Epithelial Tissue 

This is the most abundant tissue in our body. So, is the name Epithelium (Epi=upon, Thelia=grow). During embryonic development, Epithelium originates first. Power of regeneration is present/max. in this tissue. It always grows on the connective tissue. The cells are compactly packed with the little intercellular matrix. Blood vessels and lymph vessels are unable to pierce. Blood circulation is absent. Get their nutrition from underlying connective tissue
Epithelial tissue has a free surface that faces either a body fluid or outside environment and thus provides a covering or a lining. 


Cell junctions

To provide both Structural and Functional links between its individual cells, epithelial cells modify to form the following structures:-
Four types of cell junctions:-
  1. Interdigitations
  2. Tight junctions
  3. Desmosomes
  4. Gap junctions

Interdigitations

Finger-like processes of the plasma membrane enter into the cytoplasm of the adjacent cell. Mainly found in Stretchable/Transitional Epithelium.

Tight junctions

Stop substances from leaking across a tissue

Desmosomes
Perform cementing to keep neighboring cells together. Mostly found in Non-stretchable epithelium. Provide mechanical support to Non-stretchable epithelium. Also called Macula Adherens.Contain intermediate fibers known as Tonofibrils.

Gap junctions

Facilitate the cells to communicate with each other. Rapid transfer of ions, small molecules, and sometimes big molecules.



INTERCELLULAR JUNCTIONS 



The plasma membrane of the free end modified into three types of functional structures
  • MICROVILLI
  1. Minute process
  2. Non-motile
  3. Non-contractile
  4. Functions:- Increase surface area for absorption and secretion 
  5. Examples:- Wall of the intestine, Gall's bladder, PCT of the nephron.

  • CILIA/KINOCILIA
  1. Long cylindrical process (uniform diameter from base to apex)
  2. Motile
  3. Contractile
  4. Functions:- Movement and locomotion Movement is in a uniform direction 
  5. Examples:- Trachea, bronchus, and bronchioles, Fallopian tubes, Ependymal epithelium
  • STEREOCILIA
    1. Conical shape
    2. Non-motile
    3. Non-contractile
    4. Functions:- Increase surface area
    5. Examples:- Epididymis, Vas deferens, Macula & Crista of the inner ear




    TYPES OF EPITHELIUM TISSUE

    • SIMPLE EPITHELIUM
    1. Composed of a single layer of cells.
    2. Functions as lining for body cavities, ducts, and tubes
    • BASED ON SHAPE/STRUCTURAL MODIFICATIONS OF CELLS
    1. Simple Squamous
    2. Simple Cuboidal
    3. Simple Columnar
    • COMPOUND EPITHELIUM (STRATIFIED) 
    1. Consists of two or more cell layers.
    2. Protective function as it does in our skin
    • BASED ON THE PRESENCE OR ABSENCE OF STRETCHING ABILITY 
    1. NON STRETCHABLE
    • Stratified squamous 
    • Stratified cuboidal
    • Stratified columnar

    2. STRETCHABLE

    • Transitional epithelium
    SIMPLE EPITHELIUM 


    A. SIMPLE SQUAMOUS EPITHELIUM TISSUE
    1. Single-layer of flat, scale-like cells with irregular boundaries.
    2. The nucleus is present at the center and is round & flat.
    3. Pavement epithelium - due to tile like appearance
    4. Tesselated epithelium - due to wavy appearance
    5. Examples – Filtration, Diffusion


    B. SIMPLE CUBOIDAL EPITHELIUM TISSUE
    1. Cells are cube-like in shape
    2. The nucleus is spherical and present at the center.
    3. Also called the germinal epithelium
    4. Functions: Absorption , secretion & excretion
    5. Examples - most of the glands of our body are made Examples of cuboidal epithelium, Germinal epithelium in GONADS (OVARY & TESTIS), In PCT(Brushbordered)  

        C. SIMPLE COLUMNAR EPITHELIUM TISSUE

        1. Cells are pillar-like in shape. (Tall and slender). An elongated nucleus is present at the base of the cell. 
        2. Function: It helps in the absorption and secretion
        3. They are found in the stomach and intestine and help in the secretion and absorption
        4. ABSORPTION- MICROVILLI (BRUSH BORDERED)
        5. MOVEMENT - CILIA (CILIATED)
        6. SECRETION - GLAND (GLANDULAR)
        7. Examples:- LIVER, BILE DUCT, UTERUS



        PSEUDOSTRATIFIED EPITHELIUM 

        Their cells are pillar-like in shape, so it is also a modification of columnar epithelium. In this epithelium, 2 types of cells are present i.e. Tall cell and short cell. The nucleus in both is present at different levels, so it appears bilayered. All cells are present on a single basement membrane, so it is unilayered.







        Examples:- Respiratory epithelium of nasal  chambers, Trachea, Bronchi 


        COMPOUND/STRATIFIED EPITHELIUM



        Multilayered ( more than one layer). Limited role in secretion and absorption.Protection against chemical and mechanical stress 


        STRETCHABLE EPITHELIUM/TRANSITIONAL EPITHELIUM 

        • Multilayered: 4-6 layers of cells are present
        • Innermost layer - cube-like cell
        • Middle 2-4 layers - pear-shaped or umbrella-shaped cells/
        • Outermost 1-2 layers - oval-shaped cells  
        • In resting conditions a thin basement membrane is present, but on stretching the basement membrane almost gets disappeared.
        • The different shapes of cells appear only in the resting stage. When this tissue is stretched, all the cells become flattened.
        • Cells are interconnected by interdigitation
        • A thin cuticle line at the outermost layer makes this tissue waterproof 
        • Examples:- Renal pelvis, Ureter, Urinary bladder, Proximal part of the male urethra

            
        Note: Mainly present in the urinary system. So also called Urothelium


        NON-STRETCHABLE EPITHELIUM 

        Cells are interconnected by desmosomes  


        GLANDS

        A cell or a group of cells that secretes chemical substances is known as a gland. All Glands are composed of epithelial tissue. Some columnar or cuboidal cells get specialized for secretion known as Glandular Epithelium. Glands can originate from all three germinal layers.
        ECTODERM – Sweat gland 
        MESODERM – Gonads 
        ENDODERM – Thyroid gland 


        CLASSIFICATION OF GLANDS

        (A) BASED ON THE NUMBER OF CELLS
        1. Unicellular Glands:-  Examples: Goblet cells, Paneth cells 

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        2. Multicellular Glands:-         Examples All glands except goblet cells and paneth cells.
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        (B) BASED ON METHOD OF SECRETION  
        • Endocrine Glands:- 
        1. Secretory duct- Absent
        2. Their product called hormones are secreted directly into the fluid bathing the Gland.
        3. Thyroid, Pituitary, Thymus Pineal, Parathyroid, Adrenal

        • Exocrine Glands:- 
        1. Secretory duct- Present
        2. Secretes Mucus, Saliva, Earwax, Oil, Milk, Enzymes, and other cell products
          NOTE: HETEROCRINE / MIXED GLAND: Both endocrine & exocrine parts are present. Examples: Pancreas, Gonads


          (C) BASED ON THE NATURE OF SECRETION  
          • Acrine/Merocrine
          • Apocrine
          • Holocrine


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