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In biology, tissue is a cellular organisational 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 then formed by the functional grouping together of multiple tissues.
Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution. Despite the complexity of the science, there are certain unifying concepts that consolidate it into a single, coherent field. Biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the creation and extinction of species. Living organisms are open systems that survive by transforming energy and decreasing their local entropy to maintain a stable and vital condition defined as homeostasis.
The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.
Organs are groups of tissues with similar functions. Plant and animal life relies on many organs that coexist in organ systems.
The English word "tissue" is derived from the French "tissu", meaning something that is "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 then sectioned, the histological stain, and the optical microscope. In the last couple of decades,[ clarification needed ] developments in electron microscopy, immunofluorescence, and 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.
Histology, also microanatomy, is the branch of biology which studies the microscopic anatomy of animal and plant tissues. It is commonly studied using a light microscope or electron microscope, the specimen having been fixed, sectioned, stained, and mounted on a microscope slide. Histological studies may be conducted using tissue culture, where live animal cells are isolated and maintained in an artificial environment for various research projects. The ability to visualize or differentially identify microscopic structures is frequently enhanced through the use of staining. Histology is one of the major preclinical subjects in medical school. Medical students are expected to be familiar with the morphological features and function of all cells and tissues of the human body from an early stage of their studies, so histology often stretches over several semesters.
Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease. Specifically, in clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. In contrast, cytopathology examines (1) free cells or (2) tissue micro-fragments.
Plant anatomy or phytotomy is the general term for the study of the internal structure of plants. Originally 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 frequently investigated at the cellular level, and often involves the sectioning of tissues and microscopy.
Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues joined in units to serve a common function compose organs. While all eumetazoans (except [[Poricontain 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.
Connective tissue (CT) is one of the four basic types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes that envelop the brain and spinal cord are composed of connective tissue. They support and protect the body. All connective tissue consists of three main components: fibers, ground substance and cells. Not all authorities include blood or lymph as connective tissue because they lack the fiber component. All are immersed in the body water.
Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. This is opposed to other components or tissues in muscle such as tendons or perimysium. It is formed during embryonic development through a process known as myogenesis.
Nervous tissue, also 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 (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, or nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.
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, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.
Ectoderm is one of the three primary germ layers in the very early embryo. The other two layers are the mesoderm and endoderm, with the ectoderm as the most exterior layer. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma, meaning "skin."
Endoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm and mesoderm, with the endoderm being the innermost layer. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.
In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm and endoderm, with the mesoderm as the middle layer between them.
Connective tissues are fibrous tissues made up of cells separated by non-living material, which is called an extracellular matrix. This matrix can be liquid or rigid. For example, blood contains plasma as its matrix and bone's matrix is rigid. Connective tissue gives shape to organs and holds them in place. Blood, bone, tendon, ligament, adipose, 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, and fluid connective tissue.
In biology, the extracellular matrix (ECM) is a three-dimensional network of extracellular macromolecules, such as collagen, enzymes, and glycoproteins, that provide structural and biochemical support of surrounding cells. Because multicellularity evolved independently in different multicellular lineages, the composition of ECM varies between multicellular structures; however, cell adhesion, cell-to-cell communication and differentiation are common functions of the ECM.
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; skeletal muscle, typically attached to bones, which generate gross movement; and cardiac muscle, found in the heart, where it contracts to pump blood throughout an organism.
The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body.
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 to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.
Animal locomotion, in ethology, is any of a variety of methods that animals use to move from one place to another. Some modes of locomotion are (initially) self-propelled, e.g., running, swimming, jumping, flying, hopping, soaring and gliding. There are also many animal species that depend on their environment for transportation, a type of mobility called passive locomotion, e.g., sailing, kiting (spiders), rolling or riding other animals (phoresis).
Cells comprising the central nervous system and peripheral nervous system are classified as nervous (or neural) tissue. In the central nervous system, neural tissues form the brain and 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, and the inner lining of the digestive tract. The cells comprising an epithelial layer are linked via semi-permeable, tight junctions; hence, this tissue provides a barrier between the external environment and the organ it covers. In addition to this protective function, epithelial tissue may also be specialized to function in secretion, excretion and absorption. Epithelial tissue helps to protect organs from microorganisms, injury, and fluid loss.
Functions of epithelial tissue:
There are many kinds of epithelium, and 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 (one layer of cells) or stratified (multiple layers of cells). However, other cellular features such as cilia may also be described in the classification system. Some common kinds of epithelium are listed below:
In plant anatomy, tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
Plant tissues can also be divided differently into two types:
Meristematic tissue consists of actively dividing cells, and leads to increase in length and thickness of the plant. The primary growth of a plant occurs only in certain, specific regions, such as in the tips of stems or roots. It is in these regions that meristematic tissue is present. Cells in these tissues are roughly spherical or polyhedral, to rectangular in shape, and have thin cell walls. New cells produced by meristem are initially those of meristem itself, but as the new cells grow and mature, their characteristics slowly change and they become differentiated as components of the region of occurrence of meristematic tissues, being classified as:
The cells of meristematic tissues are similar in structure and have thin and elastic primary cell wall made up of cellulose. They are compactly arranged without inter-cellular spaces between them. Each cell contains a dense cytoplasm and a prominent nucleus. Dense protoplasm of meristematic cells contains very few vacuoles. Normally the meristematic cells are oval, polygonal or rectangular in shape.
Meristematic tissue cells have a large nucleus with small or no vacuoles, and no intercellular spaces.
Permanent tissues may be defined as a group of living or dead cells formed by meristematic tissue and have lost their ability to divide and have permanently placed at fixed positions in the plant body. Meristematic tissues that take up a specific role lose the ability to divide. This process of taking up a permanent shape, size and a function is called cellular differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissues. There are 3 types of permanent tissues:
A group of cells which are similar in origin; similar in structure and similar in function are called simple permanent tissue. They are of four types:
Parenchyma (para - 'beside'; enchyma - 'tissue') is the bulk of a substance. In plants, it consists of relatively unspecialized living cells with thin cell walls that are usually loosely packed so that intercellular spaces are found between cells of this tissue. These are generally isodiametric, in shape. This tissue provides support to plants and also stores food. In some situations, parenchyma contains chlorophyll and performs photosynthesis, in which case it is called a chlorenchyma. In aquatic plants, large air cavities are present in parenchyma to give support to them to float on water. Such a parenchyma type is called aerenchyma. Some of the parenchyma cells have metabolic waste and is known as idioblast. Spindle shape fiber also contained into this cell to support them and known as prosenchyma, succulent parenchyma also noted.
Collenchyma is Greek word where "Colla" means gum and "enchyma" means infusion. It is a living tissue of primary body like Parenchyma. Cells are thin-walled but possess thickening of cellulose, water and pectin substances (pectocellulose) at the corners where a number of cells join together. This tissue gives tensile strength to the plant and the cells are compactly arranged and have very little inter-cellular spaces. It occurs chiefly in hypodermis of stems and leaves. It is absent in monocots and in roots. Sometimes it contains chlorophyll which can help them photosynthesize.
Collenchymatous tissue acts as a supporting tissue in stems of young plants. It provides mechanical support, elasticity, and tensile strength to the plant body. It helps in manufacturing sugar and storing it as starch. It is present in the margin of leaves and resists tearing effect of the wind.
Schlerenchyma is Greek word where "Schlero-" means hard and "enchyma" means infusion. This tissue consists of thick-walled, dead cells and protoplasm is negligible. These cells have hard and extremely thick secondary walls due to uniform distribution and high secretion of lignin. They do not have intermolecular space between them. Lignin deposition is so thick that the cell walls become strong, rigid and impermeable to water which is also known as a stone cell or sclereids. These tissues are mainly of two types: sclerenchyma fiber and sclereids. Schlerenchyma cells have a narrow lumen and are long, narrow and unicellular.
The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. The entire surface of the plant has this outer layer of the epidermis. Hence it is also called surface tissue. Most of the epidermal cells are relatively flat. The outer and lateral walls of the cell are often thicker than the inner walls. The cells form a continuous sheet without intercellular spaces. It protects all parts of the plant.
The complex tissue consists of more than one type of cells which work together as a unit. Complex tissues help in the transportation of organic material, water, and minerals up and down the plants. That is why it is also known as conducting and vascular tissue. The common types of complex permanent tissue are:
Xylem and phloem together form vascular bundles.
Xylem consists of:
Xylem serves as a chief conducting tissue of vascular plants.
It is responsible for the conduction of water and mineral ions/salt. Xylem tissue is organised in a tube-like fashion along the main axes of stems and roots. It consists of a combination of parenchyma cells, fibers, vessels, tracheids, and ray cells. Longer tubes made up of individual cells are vessels tracheids, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The tracheids end overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell.
Though most conduction in xylem tissue is vertical, lateral conduction along the diameter of a stem is facilitated via rays.Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium. In trees and other woody plants, rays radiate out from the center of stems and roots and appear like spokes on a wheel in cross section. Rays, unlike vessel members and tracheids, are alive at functional maturity.
Phloem consists of:
Phloem is an equally important plant tissue as it also is part of the 'plumbing system' of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls. The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers, parenchyma and ray cells. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Sieve-tube members that are alive contain a polymer called callose, a carbohydrate polymer, forming the callus pad/callus, the colourless substance that covers the sieve plate. Callose stays in solution as long as the cell contents are under pressure. Phloem transports food and materials in plants upwards and downwards as required.
Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Such tissues may be found in both plants and animals, as well as algae. Typically these tissues form a protective shield against predation or provide structural support.
The term was introduced in anatomy by Marie François Xavier Bichat in 1801.He argued that the body functions would be better understood taking as unity of study the tissues, and not the organs. Bichat distinguished 21 types of elementary tissues for the human body, a number later reduced by other authors.
Plant cells are eukaryotic cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.
Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word "xylem" is derived from the Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout a plant. The term was introduced by Carl Nägeli in 1858.
In vascular plants, phloem is the living tissue that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλοιός (phloios) meaning "bark". The term was introduced by Nägeli in 1858.
Vascular plants, also known as tracheophytes, form a large group of plants that are defined as those land plants that have lignified tissues for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms and angiosperms. Scientific names for the group include Tracheophyta, Tracheobionta and Equisetopsida sensu lato. The term higher plants should be avoided as a synonym for vascular plants as it is a remnant of the abandoned concept of the great chain of being.
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm. The outer bark in older stems includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the last formed periderm is also called the rhytidome.
The vascular cambium is the main growth layer in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, and gymnosperms such as pine trees. It produces xylem on the inside and phloem on the outside. In herbaceous plants, it occurs in the vascular bundles which are often arranged like beads on a necklace forming an interrupted ring inside the stem. In woody plants, it forms a continuous ring and grows new wood on the inside.
Cork cambium is a tissue found in many vascular plants as part of the epidermis. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems. It is found in woody and many herbaceous dicots, gymnosperms and some monocots. It is one of the plant's meristems – the series of tissues consisting of embryonic disk cells from which the plant grows. It is one of the many layers of bark, between the cork and primary phloem. The function of cork cambium is to produce the cork, a tough protective material.
A meristem is the tissue in most plants containing undifferentiated cells, found in zones of the plant where growth can take place. Meristematic cells give rise to various organs of a plant and are responsible for growth.
Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified cellular walls that form small bundles of durable layers of tissue in most plants. The presence of numerous sclereids form the cores of apples and produce the gritty texture of guavas.
Lepidodendron — also known as the scale trees — is an extinct genus of primitive, vascular, tree-like plants related to the lycopsids. They were part of the coal forest flora. They sometimes reached heights of over 30 metres (100 ft), and the trunks were often over 1 m (3.3 ft) in diameter. They thrived during the Carboniferous Period before going extinct. Sometimes erroneously called "giant club mosses", the genus was actually more closely related to modern quillworts than to modern club mosses.
The pericycle is a cylinder of parenchyma or sclerenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.
Sieve elements are specialized cells that are important for the function of phloem, which is highly organized tissue that transports organic compounds made during photosynthesis. Sieve elements are the major conducting cells in phloem. Conducting cells aid in transport of molecules especially for long-distance signaling. In plant anatomy, there are two main types of sieve elements which are sieve cells and sieve tube members. Sieve cells are specialized cells in the phloem tissue of flowering plants. Companion cells and Sieve cells originate from meristems, which are tissues that actively divide throughout a plant's lifetime. They are similar to the development of xylem, a water conducting cells in plants whose main function is also transportation in the plant vascular system. Sieve elements' major function includes transporting sugars over long distance through plants by acting as a channel. Sieve elements elongate cells containing sieve areas on their walls. Pores on sieve areas allow for cytoplasmic connections to neighboring cells, which allows for the movement of photosynthetic material and other organic molecules necessary for tissue function. Structurally, they are elongated and parallel to the organ or tissue that they are located in. Sieve elements typically lack a nucleus and contain none to a very small number of ribosomes. The two types of sieve elements, sieve tube members and sieve cells, have different structure. Sieve tube members are shorter and wider with greater area for nutrient transport while sieve cells tend to be longer and narrower with smaller area for nutrient transport. Although the function of both of these kinds of sieve elements is the same, sieve cells are found in gymnosperms, non-flowering vascular plants, while sieve tube members are found in angiosperms, flowering vascular plants.
The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls.
A hydroid is a type of vascular cell that occurs in certain bryophytes. In some mosses such as members of the Polytrichaceae family, hydroids form the innermost layer of cells in the stem. At maturity they are long, colourless, thin walled cells of small diameter, containing water but no living protoplasm. Collectively, hydroids function as a conducting tissue, known as the hydrome, transporting water and minerals drawn from the soil. They are surrounded by bundles of living cells known as leptoids which carry sugars and other nutrients in solution. The hydroids are analogous to the tracheids of vascular plants but there is no lignin present in the cell walls to provide structural support.
A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in vascular tissue, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues.
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.
In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of stems and roots, causing them to elongate, and gives rise to primary tissue. Secondary growth occurs in most seed plants, but monocots usually lack secondary growth. If they do have secondary growth, it differs from the typical pattern of other seed plants.
A stem is one of two main structural axes of a vascular plant, the other being the root. The stem is normally divided into nodes and internodes:
Plant stem cells are innately undifferentiated cells located in the meristems of plants. Plant stem cells serve as the origin of plant vitality, as they maintain themselves while providing a steady supply of precursor cells to form differentiated tissues and organs in plants. Two distinct areas of stem cells are recognised: the apical meristem and the lateral meristem.
A cambium, in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. It forms parallel rows of cells, which result in secondary tissues.