Nervous tissue
Nervous tissue called neural tissue or nerve tissue, is the main tissue component of the nervous system. The nervous system regulates and controls bodily functions and activity and consists of two parts: the central nervous system comprising the brain and spinal cord, the peripheral nervous system comprising the branching peripheral nerves, it is composed of neurons, or nerve cells, which receive and transmit impulses, neuroglia known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons. Nervous tissue is made up of different types of nerve cells. An axon is the long stem-like part of the cell. Bundles of axons make up the nerves in the PNS and tracts in the CNS. Functions of the nervous system are sensory input, control of muscles and glands and mental activity. Nervous tissue is composed of neurons called nerve cells, neuroglial cells. Four types of neuroglia found in the CNS are astrocytes, microglial cells, ependymal cells and oligodendrocytes.
Two types of neuroglia found in the PNS are Schwann cells. In the central nervous system, the tissue types found are white matter; the tissue is categorized by its neuroglial components. Neurons are cells with specialized features that allow them to receive and facilitate nerve impulses, or action potentials, across their membrane to the next neuron, they possess a large cell body, with cell projections called an axon. Dendrites are thin, branching projections that receive electrochemical signaling to create a change in voltage in the cell. Axons are long projections that carry the action potential away from the cell body toward the next neuron; the bulb-like end of the axon, called the axon terminal, is separated from the dendrite of the following neuron by a small gap called a synaptic cleft. When the action potential travels to the axon terminal, neurotransmitters are released across the synapse and bind to the post-synaptic receptors, continuing the nerve impulse. Neurons are classified both structurally.
Functional classification: Sensory neurons: Relay sensory information in the form of an action potential from the PNS to the CNS Motor neurons: Relay an action potential out of the CNS to the proper effector Interneurons: Cells that form connections between neurons and whose processes are limited to a single local area in the brain or spinal cordStructural classification: Multipolar neurons: Have 3 or more processes coming off the soma. They include interneurons and motor neurons. Bipolar neurons: Sensory neurons that have two processes coming off the soma, one dendrite and one axon Pseudounipolar neurons: Sensory neurons that have one process that splits into two branches, forming the axon and dendrite Unipolar brush cells: Are excitatory glutamatergic interneurons that have a single short dendrite terminating in a brush-like tuft of dendrioles; these are found in the granular layer of the cerebellum. Neuroglia encompasses the non-neural cells in nervous tissue that provide various crucial supportive functions for neurons.
They are smaller than neurons, vary in structure according to their function. Neuroglial cells are classified as follows: Microglial cells: Microglia are macrophage cells that make up the primary immune system for the CNS, they are the smallest neuroglial cell. Astrocytes: Star-shaped macroglial cells with many processes found in the CNS, they are the most abundant cell type in the brain, are intrinsic to a healthy CNS. Oligodendrocytes: CNS cells with few processes, they form myelin sheaths on the axons of a neuron, which are lipid-based insulation that increases the speed at which the action potential, can travel down the axon. NG2 glia: CNS cells that are distinct from astrocytes and microglia, serve as the developmental precursors of oligodendrocytes Schwann cells: The PNS equivalent of oligodendrocytes, they help maintain axons and form myelin sheaths in the PNS. Satellite glial cell: Line the surface of neuron cell bodies in ganglia Enteric glia: Found in the enteric nervous system, within the gastrointestinal tract.
In the central nervous system: Grey matter is composed of cell bodies, unmyelinated axons, protoplasmic astrocytes, satellite oligodendrocytes and few myelinated axons. White matter is composed of myelinated axons, fibrous astrocytes, myelinating oligodendrocytes, microglia. In the Peripheral Nervous System: Ganglion tissue is composed of cell bodies and satellite glial cells. Nerves are composed of myelinated and unmyelinated axons, Schwann cells surrounded by connective tissue; the three layers of connective tissue surrounding each nerve are: Endoneurium. Each nerve axon, or fiber is surrounded by the endoneurium, called the endoneurial tube, channel or sheath; this is a thin, protective layer of connective tissue. Perineurium; each nerve fascicle containing one or more axons, is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. This plays a important role in the protection and support of the nerve fibers and serves to prevent the passage of large molecules from the epineurium into a fascicle.
Epineurium. The epineurium is the outermost layer of dense connective tissue enclosing the nerve; the function of nervous tissue is to form the communication network
Force
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass i.e. to accelerate. Force can be described intuitively as a push or a pull. A force has both direction, making it a vector quantity, it is measured in the SI unit of newtons and represented by the symbol F. The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, is inversely proportional to the mass of the object. Concepts related to force include: thrust. In an extended body, each part applies forces on the adjacent parts; such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate.
Stress causes deformation of solid materials, or flow in fluids. Philosophers in antiquity used the concept of force in the study of stationary and moving objects and simple machines, but thinkers such as Aristotle and Archimedes retained fundamental errors in understanding force. In part this was due to an incomplete understanding of the sometimes non-obvious force of friction, a inadequate view of the nature of natural motion. A fundamental error was the belief that a force is required to maintain motion at a constant velocity. Most of the previous misunderstandings about motion and force were corrected by Galileo Galilei and Sir Isaac Newton. With his mathematical insight, Sir Isaac Newton formulated laws of motion that were not improved for nearly three hundred years. By the early 20th century, Einstein developed a theory of relativity that predicted the action of forces on objects with increasing momenta near the speed of light, provided insight into the forces produced by gravitation and inertia.
With modern insights into quantum mechanics and technology that can accelerate particles close to the speed of light, particle physics has devised a Standard Model to describe forces between particles smaller than atoms. The Standard Model predicts that exchanged particles called gauge bosons are the fundamental means by which forces are emitted and absorbed. Only four main interactions are known: in order of decreasing strength, they are: strong, electromagnetic and gravitational. High-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction. Since antiquity the concept of force has been recognized as integral to the functioning of each of the simple machines; the mechanical advantage given by a simple machine allowed for less force to be used in exchange for that force acting over a greater distance for the same amount of work. Analysis of the characteristics of forces culminated in the work of Archimedes, famous for formulating a treatment of buoyant forces inherent in fluids.
Aristotle provided a philosophical discussion of the concept of a force as an integral part of Aristotelian cosmology. In Aristotle's view, the terrestrial sphere contained four elements that come to rest at different "natural places" therein. Aristotle believed that motionless objects on Earth, those composed of the elements earth and water, to be in their natural place on the ground and that they will stay that way if left alone, he distinguished between the innate tendency of objects to find their "natural place", which led to "natural motion", unnatural or forced motion, which required continued application of a force. This theory, based on the everyday experience of how objects move, such as the constant application of a force needed to keep a cart moving, had conceptual trouble accounting for the behavior of projectiles, such as the flight of arrows; the place where the archer moves the projectile was at the start of the flight, while the projectile sailed through the air, no discernible efficient cause acts on it.
Aristotle was aware of this problem and proposed that the air displaced through the projectile's path carries the projectile to its target. This explanation demands a continuum like air for change of place in general. Aristotelian physics began facing criticism in medieval science, first by John Philoponus in the 6th century; the shortcomings of Aristotelian physics would not be corrected until the 17th century work of Galileo Galilei, influenced by the late medieval idea that objects in forced motion carried an innate force of impetus. Galileo constructed an experiment in which stones and cannonballs were both rolled down an incline to disprove the Aristotelian theory of motion, he showed that the bodies were accelerated by gravity to an extent, independent of their mass and argued that objects retain their velocity unless acted on by a force, for example friction. Sir Isaac Newton described the motion of all objects using the concepts of inertia and force, in doing so he found they obey certain conservation laws.
In 1687, Newton published his thesis Philosophiæ Naturalis Principia Mathematica. In this work Newton set out three laws of motion that to this day are t
Secretion
Secretion is the movement of material from one point to another, e.g. secreted chemical substance from a cell or gland. In contrast, excretion, is the removal of certain substances or waste products from a cell or organism; the classical mechanism of cell secretion is via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell. Secretion in bacterial species means the transport or translocation of effector molecules for example: proteins, enzymes or toxins from across the interior of a bacterial cell to its exterior. Secretion is a important mechanism in bacterial functioning and operation in their natural surrounding environment for adaptation and survival. Eukaryotic cells, including human cells, have a evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum.
As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome; the vesicles containing the properly folded proteins enter the Golgi apparatus. In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur; the proteins are moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles. There is vesicle fusion with the cell membrane at a structure called the porosome, in a process called exocytosis, dumping its contents out of the cell's environment. Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, the cis-golgi has a pH of 6.5.
Secretory vesicles have pHs ranging between 5.0 and 6.0. There are many proteins like FGF2, interleukin-1 etc. which do not have a signal sequence. They do not use the classical ER-golgi pathway; these are secreted through various nonclassical pathways. At least four nonclassical protein secretion pathways have been described, they include 1) direct translocation of proteins across the plasma membrane through membrane transporters, 2) blebbing, 3) lysosomal secretion, 4) release via exosomes derived from multivesicular bodies. In addition, proteins can be released from cells by mechanical or physiological wounding and through nonlethal, transient oncotic pores in the plasma membrane induced by washing cells with serum-free media or buffers. Many human cell types have the ability to be secretory cells, they have a well-developed endoplasmic reticulum and Golgi apparatus to fulfill their function. Tissues in humans that produce secretions include the gastrointestinal tract which secretes digestive enzymes and gastric acid, the lung which secretes surfactants, sebaceous glands which secrete sebum to lubricate the skin and hair.
Meibomian glands in the eyelid secrete sebum to protect the eye. Secretion is not unique to eukaryotes alone - it is present in bacteria and archaea as well. ATP binding cassette type transporters are common to all the three domains of life; the Sec system constituting the Sec Y-E-G complex is another conserved secretion system, homologous to the translocon in the eukaryotic endoplasmic reticulum and the Sec 61 translocon complex of yeast. Some secreted proteins are translocated across the cytoplasmic membrane by the Sec translocon, which requires the presence of an N-terminal signal peptide on the secreted protein. Others are translocated across the cytoplasmic membrane by the twin-arginine translocation pathway. Gram-negative bacteria have two membranes. There are at least six specialized secretion systems in gram-negative bacteria. Many secreted proteins are important in bacterial pathogenesis. Type I secretion is a chaperone dependent secretion system employing the Tol gene clusters; the process begins as a leader sequence HlyA binds HlyB on the membrane.
This signal sequence is specific for the ABC transporter. The HlyAB complex stimulates HlyD which begins to uncoil and reaches the outer membrane where TolC recognizes a terminal molecule or signal on HlyD. HlyD recruits TolC to the inner membrane and HlyA is excreted outside of the outer membrane via a long-tunnel protein channel. Type I secretion system transports various molecules, from ions, drugs, to proteins of various sizes; the molecules secreted vary in size from the small Escherichia coli peptide colicin V, to the Pseudomonas fluorescens cell adhesion protein LapA of 520 kDa. The best characterized are the lipases. Type I secretion is involved in export of non-proteinaceous substrates like cyclic β-glucans and polysaccharides. Proteins secreted through the type II system, or main terminal branch of the general secretory pathway, depend on the Sec or Tat system for initial transport into the periplasm. Once there, they pass through the outer membrane via a multimeric complex of pore forming secretin proteins.
In addition to the secretin protein, 10–15 other inner and outer memb
Microtome
A microtome is a tool used to cut thin slices of material, known as sections. Important in science, microtomes are used in microscopy, allowing for the preparation of samples for observation under transmitted light or electron radiation. Microtomes use steel, glass, or diamond blades depending upon the specimen being sliced and the desired thickness of the sections being cut. Steel blades are used to prepare sections of plant tissues for light microscopy histology. Glass knives are used to slice sections for light microscopy and to slice thin sections for electron microscopy. Industrial grade diamond knives are used to slice hard materials such as bone and plant matter for both light microscopy and for electron microscopy. Gem quality diamond knives are used for slicing thin sections for electron microscopy. Microtomy is a method for the preparation of thin sections for materials such as bones and teeth, an alternative to electropolishing and ion milling. Microtome sections can be made thin enough to section a human hair across its breadth, with section thickness between 50 nm and 100 µm.
In the beginnings of light microscope development, sections from plants and animals were manually prepared using razor blades. It was found that to observe the structure of the specimen under observation it was important to make clean reproducible cuts on the order of 100 µm, through which light can be transmitted; this allowed for the observation of samples using light microscopes in a transmission mode. One of the first devices for the preparation of such cuts was invented in 1770 by George Adams, Jr. and further developed by Alexander Cummings. The device was hand operated, the sample held in a cylinder and sections created from the top of the sample using a hand crank. In 1835, Andrew Prichard developed a table based model which allowed for the vibration to be isolated by affixing the device to the table, separating the operator from the knife. Attribution for the invention of the microtome is given to the anatomist Wilhelm His, Sr. In his Beschreibung eines Mikrotoms, Wilhelm wrote: The apparatus has enabled a precision in work by which I can achieve sections that by hand I cannot create.
Namely it has enabled the possibility of achieving unbroken sections of objects in the course of research. Other sources further attribute the development to a Czech physiologist Jan Evangelista Purkyně. Several sources describe the Purkyne model as the first in practical use; the obscurities in the origins of the microtome are due to the fact that the first microtomes were cutting apparatuses, the developmental phase of early devices is undocumented. At the end of the 1800s, the development of thin and thin samples by microtomy, together with the selective staining of important cell components or molecules allowed for the visualisation of microscope details. Today, the majority of microtomes are a knife-block design with a changeable knife, a specimen holder and an advancement mechanism. In most devices the cutting of the sample begins by moving the sample over the knife, where the advancement mechanism automatically moves forward such that the next cut for a chosen thickness can be made; the section thickness is controlled by an adjustment mechanism.
The most common applications of microtomes are: Traditional Histology Technique: tissues are hardened by replacing water with paraffin. The tissue is cut in the microtome at thicknesses varying from 2 to 50 µm. From there the tissue can be mounted on a microscope slide, stained with appropriate aqueous dye after prior removal of the paraffin, examined using a light microscope. Frozen section procedure: water-rich tissues are hardened by freezing and cut in the frozen state with a freezing microtome or microtome-cryostat; this technique is much faster than traditional histology and is used in conjunction with medical procedures to achieve a quick diagnosis. Cryosections can be used in immunohistochemistry as freezing tissue stops degradation of tissue faster than using a fixative and does not alter or mask its chemical composition as much. Electron Microscopy Technique: after embedding tissues in epoxy resin, a microtome equipped with a glass or gem grade diamond knife is used to cut thin sections.
Sections are stained with an aqueous solution of an appropriate heavy metal salt and examined with a transmission electron microscope. This instrument is called an ultramicrotome; the ultramicrotome is used with its glass knife or an industrial grade diamond knife to cut survey sections prior to thin sectioning. These survey sections are 0.5 to 1 µm thick and are mounted on a glass slide and stained to locate areas of interest under a light microscope prior to thin sectioning for the TEM. Thin sectioning for the TEM is done with a gem quality diamond knife. Complementing traditional TEM techniques ultramicrotomes are found mounted inside an SEM chamber so the surface of the block face can be imaged and removed with the microtome to uncover the next surface for imaging; this technique is called Serial Block-Face Scanning Electron Microscopy. Botanical Microtomy Technique: hard materials like wood and leather require a sledge microtome; these microtomes can not cut as thin as a regular microtome.
Spectroscopy Technique: thin polymer sections are needed in order that the infra-red beam will penetrate the sample under examination. It is normal to cut
Organ (anatomy)
Organs are groups of tissues with similar functions. Plant and animal life relies on many organs. Organs are composed of main tissue, "sporadic" tissues, stroma; the main tissue is that, unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerves, blood vessels, connective tissues. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally-related organs cooperate to form whole organ systems. Organs exist in most multicellular organisms. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. In plants there are three main organs. A hollow organ is an internal organ that forms a hollow tube, or pouch such as the stomach, intestine, or bladder. In the study of anatomy, the term viscus is used to refer to an internal organ, viscera is the plural form. 79 organs have been identified in the human body. In biology, tissue is a cellular organizational level between complete organs.
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 study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant morphology. Classical tools for studying tissues include 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. Two or more organs working together in the execution of a specific body function form an organ system called a biological system or body system.
The functions of organ systems share significant overlap. For instance, the nervous and endocrine system both operate via the hypothalamus. For this reason, the two systems are studied as the neuroendocrine system; the same is true for the musculoskeletal system because of the relationship between the muscular and skeletal systems. Common organ system designations in plants includes the differentiation of root. All parts of the plant above ground, including the functionally distinct leaf and flower organs, may be classified together as the shoot organ system. Animals such as humans have a variety of organ systems; these specific systems are widely studied in human anatomy. Cardiovascular system: pumping and channeling blood to and from the body and lungs with heart and blood vessels. Digestive system: digestion and processing food with salivary glands, stomach, gallbladder, intestines, colon and anus. Endocrine system: communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal body or pineal gland, thyroid and adrenals, i.e. adrenal glands.
Excretory system: kidneys, ureters and urethra involved in fluid balance, electrolyte balance and excretion of urine. Lymphatic system: structures involved in the transfer of lymph between tissues and the blood stream, the lymph and the nodes and vessels that transport it including the Immune system: defending against disease-causing agents with leukocytes, adenoids and spleen. Integumentary system: skin and nails of mammals. Scales of fish and birds, feathers of birds. Muscular system: movement with muscles. Nervous system: collecting and processing information with brain, spinal cord and nerves. Reproductive system: the sex organs, such as ovaries, fallopian tubes, vulva, testes, vas deferens, seminal vesicles and penis. Respiratory system: the organs used for breathing, the pharynx, trachea, bronchi and diaphragm. Skeletal system: structural support and protection with bones, cartilage and tendons; the study of plant organs is referred to as plant morphology, rather than anatomy – as in animal systems.
Organs of plants can be divided into reproductive. Vegetative plant organs include roots and leaves; the reproductive organs are variable. In flowering plants, they are represented by the flower and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions of plants, the reproductive organs are called strobili, in Lycopodiophyta, or gametophores in mosses; the vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction. However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants. Many societies have a system for organ donation, in which a living or deceased donor's organ is transplanted into a person with a failing organ; the transplantation of larger solid organs requires immunosuppression to prevent organ rejection or graft-versus-host disease.
There is considerable interest throughout the world in creating laboratory-grown or artificial organs. The English word "organ" dates back in reference to any musical instrument. By the late 14th
Plant anatomy
Plant anatomy or phytotomy is the general term for the study of the internal structure of plants. It included plant morphology, the description of the physical form and external structure of plants, but since the mid-20th century plant anatomy has been considered a separate field referring only to internal plant structure. Plant anatomy is now investigated at the cellular level, involves the sectioning of tissues and microscopy. Although some plant anatomy studies utilize a systems approach, such as the study of vascular tissues, plant anatomy is more classically divided into the following structural categories: Flower anatomy Calyx Corolla Androecium Gynoecium Leaf anatomy Epidermis Palisade cells Stem anatomy Stem structure Fruit/Seed anatomy Ovule Seed structure Pericarp Accessory fruit Wood anatomy Bark Cork Phloem Vascular cambium Heartwood and sapwood branch collar Root anatomy Root structure About 300 BC Theophrastus wrote a number of plant treatises, only two of which survive, Enquiry into Plants, On the Causes of Plants.
He developed concepts of plant morphology and classification, which did not withstand the scientific scrutiny of the Renaissance. A Swiss physician and botanist, Gaspard Bauhin, introduced binomial nomenclature into plant taxonomy, he published Pinax theatri botanici in 1596, the first to use this convention for naming of species. His criteria for classification included natural relationships, or'affinities', which in many cases were structural, it was in the late 1600s. Italian doctor and microscopist, Marcello Malpighi, was one of the two founders of plant anatomy. In 1671 he published his Anatomia Plantarum, the first major advance in plant physiogamy since Aristotle; the other founder was the British doctor Nehemiah Grew. He published An Idea of a Philosophical History of Plants in 1672 and The Anatomy of Plants in 1682. Grew is credited with the recognition of plant cells, although he called them'vesicles' and'bladders', he identified and described the sexual organs of plants and their parts.
In the eighteenth century, Carl Linnaeus established taxonomy based on structure, his early work was with plant anatomy. While the exact structural level, to be considered to be scientifically valid for comparison and differentiation has changed with the growth of knowledge, the basic principles were established by Linnaeus, he published his master work, Species Plantarum in 1753. In 1802, French botanist Charles-François Brisseau de Mirbel, published Traité d'anatomie et de physiologie végétale establishing the beginnings of the science of plant cytology. In 1812, Johann Jacob Paul Moldenhawer published Beyträge zur Anatomie der Pflanzen, describing microscopic studies of plant tissues. In 1813 a Swiss botanist, Augustin Pyrame de Candolle, published Théorie élémentaire de la botanique, in which he argued that plant anatomy, not physiology, ought to be the sole basis for plant classification. Using a scientific basis, he established structural criteria for defining and separating plant genera.
In 1830, Franz Meyen published the first comprehensive review of plant anatomy. In 1838 German botanist Matthias Jakob Schleiden, published Contributions to Phytogenesis, stating, "the lower plants all consist of one cell, while the higher plants are composed of individual cells" thus confirming and continuing Mirbel's work. A German-Polish botanist, Eduard Strasburger, described the mitotic process in plant cells and further demonstrated that new cell nuclei can only arise from the division of other pre-existing nuclei, his Studien über Protoplasma was published in 1876. Gottlieb Haberlandt, a German botanist, studied plant physiology and classified plant tissue based upon function. On this basis, in 1884 he published Physiologische Pflanzenanatomie in which he described twelve types of tissue systems. British paleobotanists Dunkinfield Henry Scott and William Crawford Williamson described the structures of fossilized plants at the end of the nineteenth century. Scott's Studies in Fossil Botany was published in 1900.
Following Charles Darwin's Origin of Species a Canadian botanist, Edward Charles Jeffrey, studying the comparative anatomy and phylogeny of different vascular plant groups, applied the theory to plants using the form and structure of plants to establish a number of evolutionary lines. He published his The Anatomy of Woody Plants in 1917; the growth of comparative plant anatomy was spearheaded by British botanist Agnes Arber. She published Water Plants: A Study of Aquatic Angiosperms in 1920, Monocotyledons: A Morphological Study in 1925, The Gramineae: A Study of Cereal and Grass in 1934. Following World War II, Katherine Esau published, Plant Anatomy, which became the definitive textbook on plant structure in North American universities and elsewhere, it was still in print as of 2006, she followed up with her Anatomy of seed plants in 1960. Plant morphology Plant physiology Eames, Arthur Johnson. An Introduction to Plant Anatomy 2nd ed. McGraw-Hill, New York, link. Esau, Katherine. Plant Anatomy 2nd ed. Wiley, New York.
Meicenheimer, R. History of Plant Anatomy. Miami University, link. Cutler, D. F.. Anatomy of the Monocotyledons. 10 vols. Oxford University Press. Goffinet, B.. Morphology and classification of the Bryophyta. In: Goffinet, B.. Bryophyte Biology, 2nd ed. Cambridge University Press, pp. 55-138. Jeffrey, E. C.. The anatomy of w
Tight junction
Tight junctions known as occluding junctions or zonulae occludentes are multiprotein junctional complexes whose general function is to prevent leakage of transported solutes and water and seals the paracellular pathway. Tight junctions may serve as leaky pathways by forming selective channels for small cations, anions, or water. Tight junctions are present only in vertebrates; the corresponding junctions that occur in invertebrates are septate junctions. Tight junctions are composed of a branching network of sealing strands, each strand acting independently from the others. Therefore, the efficiency of the junction in preventing ion passage increases exponentially with the number of strands; each strand is formed from a row of transmembrane proteins embedded in both plasma membranes, with extracellular domains joining one another directly. There are at least 40 different proteins composing the tight junctions; these proteins consist of both cytoplasmic proteins. The three major transmembrane proteins are occludin and junction adhesion molecule proteins.
These associate with different peripheral membrane proteins such as ZO-1 located on the intracellular side of plasma membrane, which anchor the strands to the actin component of the cytoskeleton. Thus, tight junctions join together the cytoskeletons of adjacent cells. Transmembrane proteins: Occludin was the first integral membrane protein to be identified, it has a molecular weight of ~60kDa. It consists of four transmembrane domains and both the N-terminus and the C-terminus of the protein are intracellular, it forms one intracellular loop. These loops help regulate paracellular permeability. Occludin plays a key role in cellular structure and barrier function. Claudins are a family of 24 different mammalian proteins, they have a molecular weight of ~20kDa. They have a structure similar to that of occludin in that they have four transmembrane domains and similar loop structure, they are understood to be the backbone of tight junctions and play a significant role in the tight junction's ability to seal the paracellular space.
Different claudins are found in different locations throughout the human body. Junction Adhesion Molecules are part of the immunoglobulin superfamily, they have a molecular weight of ~40kDa. Their structure differs from that of the other integral membrane proteins in that they only have one transmembrane protein instead of four, it helps to regulate the paracellular pathway function of tight junctions and is involved in helping to maintain cell polarity. They perform vital functions: They hold cells together. Barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes such as material transport and maintenance of osmotic balance: Tight junctions help to maintain the polarity of cells by preventing the lateral diffusion of integral membrane proteins between the apical and lateral/basal surfaces, allowing the specialized functions of each surface to be preserved; this aims to preserve the transcellular transport. Tight junctions prevent the passage of molecules and ions through the space between plasma membranes of adjacent cells, so materials must enter the cells in order to pass through the tissue.
Investigation using freeze-fracture methods in electron microscopy is ideal for revealing the lateral extent of tight junctions in cell membranes and has been useful in showing how tight junctions are formed. The constrained intracellular pathway exacted by the tight junction barrier system allows precise control over which substances can pass through a particular tissue. At the present time, it is still unclear whether the control is active or passive and how these pathways are formed. In one study for paracellular transport across the tight junction in kidney proximal tubule, a dual pathway model is proposed: large slit breaks formed by infrequent discontinuities in the TJ complex and numerous small circular pores. In human physiology there are two main types of epithelia using distinct types of barrier mechanism. Epidermal structures such as skin form a barrier from many layers of keratinized squamous cells. Internal epithelia on the other hand more rely on tight junctions for their barrier function.
This kind of barrier is formed by only one or two layers of cells. It was long unclear whether tight cell junctions play any role in the barrier function of the skin and similar external epithelia but recent research suggests that this is indeed the case. Epithelia are classed as "tight" or "leaky", depending on the ability of the tight junctions to prevent water and solute movement: Tight epithelia have tight junctions that prevent most movement between cells. Examples of tight epithelia include the distal convoluted tubule, the collecting duct of the nephron in the kidney, the bile ducts ramifying through liver tissue. Leaky epithelia do not have less complex tight junctions. For instance, the tight junction in the kidney proximal tubule, a leaky epithelium, has only two to three junctional strands, these strands exhibit infrequent large slit breaks. Cadherin Gap junction Tight junction protein Zonulin An Overview of the Tight Junction at Zonapse. Net Occludin in Focus at Zonapse. Net Tight+Junctions at the US National Library of Medicine Medical Subject Headings Histology image: 20502loa – Histology Learning System at Boston University
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