrate for the process.
The fungi produce a hyphal matrix known as mycelium, that localizes and accumulates the uranium minerals that have been precipitated. Although uranium is deemed as toxic towards living organisms, certain fungi such as Aspergillus niger and Paecilomyces javanicus can tolerate it. Though minerals can be produced by fungi, they can be degraded. Oxalic acid production is increased in the presence of glucose for three organic acid producing fungi – Aspergillus niger, Serpula himantioides, Trametes versicolor; these fungi have been found to corrode galena minerals. Degradation of minerals by fungi is carried out through a process known as neogenesis; the order of most to least oxalic acid secreted by t
In chemistry, the term transition metal has three possible meanings: The IUPAC definition defines a transition metal as "an element whose atom has a filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell". Many scientists describe a "transition metal" as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. In actual practice, the f-block lanthanide and actinide series are considered transition metals and are called "inner transition metals". Cotton and Wilkinson expand the brief IUPAC definition by specifying; as well as the elements of groups 4 to 11, they add scandium and yttrium in group 3, which have a filled d subshell in the metallic state. Lanthanum and actinium in group 3 are, classified as lanthanides and actinides respectively. English chemist Charles Bury first used the word transition in this context in 1921, when he referred to a transition series of elements during the change of an inner layer of electrons from a stable group of 8 to one of 18, or from 18 to 32.
These elements are now known as the d-block. In the d-block the atoms of the elements have between 10 d electrons; the elements of groups 4–11 are recognized as transition metals, justified by their typical chemistry, i.e. a large range of complex ions in various oxidation states, colored complexes, catalytic properties either as the element or as ions. Sc and Y in group 3 are generally recognized as transition metals. However, the elements La–Lu and Ac–Lr and group 12 attract different definitions from different authors. Many chemistry textbooks and printed periodic tables classify La and Ac as group 3 elements and transition metals, since their atomic ground-state configurations are s2d1 like Sc and Y; the elements Ce–Lu are considered as the "lanthanide" series and Th–Lr as the "actinide" series.</ref> The two series together are classified as f-block elements, or as "inner transition elements". Some inorganic chemistry textbooks include Ac with the actinides; this classification is based on similarities in chemical behaviour and defines 15 elements in each of the two series though they correspond to the filling of an f subshell, which can only contain 14 electrons.
A third classification defines the f-block elements as La–Yb and Ac–No, while placing Lu and Lr in group 3. This is based on the Aufbau principle for filling electron subshells, in which 4f is filled before 5d, so that the f subshell is full at Yb, while Lu has an s2f14d1 configuration; however La and Ac are exceptions to the Aufbau principle with electron configuration s2d1, so it is not clear from atomic electron configurations whether La or Lu should be considered as transition metals. Zinc and mercury are excluded from the transition metals, as they have the electronic configuration d10s2, with no incomplete d shell. In the oxidation state +2 the ions have the electronic configuration …d10. However, these elements can exist in other oxidation states, including the +1 oxidation state, as in the diatomic ion Hg2+2; the group 12 elements Zn, Cd and Hg may therefore, under certain criteria, be classed as post-transition metals in this case. However, it is convenient to include these elements in a discussion of the transition elements.
For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to include the elements calcium and zinc, as both Ca2+ and Zn2+ have a value of zero, against which the value for other transition metal ions may be compared. Another example occurs in the Irving–Williams series of stability constants of complexes; the recent synthesis of mercury fluoride has been taken by some to reinforce the view that the group 12 elements should be considered transition metals, but some authors still consider this compound to be exceptional. Although meitnerium and roentgenium are within the d-block and are expected to behave as transition metals analogous to their lighter congeners iridium and gold, this has not yet been experimentally confirmed. Early transition metals are on the left side of the periodic table from group 3 to group 7. Late transition metals are on the right side of the d-block, from group 8 to 11; the general electronic configuration of the d-block elements is d1–10n s0–2.
The period 6 and 7 transition metals add f0–14 electrons, which are omitted from the tables below. The Madelung rule predicts that the typical electronic structure of transition metal atoms can be written as ns2dm where the inner d orbital is predicted to be filled after the valence-shell's s orbital is filled; this rule is however only approximate – it only holds for some of the transition elements, only in their neutral ground state. The d-sub-shell is denoted as d - sub-shell; the number of s electrons in the outermost s sub-shell is one or two except palladium, with no electron in that s-sub shell in its ground state. The s-sub-shell in the valence shell is represented as e.g. 4s. In the periodic table, the transition metals are present in eight groups, with some authors including some elements in groups 3 o
In anatomy, soft tissue includes the tissues that connect, support, or surround other structures and organs of the body, not being hard tissue such as bone. Soft tissue includes tendons, fascia, fibrous tissues and synovial membranes, muscles and blood vessels, it is sometimes defined by. Soft tissue has been defined as "nonepithelial, extraskeletal mesenchyme exclusive of the reticuloendothelial system and glia"; the characteristic substances inside the extracellular matrix of this kind of tissue are the collagen and ground substance. The soft tissue is hydrated because of the ground substance; the fibroblasts are the most common cell responsible for the production of soft tissues' fibers and ground substance. Variations of fibroblasts, like chondroblasts, may produce these substances. At small strains, elastin stores most of the strain energy; the collagen fibers are comparatively inextensible and are loose. With increasing tissue deformation the collagen is stretched in the direction of deformation.
When taut, these fibers produce a strong growth in tissue stiffness. The composite behavior is analogous to a nylon stocking, whose rubber band does the role of elastin as the nylon does the role of collagen. In soft tissues, the collagen protects the tissues from injury. Human soft tissue is deformable, its mechanical properties vary from one person to another. Impact testing results showed that the stiffness and the damping resistance of a test subject’s tissue are correlated with the mass and size of the striking object; such properties may be useful for forensics investigation. When a solid object impacts a human soft tissue, the energy of the impact will be absorbed by the tissues to reduce the effect of the impact or the pain level. Soft tissues have the potential to undergo large deformations and still return to the initial configuration when unloaded, i.e. they are hyperelastic materials, their stress-strain curve is nonlinear. The soft tissues are viscoelastic and anisotropic; some viscoelastic properties observable in soft tissues are: relaxation and hysteresis.
In order to describe the mechanical response of soft tissues, several methods have been used. These methods include: hyperelastic macroscopic models based on strain energy, mathematical fits where nonlinear constitutive equations are used, structurally based models where the response of a linear elastic material is modified by its geometric characteristics. Though soft tissues have viscoelastic properties, i.e. stress as function of strain rate, it can be approximated by a hyperelastic model after precondition to a load pattern. After some cycles of loading and unloading the material, the mechanical response becomes independent of strain rate. S = S → S = S Despite the independence of strain rate, preconditioned soft tissues still present hysteresis, so the mechanical response can be modeled as hyperelastic with different material constants at loading and unloading. By this method the elasticity theory is used to model an inelastic material. Fung has called this model as pseudoelastic to point out that the material is not elastic.
In physiological state soft tissues present residual stress that may be released when the tissue is excised. Physiologists and histologists must be aware of this fact to avoid mistakes when analyzing excised tissues; this retraction causes a visual artifact. Fung developed a constitutive equation for preconditioned soft tissues, W = 1 2 with q = a i j k l E i j E k l Q = b i j k l E i j E k l quadratic forms of Green-Lagrange strains E i j and a i j k l, b i j k l and c material constants. W is the strain energy function per volume unit, the mechanical strain energy for a given temperature; the Fung-model, simplified with isotropic hypothesis. This written in respect of the principal stretches: W = 1 2 [ a ( λ 1 2 + λ 2 2 + λ 3
The mollusc shell is a calcareous exoskeleton which encloses and protects the soft parts of an animal in the phylum Mollusca, which includes snails, tusk shells, several other classes. Not all shelled; the ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid and some smaller groups such as the caudofoveata and solenogastres, the derived Xenoturbella. Today, over 100,000 living species bear a shell. Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, this is called conchology—although these terms used to be, to a minor extent still are, used interchangeably by scientists. Within some species of molluscs, there is a wide degree of variation in the exact shape, pattern and color of the shell. A mollusc shell is formed and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are reflected in the shape and form and color of the shell.
When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced; the mantle edge secretes a shell. The organic constituent is made up of polysaccharides and glycoproteins; this organic framework controls the formation of calcium carbonate crystals, dictates when and where crystals start and stop growing, how fast they expand. The shell formation requires certain biological machinery; the shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle; the periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur.
The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills and epithelium, transported by the haemolymph to the calcifying epithelium, stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required; the organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition. Nucleation is endoepithelial in Neopilina and Nautilus, but exoepithelial in the bivalves and gastropods; the formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are conserved, but the proteins in the secretome are derived and evolving. Engrailed serves to demark the edge of the shell field.
In gastropod embryos, Hox1 is expressed. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – though diatoms use silica, not calcite, to form their tests! The shell-secreting area is differentiated early in embryonic development. An area of the ectoderm thickens invaginates to become a "shell gland"; the shape of this gland is tied to the form of the adult shell. The gland subsequently evaginates in molluscs. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland