Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.
The study of tissue is known as histology or, in connection with disease, histopathology.
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, 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 clinical diagnosis and prognosis.
Based on morphology, animal tissues can be grouped into four basic types. Multiple tissue types comprise organs and body structures. While all animals 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 animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm which forms the endothelium. By contrast, a true connective tissue is present only in triploblastic animals; diploblastic animals such as sponges, jellyfish and coral in effect do not have connective tissue but rather an acellular gel known as the mesoglea separates different tissues types in these animals. The mesoglea serves the function of a connective tissue but is derived from ectoderm.
Epithelium is 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. Endothelium, which comprises the vasculature, is a specialized type of epithelium.
Connective tissue is comprised of cells separated by non-living material, which is called extracellular matrix. Connective tissue holds other tissues together such as in the formation of organs, and has the ability to stretch and contract passively. Bone, often referred to as osseous tissue, and blood are examples of specialized connective tissues.
Muscle cells form the active contractile tissue of the body known as muscle 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, which is found in the inner linings of organs; skeletal muscle, which is found attached to bone providing for gross movement; and cardiac muscle which is found in the heart, allowing it to contract and pump blood throughout an organism.
Cells comprising the central nervous system and peripheral nervous system are classified as neural tissue. In the central nervous system, neural tissue forms the brain, cranial nerves and spinal cord and, in the peripheral nervous system, peripheral nerves inclusive of the motor neurons.
Examples of tissue in other multicellular organisms are vascular tissue in plants, such as xylem and phloem. Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue. Together they are often referred to as biomass.
Plant tissues can also be divided differently into two types:
- MERISTEMATIC TISSUES
- PERMANENT TISSUES
The growth of plant occurs only in certain specific regions. At these regions, the meristimatic tissues are present. New cells produced by meristem are initially those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues. Depending on the region of occurrence of meristimatic tissues they are classified as:
- a) Apical Meristem - It is present at the growing tips of stems and roots and increases the length of the stem and root. The girth of the stem or root increases due to lateral meristem (cambium).This meristem is responsible for the linear growth of an organ.
- b) Lateral Meristem - This meristem consist of cells which mainly divide in one plane and cause the organ to increase in diameter and girth. 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.
- c) 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.
Characteristics of meristematic tissues
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 molecular spaces between them. Each cell contains a dense cytoplasm and a prominent nucleus. Dense protoplasm of meristematic cells contains very few vacuoles. Normally the meristimatic cells are oval, polygonal or rectangular in shape.
The meristematic tissues that take up a specific role and 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 tissue. There are 2 type of permanent tissues:
Simple permanent tissues
These tissues are called simple because they are composed of similar types of cells which have common origin and function. They are further classified into:
Parenchyma is Greek word where "parn" means besides and "enchien" means to pour. Parenchyma is the most specialized primitive tissue. It mainly consist of thin-walled cells which have intermolecular spaces between them. The cell wall is made up of cellulose. Each parenchymatous cell is iso-diametric, spherical, or oval in shape. It is widely distributed in various plant organs like root, stem, leaf, flowers and fruits. They mainly occur in cortex epidermis, pith and mesophyll of leaves.
The main function of parenchymatous tissue is assimilation and storage of reserve food materials like starch, fats and proteins. They also store waste products such as gums, resins, and inorganic waste materials.
Collenchyma is Greek word where "Collen" 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 and pectin substances 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 do not have intermolecular spaces. It occurs chiefly in hypodermis of stems and leaves. It is absent in monocots and in roots.
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 margin of leaves and resist tearing effect of the wind.
Sclerenchyma is Greek word where "Sclrenes" means hard and "enchyma" means infusion. This tissue consists of thick-walled, dead cells. These cells have hard and extremely thick secondary walls due to uniform distribution of lignin. Lignin deposition is so thick that the cell walls become strong, rigid and impermeable to water. Sclerenchymatous cells are closely packed without intra-cellular spaces between them. Thus, they appear as hexagonal net in transverse section. The cells are cemented with the help of lamella. The middle lamella is a wall that lies between adjacent cells. Sclerenchymatous cells mainly occur in hypodermis, pericycle, secondary xylem and phloem. They also occur in endocorp of almond and coconut. It is made of pectin, lignin, protein. The cells of sclerenchyamtous cells can be classified as :
- Fibres- Fibres are long, elongated sclerenchyamtous cells with pointed ends.
- Sclerides- Sclerenchymatous cells which are short and possess extremely thick, lamellated, lignified walls with long singular piths. They are called sclerides.
The main function of Sclerenchymatous tissues is to give support to the plant.
Complex permanent tissue
A complex permanent tissue may be classified as a group of more than one type of tissue having a common origin and working together as a unit to perform a function. These tissues are concerned with transportation of water, mineral, nutrients and organic substances. The important complex tissues in vascular plants are xylem, phloem.
Xylem is a chief, conducting tissue of vascular plants. It is responsible for conduction of water and inorganic solutes.
- Tracheids- Trachids are elongated, tube-like dead cells with elongated end-walls. End walls remain intact and possess piths. In transverse section, They usually occur as polygonal cells and lignified walls.
- Vessels - Vessels are placed one upon another. Their end walls are perforated. They form long tubes or channels for conduction of water and minerals.
- Xylem Parenchyma - Xylem Parenchymatous cells are living cells present in xylem. They help in lateral conduction of organic solutes and storage reserves.
- Xylem Fibres - Xylem Fibres are lignified fibres present in xylem which provide mechanical strength to the plant body.
Xylem is a major conducting tissue of vascular plants. It serves in upward movement of water and minerals from root to different parts of plant.
Phloem is a chief conducting tissue of vascular plants. It is regarded as a living tissue responsible for translocation of organic solutes.
- Sieve tube - Sieve tubes are long tubular structures composed of elongated sieve tube elements placed one above other forming a continuous tube.
- Companion cell - Companion cells are living cells always associated with sieve tubes. Sieve tube elements and companion cells arrive from the same, initial cell and therefore forms a single functional unit. Each companion cell shows presence of fine piths with all the living components of the cell.
- Phloem Parenchyma - These cells are living parenchymatous cells that are present in phloem. These cells help in storage of food.
- Phloem Fibres - Phloem fibres are formed by dead, sclerenchymatous fibres.
The main function of phloem is translocation of organic solutes from the leaves to the storage organ and later from the storage organ to the growing part. Sieve tube allow free diffusion of soluble, organic substances across sieve plates due to the presence of large number of sieve pores.