Epithelial Tissue

Epithelial tissue, the simplest tissue type, is the animal body's protective layer. It envelops most of the body's organs and internal cavities, effectively forming a barrier that separates different bodily systems. Epithelial tissue comprises the skin and the linings of the mouth, blood vessels, lung alveoli, and kidney tubules. 

1.0What is Epithelial Tissue?

Word epithelium is derived from two words – epi and thelia, where Epi – upon, thelia–growth

• These tissues develop first during embryonic development.
• The power of regeneration is present in this tissue, while the power of regeneration is absent, and the least power is found in nervous tissue.
• Epithelium cells are compactly packed with little intercellular matrix.
• Because of very few intercellular spaces, blood and lymph capillaries are absent.
• It always rests upon underlying connective tissue.
• Cells of epithelial tissues derive their nutrition from underlying connective tissue.
• A basement membrane is present between epithelium and connective tissue.
• Basement membrane: Between epithelium and connective tissue, a thin, nonliving, acellular basement membrane is present. This membrane is highly permeable and consists of 2 layers.
• Basal lamina: Towards the epithelium, which is made up of glycoprotein secreted by epithelium cells.
• Fibrous lamina: This is toward connective tissue, in which collagen and reticular fibers are suspended in mucopolysaccharide, a connective tissue matrix. Mucopolysaccharide is present in the form of Hyaluronic acid. So, the basement membrane is secreted by both epithelium and connective tissue.
• This tissue has a free surface that faces either a body fluid (for example, the epithelium of blood vessels, the epithelium of nephron, the epithelium of stomach and intestine, etc.) or the outside environment (for example, nails, horns, hoof, feathers, scales) and thus provides a covering or a lining for some part of the body.

2.0Modifications of Plasma Membrane and Cell Junction

• Epithelium/Epithelial Tissue
• Modification of Plasma Membrane

Microvilli

• Minute process, Non-motile, Non-contractile.
• Function- Increase surface area for absorption & secretion.
• Examples - Wall of the intestine, Gallbladder, and PCT of the nephron. 

Cilia/Kinocilia

• Long cylindrical process (same diameter from base to apex). Motile and Contractile.
• It moves particles or mucus in a specific direction over the epithelium. (Movement is in uniform direction).
• Function- Helps in movement & locomotion.
• Example- Inner surface of hollow organs like bronchioles & fallopian tube, Uterus, Trachea, Ependymal epithelium.

Stereocilia

• Conical shape, Non-motile, Non-Contractile.
• Function- Increase surface area.
• Examples are epididymis & Vas deferens.

3.0Types of Cell Junction

Four types of cell junctions

• Interdigitations
• Tight junctions
• Desmosomes
• Gap junctions
• Cell Junction 

Cell Junction 

• Cell junctions are found in epithelium and other tissues.
• These specialized junctions provide both structural & functional links between its individual cells.
• Interdigitations - Finger-like processes of the plasma membrane, which enter into the cytoplasm of an 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. Primarily found in Non-stretchable epithelium.
• Provide mechanical support to Non-stretchable epithelium. It is also called Macula Adherens.
• Contains intermediate fibers known as Tonofibrils.
• Gap junctions - Enable communication between cells.
• Rapid transfer of ions, small molecules and sometimes big molecules.

4.0Classification of Epithelial Tissue

5.0Simple Epithelial Tissue and Function

Simple Epithelium

• Made up of a single layer of cells.
• All the cells rest upon the basement membrane.
• Functions as lining for body cavities, ducts and tubes.
• All the cells of simple epithelium rest upon the basement membrane.
• Only the cells of the lowermost layer rest upon the basement membrane. 

Simple Squamous Epithelium 

• It consists of a single-cell layer. The basement membrane supports every cell and functions as a protective layer for body cavities, ducts, and tubes.
• All the cells of simple epithelium rest upon the basement membrane.
• Only the cells of the lowermost layer rest upon the basement membrane.
• The squamous epithelium comprises a single layer of thin, flat cells with irregular edges. These cells form a lining along the walls of blood vessels and air sacs in the lungs, creating a barrier that aids diffusion.
• It is also called pavement epithelium due to its tile-like appearance.
• It is also called Tessellated epithelium due to its wavy appearance in blood vessels and coelom.
• This epithelium is associated with filtration & diffusion.

Examples

• This epithelium is found on the outer surface of the stomach and intestine.
• Bowman's capsule (Podocyte)
• Alveoli of lungs (Pneumocytes type-I) (air sacs of lungs)
• Endothelium – Inner lining of blood vessels and lymph vessels.
• Mesothelium – The covering of the coelom is called mesothelium.
• Endocardium – The innermost lining of the heart is also made up of simple squamous epithelium.
• The parietal layer of the pericardium and epicardium of the heart is also made up of simple squamous epithelium.
• The thin part of the loop of Henle of the nephron.
• Pleura – Covering of lungs is lined by simple squamous epithelium.

Simple Cuboidal Epithelium

• The cuboidal epithelium consists of a single layer of cube-shaped cells and is typically found in the ducts of glands and the tubular sections of nephrons in the kidneys.
• Its main functions are secretion and absorption.

6.0Modification of Simple Epithelial Tissue

Brush bordered cuboidal epithelium

• When microvilli are present on the free end of cuboidal cells, they are called brush-bordered cuboidal epithelium. This type is found in the PCT of the nephron.

Ciliated cuboidal epithelium

• When cilia are present on the free end of cuboidal cells, it is known as ciliated cuboidal epithelium. It is usually found in the collecting duct.
• This epithelium is also called germinal epithelium because, in gonads (testis and ovaries), cuboidal cells divide to form eggs and sperm.

Simple Columnar Epithelium

• The columnar epithelium consists of a single layer of cylindrical cells, each with an elongated nucleus at the cell's base.
• Cells are more extended and less wide, so they appear pillar-shaped in vertical sections.
• Function: It helps in absorption and secretion.

Modifications of Simple Columnar Epithelium 

1. Brush Border Columnar epithelium: When microvilli are present on the free end of columnar epithelium. e.g. Gallbladder

2. Glandular columnar epithelium : Unicellular mucous-secreting goblet cells are also present in between columnar cells. e.g. Stomach, Colon, Rectum and Uterus 

3. Glandular brush border columnar epithelium : When microvilli are present on the free end of columnar cells and in between these cells, goblet cells are also present. e.g. Duodenum, jejunum and Ileum 

4. Simple columnar ciliated epithelium : When cilia are present on the free end of columnar cells. e.g. Fallopian tube, Ependymal epithelium of brain ventricles.

5. Simple columnar stereo ciliated epithelium : When stereocilia are present on the free end of columnar cells. e.g. vas deferens, epididymis, internal ear

Pseudostratified Epithelium

• These cells are pillar-like in shape, so they are also modified to a columnar epithelium.
• In this epithelium, two types of cells are present: i.e. Long and Short cells.
• The nucleus in both cells is present on different levels, so it appears bilayered because few cells are too short to reach the top surface.
• However, all cells are on a single basement membrane, so it is unilayered.

Pseudostratified Epithelium Columnar Ciliated Glandular Epithelium

• Cilia are present on the free end of long cells in this epithelium, and mucus-secreting cells are also present in this epithelium.
• e.g., nasopharynx, trachea, bronchus, bronchioles, and respiratory epithelium of nasal chambers.

7.0Compound Epithelial Tissue and Function

• It is multi-layered and has a limited role in secretion and absorption.
• It protects against chemical & mechanical stress. 

Transitional Epithelium (Urothelium)

• In this epithelium, the basement membrane becomes invisible during stretching.
• In this epithelium, 4-6 layers of cells are present. The innermost layer is composed of cube-like cells.
• The middle 2-4 layers are composed of pear-shaped/Polyhedral cells.
• The outermost 1 or 2 layers are of oval-shaped cells.
• This different shape of cells appears only in the resting stage. When this tissue is stretched, all the cells become flattened.
• Cells are interconnected by interdigitation.
• Stretchable & waterproof.
• e.g., Renal pelvis, Ureter, Urinary bladder, Proximal part of the male urethra.

Non Stretchable Epithelium

On the basis of the shape of the cells of the outermost layer it is of three types.

• Stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium 

Stratified Squamous Epithelium

• The innermost layer of cells is of cuboidal or columnar shape.
• They divide to form cells of the epithelium's outer layer, which is called the Germinativum layer.
• The intermediate layers consist of polygonal-shaped cells interconnected through desmosomes that contribute to their mechanical stability and support. The cells in the outermost layer are flat and resemble scales. Depending on whether the outermost cells contain the keratin protein, the epithelium can be categorized into two types.

Stratified Cuboidal Epithelium 

• In this epithelium, 4-6 layers of cells are present. The innermost cells are composed of cube-like cells.
• The middle 2-4 layers are composed of pear-shaped/Polyhedral cells.
• Cells of the outermost layer are cube-like, nucleated and living.

Stratified Columnar Epithelium

• In this epithelium, 4-6 layers of cells are present. The innermost layer is composed of cube-like cells.
• The middle 2-4 layers are composed of pear-shaped/Polyhedral cells.
• The cell of the outermost layer is columnar, nucleated and living.

8.0Glands

• A cell or a group of cells which secretes chemical substances are called glands.
• All glands are composed of epithelial tissue.
• Glands can originate from all the three germinal layers.
•  Ectoderm – Sweat gland
• Mesoderm – Gonads
• Endoderm – Thyroid gland
• Some of the columnar or cuboidal cells get specialized for secretion and are called glandular epithelium (Figure). They are mainly of two types: unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal), and multicellular, consisting of clusters of cells (salivary gland).

On the basis of the nature of secretion :- 3 types of glands are

Endocrine glands :- Secretory duct absent.

e.g. Thyroid, pituitary, thymus, pineal, parathyroid, adrenal

Exocrine gland :- Secretory duct present 

E.g. Liver (largest gland), Salivary gland, Gastric glands, intestinal glands etc. Exocrine glands secrete mucus (Goblet cells), saliva (Salivary glands), earwax (Ceruminous glands), oil (Sebaceous glands), milk (Mammary glands), digestive enzymes (Gastric glands, Intestinal glands) and other cell products.

Heterocrine/mixed gland :- Both endocrine  & exocrine parts are present. 

e.g. Pancreas, Gonads etc.