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ISC 12>CONTENT>STRUCTURE AND FUNCTION OF ANIMALS> 1. ANIMAL TISSUES

THE SCOPE OF SYLLABUS
Animal tissues
Epithelial, connective, muscular and nervous tissues to be taught with the help of diagrams. 
Location, structure and functions of epithelial tissues (simple, stratified, pseudostratified, specialised – transitional, neurosensory and pigmented) with examples,
Types of junctions (tight, adhering and gap junctions)
Location and general structure of areolar tissue - functions of different types of cells (fibroblasts, macrophages, Mast cells, plasma cells, adipocytes); Fibrous connective tissue (ligaments and tendon); difference between bone and cartilage; types of cartilage (hyaline, white fibrous, yellow elastic and calcified); T.S. of hyaline cartilage, T.S and L.S. of mammalian bone(to be taught with the help of diagrams);
Different types of muscles and their functions;
Structure of a neuron (types
– unipolar, bipolar, multipolar, myelinated, non- myelinated). Neuroglial cells. 
eight-types-of-epithelial-tissue

ANIMAL TISSUES

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EPITHELIAL TISSUE
It covers the body surface and lines the cavities.
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CONNECTIVE TISSUE
Binds and supports body parts
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MUSCLE TISSUE
Enables locomotion and movement of body parts
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NERVOUS TISSUE
Helps in response to stimuli and coordination of body functions

EPITHELIAL TISSUE

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
  • The cells are compactly arranged and form continuous sheets.
  • Intercellular spaces and intercellular matric absent.
  • Cells of lowermost layer always rest on a non living basement membrane.
  • The cells are cemented together by a cementing substance made of glycoprotein.
  • These cells are capable of division and regeneration throughout the life.
  • Free surface of the cells may have fine hair cilia or microvilli or may be smooth.
  • Epithelium is avascular.
  • Due to absence/less of intercellular spaces blood vessels, lymph vessels are unable to pierce this tissue so blood circulation is absent in epithelium.
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Basement membrane

A thin non living acellular basement membrane is present between the epithelial tissue and the connective tissue which is highly permeable.
​

The basement membrane has
  • ​Basal lamina – composed of mucopolysaccharides and glycoprotein that is secreted by epithelial cells.
  • Mucopolysaccharide is present in the form of Hyaluronic acid which is composed of 2 components–N acetyl glucosamine & glucuronic acid. Both these components are found in alternate form.
  • Fibrous or reticular lamina- thick layer of collagen or reticulate fibers​
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Location of epithelial tissues

  • Covers internal and external surfaces of the body organs.
  • Lines body cavities (mesothelium) and cavities of hollow body organs
  • Inner lining of blood vessels (endothelium)

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.


→ 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).



→ 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 membranes. They make contact with one another.


→ 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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SPECIALIZED FUNCTIONAL STRUCTURES 

Plasma membrane of free end get modified to form 3 types of functional structures.

MICROVILLI
KINOCILIA
STEREOCILIA
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These are minute protoplasmic process that are invaginations of the cell membrane which are non motile, non contractile.
They do not contain microtubules.
​
Function-
  • Help in absorption, secretion, excretion
  • Increase surface area more than 20 times.
Location-
Present in the wall of Intestine, Gall bladder, Proximal convoluted tubule etc.
Picture
These are long cylindrical protoplasmic process, that are motile and contractile. 
Kinocilia originate from basal granule or kinetosome.
Function-
  • They helps in conduction of substances
Location:
 Fallopian tube, Trachea, Uterus, Ependymal epithelium.
Picture
These are long cytoplasmic process, these are non motile and non contractile.
The basal granule is absent, and plasma membrane is thick & rigid.

Function:
They increase surface area.

Location:
Epididymis,  Vasa deferens etc.

KinociliaStereocilia
1Kinocilia are motileStereocilia are non motile
2Kinocilia arise from basal granulesStereocilia are extensions of the plasma membrane of epithelial cells
3Their movement is controlled by basal granules.These move with the fluid flowing through their tubes
4They occur in the cells of respiratory tract and reproductive organsThey occur on the epithelial cells of vas deferens and epididymis.

Functions of epithelium

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.

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SIMPLE EPITHELIUM

Epithelium on the basis of shape

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SQUAMOUS EPITHELIUM
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CUBOIDAL EPITHELIUM
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COLUMNAR EPITHELIUM

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
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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. 
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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.
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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. 
Picture
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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". 
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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

  • The cells are flat, scale-like, and can be multi-layered.
  • The basal layer is in contact with basal lamina (the collagen membrane between the epithelia and capillaries)
  • It is selectively permeable and helps in filtration/diffusion
  • It provide protection against:
    • mechanical friction - rubbing and physical trauma from external sources
    • chemical damage - environmental and internal chemicals/compounds degrading epithelial lining
  • In the alveoli, it forms a cell layer essential for gas exchange, also a protective layer against air-born pathogens/toxic gases
  • In the lining of the esophagus it helps in protection of the esophagus from stomach acid, non-keratinized, mucousal cell layer.
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KERATINISED AND NON KERATINISED EPITHELIUM
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STRATIFIED CUBOIDAL 

  • These type of cells occur rarely, and is cube-shaped.
  • They commonly make up two layers of cells.
  • They help in protection of larger ducts -cell layers surround /protect gland ducts
They are found in
  • sweat glands.
  • mammary gland.
  • salivary gland.
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STRATIFIED COLUMNAR

  • These tissues occur rarely, and are column-shaped
  • The main function is secretion and protection.
Location 
  • Between simple columnar epithelia and stratified squamous epithelia.
  • Near salivary glands - cell lining protects the salivary duct, goblet cells found between the cells, also form layers in ocular conjunctiva and the linings of the pharynx, anus, uterus, urethra
Function 
  • In vas deferens where it protects and aids in secretion of glands.
  • Intestinal lining - ciliated to help move nutrients and increase absorption, also protects against pathogens/bacteria
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Transitional epithelium

  • These cells show great amount of flexibility.
  • without distention (relaxed), the cells appear cuboidal-shaped
  • when distented (tense), the cells appear squamous-shaped
  • impermeable to salts and water
  • found in urinary bladder, ureter, and pelvis of kidney
    • urine/fluid builds up in these cavities, the transitional layers extend to relieve the stress 
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GLANDULAR TISSUE

Glands are specialised cells that give out specific secretions. There are basically two types of glands-
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 
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The exocrine glands can be classified on the basis of-

1. Number of cells

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 
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2. Type of secretion

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

MEROCRINE
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Expelled by exocytosis 
Only secretory product is lost 
Eg. sweat gland, salivary gland
APOCRINE
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Uses membrane vesicles 
Secretory product and some membrane lost 
Eg. mammary gland
HOLOCRINE
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Entire secretory cell lost, as plasma membrane breaks to release product 
Eg. Sebaceous gland

4. Shape of secretory unit

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.
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CONNECTIVE TISSUE>

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