This article is about biological tissue. For the moth, see Triphosa dubitata. For other uses, see Tissue (disambiguation).

Cross section of sclerenchyma fibers in plant ground tissue.

Microscopic view of a histologic specimen of human lung tissue stained with hematoxylin and eosin.

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 then formed by the functional grouping together of multiple tissues.

The English word 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.

Contents

• 1Animal tissues
  • 1.1Connective tissue
  • 1.2Muscular tissue
  • 1.3Nervous tissue
  • 1.4Epithelial tissue
• 2Plant tissues
  • 2.1Meristematic tissues
  • 2.2Permanent tissues
    • 2.2.1Simple tissues
      • 2.2.1.1Parenchyma
      • 2.2.1.2Collenchyma
      • 2.2.1.3Sclerenchyma
      • 2.2.1.4Epidermis
    • 2.2.2Complex permanent tissue
      • 2.2.2.1Xylem
      • 2.2.2.2Phloem
• 3Mineralized tissues
• 4History of the concept
• 5See also
• 6References
• 7External links

Tissue (biology)   

Animal tissues

PAS diastase showing the fungus Histoplasma.

Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues joined in structural units to serve a common function compose organs. While all eumetazoan animals (i.e. except Porifera) can generally 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, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.

Connective tissue

Main article: Connective tissue

Connective tissues are fibreous tissues. They are 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.

Muscular tissue

Main article: Muscle 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; 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.

Nervous tissue

Main article: Nervous tissue

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.

Epithelial tissue

Main article: Epithelium

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:

• 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 elimination of waste.
• Epithelial tissues secrete enzymes and/or hormones in the form of glands.
• Some epithelial tissue perform secretory functions. they secrete a variety of substance such as sweat,saliva(mucus),enzymes,etc.

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:

• Simple squamous epithelium
• Stratified squamous epithelium
• Simple cuboidal epithelium
• Transitional epithelium
• Pseudostratified columnar epithelium (also known as Ciliated columnar epithelium)
• Columnar epithelium
• Glandular epithelium
• Ciliated columnar epithelium

Plant tissues

Cross-section of a flax plant stem with several layers of different tissue types:
1. Pith
2. Protoxylem
3. Xylem I
4. Phloem I
5. Sclerenchyma (bast fibre)
6. Cortex
7. Epidermis

In plant anatomy, tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and 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 fluids and nutrients internally.
• Ground tissue - Ground tissue is less differentiated than other tissues. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients.

Plant tissues can also be divided differently into two types:

1. Meristematic tissues
2. Permanent tissues.

Meristematic tissues

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 meristimatic tissues, they are classified as:

• Apical Meristem - It is present at the growing tips of stems and roots and increases the length of the stem and root. They form growing parts at the apices of roots and stems and are responsible for increase in length, also called primary growth. This meristem is responsible for the linear growth of an organ.
• Lateral Meristem
• 
  
•  - This meristem consist of cells which mainly divide in one plane and cause the organ to increase in diameter and growth. Lateral meristem usually occurs beneath the bark of the tree in the form of Cork Cambium and in vascular bundles of dicots in the form of vascular cambium. The activity of this cambium results in the formation of secondary growth.
• Intercalary Meristem - This meristem is located in between permanent tissues. It is usually present at the base of node, inter node and on leaf base. They are responsible for growth in length of the plant and increasing the size of the internode, They result in branch formation and growth.

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.

Meristemetic tissue cells have a large nucleus with small or no vacuoles, they have no inter cellular spaces.

Permanent tissues

Permanent tissues may be defined as 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:

1. simple permanent tissues
2. complex permanent tissues
3. special or secretory tissues (glandular).

Simple tissues[edit]

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:

1. Parenchyma
2. Collenchyma
3. Sclerenchyma
4. Epidermis (botany)

Parenchyma[edit]

Parenchyma (para - 'beside'; enchyma - 'tissue') is the bulk of a substance. In plants, it consists of relatively unspecialised 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 isodiameteric, in shape. This tissue provides support to plants and also stores food. In some situations, a 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 parenchyma cells have metabolic waste and is known as idioblast. Spindle shape fibre also contained into this cell to support them and known as prosenchyma , succulent parenchyma also noted.

Collenchyma[edit]

Cross section of collenchyma cells

Collenchyma is Greek word where "Collen" means gum and "chyma" 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 number of cells join together. This tissue gives a 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 photosynthesis.

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 resist tearing effect of the wind.

Sclerenchyma[edit]

Sclerenchyma is Greek word where "Sclerena" means hard and "chyma" 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 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.

Epidermis[edit]

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 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 forms a continuous sheet without inter cellular spaces. It protects all parts of the plant.

Complex permanent tissue[edit]

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 or wood
• Phloem or bast.

Xylem and phloem together form vascular bundles.

Xylem[edit]

Xylem consists of:

• Tracheid
• Vessel members
• Xylem fibres
• Xylem parenchyma

Xylem serves as a chief conducting tissue of vascular plants. It is responsible for the conduction of water and mineral ions/salt.

Cross section of 2 year old Tilia Americana, highlighting xylem ray shape and orientation.

Xylem tissue is organized 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 (tracheae), 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 ends 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.^[1] 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.^[2]

Phloem[edit]

Phloem consists of:

• Sieve tube
• Companion cell
• Phloem fibre
• Phloem parenchyma.

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

Main article: Mineralized tissues

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.

History of the concept[edit]

The term was introduced in anatomy by Marie François Xavier Bichat in 1801.^[3] 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.

Tissue, in physiology, a level of organization in multicellular organisms; it consists of a group of structurally and functionally similar cells and their intercellular material.

By definition, tissues are absent from unicellular organisms. Even among the simplest multicellular species, such as sponges, tissues are lacking or are poorly differentiated. But multicellular animals and plants that are more advanced have specialized tissues that can organize and regulate an organism’s response to its environment.

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Plants

Bryophytes (liverworts, hornworts, and mosses) are nonvascular plants; i.e., they lack vascular tissues (phloem and xylem) as well as true leaves, stems, and roots. Instead bryophytes absorb water and nutrients directly through leaflike and stemlike structures or through cells comprising the gametophyte body.

In vascular plants, such as angiosperms and gymnosperms, cell division takes place almost exclusively in specific tissues known as meristems. Apical meristems, which are located at the tips of shoots and roots in all vascular plants, give rise to three types of primary meristems, which in turn produce the mature primary tissues of the plant. The three kinds of mature tissues are dermal, vascular, and ground tissues. Primary dermal tissues, called epidermis, make up the outer layer of all plant organs (e.g., stems, roots, leaves, flowers). They help deter excess water loss and invasion by insects and microorganisms. The vascular tissues are of two kinds: water-transporting xylem and food-transporting phloem. Primary xylem and phloem are arranged in vascular bundles that run the length of the plant from roots to leaves. The ground tissues, which comprise the remaining plant matter, include various support, storage, and photosynthetic tissues.

Secondary, or lateral, meristems, which are found in all woody plants and in some herbaceous ones, consist of the vascular cambium and the cork cambium. They produce secondary tissues from a ring of vascular cambium in stems and roots. Secondary phloem forms along the outer edge of the cambium ring, and secondary xylem (i.e., wood) forms along the inner edge of the cambium ring. The cork cambium produces a secondary dermal tissue (periderm) that replaces the epidermis along older stems and roots.

Animals

Early in the evolutionary history of animals, tissues became aggregatedinto organs, which themselves became divided into specialized parts. An early scientific classification of tissues divided them on the basis of the organ system of which they formed a part (e.g., nervous tissues). Embryologists have often classified tissues on the basis of their origin in the developing embryo; i.e., ectodermal, endodermal, and mesodermal tissues. Another method classified tissues into four broad groups according to cell composition: epithelial tissues, composed of cells that make up the body’s outer covering and the membranous covering of internal organs, cavities, and canals; endothelial tissues, composed of cells that line the inside of organs; stroma tissues, composed of cells that serve as a matrix in which the other cells are embedded; and connective tissues, a rather amorphous category composed of cells and an extracellular matrix that serve as a connection from one tissue to another.

The most useful of all systems, however, breaks down animal tissues into four classes based on the functions that the tissues perform. The first class includes all those tissues that serve an animal’s needs for growth, repair, and energy; i.e., the assimilation, storage, transport, and excretion of nutrients and waste products. In humans, these tissues include the alimentary (or digestive) tract, kidneys, liver, and lungs. The digestive tract leads (in vertebrates) from the mouth through the pharynx, stomach, and intestines to the anus. In vertebrates and some larger invertebrates, oxygen and the nutrients secured by the alimentary tissues or liberated from storage tissues are transported throughout the body by the blood and lymph, which are themselves considered by many to be tissues. Tissues that secure oxygen and excrete carbon dioxide are extremely variable in the animal kingdom. In many invertebrates, gas exchange takes place through the body wall or external gills, but in species adapted to a terrestrial life, an internal sac capable of expansion and contraction served this purpose, and gradually became more complex over evolutionary time as animals’ demand for oxygen increased.

The second class of tissues consists of those used in coordination. There are basically two types: physical (nervous and sensory tissues), which operate via electrical impulses along nerve fibres; and the chemical (endocrine tissues), which release hormones into the bloodstream. In invertebrates, both physical and chemical coordination are performed by the same tissues, because the nervous tissues also serve as hormone sources. In vertebrates, most endocrine functions are isolated in specialized glands, several of which are derived from nervous tissue.

The basic unit of all nervous tissue is the neuron, aggregations of which are called ganglia. The bundles of axons along which neurons transmit and receive impulses are called nerves. By comparison, chemical control by hormones is much slower and longer-acting. In many invertebrates, chemical stimulators are secreted by the neurons themselves and then move to their site of action along the axon. In higher vertebrates, the principal endocrine tissues are the thyroid, parathyroid, pituitary, and endocrine constituents of the pancreas and adrenal glands.

The third class of tissues includes those contributing to the body’s support and movement. The connective tissues proper surround organs, bones, and muscles, helping to hold them together. Connective tissues proper consist of cells embedded in a matrix composed of an amorphous ground substance and collagen, elastic, and reticular fibres. Tendons and ligaments are examples of extremely strong connective tissues proper. The other major structural tissues are cartilage and bone, which, like connective tissues proper, consist of cells embedded in an intercellular matrix. In cartilage the matrix is firm but rubbery; in bone the matrix is rigid, being impregnated by hard crystals of inorganic salts. Muscle tissue is primarily responsible for movement; it consists of contractile cells. There are two general types of muscle: striated muscle, which moves the skeleton and is under voluntary control; and smooth muscle, which surrounds the walls of many internal organs and cannot normally be controlled voluntarily.

A fourth class of tissues includes reproductive tissues, hemopoietic tissues, and tissue fluids. The most important reproductive tissues are the gonads (ovaries and testes), which produce the gametes (eggs and sperm, respectively). Hemopoietic tissues produce the cellular components of the blood. Among the important tissue fluids are lymph, cerebrospinal fluid, and milk (in mammals).

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  transplant: Tissue transplants
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  The majority of skeletal and muscular dimensions follow approximately the growth curve described for height, and so also do the dimensions of the internal organs such as the liver, the spleen, and the kidneys. But some exceptions exist, most…

• Examination of Tissues

  A tissue is one of the building blocks of an organism--either animal or plant. An organism is comprised of tissues, which are made up of individual cells. These cells share a similar structure (how they're built) and function (what they do). The study of tissues is a field known as histology.

  You may be asking why tissues are important. . . it's simple! Tissues are what makes up our bodies: everything from our bones to organs like our heart and brain! Even blood is considered to be a tissue!

  The Four Basic Human Tissue Types

  In humans, there are four basic tissue types, which are: epithelial tissue, connective tissue, muscle tissue and nervous tissue.

  Within each general tissue type, there are specific tissue types. Think of this like a football team. There are individual players, each with their own 'job' on the field. These are the specific tissue types. The football players are then categorized into either an offense or a defensive team. These teams would be the basic tissue types. The offense and defensive teams are combined into one football team, or an organism.

  Epithelial and Connective Tissue

  The easiest way to describe an epithelial tissue is to say that it covers something; typically an organ. The skin of our bodies is just one big piece of epithelial tissue!

  All epithelial tissues share one thing in common: a basement membrane. A membrane is a very thin layer of tissue. It's so thin that you can typically see through it! So, how is a basement membrane different from a 'regular' membrane? Where in your house do you find the basement? The very bottom level of your house, right? A basement membrane is exactly that: the very bottom layer of a tissue.

  Different types of epithelial tissues are defined by two things:

  1. The cell shape they have
  2. How many cell layers thick they are

  Epithelial tissues have one of three main cell shapes: columnar, cuboidal, and squamous. Columnar cells look like columns, and cuboidal cells look like cubes. Squamous cells are scale-like.

  The number of cell layers each epithelial tissue has falls into one of two categories: simple, which means 1-cell layer thick, or stratified, which means more than 1-cell layer thick.

  Specific epithelial tissue types consist of the combination of the cell shape and number of cell layers. For example, simple cuboidal epithelial tissue describes tissue that is one cell layer thick, with cells that are cube-like in appearance. This tissue, in part, is what makes up our kidneys!

  Connective tissue does just that--it connects things in our body together! This tissue has many different specific types, which range from bone and cartilage, which make up the human skeleton, to blood and fat (also known as adipose connective tissue).

  Bone Connective Tissue

  Muscle and Nervous Tissue

  There are three different types of muscle tissue. They are smooth, skeletal and cardiac muscle tissue.

  Smooth muscle and cardiac muscle are under involuntary control. This means, we can't tell these muscles to work. This comes in handy considering where these tissues are found - smooth muscle makes up the organs of our digestive tract, our stomach and intestines, and cardiac muscle is found in our heart. Can you imagine having to 'tell' your stomach to digest your food or your heart to beat?

  Skeletal muscle, on the other hand, is under voluntary control, which means we need to tell our muscles to work.

  
  

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