Epithelial Tissue

Every cell in our body is specialized to perform a certain function. Similar kinds of cells form a tissue, which is classified based on its structure and functions. The human body has four main types of tissue: epithelial, connective, neural, and muscular.

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity. It is made up of cells that are closely packed together and joined by cell junctions. Epithelial tissue serves several functions, including protection, secretion, absorption, excretion, filtration, and sensory reception.

Epithelial tissue or epithelium is the outer covering of the skin and also lines the body cavity. It is found in the lining of the respiratory, digestive, reproductive and excretory tracts and performs functions such as absorption, protection, sensation and secretion.

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Structure of Epithelial Tissue

Epithelial tissue is composed of a continuous layer of tightly packed cells, with one surface exposed to either the external environment or body fluid, and the other surface attached to other tissue by a membrane composed of fibres and polysaccharides secreted by the epithelial cells.

There is minimal intercellular material between cells; however, specialized junctions are present between the cells of the epithelium that connect individual cells.

Tight junctions prevent leakage of substances across tissues.

Adhering Junctions: Keep the neighboring tissues firmly connected together.

Gap junctions facilitate the movement of ions and molecules across the tissue.

Epithelial cells are responsible for forming membranes. These membranes consist of a layer of epithelial tissue and have underlying connective tissue beneath them. There are two types of epithelial membranes, namely the mucous membrane and the serous membrane.

Mucous Membrane: Also referred to as mucosa, it contains Goblet Cells which secrete mucus. This mucus helps to lubricate, protect and enable the movement of materials. It also prevents tissues from drying and lines the body cavities such as respiratory and digestive tracts which open to the exterior of the body.

Serous Membrane: The serous membrane lines the body cavities that do not open to the exterior of the body, such as the pleural and pericardial cavities. This membrane is composed of simple squamous epithelium and produces the fluid within the cavity.

Glands are composed of epithelial cells, and can be divided into two categories: exocrine and endocrine. Exocrine glands release their products through a duct, such as goblet cells and sweat glands. Endocrine glands, also known as ductless glands, secrete their products directly into the blood or intestinal fluid, for example, hormones.

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Epithelial Tissue Functions

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity. It serves several important functions, including protection, secretion, absorption, excretion, filtration, and diffusion.

Protection: The body’s surface is completely covered by the skin, making it the first line of defense against mechanical trauma, chemical exposure, excessive fluid loss, and infections. Additionally, cilia, which are projections found in the nose and upper respiratory tract, capture dust particles and prevent them from entering the body.

Absorption: The epithelial lining of the digestive tract takes in water and nutrients.

Exchange of Substances: Epithelial tissue plays an important role in regulating the exchange of substances between the body and the external environment, as well as the exchange of substances between different parts of the body. All substances that enter the body or enter the bloodstream through absorption must first cross the epithelial barrier.

Sensation: Sensory receptors located in the epithelial tissue of the nose, eyes, ears, taste buds, etc. help to transmit signals from external stimuli to the brain.

Secretion: Various glands composed of epithelial cells release hormones, enzymes, saliva, mucus, sweat, and more.

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| MCQs on Epithelial Tissue |

| MCQs on Connective Tissue |

Types of Epithelial Tissue

There are three types of epithelial cells, each of which has a distinct shape and function.

Squamous Epithelium - They are thin and flat cells.

Cuboidal Epithelium – They are short cylindrical cells with hexagonal cross-sections.

Columnar Epithelium - They are cylindrical cells which are long or column-like in shape, and have a nucleus present at the base.

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Range of Epithelial Cells

On the basis of the number of layers present, epithelial tissue is divided into simple epithelium and stratified or compound epithelium.

Simple Epithelium: It is composed of a single layer of cells and typically serves either a secretory or absorptive function.

Compound (Stratified) Epithelium: It is composed of two or more layers of cells and typically serves a protective function.

The glandular epithelium is composed of cuboidal or columnar cells, which are specialized for secretion.

Unicellular: isolated glandular cells, e.g. goblet cells

Multicellular - a cluster of cells, e.g. salivary glands.

Epithelium that is columnar or cuboidal in shape and bears cilia is known as Ciliated Epithelium. This type of epithelium is particularly useful in the movement of mucus in a specific direction, such as in the fallopian tubes and bronchioles.

Types of Epithelial Tissue	Location	Structure	Function
Simple Squamous Epithelium	Lungs	Flat cells	Diffusion
Stratified Squamous Epithelium	Skin	Layered cells	Protection
Simple Cuboidal Epithelium	Kidneys	Cube-shaped cells	Secretion
Simple Columnar Epithelium	Stomach	Tall cells	Absorption
Pseudostratified Columnar Epithelium	Airways	Tall cells	Protection

| Simple Squamous | Blood vessel lining, air sac lining of lungs | A single layer of flat cells having irregular boundaries | Transport by diffusion and where minimal protection is required |

Simple Cuboidal Epithelium
The tubular lining of kidneys, glandular ducts
A single layer of short cylindrical cells. It may have microvilli as in proximal convoluted tubules
Function: Absorption and secretion

Type of Epithelium	Location	Description	Function
Simple Columnar Epithelium	Digestive tract and upper respiratory tract lining	A single layer of columnar cells (tall and slender) and often ciliated	Protection, absorption, mucus secretion and movement in a specific direction

| Stratified Squamous Epithelium | The lining of the mouth and vagina | Made up of several layers of cells, continuously sloughed off and regenerated. The older layer of cells is pushed upwards and becomes flat. The lower layer is columnar and metabolically active | Protection |

Stratified Cuboidal	Protection of Ducts of Various Glands
Mammary Glands	Sweat Glands
Salivary Glands	Upper Layer is Cuboid
	Other Layers May be Cuboidal or Other Types

| Stratified Columnar | Male Urethra and Lobar Ducts of Salivary Glands | Layer of Columnar Cells present on Squamous, Columnar, or Cuboidal Epithelial Cells | Protection and Secretion |

| Pseudostratified Columnar | Respiratory passage and ducts of many glands | Similar to columnar epithelium but all the cells are not of equal height | Protection, secretion and movement of mucous |

Type of Epithelia	Location	Description	Function
Transitional Epithelia (Urothelium)	Urinary Bladder, Urethra, Ureter	Stratified Epithelium which can contract or expand; Cells are cuboidal when not stretched, but when the organ stretches, cells become irregular and squamous-shaped	Stretch readily to accommodate different volumes of liquids

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