ISC 12>CONTENT>STRUCTURE AND FUNCTION OF ANIMALS> 1. ANIMAL TISSUES
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THE SCOPE OF SYLLABUS
Epithelial; connective; muscular; nervous. Location, structure and functions of epithelial tissues with examples, location. General structure of areolar tissue - functions of different types of cells; Difference between collagen and elastin fibres; Difference between bone and cartilage; hyaline cartilage, T.S and L.S of bone, lymph and blood, Different types of muscles and their functions; Structure of a neuron. |
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ANIMAL TISSUES
EPITHELIAL TISSUE
It covers the body surface and lines the cavities. |
CONNECTIVE TISSUE
Binds and supports body parts |
MUSCLE TISSUE
Enables locomotion and movement of body parts |
NERVOUS TISSUE
Helps in response to stimuli and coordination of body functions |
EPITHELIAL TISSUE
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Epithelial tissue covers the body surface. It is made up of closely packed cells that are arranged in one or more layers.
It is the only tissue which originated from all the three primordial germinal layers. (i) Ectodermal – Epidermis (stratified squamous epitheliium) (ii) Mesodermal – Mesothelium (simple squamous Epithelium) (iii) Endodermal – Endothelium (simple squamous Epithelium) Features of the epithelial cells
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Competitive focus- Junctions between epithelial cells
Cell junctions are found in eukaryotic animal cells. They are found on the cell membrane.
To provide mechanical support for the tissue plasma membrane of adjacent epithelial cells modified to form following structures -
→ Tight junctions (Zonula occludens) : help to prevent substances from leaking across the tissue. Plasma membranes in the apical parts become tightly packed together or are even fused.
→ Interdigitations : These are interfitting, finger like processes of the cell membranes of the adjacent cells.
→ Intercellular Bridges : These are minute projections that arise from adjacent cell membrances. They make contact with one anther.
→ 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.
→ Intermediate Junctions (= Zonula adherens) : These usually occur just below tight junctions. The intercellular space at these places contains a clear, low electron density fluid. There is a dense plaque like structure on cytoplasmic side of each plasma membrane from which fine microfilaments of actin (protein) extend into the cytoplasm. There is no intercellular filaments between the adjacent cell membranes. There is an adhesive material at this point. They probably serve anchoring functions.
→ Desmosomes ( =Macula adherens) : Perform cementing to keep the neighbouring cells together. These are like zonula adherens but are thicker and stronger and are disc like junctions. They have intercellular protein. The plaque-like structures (= protein plate) are much thicker. The microfilaments
which extend from microfilaments are called tonofibrils. Desmosomes serve anchoring function.
Hemidesmosomes (single sided desmosomes) are similar to desmosomes, but the thickening of cell membrane is seen only on one side. Hemidesmosomes join epithelial cells to basal lamina (outer layerof basement membrane).
To provide mechanical support for the tissue plasma membrane of adjacent epithelial cells modified to form following structures -
→ Tight junctions (Zonula occludens) : help to prevent substances from leaking across the tissue. Plasma membranes in the apical parts become tightly packed together or are even fused.
→ Interdigitations : These are interfitting, finger like processes of the cell membranes of the adjacent cells.
→ Intercellular Bridges : These are minute projections that arise from adjacent cell membrances. They make contact with one anther.
→ 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.
→ Intermediate Junctions (= Zonula adherens) : These usually occur just below tight junctions. The intercellular space at these places contains a clear, low electron density fluid. There is a dense plaque like structure on cytoplasmic side of each plasma membrane from which fine microfilaments of actin (protein) extend into the cytoplasm. There is no intercellular filaments between the adjacent cell membranes. There is an adhesive material at this point. They probably serve anchoring functions.
→ Desmosomes ( =Macula adherens) : Perform cementing to keep the neighbouring cells together. These are like zonula adherens but are thicker and stronger and are disc like junctions. They have intercellular protein. The plaque-like structures (= protein plate) are much thicker. The microfilaments
which extend from microfilaments are called tonofibrils. Desmosomes serve anchoring function.
Hemidesmosomes (single sided desmosomes) are similar to desmosomes, but the thickening of cell membrane is seen only on one side. Hemidesmosomes join epithelial cells to basal lamina (outer layerof basement membrane).
SPECIALIZED FUNCTIONAL STRUCTURES
Plasma membrane of free end get modified to form 3 types of functional structures.
MICROVILLI
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These are minute protoplasmic process which are non motile, non contractile.
Function-
Present in the wall of Intestine, Gall bladder, Proximal convoluted tubule etc. |
KINOCILIA
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These are long cylindrical protoplasmic process, that are motile and contractile.
The movement of cilia is always in uniform direction. Kinocilia originate from basal granule or kinetosome. Function-
Fallopian tube, Trachea, Uterus, Ependymal epithelium : (Inner lining of ventricles of brain & central canal of spinal cord. Function of cilia is to conduct substances in CSF.) |
STEREOCILIA
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These are long cytoplasmic process, these are non motileand non contractile.
The basal granule is absent, and plasma membrane is thick & rigid. Base of stereocilia is broad & apical part is narrow so they are conical in shape. Function: They increase surface area. Location: Epididymis, Vasa deferens etc. |
| Kinocilia | Stereocilia | |
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| 1 | Kinocilia are motile | Stereocilia are non motile |
| 2 | Kinocilia arise from basal granules | Stereocilia are extensions of the plasma membrane of epithelial cells |
| 3 | Their movement is controlled by basal granules. | These move with the fluid flowing through their tubes |
| 4 | They occur in the cells of respiratory tract and reproductive organs | They occur on the epithelial cells of vas deferens and epididymis. |
Functions of epithelium
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1. Protection- protects the underlying tissues from- mechanical injury, drying up, from harmful chemical and infection.
2. Absorption- Epithelial lining of the intestine absorbs digested food. 3. Excretion- Epithelial lining of the uriniferous tubules in the kidney helps in excretion of substances. 4. Conduction- ciliated epithelium helps in conduction of substances. 5. Secretion- Lining of the glands secretes substances. 6. Exoskeleton- Nails, hairs, claws, feathers etc. are formed form the epithelial cells. 7. Respiration- Lining of the lung brings about exchange of gases. 8. Sensory function- epithelium of sense organs receives stimuli and convey nerve impulses. |
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Types of epithelium
These are classified on the basis of shape and number of layers.
SIMPLE EPITHELIUM
Epithelium on the basis of shape
SQUAMOUS EPITHELIUM
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CUBOIDAL EPITHELIUM
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COLUMNAR EPITHELIUM
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SQUAMOUS EPITHELIUM
Squamous cells appear like thin, flat plates/scales.
They have horizontal flattened, elliptical nuclei because of the thin flattened form of the cell.
Location
They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.
Function-
A single layer of flat cells which forms a thin boundary of tissue that easily allows materials to diffuse through it, such as is found in the alveolar sacs in the lungs, where the exchange of CO2 and O2 occur. It is also associated with excretion, protection and secretion
They have horizontal flattened, elliptical nuclei because of the thin flattened form of the cell.
Location
They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.
Function-
A single layer of flat cells which forms a thin boundary of tissue that easily allows materials to diffuse through it, such as is found in the alveolar sacs in the lungs, where the exchange of CO2 and O2 occur. It is also associated with excretion, protection and secretion
SIMPLE SQUAMOUS EPITHELIUM OF LUNG
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CUBOIDAL EPITHELIUM
Cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre.
Location:
Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. It is found in peripheral region of ovary & seminiferous tubules in Testis.
Function-
In gonads this epithelium is also called as Germinal epithelium (testis & ovaries) where cuboidal cells divide to form egg & sperm.
In the kidney these cells excrete the waste products of cell metabolism and reabsorb any needed materials from urine before it's excreted.
Location:
Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. It is found in peripheral region of ovary & seminiferous tubules in Testis.
Function-
In gonads this epithelium is also called as Germinal epithelium (testis & ovaries) where cuboidal cells divide to form egg & sperm.
In the kidney these cells excrete the waste products of cell metabolism and reabsorb any needed materials from urine before it's excreted.
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COLUMNAR EPITHELIUM
Columnar epithelial cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells.
Location
Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialised for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum.
Function-
They secrete mucus or slime, a lubricating substance which keeps the surface smooth. This tissue lines the intestine, producing digestive enzymes and absorbing digested nutrients by diffusion, active transport, or facilitated diffusion. Columnar cells with microvilli increases surface area for absorption.
Location
Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialised for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum.
Function-
They secrete mucus or slime, a lubricating substance which keeps the surface smooth. This tissue lines the intestine, producing digestive enzymes and absorbing digested nutrients by diffusion, active transport, or facilitated diffusion. Columnar cells with microvilli increases surface area for absorption.
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CILIATED EPITHELIUM
If the columnar or cuboidal cells bear cilia on their free surface they are called ciliated epithelium.
Location
They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.
Function
Their function is to move particles or mucus in a specific direction over the epithelium.
Location
They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.
Function
Their function is to move particles or mucus in a specific direction over the epithelium.
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PSEUDOSTRATIFIED EPITHELIUM
The pseudostratified epithelium is "falsely stratified".
Cells are columnar but tall and thin. All cells rest on the basement membrane. The unique appearance of pseudostratified epithelia occurs because the tall, thin cells intertwine. Nuclei appear at various levels but there is no distinct layering. Pseudostratified epithelia are most prevalent in the upper or lower respiratory tract as ciliated types.
Non-ciliated pseudostratified epithelium can be found in the ducts of larger glands or the male urethra but these may be transition zones between epithelial types.
The cilia of respiratory pseudostratified epithelia beat in a rhythmic manner to propel mucous along the surfaces of cells. This mucous layer, a product of goblet cells, entraps dust, debris, and microbes inhaled into the tract. The resultant movement of mucous from the lower tract via ciliary movements is called the "ciliary escalator".
Cells are columnar but tall and thin. All cells rest on the basement membrane. The unique appearance of pseudostratified epithelia occurs because the tall, thin cells intertwine. Nuclei appear at various levels but there is no distinct layering. Pseudostratified epithelia are most prevalent in the upper or lower respiratory tract as ciliated types.
Non-ciliated pseudostratified epithelium can be found in the ducts of larger glands or the male urethra but these may be transition zones between epithelial types.
The cilia of respiratory pseudostratified epithelia beat in a rhythmic manner to propel mucous along the surfaces of cells. This mucous layer, a product of goblet cells, entraps dust, debris, and microbes inhaled into the tract. The resultant movement of mucous from the lower tract via ciliary movements is called the "ciliary escalator".
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COMPOUND/ STRATIFIED EPITHELIUM
It consists of more than one layer of cells. since it has more than one layer it provides protection to underlying tissues against mechanical, chemical or thermal stress. Stratified epithelia are normally found in places where there is a
great deal of wear and tear. Frequently, the outer surface of these
epithelia is sloughed off and replaced by cells below it. Thus, the
deeper layers of such epithelia (those nearest the basement membrane)
are mitotically active.
Stratified squamous epithelium
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KERATINISED AND NON KERATINISED EPITHELIUM
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STRATIFIED CUBOIDAL
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STRATIFIED COLUMNAR
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Transitional epithelium
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GLANDULAR TISSUE
Glands are specialised cells that give out specific secretions. There are basically two types of glands-
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ENDOCRINE
No duct system
Secretions directed into the extracellular fluid (basal side), move into vascular system |
EXOCRINE
Ducts to release products
Secretions released to the apical cell surface, move out of ducts to outside environment |
The exocrine glands can be classified on the basis of-
1. Number of cells
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UNICELLULAR
Made of only one glandular epithelium cell; called intraepithelial cells
Goblet cells in humans |
MULTICELLULAR
Multiple cells make up one gland; called extraepithelial cells
Many examples, including secretory sheets in the human stomach |
2. Type of secretion
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SEROUS
Thin, watery protein-rich secretion
Eg. sweat glands, intestinal glands |
MUCOUS
Viscous slimy secretion
Eg. Goblet cells of gut |
MIXED
serous and mucus secretion
Eg. gastric glands, salivary glands |
3. Mode of secretion
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MEROCRINE
Expelled by exocytosis
Only secretory product is lost Eg. sweat gland, salivary gland |
APOCRINE
Uses membrane vesicles
Secretory product and some membrane lost Eg. mammary gland |
HOLOCRINE
Entire secretory cell lost, as plasma membrane breaks to release product
Eg. Sebaceous gland |
4. Shape of secretory unit
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Tubular gland
Secretory units are tube like 
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Acinar gland
Secretory units are round 
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Alveolar gland
Secretory units are flask shaped 
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Tubuloalveolar gland
Secretory units are mixed. 
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When the secretory units are branched it is called compound gland.
