Tissue: Definition, Types, and Examples

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Tissue Definition

The group or aggregation of cells having a similar function is called tissue. All multicellular organisms are made up of tissues. The body functions in these organisms are not carried out by cells independently thus they form tissues. Different types of cells combine to form a tissue-based upon the requirement of tissue function. The study of tissues is called histology. They can be only seen under microscopes.

Type of Animal Tissue

In animals, the cells combine to perform a particular function, this group of cells is called animal tissue. Based upon the structure and function, the animal tissues are further classified into four types-

i. Epithelial Tissue

ii. Connective Tissue

iii. Muscular Tissue

iv Nervous Tissue.

i. Epithelial Tissue

It is the most abundant human tissue that covers the whole surface of the body. It helps in the formation of the epidermis of the skin, which is the outermost layer and the lining of mucus, digestive system is also formed by epithelial tissues. Thus, it plays an important role in the protection of organs from injury and pathogens. The synthesis and excretion of wastes from the body are carried out by glandular epithelial tissues. The saliva, mucus, and sweat are also secreted by epithelial tissues. A single layer or multilayer of epithelial cells or simple epithelium from the epithelial tissue.

The epithelial cells have three surfaces

(1) an apical surface

(2) a basal surface and

(3) a lateral surface.

The exchange of material within the cell is carried out by the apical surface. The apical surface of mucus-moving cells contains cilia whereas microvilli are present in the epithelial cells of the inner linings of the small intestine. The bottom layer of the epithelial tissue is connected with the basement membrane with the help of the basal surface. The basement membrane is made up of proteins like collagen. The plasma membrane of epithelial cells contains integrins that connect them to the basement membrane. The intracellular connections among epithelial cells are mediated by lateral surfaces.

ii. Connective Tissue

Most of the part of connective tissue is made up of non-living extracellular material. A few living cells are embedded in the extracellular matrix. Except for fat tissue cells or adipocytes, the cells of connective tissues are often far from each other. The functions of connective tissue include connecting different cells and tissues forming, protecting and supporting soft tissues, transmit mechanical forces, and production of carbohydrates and proteins. There are two types of connective tissues;

a) Connective Tissue Proper

According to the structure and function of the connective tissue, the cells may vary in their structure. For example- the cells of fibroblast have abundant endoplasmic reticulum and Golgi apparatus. The fibroblasts are responsible for the secretion of fibers such as collagen, reticular fibers, and elastic fibers. They also provide a structural framework for many tissues.

The mesenchymal cells differentiate into different types during the embryonic stage. The bones are formed by osteoblasts whereas chondroblasts forming cartilages. Another important connective is blood. The blood cells are of three types named red blood cells, white blood cells, and platelets. Sometimes platelets do not consider blood cells and are defined as cellular fragments. The red blood cells are also called erythrocytes that carry oxygen throughout the body. White blood cells or leukocytes have the main function of protection against pathogens and infections.

Dense Connective Tissue: they are further divided into two types: Dense regular connective tissue: the collagen fibers are arranged in a single direction. Example- ligaments and tendons. Dense irregular connective tissue: the collagen fibers of these tissues are arranged in multiple directions. Example- the white layer of the eyeball.

a. Loose connective tissue: The fibers are present with some other ground substances.

b. Special connective tissues: these tissues are not fibrous. Examples of special connective tissue include adipose tissue, cartilage, bone, and blood.

b) Connective Tissue Fibers

The connective tissue consists of different types of fibers such as collagen fiber, elastic fiber, and reticular fiber. Collagen form the collagen fibers found in tendons and ligaments. The elastic fibers are made up of elastin that has an elastic property. Due to their elastic property, they can obtain their original shape after deformation. These tissues are found in lungs, skin, and large arteries and respond to different stimuli. Type III collagen forms the reticular fibers. It is secreted by the reticular cells of the body.

c) Ground Substance

Proteoglycans, glycosaminoglycans, glycoproteins, water, and ions made the ground substance of connective tissues. They are responsible for lubrication, space-filling, and limiting the movement of bacteria in connective tissues.

iii. Muscular Tissue

The contractile tissues form muscles in the body. They are divided into three types:

(1) Skeletal Muscle

(2) Smooth Muscles

(3) Cardiac Muscles

The voluntary movement of the body occurs by skeletal muscles whereas cardiac muscles are responsible for involuntary movement. The cardiac muscles possess intercalated discs that are absent in the other two muscular tissues.

(1) Smooth Muscle Tissue: The striations are not present in the smooth muscles. The cells are narrow, spindle-shaped, and have a centrally located nucleus. The striations are not present due to the disorganized manner of actin and myosin filaments. It enables the cell to contract that can be seen in uterine muscles during childbirth. Both smooth and cardiac muscles cells contract involuntarily, which means that they are not controlled by the demand of an individual.

(2) Skeletal Muscle Tissue: These tissues are formed by the fusion of multinucleated cells that form myotubes. They produce proteins and RNA in large amounts that stimulate the muscles to contract after the organization of proteins into sarcomeres. The basal lamina is secreted by the skeletal muscles that surround the muscle and helps in the transmission. Some special cells, called satellite cells are also present in between the basal lamina that helps in the development of muscle fibers. The skeletal muscles are divided into two types- fast and slow muscles.

(3) Cardiac Muscle Tissue: Cardiac muscles are striated muscles that regulate involuntary muscles. They consist of a single nucleus and are rich in blood vessels and mitochondria to get enough energy for continuous contraction. They usually present in the heart and pump blood thus they contract at the same time. The contraction occurs with the help of junctions that link the muscle to the next one and helps in signal transmission. They are also known as cardiomyocytes. Unlike the skeletal cells, cardiac cells do not contain satellite cells thus they do not divide and only grow by increasing their size only.

iv. Nervous Tissue

The central nervous system and peripheral nervous system are formed by the nervous tissues. The brain and spinal cord are formed by the central nervous system whereas sensory, motor, and autonomic nerves are formed by the peripheral nervous system.

Neurons are the unit of the nervous system that include dendrites or axons as cell processes. These axons help in signal transduction across different neurons. The dendrites are responsible to carry signals from the periphery to the cell body. As compare to the cell body, the axons contain a large amount of cytoplasm. The microfilaments and microtubules present in the cell help in signal transport.

The branching of axons is called presynaptic terminals. At this point, the electrical impulse is converted into chemical neurotransmitters and send signals to the next neuron. The main function of nervous tissue is to transmit nerve impulses across different nerve cells.

Plant Tissue

The plant body is also made up of different tissues that perform specific functions in the plant. Some examples of plant tissues are plant epidermis, vascular tissues, etc.

Plant Epidermal Tissue: The outermost layer of the plant is made of epidermal tissue (epidermis). It protects the inner linings of plants from the outer environment. A layer of cuticle covers the epidermis and makes it impermeable. The plant is also protected by the ultraviolet radiation of the sun with the help of the epidermis. The gaseous exchange occurs via stomata present on the lower surface of the leaves. These stomata are also an example of epidermal tissue that consists of two kidney-shaped guard cells. They absorb water and expand which results in the opening of stomata and permits the exchange of gases. Trichomes are also another example of plant epidermal tissue. They secrete toxins to protect the plant from herbivores and insects. The root epidermis is also composed of epidermal tissues that help in absorbing nutrients. Unlike the shoot epidermis, it permits the passage of nutrients.

Ground Tissue: There are three types of ground tissues: parenchyma, collenchyma, and sclerenchyma. The parenchyma cells are composed of the thin cell wall and occupy most of the cortex. The major function consists storage of proteins, starch, and oils in the plant. The collenchyma cells also have a thin cell wall. It provides turgidity to the cell. The sclerenchyma cells differ from the other two types in having thick cell walls containing lignin. It provides support to the plant. The sclerenchyma cells are rigid, and dead at maturity.

Vascular Tissue: The higher plants contain vascular tissues for transportation. There are two types of vascular tissues named xylem and phloem. All the parts of the plant contain vascular tissues in the form of vascular bundles. The upward transport of water and minerals is carried out by the xylem while phloem transports metabolites, sugar, and ions from the leaves of the plant. The phloem is made up of several components named companion cells, phloem parenchyma cells, sieve tubes, and phloem fibers. The sieve tubes consist of sieve pores that are also called sieve elements. The sieve elements are living cells, however, they lack nuclei but contain plasmalemma and proteins. The metabolites are transported by sieve elements and loaded by companion cells in and out of sieve elements. The xylem is made up of xylem vessels, tracheids, xylem parenchymal cells, and xylem fibers. Xylem can only transport laterally through pores or plates.

Tissue Engineering

The method of producing tissue outside the body is called tissue engineering. Tissues can be prepared artificially by various engineering methods. The tissue culture is an important method to repair damaged tissues, burns, trauma, or diseases. However, there are various difficulties also with this such as creating artificial nutritional blood vessels and supplying tissues with circulating blood. The use of bioreactors, use the right amount of nutrients, enzymes, and cellular components are also some challenges with this technique. Tissue culture or tissue engineering is an emerging science that has various important roles in the field of science.

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