The nuclear lamina is a dense fibrillar network inside the nucleus of most cells. It is composed of membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope; the nuclear lamina is associated with the inner face of the double bilayer nuclear envelope, whereas the outer face is continuous with the endoplasmic reticulum. The nuclear lamina is similar in structure to the nuclear matrix, but the latter extends throughout the nucleoplasm; the nuclear lamina consists of two components and nuclear lamin-associated membrane proteins. The lamins are type V intermediate filaments which can be categorized as either A-type or B-type according to homology of their DNA sequences, biochemical properties and cellular localization during the cell cycle. Type V intermediate filaments differ from cytoplasmic intermediate filaments in the way that they have an extended rod domain, that they all carry a nuclear localization signal at their C-terminus and that they display typical tertiary structures.
Lamin polypeptides have an complete α-helical conformation with multiple α-helical domains separated by non-α-helical linkers that are conserved in length and amino acid sequence. Both the C-terminus and the N-terminus are non α-helical, with the C-terminus displaying a globular structure, their molecular weight ranges from 60 to 80 kilodaltons. In the amino acid sequence of a nuclear lamin, there are two phosphoacceptor sites present, flanking the central rod domain. A phosphorylation event at the onset of mitosis leads to a conformational change which causes the disassembly of the nuclear lamina. In the vertebrate genome, lamins are encoded by three genes. By alternative splicing, at least seven different polypeptides are obtained, some of which are specific for germ cells and play an important role in the chromatin reorganisation during meiosis. Not all organisms have the same number of lamin encoding genes; the presence of lamin polypeptides is an exclusive property of Metazoan organisms. The nuclear lamin-associated membrane proteins are either peripheral membrane proteins.
The most important are lamina associated polypeptides 1 and 2, lamin B-receptor, otefin and MAN1. Due to their positioning within or their association with the inner membrane, they mediate the attachment of the nuclear lamina to the nuclear envelope; the nuclear lamina is assembled by interactions of two lamin polypeptides in which the α-helical regions are wound around each other to form a two stranded α-helical coiled-coil structure, followed by a head-to-tail association of the multiple dimers. The linearly elongated polymer is extended laterally by a side-by-side association of polymers, resulting in a 2D structure underlying the nuclear envelope. Next to providing mechanical support to the nucleus, the nuclear lamina plays an essential role in chromatin organization, cell cycle regulation, DNA replication, DNA repair, cell differentiation and apoptosis; the non-random organization of the genome suggests that the nuclear lamina plays a role in chromatin organization. It has been shown that lamin polypeptides have an affinity for binding chromatin through their α-helical domains at specific DNA sequences called matrix attachment regions.
A MAR has a length of 300–1000 bp and has a high A/T content. Lamin A and B can bind core histones through a sequence element in their tail domain. Chromatin that interacts with lamina forms lamina-associated domains; the average length of human LADs is 0.1–10 MBp. LADs are flanked by CTCF-binding cites. At the onset of mitosis, the cellular machinery is engaged in the disassembly of various cellular components including structures such as the nuclear envelope, the nuclear lamina and the nuclear pore complexes; this nuclear breakdown is necessary to allow the mitotic spindle to interact with the chromosomes and to bind them at their kinetochores. These different disassembly events are initiated by the cyclin B/Cdk1 protein kinase complex. Once this complex is activated, the cell is forced into mitosis, by the subsequent activation and regulation of other protein kinases or by direct phosphorylation of structural proteins involved in this cellular reorganisation. After phosphorylation by cyclin B/Cdk1, the nuclear lamina depolymerises and B-type lamins stay associated with the fragments of the nuclear envelope whereas A-type lamins remain soluble throughout the remainder of the mitotic phase.
The importance of the nuclear lamina breakdown at this stage is underlined by experiments where inhibition of the disassembly event leads to a complete cell cycle arrest. At the end of mitosis, there is a nuclear reassembly, regulated in time, starting with the association of'skeletal' proteins on the surface of the still condensed chromosomes, followed by nuclear envelope assembly. Novel nuclear pore complexes are formed through which nuclear lamins are imported by use of their NLS; this typical hierarchy raises the question whether the nuclear lamina at this stage has a stabilizing role or some regulative function, for it is clear that it plays no essential part in the nuclear membrane assembly around chromatin. The presence of lamins in embryonic development is observed in various model o
A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, covered by complex soft tissue and keratinised structures. Since a single digit must bear the full proportion of the animal's weight, borne by that limb, the hoof is of vital importance to the horse; the phrase "no hoof, no horse" underlines how much the health and the strength of the hoof is crucial for horse soundness. Both wild and feral equid hooves have enormous strength and resilience, allowing any gait on any ground. A common example of the feral horse type is the Mustang; the Mustang is, in part, descended from the Iberian horses brought to the Americas by the Spanish, but most herds have ancestry from other breeds. Therefore, the famous Mustang hoof strength is in part a result of natural selection and environment. Thus, it is proposed that other domestic breeds could develop similar hooves if raised under similar conditions; the recent barefoot movement claims that such a strength can be completely restored to domesticated horses, when appropriate trimming and living conditions are applied, to such an extent that horseshoes are no longer necessary in any horse.
If true, it would undermine the belief that "the horseshoe is a necessary evil." The barefoot management system has not, gained a foothold among serious equine professionals, due to three factors: 1) increased strain placed on the hoof in sports, such as eventing and endurance riding, 2) the added weight of the rider and saddle, 3) man-made surfaces, such as concrete and gravel, which can wear the walls down to the sensitive tissue over time. The hoof is made up by an outer part, the hoof capsule and an inner, living part, containing soft tissues and bone; the cornified material of the hoof capsule is different in structure and properties in different parts. Dorsally, it covers and supports P3. Palmarly/plantarly, it protects specialised soft tissues; the upper circular limit of the hoof capsule is the coronet, having an angle to the ground of similar magnitude in each pair of feet. These angles may differ from one horse to another, but not markedly; the walls originate from the coronet band. Walls are longer in the dorsal portion of the hoof, intermediate in length in the lateral portion and short in palmar/plantar portion.
Heels are separated by an elastic, resilient structure named the'frog'. In the palmar/plantar part of the foot, above the heels and the frog, there are two oval bulges named the'bulbs'; when viewed from the lower surface, the hoof wall's free margin encircles most of the hoof. The triangular frog occupies the center area. Lateral to the frog are two grooves, deeper in their posterior portion, named'collateral grooves'. At the heels, the palmar/plantar portion of the walls bend inward following the external surface of collateral grooves to form the bars; the lower surface of the hoof, from the outer walls and the inner frog and bars, is covered by an exfoliating keratinised material, called the'sole'. Just below the coronet, the walls are covered for about an inch by a cornified, opaque'periople' material. In the palmar/plantar part of the hoof, the periople is thicker and more rubbery over the heels, it merges with frog material. Not all horses have the same amount of periople. Dry feet tend to lack this substance.
The walls are considered as a protective shield covering the sensitive internal hoof tissues, as a structure devoted to dissipating the energy of concussion, as a surface to provide grip on different terrains. They are elastic and tough, vary in thickness from 6 to 12 mm; the walls are composed of three distinct layers: the pigmented layer, the water line and the white line. The pigmented layer is generated by the coronet, its color is just like that of the coronet skin from which it is derived. If the coronet skin has any dark patch, the walls show a corresponding pigmented line, from the coronet to the ground, showing the wall's growth direction; this layer has predominately protective role, is not as resistant to ground contact, where it can break and flake away. The water line is built up by the wall's corium, its thickness increases proportionally to the distance from the coronet and, in the lower third of the walls, is thicker than the pigmented layer. It is resistant to contact to the ground, it serves a support function.
The white line is the inner layer of the wall. It is fibrous in structure and light in color. From the underside of the healthy hoof, it is seen as a thin line joining the walls; the white line grows out from the laminar connections. Any visible derangement of the white line indicates some important derangement of laminar connections that fix the walls to the underlying P3 bone. Since the white line is softer than both the walls and the sole, it wears fast where it appears on the surface; the three layers of the wall merge in a single mass and they grow downwards together. If the wall does not wear from sufficient movement on abrasive terrains, th
The lamina or blade in macroscopic algae like seaweed is a flattened structure that forms the principal bulk of the thallus. It is developed into specialised organs such as flotation bladders and reproductive organs; the lamina is an expansion of the stipe which in term is attached to the substrate by the holdfast
In the vertebrate spinal column, each vertebra is an irregular bone with a complex structure composed of bone and some hyaline cartilage, the proportions of which vary according to the segment of the backbone and the species of vertebrate. The basic configuration of a vertebra varies; the upper and lower surfaces of the vertebra body give attachment to the intervertebral discs. The posterior part of a vertebra forms a vertebral arch, in eleven parts, consisting of two pedicles, two laminae, seven processes; the laminae give attachment to the ligamenta flava. There are vertebral notches formed from the shape of the pedicles, which form the intervertebral foramina when the vertebrae articulate; these foramina are the exit conducts for the spinal nerves. The body of the vertebra and the vertebral arch form the vertebral foramen, the larger, central opening that accommodates the spinal canal, which encloses and protects the spinal cord. Vertebrae articulate with each other to give strength and flexibility to the spinal column, the shape at their back and front aspects determines the range of movement.
Structurally, vertebrae are alike across the vertebrate species, with the greatest difference seen between an aquatic animal and other vertebrate animals. As such, vertebrates take their name from the vertebrae; each vertebra is an irregular bone. The size of the vertebrae varies according to placement in the vertebral column, spinal loading and pathology. Along the length of the spine the vertebrae change to accommodate different needs related to stress and mobility; every vertebra has a body, which consists of a large anterior middle portion called the centrum and a posterior vertebral arch called a neural arch. The body is composed of cancellous bone, the spongy type of osseous tissue, whose micro-anatomy has been studied within the pedicle bones; this cancellous bone is in turn, covered by a thin coating of cortical bone, the hard and dense type of osseous tissue. The vertebral arch and processes have thicker coverings of cortical bone; the upper and lower surfaces of the body of the vertebra are flattened and rough in order to give attachment to the intervertebral discs.
These surfaces are the vertebral endplates which are in direct contact with the intervertebral discs and form the joint. The endplates are formed from a thickened layer of the cancellous bone of the vertebral body, the top layer being more dense; the endplates function to contain the adjacent discs, to evenly spread the applied loads, to provide anchorage for the collagen fibers of the disc. They act as a semi-permeable interface for the exchange of water and solutes; the vertebral arch is formed by pedicles and laminae. Two pedicles extend from the sides of the vertebral body to join the body to the arch; the pedicles are short thick processes that extend, one from each side, from the junctions of the posteriolateral surfaces of the centrum, on its upper surface. From each pedicle a broad plate, a lamina, projects backwards and medialwards to join and complete the vertebral arch and form the posterior border of the vertebral foramen, which completes the triangle of the vertebral foramen; the upper surfaces of the laminae are rough to give attachment to the ligamenta flava.
These ligaments connect the laminae of adjacent vertebra along the length of the spine from the level of the second cervical vertebra. Above and below the pedicles are shallow depressions called vertebral notches; when the vertebrae articulate the notches align with those on adjacent vertebrae and these form the openings of the intervertebral foramina. The foramina allow the entry and exit of the spinal nerves from each vertebra, together with associated blood vessels; the articulating vertebrae provide a strong pillar of support for the body. There are seven processes projecting from the vertebra. A major part of a vertebra is a backward extending spinous process; this process points caudally from the junction of the laminae. The spinous process serves to attach ligaments; the two transverse processes, one on each side of the vertebral body, project from either side at the point where the lamina joins the pedicle, between the superior and inferior articular processes. They serve for the attachment of muscles and ligaments, in particular the intertransverse ligaments.
There is a facet on each of the transverse processes of thoracic vertebrae which articulates with the tubercle of the rib. A facet on each side of the thoracic vertebral body articulates with the head of the rib. There are superior and inferior articular facet joints on each side of the vertebra, which serve to restrict the range of movement possible; these facets are joined by a thin portion of the vertebral arch called the pars interarticularis. The transverse process of a lumbar vertebra is sometimes called the costal or costiform process because it corresponds to a rudimentary rib which, as opposed to the thorax, is not developed in the lumbar region. Vertebrae take their names from the regions of the vertebral column. There are thirty-three vertebrae in the human vertebral column—seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five fused sacral vertebrae forming the sacrum and three to five coccygeal vertebrae, forming the coccyx; the regional vertebrae increase in size as they become smaller in the coccyx.
There are seven cervical vertebrae
In mathematics, a planar lamina is a closed set in a plane of mass m and surface density ρ such that: m = ∫ ∫ ρ d x d y, over the closed set. The center of mass of the lamina is at the point where M y moment of the entire lamina about the x-axis and M x moment of the entire lamina about the y-axis. M y, over the closed surface. M x = lim m, n → ∞ ∑ i = 1 m ∑ j = 1 n y i j ∗ ρ Δ A = ∬ y ρ d x d y, over the closed surface. Example 1. Find the center of mass of a lamina with edges given by the lines x = 0, y = x and y = 4 − x where the density is given as ρ = 2 x + 3 y + 2. M = ∫ 0 2 ∫ x 4 − x d y d x Integrate 2x + 3y + 2 with respect to y and substitute the limits 4-x and x m = ∫ 0 2 | x 4 − x d x m = ∫ 0 2 d x m = ∫ 0 2 d x m = ∫ 0 2 ( 8 x − 2 x 2 +
Toxopidae is a small family of araneomorph spiders, first described in 1940. For many years it was sunk into Desidae as a subfamily, although doubts were expressed as to whether this was correct. A large-scale molecular phylogenetic study in 2016 led to the family being revived
Lamia (Basque mythology)
The lamia is a siren- or nereid-like creature in Basque mythology. Lamiak, laminak or amilamiak live in the river, they are beautiful, stay at the shore combing their long hair with a golden comb. They have duck feet. In coastal areas, some believed that there were itsaslamiak in the sea, who had fish tails—a kind of mermaid. Lamiak help those. In some places, bridges were believed to have been built at night by lamiak: Ebrain, Urkulu, Liginaga-Astüe. In some places lamiak had to go away if the bridge they were building at night was left unfinished at cockcrow. People believed. Most lamiak disappeared. A lamia is at the other side of the rainbow combing her hair; when the sun lights her hair, the rainbow opens. In some places male lamiak exist. Sometimes they can enter a house, they are given different names: Maideak, Mairuak, Intxixuak (in Oiartzun, Saindi Maidi. Many toponyms are related to lamiak: Lamikiz, Lamitegi, Lamusin, Lamiñosin. Basajaun Sorginak Mari