$\displaystyle \small \bullet$ In multicellular animals, a group of similar cells along with intercellular substances perform a specific function. Such an organization is called tissue.
$\displaystyle \small \bullet$ These tissues are organized in specific proportion and pattern to form an organ like stomach, lung, heart and kidney.
$\displaystyle \small \bullet$ When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system.
ANIMAL TISSUES
The structure of the cells varies according to their function.
Types of Tissues:
i) Epithelial
ii) Connective
iii) Muscular
iv) Neural
Epithelial Tissue
$\displaystyle \small \bullet$ This tissue has a free surface, which faces either a body fluid or the outside environment. Thus provides a covering or a lining for some part of the body.
$\displaystyle \small \bullet$ This tissue consist either single layer or many layers of compactly arranged cells without intercellular space and arranged in continuous layer.
$\displaystyle \small \bullet$ The epithelium do not have direct blood supply, hence it is avascular.
$\displaystyle \small \bullet$ It can produce or replace the old layers
$\displaystyle \small \bullet$ They may be derived from ectoderm or mesoderm.
$\displaystyle \small \bullet$ Types of epithelial tissues:
a. Simple epithelium
b. Compound epithelium.
Simple Epithelium
Simple epithelium is composed of single layer of cells and functions as lining of body cavities, ducts and tubes.
Classification based on their structure
a) Squamous epithelium or pavement tissue
b) Cuboidal epithelium
c) Columnar epithelium
Squamous Epithelium
$\displaystyle \small \circ$ The squamous epithelium is made up of single layer of flattened cells with irregular boundaries.
$\displaystyle \small \circ$ They are present in lining of blood vessels, air sacs of lungs.
$\displaystyle \small \circ$ They are also known as pavement epithelium because of the resemblance of its compact structure to tiles on floor.
$\displaystyle \small \circ$ The cells are polygonal and large in size.
$\displaystyle \small \circ$ The nucleus is thicker than the cytoplasm, hence bulged in centre.
$\displaystyle \small \circ$ They are involved in the function like formation of diffusion boundary.
Cuboidal epithelium
$\displaystyle \small \circ$ Cuboidal epithelium is made up of single layered cube-like cells.
$\displaystyle \small \circ$ They are found in ducts of glands and tubular part of nephrons of kidney.
$\displaystyle \small \circ$ They appear as squares in section, but on free surface they appear hexagonal
$\displaystyle \small \circ$ They are involved in absorption, excretion and secretion.
$\displaystyle \small \circ$ They also provide mechanical support.
$\displaystyle \small \circ$ Single spherical nucleus is present in the centre.
Columnar epithelium
$\displaystyle \small \circ$ Columnar epitheliums are made up of tall and slender cells.
$\displaystyle \small \circ$ The nuclei are located at the base.
$\displaystyle \small \circ$ In some cells the free surface have finger like projections called microvilli.
$\displaystyle \small \circ$ These are found in lining of stomach and intestine.
$\displaystyle \small \circ$ They are helpful in secretion and absorption.
Ciliated epithelium
$\displaystyle \small \circ$ These are modified cuboidal or columnar epithelium.
$\displaystyle \small \circ$ The cells at free end has numerous fine hairs like cilia.
$\displaystyle \small \circ$ The cilia are capable of movement, which helps to move the particles.
$\displaystyle \small \circ$ These are found in kidney tubules as brush border and in trachea, oviduct, bronchioles, fallopian tube etc.
Glandular epithelium
$\displaystyle \small \circ$ Some of the columnar or cuboidal cells get specialized for secretion and are called glandular epithelium.
$\displaystyle \small \circ$ Types of Glandular epithelium:
i) Unicellular: Consists of isolated glandular cells (goblet cells of the alimentary canal).
ii) Multicellular: Consists of cluster of cells (salivary gland).
$\displaystyle \small \circ$ Classification on the basis of the mode of pouring of their secretions
i) Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products.
ii) Endocrine glands do not have ducts. Their products called hormones are secreted directly into the fluid bathing the gland.
Compound epithelium (Stratified epithelium)
$\displaystyle \small \bullet$ This epithelium consists several layers of cells.
$\displaystyle \small \bullet$ The cells are arranged in strata. The cells in basal layer are round or elongated, called basal cells.
$\displaystyle \small \bullet$ They mainly function as protection against mechanical and chemical stress.
$\displaystyle \small \bullet$ Classification based on cell shape in surface layer
a) Stratified Squamous epithelium: This is an epithelium consisting several layers, in which outer layer has squamous epithelial cells. In the skin, it consist the outer hard dead cells due to deposition of keratin called keratinization or cornification, which helps in protection.
b) Stratified cuboidal epithelium: It is the stratified epithelium with cuboidal cells at surface. These are at sweat glands, ducts of salivary glands, pancreas etc.
c) Stratified columnar epithelium: It is the epithelium found in the wall of ducts of mammary glands and urinary passage in males.
$\displaystyle \small \blacktriangle$ All cells in epithelium are held together with little intercellular material.
$\displaystyle \small \blacktriangle$ All animal tissues have provided both structural and functional links between its individual cells.
$\displaystyle \small \blacktriangle$ Three types of cell junctions are found in the epithelium and other tissues.
i) Tight junctions help to stop substances from leaking across a tissue.
ii) Adhering junctions perform cementing to keep neighbouring cells together.
iii) Gap junctions facilitate the cells to communicate with each other, by connecting the cytoplasm of adjoining cells for rapid transfer of ions, small molecules and sometimes big molecules.
$\displaystyle \small \bullet$ These tissues are organized in specific proportion and pattern to form an organ like stomach, lung, heart and kidney.
$\displaystyle \small \bullet$ When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system.
ANIMAL TISSUES
The structure of the cells varies according to their function.
Types of Tissues:
i) Epithelial
ii) Connective
iii) Muscular
iv) Neural
Epithelial Tissue
$\displaystyle \small \bullet$ This tissue has a free surface, which faces either a body fluid or the outside environment. Thus provides a covering or a lining for some part of the body.
$\displaystyle \small \bullet$ This tissue consist either single layer or many layers of compactly arranged cells without intercellular space and arranged in continuous layer.
$\displaystyle \small \bullet$ The epithelium do not have direct blood supply, hence it is avascular.
$\displaystyle \small \bullet$ It can produce or replace the old layers
$\displaystyle \small \bullet$ They may be derived from ectoderm or mesoderm.
$\displaystyle \small \bullet$ Types of epithelial tissues:
a. Simple epithelium
b. Compound epithelium.
Simple Epithelium
Simple epithelium is composed of single layer of cells and functions as lining of body cavities, ducts and tubes.
Classification based on their structure
a) Squamous epithelium or pavement tissue
b) Cuboidal epithelium
c) Columnar epithelium
Squamous Epithelium
$\displaystyle \small \circ$ The squamous epithelium is made up of single layer of flattened cells with irregular boundaries.
$\displaystyle \small \circ$ They are present in lining of blood vessels, air sacs of lungs.
$\displaystyle \small \circ$ They are also known as pavement epithelium because of the resemblance of its compact structure to tiles on floor.
$\displaystyle \small \circ$ The cells are polygonal and large in size.
$\displaystyle \small \circ$ The nucleus is thicker than the cytoplasm, hence bulged in centre.
$\displaystyle \small \circ$ They are involved in the function like formation of diffusion boundary.
Cuboidal epithelium
$\displaystyle \small \circ$ Cuboidal epithelium is made up of single layered cube-like cells.
$\displaystyle \small \circ$ They are found in ducts of glands and tubular part of nephrons of kidney.
$\displaystyle \small \circ$ They appear as squares in section, but on free surface they appear hexagonal
$\displaystyle \small \circ$ They are involved in absorption, excretion and secretion.
$\displaystyle \small \circ$ They also provide mechanical support.
$\displaystyle \small \circ$ Single spherical nucleus is present in the centre.
Columnar epithelium
$\displaystyle \small \circ$ Columnar epitheliums are made up of tall and slender cells.
$\displaystyle \small \circ$ The nuclei are located at the base.
$\displaystyle \small \circ$ In some cells the free surface have finger like projections called microvilli.
$\displaystyle \small \circ$ These are found in lining of stomach and intestine.
$\displaystyle \small \circ$ They are helpful in secretion and absorption.
Ciliated epithelium
$\displaystyle \small \circ$ These are modified cuboidal or columnar epithelium.
$\displaystyle \small \circ$ The cells at free end has numerous fine hairs like cilia.
$\displaystyle \small \circ$ The cilia are capable of movement, which helps to move the particles.
$\displaystyle \small \circ$ These are found in kidney tubules as brush border and in trachea, oviduct, bronchioles, fallopian tube etc.
$\displaystyle \small \circ$ Some of the columnar or cuboidal cells get specialized for secretion and are called glandular epithelium.
$\displaystyle \small \circ$ Types of Glandular epithelium:
i) Unicellular: Consists of isolated glandular cells (goblet cells of the alimentary canal).
ii) Multicellular: Consists of cluster of cells (salivary gland).
i) Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products.
ii) Endocrine glands do not have ducts. Their products called hormones are secreted directly into the fluid bathing the gland.
Compound epithelium (Stratified epithelium)
$\displaystyle \small \bullet$ This epithelium consists several layers of cells.
$\displaystyle \small \bullet$ The cells are arranged in strata. The cells in basal layer are round or elongated, called basal cells.
$\displaystyle \small \bullet$ They mainly function as protection against mechanical and chemical stress.
a) Stratified Squamous epithelium: This is an epithelium consisting several layers, in which outer layer has squamous epithelial cells. In the skin, it consist the outer hard dead cells due to deposition of keratin called keratinization or cornification, which helps in protection.
b) Stratified cuboidal epithelium: It is the stratified epithelium with cuboidal cells at surface. These are at sweat glands, ducts of salivary glands, pancreas etc.
c) Stratified columnar epithelium: It is the epithelium found in the wall of ducts of mammary glands and urinary passage in males.
$\displaystyle \small \blacktriangle$ All cells in epithelium are held together with little intercellular material.
$\displaystyle \small \blacktriangle$ All animal tissues have provided both structural and functional links between its individual cells.
$\displaystyle \small \blacktriangle$ Three types of cell junctions are found in the epithelium and other tissues.
i) Tight junctions help to stop substances from leaking across a tissue.
ii) Adhering junctions perform cementing to keep neighbouring cells together.
iii) Gap junctions facilitate the cells to communicate with each other, by connecting the cytoplasm of adjoining cells for rapid transfer of ions, small molecules and sometimes big molecules.
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