The word tissue means a group of cell similar in origin, structure and function.
In unicellular organisms a single cell performs all the functions like respiration, digestion, excretion etc. Similarly, in multicellular organisms all such activities are performed by a specific group of cells, tissue. All the organs in multicellular organisms are composed of tissues.
Based on the location and function, animal tissues can be classified into four different types:
1. Epithelial tissues.
2. Connective tissues.
3. Muscular tissues.
4. Nervous tissues.
A. EPITHELIAL TISSUES:
The basic functions of epithelial tissues are protection, secretion and absoption. The cells of epithelial tissues are located very close to each other. The cells are separated from each other by thin films of extracellular materials. These tissues lies on a non-cellular basement membrane which separates it from the underlying connective tissues. The basement membrane has two layers; upper layer called the basal lamina made up of glycoproteins and mucopolysaccharides secreted by the epithelial cells and the lower layer called the reticular lamina made up of reticular fibres and collagen fibres. These fibres are the part of the underlying connective tissues. Epithelial tissues lack blood vessels and the materials are exchanged by diffusion between them and vessels of connective tissues.
There are three types of epithelial tissues; simple, compound and glandular.
1. Simple epithelium: These tissues are present at the secretory and absorptive surfaces. It helps in nutrition, secretion and excretion but not in the protection of the underlying cells. They are made up of a single layer and the adjacent cells are held together by desmosomes, lying on the basement membrane.
There are further five types of simple epithelium:
a) Squamous epithelium: These are present at the inner linings of the lung alveoli and blood vessels. The constituent cells are thin, flat, scaly with a prominent nuclei and irregular boundaries which fits with the neighbouring cells perfectly. These are also called as the pavement membrane and the tesselated epithelial.
b) Cubical epithelium: These are present at the small salivary and pancreatic ducts and thyroid
vesicles. The cells are polygonal in shape but appears cubical in vertical section hence it is called as cubical epithelial. These cells helps in secretion, absorption and excretion. The epithelial associated with the absorption contain microvilli on the absorptive surface which great increases the absorptive area. These micreovilli gives them a brush-like appearance and hence they are also called as the brush bordered cubical epithelial cells. Such kind of cells is present at the proximal tubules of the kidney.
c) Columnar epithelium: These are present at the surface of intestine, stomach, gall bladder, gastric and intestinal glands. Its main function is secretion and absorption. The cells are tall and polygonal in shape and forms columns of cells.
d) Ciliated epithelium: These are present at the falopian tube, branchioles, and small bronchi. These consists of either cubical or columnar cells bearing cilia on the free surfaces.Their function is to move the particles, free cells or mucus in a specific direction. The ciliated columnar epithelial present in the brain and spinal cord is called the ependyma.
The cilia is of two types:
1. Kinocilia: These are motile
2. Stereocilia: Non-motile. It is found in some parts of male reproductive tracts.
e) Pseudo-stratified epithelium: These are present at the inner linings of trachea and large bronchi. The cell are columnar and although made up of single layer of cells but appears two layered as some cells are shorter than the others and hence have their nuclei at different level. The shorter cells are non-ciliated and secretes mucous that traps particles and the longer cells are ciliated. Pseudostratified non ciliated columnar epithelial tissue is found in urethra of male and parotid salivary glands.
2. Compound epithelium: It consists of more than one layer and have little role in secretion and absorption. Its main function is protection of the underlying cells from the mechanical, chemical, thermal or osmotic stress.
The compound epithelial are of two types; stratified and transitional.
a) Stratified epithelium: They are made up of many layer and the deepest layer of cells is composed of cuboidal epithelial cells. It is present at the inner surfaces of larger salivary and pancreatic ducts. Stratified non keratinised sqamous epithelium covers moist surfaces such as buccal cavity, oesophagus etc. and stratified keratinised sqamous epithelium are present at the dry surface of the skin. It has many layer of horny, scaly like dead remaining of the sqamous cells. Deposits of keratin at the dead remaining of sqamous cells makes the epithelium impermeable to water and resistant to mechanical abrasions. And non-keratinised epithilium is permeable to water and provides lesser protection.
b) Transitional epithelium: It lines the inner surface of urethra and urinary bladder. It is much more thinner and stretchable than straited epithelium. It allows the expansion of these organs to store urine.
Two more kinds of epithelium tissues have been identified:
1. Neuroepithelium: Specialised in sensory functions like cells of taste buds.
2. Myoepithelium: Have branched cells that contains muscle proteins like actin and myosin.
3. Glandular epithelium: The cells are either cuboidal or columnar. It is present in the glands. It can be unicellular (consisting of isolated glandular cells) or multicellular (consisting of cluster of cells).
B. CONNECTIVE TISSUES:
These are of three types; connective tissue proper, supportive connective and fluid connective tissues.
1. Connective tissue proper:
a) Loose connective tissues: It consists of cells scattered within an amorphous mass of proteins that forms a ground substance. The gelatinous material is strengthened by loose scattering of protein fibres such as collagen, elastin which makes tissue elastic and reticulin.
1) Adipose tissue: These are present under the skin, kidney and bone-marrow. These are the fat storing tissues which stores fat for emergency conditions. Hence they are also called the fat cells or adipocytes. They synthesis, store and metabolizes fat. Their functions is to prevent heat losses as they are heat insulators, shock absorbent and acts as food reserves.
2) Areolar tissues: It occurs beneath the epithelia of many hollow visceral organs, skin and in the walls of arteries and veins. These contains many kinds of cells:
i) Fibroblasts: These are the principle cells of these tissues. These produces two types of proteins; collagen and elastin. They are irregular, flat cells with long protoplasmic processes. Fibroblasts secrete the major amount of matrix.
ii)Macrophage/ histiocytes/ clasmatocytes: These are phagocytic in nature i.e. they eat up the bacteria and other foreign bodies that enter our body.
iii) Mast cells/ mastocytes: These produces anti-allergic substances like histamine, heparin and serotonin in case of allergies.
iv) Plasma cells/ cart wheel cells: These synthesizes antibodies.
The areolar tissues joins different tissues and forms the packing between them and helps to keep the organs in place and in normal shape.
b) Dense connective tissues: The fibre are closely packed in dense connective tissues. These includes the following:
i) White fibrous tissues: It carries only a few fibroblasts scattered at the middle of the dense network of thick collagen fibre bundles. The presence of white fibrous tissues at the joints between skull bones makes them immovable.
ii) Tendons: It is a very dense, strong and fibrous connective tissue with thick parallel bundles of collagen fibres. A few flat, elongated tendon cells lie in a single row between the fibre bundles. Tendon forms the strong inextensible attachment of a skeletal muscle to a bone.
iii) Ligaments: These connects bones at the joints and hold them in position. They are made up of bundles of elastic fibres and few collagen fibres.
iv) Reticular tissues: It consists of star shaped reticular cells whose protoplasmic processes form a network. These cells are present at the spleen, lymph nodes, bone marrows etc.
2. Supportive connective tissues:
1. Cartilage: It is a solid but semi-rigid and flexible connective tissue. They occur in cluster of 2 or 3 cells in small spaces scattered in the matrix.
Types of cartilages:
i) Hyaline cartilage: In this, the matrix is fibreless and glass like but translucent. It occurs in the larynx, nasal septum, tracheal rings and ribs. It gives these structures a definite form.
ii) Elastic cartilage: These contains dense network of elastic fibres. It is present in the Eustachian tube, epiglottis and pinna of ear. The elastic fibres make those organs elastic and pliable.
iii) Calcified cartilage: Initially it is like hyaline cartilage but later on it gets hardened like bone due to deposition of calcium salts.
2.Bone: It is a solid, rigid connective tissue. The matrix of the bone has the deposition of apatite salts of calcium and phosphates. 60-70% of the bones are made up of inorganic matters and the rest by organic matters. Osteoblasts are the bone forming cells. Bones gives the basic structure of our body and together all the bones constitutes the skeleton.
Types of bones:
i) Cartilage bones/ endochondrial/ replacing bones:. They are formed by the replacement of cartilage by the bones like humerus, femur vertebrae etc.
ii) Membrane/ investing bone/ dermal: These bones are formed in the dermis of the skin and are invested over the already present cartilage like skull bones.
iii) Sesmoid bones: They are formed by the ossification of the tendon by bone. e.g. patella,
iv) Visceral bones: They are those bones which get detached from the skeleton and come to lie int he visceral organs. e.g OS cordis in septum of heart of deer etc.
3. Fluid connective tissues:
Blood is the fluid connective tissue. Its cells are different from the other connective tissues both in function and structure.
.a) Plasma: It contains three types of proteins i.e. serum albumin, serum globulins and fibrinogen. These proteins serves as the source of proteins to the tissue cells. Albumin and globulin retain water in blood plasma by their osmotic effects. They also transport many substances like thyroxine etc. A type of globulin called immunoglobin acts as antibodies. These proteins also mains the pH of blood. Plasma is slightly alkaline and constitutes about 60% of the blood.
b) Blood glucose: Glucose is mainly absorbed in the liver and after absorption it reaches the blood. Excess of glucose is converted into glycogen by insuline in the liver and muscles which is kept as stored food. If the blood glucose level exceeds 180 mg per 100 ml, it starts appearing in urine and is called glucosuria. Glucose level after breakfast is 110-140 mg and if it is higher then it causes diabetes mellitus.
c) Blood cholestrol: Cholestrol is needed in a very little amount in our body. It is required for the synthesis of biomembranes, vitamin D, bile salts and steroid hormones. But excess of cholestrol leads to the deposition of cholestrol in the veins and arteries which results in heart diseases. It reaches the blood either by intestinal absorption or synthesis by liver or both.
.d) Blood cells:
i) RBC (Red blood cells/corpuscles): These are formed in the maximum amount in the blood. It is red due to the present of the oxygen carrying pigments haemoglobin. Their main function is to transport oxygen and food to each part of the body and to remove some nitrogenous and other wastes from the body.
ii)WBC (White blood cells/corpuscles): These are also called leukocytes and they lack haemoglobin. It is mainly associated with the defence mechanism of the body. Rise in the level of WBC lead to leukemia (blood cancer).
Types of WBC:
a) Neutrophils:. They are maximum in number. These helps in phagocytosis. The pus contains dead neutrophiles and living virus/bacterias.
b) Eosinophiles: They helps in protecting from allergic reactions.
c) Basophil: These releases heparin and histamine which are anti-allergens.
d)Lymphocytes: There are two types of lymphocytes; B-lymphocytes and T-lymphocytes. B-lymphocytes produces antibodies against the antigens and T-lymphocytes do not directly the foreign bodies but helps B-lymphocytes in providing immunity. They are also called helper T cells.
e)Monocytes: These are the largest WBC. They also helps in phagocytosis.
C. MUSCLE TISSUES:
Muscle tissues helps in the contraction and functioning of muscle responsible for locomotion and movement of organs.
Muscle tissues can be classified into:
1. Striated/ skeleton/ voluntary muscle: These are attached to bones by tendons. These are made up of straited muscle fibres. Each muscle fibre is long, branched and cylindrical cell. It can be identified by checking the alternate dark (A) and light (I) bands and in the middle of these I bands is a fine dense, dark band called the Z band and H band in the middle of A bands. The action of these muscles can be controlled by us. Hence are also called voluntary muscles.
2. Non-striated/ smooth/ involuntary muscles: They do not show cross striation and hence look smooth. Single unit smooth muscle are composed of muscle fibres closely joined together. These function automatically and rhythemically i.e. cannot be controlled by us. Smooth muscles are present on the walls of some hollow visceral organs like urinary bladder and gastrointestinal tract. Multiunit smooth muscle is composed of more independent muscle fibre which are not so closely packed. These are present at the hair root and on the walls of large blood vessels.
These can be identified by their spindle shaped cellsl. They are packed parallel to each other. These are usually shorter than striated muscle fibres.
3. Cardiac muscle: These can be found in the heart only. These are also involuntary in action and they generates their own wave of excitation. It also shows cross-striation but much fainter than in striated muscles. It is responsible for the pumping of heart.
Cardiac muscle cells are short cylindrical cells joined end to end to form rows.
D. NERVE TISSUE:
Neurons are present in the CNS which helps in passing the electrical signals to brain from different parts of the body and vice-versa. Neurons are held together by the supportive cells called the neuroganglial cells. Nerve tissue is composed of neurons and neuroganglial cells and its function is to send electrical messages.
Identification of the neurons can be done by checking a cell body with small branches called the dendrites and one long branch called axon. At the end of the axon, it joins with another neuron and forms a junction called synapse where the signals are transferred from one to another neuron.
The word tissue means a group of cell similar in origin, structure and function.
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