Detailed Guide on Tissues

Tissues are groups of cells that have a similar structure and which performs a specific function. The word tissue comes from a form of an old French verb meaning “to weave”.

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.

A tissue is an aggregate of cells in an organism that have similar structure and function. Tissues that work in unison to carry out a specific set of functions form an organ. Examples of plant tissues are meristematic tissues and vascular tissues. Examples of animal tissues are epithelial tissues, connective tissues, muscular tissues, and nervous tissues. Etymology: Middle English tissu, a rich kind of cloth, from Old French, from past participle of tistre, to weave, from Latin texere.

There are mainly two types of tissues :-

1. Plant Tissue

2. Animal Tissue

Lets see it in detail

Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissues are composed of cells that are similar and perform a specific function. Together, tissue types combine to form organs. Each organ itself is also specific for a particular function.

Plant tissue systems fall into one of two general types: meristematic tissue, and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.

Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant.

Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.

Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant.

The ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars. The vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue is a simple tissue, meaning that each ground tissue consists of only one cell type. Dermal and vascular tissues are complex tissues because they consist of multiple cell types.

Dermal Tissue

Dermal tissue covers the plant and can be found on the outer layer of roots, stems and leaves. Its main functions are transpiration, gas exchange and defense. The epidermis is an example of dermal tissue . It is composed of a single layer of epidermis cells.

Openings are called stomata (singular: stoma) which allows a plant to take up CO2 and release 02 and water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a eudicot. Each stoma is flanked by two guard cells that regulate its (b) opening and closing. The guard cells are more curved when the stoma is open compared to when it is closed. The (c) guard cells sit within the layer of epidermal cells

It may contains stomata and guard cells that allow gas exchange. It may contain root hairs that increase surface area or or trichomes used in transpiration or defense. It may contain a waxy cuticle if found on the upper surface of leaves, to aid with lowering transpiration.

In woody plants, the epidermis breaks apart into a thick periderm as secondary growth allows the plant to grow in girth. The cork cambium, which makes cork cells, the cork cells (which are dead at maturity), and the phelloderm (parenchyma cells on the inside of the cork cambium) together make up the periderm . The periderm functions as the first line of defense for the plant, protecting it from fire or heat injury, dehydration, freezing conditions, and/or disease.

Cross section of a woody stem. The periderm is composed of the cork cambium, cork cells, and phelloderm.

Ground Tissue

Often times, tissues that are not considered dermal or vascular tissue are noted as ground tissue. These cells store molecules (such as starch), photosynthesize (such as mesophyll cells), or support the plant. Ground tissue is often divided into three cell types: Collenchyma, sclerenchyma, and parenchyma.

Collenchyma is living supportive tissue that has elongated cells and an unevenly thickened primary cell wall. Its main function is the mechanical support of young stems and leaves via turgor.

Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures.

Sclerenchyma is a dead supportive tissue that consists of long sclerenchyma fibers or short, crystal-like cells (sclereids] . Sclerenchyma fibers occur in groups (bundles). Sclereids may be branched or not and occur individually or in small clusters.

Each cell has a uniformly thick secondary wall that is rich in lignin. Its main function is a support of older plant organs, and also hardening different parts of plants (for example, make fruit inedible before ripeness so no one will take the fruit before seeds are ready to be distributed).

Without sclerenchyma, if a plant isn’t watered, the leaves will droop because the vacuoles will decrease in size which lowers the turgor. Fibers inside phloem (see below) are sometimes regarded as a separate sclerenchyma.

Left to right, top to bottom: parenchyma, sclerenchyma (cross- and longitudinal sections) and collenchyma.

Parenchyma are spherical, elongated cells with a thin primary cell wall. It is a main component of young plant organs. The basic functions of parenchyma are photosynthesis and storage.

They are also important in regeneration because they are totipotent (capable of differentiating into any cell type). Parenchyma cells are widespread in plant body. They fill the leaf, frequent in stem cortex and pith and is a component of complex vascular tissues

Vascular Tissue

Vascular tissue is the plumbing system of the plant. It allows water, minerals, and dissolved sugars from photosynthesis to pass through roots, stems, leaves, and other parts of the plant. It is primary composed of two types of conducting tissue: xylem and phloem.

The veins on leaves are an example of vascular tissue, moving material through the plant in the same manner that our blood vessels carry nutrients through our body. The xylem and phloem always lie adjacent to each other. In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.

Xylem tissue transports water and minerals from the roots to different parts of the plant. The conducting cells of the xylem are called tracheary elements. Parenchyma cells are also found in the xylem, and sclerenchyma fibers and sclereids are sometimes present.

There are two type of tracheary elements: vessel elements and tracheids . Both cell types that are dead at maturity and have thickened secondary cell walls. These cells connect to one another and allow water to be transported through them. Structurally, the vessel elements are wider than tracheids and contain perforation plates between adjacent vessel elements .

Wide openings (slits or pores) in perforation plates allow water to flow vertically between vessel elements, forming a continuous tube. Both types of tracheary elements contain pits, gaps in their secondary cell walls.

Adjacent cells have pits in the same locations, forming pit pairs, which allow water and minerals to flow between adjacent cells through the pit membrane (the remaining, thin primary cell walls in these regions;

Therefore, water flows through both perforation plates and pit pairs in vessel elements but only through pit pairs in tracheids. While water can move more quickly through vessel elements, they are more susceptible to air bubbles.

An air bubble disrupts cohesion in the column of water moving up the tube of vessel elements preventing use of that particular pathway. In tracheids, an air bubble would only decommission a single tracheid rather than an entire column of vessel elements. Vessel elements are found only in angiosperms, but tracheids are found in both angiosperms and gymnosperms.

Xylem transports water and minerals through vessel elements and tracheids, which are dead at maturity and have a thin primary and thick secondary cell wall internal to the primary cell wall. In pits, the secondary wall is thin or missing, allowing water to flow laterally. The xylem of angiosperms contains both types of tracheary elements: vessel elements, and tracheids. Vessel elements are stacked up top of each other and contain perforation plates in between cells. Tracheids are thinner and lack performation plates. Pits are thinned regions in the cell wall that allow movement of water between adjacent tracheary elements.

Phloem tissue transports organic compounds such as sugars from the site of photosynthesis to rest of the plant. The conducting cells of the phloem are called sieve elements. In comparison to tracheary elements, sieve elements have only primary cell walls (and thus thinner cell walls overall) and are alive at maturity; however, they lack certain organelles, including a nucleus.

Sieve-tube elements are the sieve elements found only in angiosperms while sieve cells are found only in gymnosperms while. Both types of sieve elements have pores in their cell walls (sieve areas) that allow transfer of materials between adjacent cells, but these are concentrated at sieve plates in sieve-tube elements and evenly distributed in sieve cells.

Because they lack essential organelles, sieve elements rely on specialized parenchyma cells to support them. Companion cells support sieve-tube elements in angiosperms, and albuminous cells support sieve cells in gymnosperms. Additionally parenchyma cells and sclerenchyma cells (phloem fibers) are also found in the phloem.

phloem transports sugars and other items. In angiosperms, sieve-tube elements contain the sugar solution. Sieve-tube elements are the conducting cells of the phloem in angiosperms. Sieve plates allow sieve-tube elements stacked on top of each other to connect.

Sieve-tube cells are surrounded by various support cells. Companion cells are narrower than sieve-tube elements and each contain a nucleus. They are connected to sieve-tube elements via plasmodesmata and provide them with the molecules they need to function (energy molecules, proteins, etc.)

Some companion cells are specialized as intermediary cells, which are between the bundle sheath (see below) and sieve-tube element. Transfer cells are parenchyma cells with cell wall ingrowths, which increase surface area for transport. The bundle sheath cells form the bundle sheath, which surrounds vascular bundles.

The table below summarizes differences between xylem and phloem:

	Xylem	Phloem
Contains mostly	Dead cells	Living cells
Transports	Water & Minerals	Sugar
Direction	Up	Up and Down
Biomass	Big	Small

Meristematic Tissue

Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue.

Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.

Animal Tissue

The animal cells are grouped together to form animal tissues. These tissues vary in their structure, function, and origin. The animal tissues are divided into epithelial, connective, muscular and nervous tissues. Let us have a glimpse of each type of animal tissue in detail.

Types of Animal Tissue

A tissue is a group of cells connected to each other that collectively perform similar functions in an organism. All contents of the body including structures and various organs are made of tissues.

The animal body comprises four basic types of tissues, all of which vary in their origin and function. They are:

• Epithelial Tissues:

Made up of tightly packed cells layer together, epithelial tissues line the body surface. Their functions include protection, absorption, and secretion. Epithelial tissues can be found in the lining of the mouth and nose, digestive system lining, and the skin.

• Muscle Tissues:

These are of three types, smooth muscle tissue- found in inner linings of organs, skeletal muscle tissue- found attached to the bone and helps in body movement and cardiac muscle tissue- found in the heart. These tissues help in changing the size of a cell.

• Nervous Tissues:

Made up of neurons (nerve cells in the brain), these tissues form the entire nervous system, including the spinal cord and the brain.

• Connective Tissues:

Made of various cells that are involved in lending support to the body, connective tissues are namely the fat, bone, blood and cartilage in an animal body.

Epithelial Tissues

These tissues form the lining of body surfaces and also account for glands. The cells along these tissues are tightly connected to each other. The epithelium does not contain blood vessels and hence depends on the other connective tissues to derive its nutrients and other essentials.

It is found along the edges of the organs and has two prominent surfaces, namely, the apical surface which is on the exterior and lies open to the body cavity, and the basal surface which lies adjacent to the underlying tissue.

Epithelial cells can be either squamous, cuboidal, or columnar in shape. The number of cell layers along with the combination of cell shapes decides the classifying features of epithelial tissue.

Types of Epithelial Tissues and Their Function

Epithelial Tissues are always located on the inner or outer surfaces of organs, and their functions largely depend on the exact position of their locations. These are of the following types:

Squamous Epithelium

Closely packed with one another, these are thin and flat cells that mostly like the esophagus, blood vessels, alveoli and the inner cavities of the mouth. The squamous epithelium tissue lends protection against mechanical injuries, while also blocking any sort of germs from entering.

The squamous epithelium may also be arranged in multiple layers, in which case it is known as the stratified squamous epithelium tissue. These tissues are usually found in the lining of the esophagus and the skin.

Cuboidal Epithelium Tissue

These are cuboidal in shape, hence rightfully deriving their name. Found in kidney tubules, salivary glands, and sweat glands, the functions of the cuboidal epithelium tissue are secretion, protection and absorption.

When the cuboidal epithelium is arranged in multiple layers, it is known as the stratified cuboidal epithelium tissue, and found on the inner side of the salivary glands and pancreatic ducts.

Columnar Epithelium Tissue

Mostly with column-like or pillar-like cells, these can be found in the intestine and lining of the stomach. Important functions of the columnar epithelium tissue include secretion and absorption.

Ciliated Epithelium Tissue

The columnar epithelium tissues often have cilia; this is when they come to be known as ciliated epithelium tissues. These can be found in kidney tubules, the respiratory tract and lining of the trachea. Their function is to help in the movement of material in a given direction.

Glandular Epithelium Tissue

These are majorly modified columnar epithelial tissues whose main function is secretion. They can be found in the sweat glands and tear glands.

Muscle Tissues

Muscle tissues are specialised tissues found in animals, responsible for applying force to various parts of the body by using the method of contraction. Thin and elongated cells called muscle fibers make up the muscle tissues.

The structure of a muscle tissue contains three distinct elements- the cytoplasm in the muscle fibers, called the sarcoplasm, a membrane network known as the sarcoplasmic reticulum, and the surrounding membrane of the muscle fibers known as the sarcolemma.

Important properties of muscle tissues are as follows- Extensibility:

The ability of a muscle to stretch itself

• Contractibility:

The ability of the muscle cells to forcefully shorten themselves

• Excitability:

The ability of a muscle tissue to respond to a stimulus given by any hormone or a motor neuron

• Elasticity:

The ability of a muscle to recoil to it's usual length after being stretched

Types of Muscle Tissues and Their Functions

Mostly responsible for initiating movement of an organism, muscle tissues have a range of other functions based on their types. These are:

Skeletal Muscle Tissues

Voluntary muscle and striated in nature, skeletal muscle tissues have neatly arranged bundles and have tendons anchoring them. These have an impact on the skeletal movements of an organism that include posture and locomotion.

Smooth Muscle Tissues

These are involuntary and non-striated in nature and have tapered ends. They are mostly located in the blood vessel walls like arteries and veins, urinary tract, trachea and digestive system. Smooth muscle tissues help in peristalsis to move food up and down the alimentary canal.

Cardiac Muscle Tissues

These majorly consist of making up the heart. Involuntary and striated, these are branched out at irregular angles to help with coordinated contractions occurring inside the heart.

Nervous Tissues

Nervous tissues are the cells that form the central and peripheral nervous system. While in the central nervous system, the nervous tissues form the spinal cord and the brain, in the peripheral nervous system, the nervous tissues make up the cranial and spinal nerves, also including the motor and sensory neurons.

The most important function of the nerve tissues is to transmit and carry nerve impulses in various parts of the body. Impulses are often sent by axons and received by dendrites.

Nerve cells can be of three types- sensory neurons, motor neurons and interneurons.

Connective Tissues

These are tissues that support, connect or separate various other kinds of tissues and organs inside the body. They are made up of cells, fibers like collagen and extracellular matrix. Collective tissues can be found abundantly located inside the body in a freely arranged form or in a matrix.

Types and Functions of Connective Tissues

The various types of connective tissues include areolar, adipose, bone, cartilage and fat. All cells are involved in the secretion of collagen except for blood.

Areolar Connective Tissues

These are found under the skin, surrounding nerves and blood vessels. Their function is to repair tissues and provide support.

Adipose Tissues

These can be found in the organs and skin. Composed of fat globules, their function is to insulate the body with the fat presence.

Bones

These form the skeletal structure of the body and have a characteristic of being rich in calcium and collagen fibers. They protect the body and are the location of blood cell production.

Cartilage

These can be found in the ear tips, vertebral column, bronchi and are made of chondrocytes that are composed of flexible intercellular materials.

Blood

The functions of blood include putting up a defence system, transportation and most importantly homeostasis. Blood is composed of blood cells that include platelets, RBC and WBC along with plasma.

Relevance of knowing Types of Animal Tissues in Biology

Our body is made up of four kinds of tissues. Each of these tissues vary in their functions.

They are epithelial tissues, muscle tissues, nervous tissues and connective tissues.

Knowing about these will help all students in understanding basic facts about our bodies.

It will especially be a help to those students who intend to take up Biology as a career later on as getting the fundamentals right at an early stage will always come to one’s rescue when he/ she faces difficulty in tackling a question.

Conclusion

The article presented a complete insight about animal tissues types and functions. Learning about them will help in future.